Summer Research Fellowship Programme of India's Science Academies

Recent seismicity and Moment Tensor Solution of earthquakes (M≥4) in the Garhwal Himalaya, India: To understand the seismotectonics

Hritika Deopa

Department of Geology, Banasthali Vdyapith, Rajasthan 304022

Dr. Sushil Kumar Rohella

Scientist 'F' & Group Head, Department of Geophysics, Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand 248001


The continuous convergence and collision of the Indian and Eurasian Plates is the main cause of earthquakes occurring along the 2400 km long Himalayan arc and has resulted in the formation of the Himalayan Seismic Belt. The seismic sources can be Volcanic, Tectonic and Oceanic or artificially induced, but 99% of the earthquakes in the Himalayan region are tectonically-induced. As we know, there are 7 major and 10 minor plates all around the world, which are like passive passengers of the underlying asthenosphere. This movement of Tectonic Plates leads to their convergence, divergence and slide-past motion, which finally results into earthquake in different regions due to release of strain energy. This energy is radiated in the form of Seismic-waves, which travel in all directions through Earth's interior and are recorded by sensitive instruments, called Seismometers, placed at or near Earth's surface. These Seismometers are established at different stations to continuously monitor the Seismic-waves, which record Earth's motion as a function of time. The Elastic Rebound Theory explains the occurrence of earthquakes due to release of strain when it exceeds the rock strength. During my training period, I will read-out the raw data of different phases of seismic waves and then locate different earthquakes and find their magnitude. After this, my focus will be to locate the Moment Tensor of recent earthquakes in Uttarakhand, India. It helps us to estimate the amount of stress, a fault plane can accommodate and further, we can prepare tectonic model for the same to better understand the seismicity in the area and to mitigate the seismic hazards. The present study uses two softwares, i.e, the SEISAN Seismic Analysis System, which is a complete set of programs and databases which analyse earthquakes from Analog and Digital Data. It is very helpful to pick the events, to locate epicentres and edit them, and to determine the moment tensor. Moreover, it is also used for Seismic- Hazard Calculation. Second software used is, the ISOLA GUI using MATLAB GUIDE Tool, it helps in Green Function preparation, Waveform Inversion, Moment Tensor Determination and associated Focal Mechanism.

Keywords: Himalayan Seismic Belt, Elastic Rebound Theory, fault-plane movement, Moment Tensor, Waveform Inversion, Focal Mechanism


AD Anno Domini
BBS Broadband Stations
HFT Himalayan Frontal Thrust
MBT Main Boundary Thrust
MCT Main Central Thrust
MFT Main Frontal Thrust
MT Moment Tensor
Mya Million years ago
STD's South Tibetan Detachment System



The movement of Tectonic Plates leads to their convergence, divergence and slide-past motion, which finally results into earthquake in different regions due to release of strain energy. This energy is radiated in the form of Seismic-waves, which travel in all directions through Earth's interior and are recorded by sensitive instruments, called Seismometers, placed at or near Earth's surface. The occurrence of earthquake is explained using the Elastic rebound theory, given by Reed in 1910. According to this theory, earthquake occurs in the region undergoing deformation. The strain energy gets accumulated in the region when deformation is happening and then, this strain energy is released when it exceeds the strength of the rock. Hence, it explains the occurrence of earthquakes as a result of faulting. So, later we can find out the type of fault giving rise to the particular earthquake. After studying the propagation of seismic waves it was concluded that the system of forces generation earthquakes are Single Coupled Forces. These forces generate the compressions and dilations in the orthogonal quadrants. The first motion can be either compression or dilation and this will be helpful to infer about the fault system that caused the earthquake. Therefore it is necessary to trace back the first p-wave motion to its hypothetical focal sphere. This focal sphere is a 3-dimensional sphere and is needed to be changed into 2-dimensional figure using projection system. The first P-wave motion can also be utilized to find the Fault Plane Solution or Moment Tensor Solution of any earthquake or rupture. The Fault Plane Solutions are always given along with the size and location of the earthquake. Most of the earthquakes which are occurring in the Himalayan Seismic Belt are thrust type, in which one of the fault planes is having dip less than 100 and are reverse type faults. The Fault Plane Solution gives the idea about geometry of the fault and the physics of the source of earthquake, hence, it is used to describe the forces acting on the fault.


During my training period, I will read-out the raw data of different phases of seismic waves and then locate different earthquakes and find their magnitude. After this, the objective of my study will be to make plots in order to compare synthetic and observer seismograms, Correlation vs. Depth plot and to locate the Moment Tensor Solution of recent earthquakes in Uttarakhand, India.


This study covers seismicity, aftershock distribution, M.T Inversion for important earthquakes of 2018. It helps us to estimate the amount of stress, a fault plane can accomodate and thus it will help in predicting the chances of rupture. This is important to understand more accurate tectonic regime of the study region. Further, we can prepare tectonic model for the same to better understand the seismicity in the area and in this way geologists and seismologists can together mitigate the seismic hazards.



According to USGS Earthquake Glossary, Seismology is the ‘study of earthquakes and the structure of the Earth, by both naturally and artificially generated seismic waves.’ The first functional seismoscope was invented by the Chinese scientist Chang Heng in 132 AD, it was made to register the arrival of seismic waves and also to find the direction of these coming waves (Lowrie, 2007). It is basically the study of seismic sources like earthquakes, waves produced by them and the properties of different media through which these waves travel (Agnew, 2002).

Local Seismology is the study of waveforms upto about 200 km and the p-waves and s-waves are mainly confined to Earth’s crust. Regional Seismology studies waveforms beyond ~200 km and upto 2000 km. At these distances, the first seismic arrivals travel through the upper mantle and Teleseismic or Distant Seismology studies the waveforms beyond 2000 km (Shearer, 2009).

Seismic Waves

The interior structure of the Earth cannot be studied directly, hence, Geologists make use of seismic (or earthquake) waves to determine the depth and nature of molten and semi-molten layers within Earth. Seismic waves behave differently in all states of matter.

When seismic energy is released suddenly at any point near the surface of a homogeneous medium, part of the energy propagates through the body of the medium as seismic body waves. While the remaining part of it spreads out over the surface as a seismic surface wave.

Body Waves are emitted earlier than the surface waves and hence their frequency is higher than that of surface waves. They are less destructive and are mainly of 2 types

  • P-wave: Also known as primary or longitudinal waves. These are the fastest travelling waves and are first to arrive at any seismic station. They behave same as the sound waves. The longitudinal (or compressional) body wave passes through a medium as a series of dilatations and compressions (Lowrie, 2007). P-wave can travel through solids as well as liquid medium. Subjected to a P wave, particles move in the same direction that the wave is moving in, which is the direction that the energy is traveling in, and is sometimes called the 'direction of wave propagation.
  • S-wave: Also known as secondary or shear waves. Their seismic velocity is less than that of p-waves and can only move through solids and not through any liquid or gaseous medium. It is this property of S waves that led seismologists to conclude that the Earth's outer core is a liquid.
    Graphical representation of propagation of body waves.

    Surface wave arrives later than the body waves and since they travel along the surface of the medium, they are most destructive and damaging waves. This damage is reduced in deeper earthquakes. They are also of 2 types

    • Love Wave: The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion.
    •  Rayleigh Wave: The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving.
      Graphical representation of propagation of surface waves.


      A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismograph.


        A seismograph, or seismometer, is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage.

        Seismic Velocity

        Seismic velocities depend on the properties of the material, like, composition, mineral phases, temperature and pressure of the media through which seismic waves passes. The velocity of the seismic waves is higher through denser materials and therefore travel more quickly with depth, while, hot areas slow down the seismic waves due to low density. It is used to assess the regional tectonics and earthquake hazards and provides evidence of the evolutionary model of the Himalaya (Parija et al., 2015). The fig.4 shows negligible S-wave velocity in the outer core because it is liquid, while in the solid inner core the S-wave velocity is non-zero.

          Depth v/s velocity graph for p-waves (brown) and s-waves (purple).

          Focal Mechanism or Fault Plane Solution

          The focal mechanism solutions of earthquakes are useful to study the nature of faulting processes operating in a seismically active area (Kumar et al., 2014). It is based on the direction of the first arriving p-waves. It is used to describe the deformation in the source region that generates the seismic waves. If the event is fault-related then, it is also known as a fault-plane solution. It gives the idea about the geometry and the source of earthquake. Fault plane solutions are always given along with the size and location of the earthquake.

          Seismic Moment Tensor

           It is used as a basic tool for studying earthquake sources and to completely describe the forces on the fault. It is calculated by using the amplitudes of the seismic waves (Vavrycuk, 2012) and the location of earthquake. It is used to determine the geometry of the fault that caused the seismic event. The ‘Beachball’ diagram, graphically represents the geometry of a moment tensor derived by seismologists (Cronin, 2010).

          STUDY AREA


          Before going to discuss the methodology and techniques, we need to know the geology and the tectonic divisions of the area. It will provide so informations for basic consideration and to build a good approximation to the results. Current study is of earthquakes (M≥4) occurred in 2018 along the Gharwal region of the Western Himalayan segment, NW Himalaya, India. Basically the Himalaya is an orogenic (geotectonic) system created by collision of two continental plates, Indian and Eurasian (Dewey and Bird, 1970; Dewey and Burke, 1973). As the Himalayan Belt is major global seismic belt where earthquakes of magnitude 4.5 to 5.5 can be recorded every year; over 600 earthquakes of magnitudes of 5 (M 5) or above were recorded during the period 1950-1990 (Thakur, 2001). So, there is always fresh data available to study and compare the results and improve our knowledge about the earth subsurface or to develop new theories. Therefore, it is important for each seismologist to study the Himalaya because it will introduces diverse geologic, seismic and tectonic processes related to mountain building.

          In this chapter we will mainly focus on the tectonic evolution and divisions of Himalaya and a brief review about the tectonics of Uttarakhand Himalaya, India. The state of Uttarakhand, divided as Kumaun and Garhwal, encompasses the central sector of the Himalayan arc. Gharwal region in brief including its tectonic evolution and geology. We will also focus on the recent local earthquakes and their impacts in the region.

          Tectonic Evolution of Himalaya

          The Himalaya is a classic example of an orogenic system created by continent–continent collision (Yin, 2006). Collision resulted in a vast elevated region of the Tibetan Plateau (Asian Plate) and the Himalayan Range (of Indian Plate affinity) (Coleman, 1996), formed due to the continuous convergence and collision between Indian and Eurasian Plate approximately 50 Mya. This started after the break-up of Supercontinent Pangaea, when India was formed as a part of Southern Supercontinent ‘Gondwanaland’ 200 Mya and the Northern Supercontinent was the ‘Laurasia’. A vast ocean called Tethys lay between these two continents. Between 200 and 130 Mya. The movement of tectonic plates led to the northward drift of Indian Continental Plate towards Asia and finally led to the closing of the Tethys Ocean and initiation of upliftment of Himalayas and formation of various structures viz., Folds, Faults, Thrusts, Nappes etc. India continued its northward journey at the rate of some 10-15 cm per year (Thakur, 1992)

          As a result, the 2500-km long and 300-km wide Himalayan arc forms the southern margin of the India-Eurasia collision zone and the Tibetan Plateau. These Himalayan Mountain Range strikes WNW to ESE, bounded by 2 structural bends i.e. Nanga Parbat in NW and Namcha Barwa in NE. The Indian Plate is still moving northward by approximately 5 cm per year and most of the convergence of plates is accommodated within the Himalaya by causing movement on various thrusts and folds (Upreti, 1990). By 55 million years ago, the Tethys ocean gap was completely closed and around 50 million years ago, the Indian continent was completely lodged against the Asian continent resulting in the collision tectonics (Coward at al. 1985). Subduction resulted in the reduced spreading rates in the Indian Ocean (Le Forte, 1975). By that time the last remains of marine condition of the Tethys had disappeared. As the continental crust of India could not sink to the great depths needed to be consumed, resistance developed against the northward movement of India. So, the rate of northward movement of India decreased from 15 cm to 5 cm per year (Thakur, 1992).

          Due to such geodynamic activity, the Himalayan terrain is under lateral compression from south and southwest, resulting in continuous deformation of the rock in the form of folding, faulting, fracturing, shearing, metamorphism and igneous activities.

          Tectonic Division of the Himalaya

          Himalaya is full of diversity in geological terms, and is very complex to understand. Many workers tried to justify the classification of the Himalayas based on it’s politically, geographically, structurally, and stratigraphically correlated combinations. Himalayan mountain ranges are interchangeable (LeFort, 1975, 1996). Heim and Gansser (1939) worked in Kumaun region of NW India and divided the Himalaya into four east trending geographic belts that correspond exactly to four geologic domains which are continues zones along the entire orogen (Gansser, 1964; LeFort, 1975).

            Regional geologic map of the Himalayan orogen. Main sources are from Liu (1988), Frank et al. (1995), Fuchs and Linner (1995); Yin and Harrison (2000), Ding et al. (2001), DiPietro and Pogue (2004). All map symbols are defined below the map(after Yin, 2005).(Blue rectangle shows the study region).

            Concept of Himalayan divisions

            Based on the position and the geological characters of the regions, the entire Himalayan arc is divided into four basic subdivisions named as: the Tethyan Himalaya (marine, fossiliferous strata), the Higher Himalaya (or the Greater Himalaya), the Lower Himalaya or the Lesser Himalaya (non-fossiliferous low-grade metamorphic rocks) and Sub-Himalaya or Outer Himalaya or the Siwaliks (Tertiary strata). These blocks are separated by the faults which are the major faults (thrusts) of the Himalaya, they are named as:

            • the STD’s: It is a system or series of Low Angle Normal Faults (Dawn and Grujic, 2012) that separates the Tethys Himalaya from the Higher Himalaya,
            • the MCT: It constitutes the real boundary between the Lesser and Great Himalaya (Valdiya, 1979),
            • the MBT: It forms the boundary between Lesser and Outer Himalayas (Mugnier et al., 1994),
            • the HFT (or the MFT): It represents a discontinuous zone of active faulting between the Sub-Himalaya and the alluvial plain.

            Geographic division

            Along Strike, the Himalayan Orogen can be divided into the Western, Central, and Eastern segments. The western segment of the Himalaya covers following regions: Salt Range in the Punjab province of Pakistan, Zanskar Mountain range in Jammu and Kashmir, Lahul, Spiti and Chamba in Himachal Pradesh , Garhwal and Kumaun Himalayan Range in Uttarakhand. The central Himalayan segment occupies Nepal, Sikkim, and South Central Tibet, whereas the eastern Himalayan segment includes Bhutan, Arunachal Pradesh of NE India, and southeastern Tibet.

            Stratigraphic division

            The major lithologic units in the Himalayan orogen consist of the Neogene Siwalik Group, the Proterozoic Lesser Himalayan Sequence (LHS), the Proterozoic–Ordovician Greater Himalayan Crystalline Complex (GHC), and the Proterozoic to Eocene Tethyan Himalayan Sequence (THS) (LeFort, 1996).

            Structural division

            Geologically, the Himalayan fold thrust belt is bounded by the Indus-Tsangpo Suture to the north and the Main Frontal Thrust (MFT) to its south (Fort, 1975). The Himalayan fold thrust belt consists of several south-vergent thrust sheets and related folds. Since the collision, continued convergence has been accommodated along major fault zones, such as the South Tibetan Detachment (STD), Main Central Thrust (MCT), Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT) as its most southern continuation (Theide et al., 2015). Beside these Thrust Fault Zones, the terrain is also affected by normal as well as strike-slips faults. The continuous movement of the tectonic plates build-up stress in the rocks that gets accumulated and whenever this stress is released, earthquakes occur. Therefore, the Himalayan Belt is an earthquake prone belt.

            Temporal division

            On the basis of its evolution, there are two divisions of the Himalaya, the Eohimalayan Event that occurred during the middle Eocene to Oligocene (45–25 Mya) and the Neohimalayan Event that occurred since the early Miocene (LeFort, 1996). This division is based on the different phases of metamorphism associated with different regions of the belt.

            Local Geology of Uttarakhand

            Uttarakhand State lies between Nepal to the east and Himanchal Pradesh to the west and encompasses Garhwal and Kumaun Himalaya. Yamuna, Ganga, Ramganga, and Goriganga in Uttarakhand and Kali between Nepal and Uttarakhand are the major Himalayan Rivers. The Uttarakhand Himalaya includes a 320 km stretch of the mountain between the Kali River, forming the Indo-Nepal Border in the east and Tons-Pobar Valley from the eastern border of the Himanchal Pradesh in the west. The major seismic activity in India is concentrated along the seismo-tectonically active Himalayan arc and the state of Uttarakhand lies right in the region between epicenter of two great earthquake, namely the Kangra Earthquake of 1905 and the Bihar-Nepal Earthquake of 1934. The understanding of active faults and earthquakes in the region is relevant and important for constructing the safer earthquake-resistant infrastructure projects. The geological framework of the area is complex, in fact, the 3 main Himalayan geological zones (Sub-Himalaya, Lesser Himalaya and Central Himalayan Crystalline Complex) include a large variety of geological formations.

            NW Himalayan Region (Study Area)

            To understand earthquake source processes, mitigation and reduction of seismic hazard, the Geophysics Division of Wadia Institute of Himalayan Geology, Dehradun, India has been operating a regional seismic network in NW Himalaya since 1985. Presently, this seismic network has 55 BBS and 8 short period seismic stations. 12 BBS stations of this network are connected through VSAT to monitor regional seismicity in real time mode at central station Dehradun. The present study includes areas of North Indian states; Uttarakhand and Himachal Pradesh. There are 7 BBS stations in these two states that has been used in the study. Here is the list of stations and their geographical locations:

            : Seven BBS Stations used in the study.
            Sr. No. Station Name Latitude Longitude
            1.         Bhatwari (BHTW) 30°49.20’N 78°36.60’E
            2.         Garurganga (GRGA) 30°27.60’N 79°26.40’E
            3.         Munsiari (MUNS) 30°03.10’N 80°14.16’E
            4.         Naddi (NADI) 32°15.00’N 76°18.09’E
            5.         Pithoragarh (PITG) 29°34.38’N 80°17.16’E
            6.         Rewalsar (RWSR) 31°37.98’N 76°48.84’E
            7.         Tehri (THRI) 30°22.20’N 78°25.80’E

            This area was hit by two destructive moderate to great earthquakes since the beginning of the 20th century: The Uttarkashi earthquake, 1991 (Mb 6.5) and the Chamoli earthquake, 1999 (Mb 6.4) (Parija et al., 2018).

              Satellite Image of BBS and SMA stations of Uttarakhand and Himanchal Pradesh (yellow-used in this study).

              There are number of thrusts in the region of study, namely Ramgarh thrust, North Almora thrust, Tons thrust and Munsiari thrust; as the region was tectonically active in the past and there was continuous rupturing and folding occurring in the region so most of these minor thrusts are folded, these rupturing and folding occurred during the collision times.

              Geological Setup

               North Almora thrust (NAT)/Tons thrust (TT) divides lesser Himalaya region into Inner and Outer Lesser Himalayan sequences). The Inner Lesser Himalaya is more proximal to the south of the Indian shield than the Outer Lesser Himalaya (Ahmad et al., 2000). In the inner (northern) belt of the Lesser Himalaya, Proterozoic sediments of Damtha and Tejam groups is exposed at the vast window. The Outer Lesser Himalaya mostly consists of the Jaunsar and Mussoorie group (Valdiya 1980). Our Area of study is in the Inner Lesser Himalaya which includes Almora Group, Ramgarh Group, Sirmur Group, Mussoorie Group, Jaunsar Group, Tejam Group and Damtha Group.

              FILE OPERATION


              The SEISAN is a software used to analyze and process earthquake from analog and digital data. It is developed into a toolbox which is being used widely by many researchers, local seismic as well as volcanic observatories throughout the world. With SEISAN, it is possible to view waveforms, pick the phases from 3-component stations, locate and edit several events, and to obtain several parameters like, magnitude, depth, moment tensor, fault-plane solution and source parameters of the event (Ottemoller, 19).

              File Operation

              It is assumed that SEISAN is installed under C:\Seismo, readings and other parameters are in S-files under test database TEST in the directory named C:\Seismo\REA, waveform files are in C:\seismo\WAV, calibration files are under C:\Seismo\CAL and other parameter files under C:\Seismo\DAT (Havskov, 14).

              To plot the waveform files

              • Open command prompt.
              • Type the name of local disk folder whether it is C: or E: or D: and press enter.
              • Then type cd, give space and type the address of the particular file which have the data of the event and press enter.
              • Then type dirf *.* and press enter, some prompts will appear.
              • Then type mulplt and press enter and.
              • Give your choice and type conts for Large SEED volume and press enter.
              • Then type 01 to open the first SEED file and press enter.
              • Type the name of local disk folder whether it is C: or E: or D: and press enter.
              • Then type cd, give space and type the address of the particular file which have the data of the event and press enter.
              • Then type dirf *.* and press enter, some prompts will appear.
              • Then type mulplt and press enter and.
              • Give your choice and type conts for Large SEED volume and press enter.
              • Then type 01 to open the first SEED file and press enter.
              • Station file opens, give start time in the format yyyymmddhhmmss and press enter.
              • Then give any interval time for the window, e.g. 20 (here) and press enter.
              • Station file opens, give start time in the format yyyymmddhhmmss and press enter.
              • Then give any interval time for the window, e.g. 20 (here) and press enter.
              • Give the plot option as Multi trace plot on screen, def; for this, type 0 and press enter.
              • At last, give the low and high cut for filter, 1 5 is enough for most of the plot and press enter.
              • Waveform file window opens (fig. 10).
                Giving plotting and filter details (©Command Prompt).

                How to ZOOM

                To Zoom In, put cursor and left click the mouse button among the traces, at position from where you want the start of zoom and then click at the end position of zoom.

                To Zoom Out, do the opposite.

                Filter the Traces

                Use of different filters will eliminate the noise of certain frequency levels. On the menu, select any filter (e.g. 1-5) or use the corresponding keys (v) on the keyboard. Press Plot or r and the filtered plot appears. Plot again to remove the filters. (fig.10;blue)

                Select Stations on Plot

                Select the stations by clicking on any station name and then click on Plot. The plot with selected station will follow. A range of stations can also be selected by clicking on those station names (fig.10;yellow) and then click Plot.

                Plot Components for Selected Station

                Select any station as said above, press y and then the plot with all 3-components will appear. To return to multi-trace plot, press y or t.

                Picking an Event

                Click left mouse button at position just before the event, then, click again at point where all event waves die-out. Now, click Out O on the menu (fig.10;black) or press O to OUT the earthquake event correctly, so that its tail is visible. This event gets saved as the NSN-file.

                  Waveform Window along with an event (©SEISAN 10.3).

                  Phase Picking

                  For this, P-phase is usually best recorded in Z-component of the stations, hence, picked from there. While S-phase can be picked from any of the two components.

                  P-phase picking

                  To make it easy, plot only Z-traces of the stations. Now move the cursor to the start of p-wave and press 1 or 2 and the phase will appear on the plot. The phase gets marked with IP or EP respectively (fig.11).

                  S-phase picking

                  S-phase of the wave is best visible on its vertical components (i.e. N or E). So, pick S in the similar way and press 7 or 8 and the phase will appear on the plot. It then gets marked with IS or ES respectively (fig.11).

                  Repeat the steps to pick P and S phases from all stations and calculate Time-lag [(S wave start time)-(P wave start time)] for station where waves appeared first.

                  Picking Amplitudes

                  Place the cursor at top or bottom extremities of a wave having greater amplitude and press a. Again press a after placing cursor at other extreme. Cross-mark appears at points where extreme is marked. Amplitude reading window will appear, choose AML and then confirmation dialog box appears, press y and then enter. ‘Red-colored, arrow-like' marking will appear in the window (fig.11).

                  Registering an Event

                  Most important thing before registering any event is to create a database by giving the Operator Code name (here;WIHG) and also the Database name (here;SRFP) such that, both should have 2-4 letters. Now, open the Waveform file window and choose Regis from the menu (fig.10) or press p. From the time lag calculated in previous section, identify the event as Local (L), REGIONAL (R) or TELESEISMIC or DISTANT (D);

                  : Type of event on the basis of time-lag.
                  Difference between arrival time of p-and s-wave Type
                  ts-tp <30 sec L
                  ts-tp 30-100 sec R
                  ts-tp >100 sec D

                  Enter the abbreviation L or R or D while registering and press enter. Make all the entries including the operator code (WIHG) and the database name (SRFP). Now, confirmation tab will open, press y and then enter. In this way, particular event will get registered. Register all the events of whole month by repeating the same steps.

                  Locating an Event

                  After picking up the P and S-phases, marking the amplitude and registering an event, we move forward towards its location. Command po is used to open the waveform window of any event, from where, we will choose Locat from the menu or press l. This function will open the COMMAND PROMPT window (fig.12), which will be showing the latitude, longitude (fig.12; blue box), focal depth (fig.12; white box), error (or rms value) (fig.12; orange). After locating the event, we will reduce the rms value as much as possible (less than 1) and then give the command update and then give operator code name; wihg and update that event by pressing y and enter. This event will be saved and will summarize all the necessary details of the event in single line (fig.12; white box).

                    cmd window showing event location (blue), magnitude (green), depth (white) (©Command Prompt).

                    Counting Registered Events

                    This step will provide access to all the registered events. Open COMMAND PROMT window and type in the format; ‘eev yyyymm database name’, (e.g. eev 201806 srfp) for counting all the registered events of June, 2018. Each event can be accessed by pressing enter after every step. Particular event can be plotted using the command po.

                    After this, an excel-sheet can be prepared to summarize all the events; this can be done by giving different column-names, such as: Year, Month, Number of Local Events, Number of Regional Events, Number of Distant Events, Total number of events, etc.

                    DATA AND METHODOLOGY


                    The ISOLA(ISOLated Asperities) is a software package developed by Sokos and Zahradnik (2008). It uses the Matlab GUIDE tool and its inversion engine is written in FORTRAN, both of which are user-friendly. It helps in data processing along with Green function preparation, Waveform Inversion and plotting the result. In addition to this, few utilities are also there that helps to Inspect the data-quality using different filters, to Create folder structure and to Backup files. The Create folder structure option creates the necessary folders needed for ISOLA. It is applicable for local and regional events and uses the instrumentally corrected velocity records. The code transforms velocity into displacement, inverts the displacement and provides synthetic displacement (Sokos, 2006).

                    Running the ISOLA GUI

                    Following are the steps to run ISOLA using the Matlab GUIDE tool. If any of the steps given below are not followed, the program will show error and hence provide undesired results. Before running the ISOLA, few steps are taken into consideration for the proper handling of data. It is applicable for only those events which have already been picked up using the SEISAN Software. First step involves the conversion of NSN files into SAC files giving the input: ‘WAVETOOL’ in Command Prompt Window. The waveform files for all stations are converted into the SAC files and then transferred into the root folder in which ISOLA is working.

                    Use of Matlab command

                    Open Matlab command window, change directory using the command cd and then enter the root folder name by providing single space. Go to the folder where data is kept, type isola and press enter. This function will open the ISOLA GUI window as shown in Fig.13. Main features include Utilities, Crustal Model, Input Data, Inversion and Tools. This provides very easy handling of the data and a good platform to plot results.

                      GUI of ISOLA Source Code (©ISOLA).


                      • Create Pole Zero Files: This creates the pole and zero-files (pz-files) after entering the number of poles and number of zeroes individually for every station. The pz-file of one particular station is same for all 3-components. These files are saved as .pz file.
                      • Import Data: Data is imported in SAC and GCF (Guralp Compressed Format) by reading the files component by component (N,E and Z) and then save the file after importing data for all the stations present. These get saves as unc files in folder named DATA.
                      • Calculate SNR: This option helps in calculating the signal-to-noise ratio. We need to load the unc file and then pick p-arrival of the waves. Similar steps are repeated for each station and then it is saved as snr files.
                      • Inspect data: This is used to inspect whether the Sampling Frequency is common for all stations or not.
                      • Try Filters: First, we need to load the unc-file for single station and different filters are given. Frequency range, which gives clear picture of the data, is used; (commonly 0.06-0.08).

                       Crustal model

                      First step in the inversion process is to Define Crustal Model. After pressing this option, a window will appear (Fig.14); where we will LOAD the Crustal data (crustal.dat) for different layers of stations. Data can be given for maximum of 15 layers, where, for each layers user has to give the depth of the layer, Vp ,Vs (in km/s), Density (in g/cm3), Qp, Qs and a Crustal Model Title (here; test). In this study for the Gharwal region, the velocity model that I used is from Parija et. al 2015. Its values are shown below in fig.14 and has data for 7 layers. The SAVE option will save the data under Green and Polarity folders (as crustal.dat file) in a format that is suitable for the ISOLA Fortran and PLOT option will plot the graphs of Vp and Vs versus Depth. The automatic calculation of Density is also possible.

                        The Crustal model definition form (©ISOLA).

                         Event Info

                        After finishing with the crustal model, information regarding the event is given by pressing Event Info option. Here, we need to define all the information like Date and Time of the event, Latitude and Longitude of the epicenter, Depth, Magnitude and its Location agency (here; USGS). The window shows the data for earthquake that occurred 45km E of Uttarkashi in Uttarakhand, India on Dated: 06/06/2018, Timed: 17:41:24 UTC, Magnitude: 4.5 at a Depth of 10 km and the time window for wave form to be selected around 245.76 seconds so that the program can take all the useful information starting from the p-arrival. After setting up all the required parameters, we save the information and proceed for the next step.

                          The Event Info Window (©ISOLA).

                          Station selection

                           Next step is the selection of stations that will be used for the inversion. Here, we have to give the path to the file having all the merged station data for the event by choosing BROWSE and then click on “Make Map-Select Stations”. New window will appear that shows the map for all the stations required. In case of this event we have chosen 6 stations for the best waveform recorded, we choose only those stations for the inversion of Moment Tensor for which S/N ratio is good enough so that the synthetic seismogram and the observed seismogram has the minimum R.M.S. and high variance reduction factor.

                            Selection of stations on map (©ISOLA).
                              Selection of stations on map (©ISOLA).

                              Data preparation

                              In this section we will Load Ascii file from the DATA folder that was created initially during the ‘Import Data’ process. When the file is read, the sampling frequency is displayed (Fig.17; in red) at the top of the form, side by side with the resampling frequency of the data (Fig.17; in blue). The resampling is made automatically, based on the ‘event info’ input from the previous steps. Then, we will enter the origin time of the event, apply Instrument Correction (Inst. Correction) to the data and then press “Origin Allign”, which will show the ‘Origin Alligned Displacement Data’. After doing the corrections we click “Save Data” and the file will be saved in DAT format. The GUI will automatically creates the proper filename and it must not be changed; (e.g for station BHTW, it will be BHTWraw.dat).

                                Waveform of BHTW station’s NS, EW and Z component after the Instrument correction and origin alignment (©ISOLA).

                                Seismic source definition

                                The user has to select single source or multiple source definition. In the case of single source (Sources below epicenter), user can vary the depth only, thus the options are Starting depth, Depth step and No of Sources, the explanation of which is shown in Fig. 18. Here, in this case, all the trial positions are below the epicenter and the depth search starts at 2 km and ends at 21 km (as it uses 20 sources and a depth step of 1 km for each source).

                                  Seismic Source Definition (a) The Single Source definition form (b) The Source Preparation (©ISOLA).

                                  Green function computation

                                  After defining the velocity model, source information and source definition we generate the Green’s function using the Green Function computation button, in which we defined the maximum frequency (fmax) of the signal to be 0.15 Hz. Any frequency higher than 1 Hz will not be accepted. Press Run and then the CMD prompt pop up in which the computation is running.

                                  After the Green Function calculation is finished and the files are copied in INVERT folder, we came out of the window of Green Function by pressing Yes. Until now we have just given the inputs and generated files to proceed for the Inversion process.

                                    cmd window appear to run the inversion (©Command Prompt).


                                    Before running the inversion, the user has to select the Type of Inversion (Deviatoric MT), Number of Subevents (1) to be retrieved, and the Time Function (Delta). The same filter is automatically used for the data and synthetics. Proper values of the above parameters depend on the specific problem e.g. on the event magnitude, on signal to noise ratio, on location uncertainty etc. Thus, the user has to select the best values for his problem. The user may press the Compute weights button which automatically calculates weights to be applied in the inversion. If not, then the unit weights will be used. The automatic weights are inversely proportional to peak values of ground displacement of the individual components in the used frequency band. In fact what are computed by the Compute weights function are just the peak values for the traces and latter isola.exe takes the 1/peak value as the weight. Finally, pressing Run, the main executable FORTRAN code isola.exe is running in the system’s Command window. Type n if you don’t want to see the process step by step. When finishing with the fist subevent retrieval, the code makes a pause and asks the user to accept the automatic selection of the trial source position (that of maximum correlation), or to select some other user’s preferred trial source position. To simplify the decision, the user can plot a correlation diagram using the Plot Correlation option (Fig. 21); GMT and GSview are needed for this plot. In this 2D diagram the correlation value and focal mechanism are plotted against the trial source position and time shift for the sub-event under study. Frequencies f1,f2,f3,f4 have specific value according to the S/N ration of the signal recorded by each individual station. These value can be given individually using ‘Select Station’s Freq. Band’ option or can be taken as average for all stations, the plot of S/N vs. Frequencies for each station is given below in Fig. 21.

                                      Invert Window (©ISOLA).

                                      RESULTS AND DISCUSSION


                                      In the previous chapter we have seen the method of finding the Fault Plane Solution using ISOLA. The Moment Tensor Solution is basically the result of analysis of waveforms recorded by the seismometers which were generated during any event. After the Inversion, there are some results to discuss and these results give some important characteristics of the event of this study. In Chapter 2 we have seen the major faults present in the area. These faults are active ever since we started to record the seismic activities in the region. In this chapter we are going to state comparison of synthetic and observer seismograms, Correlation plot of Moment Tensor Inversion, Correlation vs. Depth plot and the Moment Tensor Solution and we are going to discuss them by comparison with previous studies of the same region. 

                                      Inversion Results-Plotting

                                       There are a number of files after the inversion of the data; this is done by pressing ‘Plot Results’ button on main ISOLA form. When we plot the results then code calls the plotters.m and the comparison of the solution with first motion polarities. We have comparison between observed and the synthetic seismograms (Fig.22 and Fig.23), the Correlation vs Source number plot (Fig.24 and Fig.25), Correlation vs. DC% (Fig.26 and Fig.27). We are going to discuss them one by one. If we see the results one by one then it becomes easy in interpretation.

                                      Observed and Synthetic Seismograms

                                      Let’s consider the Fig. 22 and Fig. 23 in which there are plots of the synthetic and observed seismograms, legends shows their identity. Note that some of the seismograms are not active (BHTW-NS, EW, Z; GRGA- EW,Z; MUNS- EW,Z; PITH- EW,Z; RWSR- EW in Fig.22) it is because these seismograms with negative Variance Reduction values, which simply indicates that the matching of these two type of seismograms (Observed and synthetic) is not good enough to let them go for further inversion so at the starting stage of the inversion we have already removed them, we can remove them simply by unpicking the station components when in inversion window we were setting up their values for f1, f2, f3 and f4 frequencies. Remaining other station with their respective components are giving a very good match in positive range so that their matching is good enough. This is a very important illustration without the good match the correlation factor and the value of DC% comes very poor which simply affects the Centroid solution. So we need to choose the frequency band very carefully. While choosing the frequency band for any station’s components take into the account the epicentral distances from the priory information. If the distance between the station and source is great then only low frequencies will come but if the source is near then we have to consider the fact that the attenuation is less and signal consists of higher frequency band. Our frequency band is between 0.06 to 0.08 Hz. In selecting the range directivity also play an important role.

                                        Comparison of Observed and Synthetic seismograms of the stations used for inversion (Event date: 06-06-2018).
                                          Comparison of Observed and Synthetic seismograms of the stations used for inversion (Event date: 14-06-2018).   

                                          Correlation and Source Position

                                           In previous section we have discussed about the observed and synthetic seismogram matching, the easy way of showing the best fit between the synthetic and observed seismograms is the correlation factor, the value lies between zero to one. Grid search method will generate the possible number of Fault Plane Solutions with various correlation factor and DC% to update the depth. The best value (Red one) is chosen for the centriod solution. A correlation and depth plot is given in Fig.26 and Fig.27 which simply shows those 20 sources that we wanted to check after the inversion, here the maximum DC% and Maximum Correlation factor will guide us to distinguish between the results best for the selection. The variation can be seen in the curve, the peak of the curve is the result. We can also see the correlation vs. DC% in Fig.24 and Fig.25 in which the values for correlation factor and DC% for the best Fault Plane is taken for the curve (Blue one) with best space position and point with best time position.

                                            The Correlation vs. DC% plot (Event date: 06-06-2018).
                                              The Correlation vs. DC% plot (Event date: 14-06-2018).
                                                The Correlation vs. Depth plot (Event date: 06-06-2018).
                                                  The Correlation vs. Depth plot (Event date: 14-06-2018).

                                                  Moment Tensor Solution

                                                   We have the calculated Moment Tensor Solution with the respective parameters. Here we have Trial Source number, Centroid Latitude and Longitude, Centroid Depth, Centroid Time (Resptive to origin time). Except these values we have calculated Seismic moment in Nm (Newton meter), Moment magnitude (MW), VOL%, DC%, CLVD%, Variance reduction. We also have Strikes, Dips, Rakes for both nodal planes, parameters for both pressure and tension axis i.e. their azimuth and plunge angles. Moment Tensor Solution also shows the inputs (frequency band) that we set up for each station and the component of each station being used for inversion (marked + if used, marked – if not used). 


                                                  Three earthquakes of magnitude more than equal to 4 (M≥4) occurred in Garhwal region of Uttarakhand, India in 2018. Table 3 provides the detail of two earthquakes of June, 2018. Third earthquake of magnitude=4.5 occurred on 11 November, 2018, but due to lack of data we calculated the MT Solution of only two events. These are not very deep earthquake with depth of 10 km. Using the ISOLA GUI and Grid Search Method for inversion, we have calculated the Centroid Moment Tensor for the event.

                                                  Table 4 shows the detail of MT Solution calculated during the study including DC%, CLVD % and Variance Reduction. While Table 5 shows the computed parameters for the fault plane, i.e. the Strike, Dip, Rake as well as the Azimuth and Plunge for both pressure axis and Tension axis.

                                                  : Event Information (Source: USGS).
                                                  DATE TIME LOCATION MAGNITUDE (Mw) DEPTH (km)
                                                  LATITUDE LONGITUDE
                                                  06-06-2018 17:41:24 30.7553 78.9228 4.5 10
                                                  14-06-2018 00:42:07 30.8612 78.2815 4.4 10

                                                  Moment Tensor Solution (Calculated using ISOLA).
                                                  DATE MOMENT (Nm) MAGNITUDE (Mw) DC% CLVD% V.R.
                                                  06-06-2018 1.001e+012 4.4 65.3 34.7 0.9853
                                                  14-06-2018 2.125e+012 4.2 20.1 79.9 0.5696
                                                  Geometric Parameters of the Fault plane (Calculated using ISOLA).
                                                  NODAL PLANE1 NODAL PLANE2 Pressure (P) axis Tension (T)axis
                                                  STRIKE DIP RAKE STRIKE DIP RAKE AZIMUTH PLUNGE AZIMUTH PLUNGE
                                                  56 34 -66 207 60 -105 83 71 308 13
                                                  53 43 16 312 80 132 11 23 260 40


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                                                  External Links:




                                                  I would like to give my special thanks to Dr. Kalachand Sain, Director, Wadia Institute of Himalayan Geology, Dehradun for providing me adequate facilities during the period of training and also to Dr. Rashmi Sharma, Dean School of Earth Sciences, Banasthali Vidyapith, Rajasthan for granting me permission to undergo training at Wadia Institute of Himalayan Geology, Dehradun.

                                                  I would like to express my deepest sense of gratitude to my guide Dr. Sushil Kumar Rohella, Scientist-‘F’ and Group Head-Geophysics, Wadia Institute of Himalayan Geology, Dehradun for his valuable guidance, kind support and consistent encouragement during my present study. He managed time from his busy schedule and at every point provides right direction to my work. I thank him for the systematic guidance and great effort he put into training me in the scientific field. He tried his level best to make the study really worthy.

                                                  I am also very thankful to Dr. Mahesh P. Parija, Research Associate; National Geophysical Research Institute (NGRI), Hyderabad for his timely and scholarly advice that helped me to a very great extent to accomplish this task.

                                                  I would like to thank Mr. Anil and Ms. Vaishali; Research Scholars, for their valuable guidance, kind support and consistent encouragement during my present study and helped me in generating a good sense of the topic.

                                                  I offer my sincere thanks to Mr. H.C.Pandey, Senior Technical Officer, Mr. Rajeev Kumar and Mr. Manral of Geophysics group, for their help and cooperation during my internship.

                                                  I express my heartful thanks to my professors, who offered their continuous advice and encouragement throughout the course of this internship. I am thankful to Dr. Reshmi M.R., Dr. Mamta Chauhan, Mr. Amit K. Mishra and all the faculty members of the Department of Geology, Banasthali Vidyapith, Rajasthan.

                                                  I am extremely greatful to the Summer Research Fellowship Program (SRFP) for providing me with such a golden opportunity. It would have not been possible without this program. Also, the AuthorCafe platform is extremely user-friendly and helped me in compiling my report. 

                                                  Again, it is my parents and friends whose support and care have always helped and motivated me in accomplishing any task. However, thanks to them cannot be expressed easily.

                                                  APPENDIX 1

                                                  Velocity Model

                                                  The optimal 1D velocity model is achieved by inverting the earthquake epicentres along with earlier published velocity model of P and S wave for the study region. Results are based on the least square residual error and also make sure that all the results derived clearly matched with the previous information obtained from initial locations. This velocity model divides the 30 km crustal layer into seven layers with a velocity of 5.314 km/s for the topmost layer (up to 5 km) and of 5.391 km/ for the second layer (up to 10 km). There is a slight change from 5.391 km/s at 10 km to 5.392 km/s i.e. 0.01 km/s at 15 km depth that is significantly low but there is a major change in velocity from 5.392 km/s at 15 km to 5.964 km/s at 20 km depth suggesting that Conrad discontinuity is at 18 km depth within the crust i.e. a sharp velocity contrast thus marking the boundary between Sial and Sima. This generally matches with the tectonics of the area which suggests that crust has shallow seismic events that are mainly confined at upper crustal layer. The comparision of present model (Parija et al. 2016), Kumar et al. 2009 (green line) and Kamble et al., 1974 (blue line) is given in Fig.A and Fig. B and C gives the detailed parameters for Kumar et al., 2009 and present model (Parija et al. 2016).

                                                    Fig.A : The minimum 1D velocity model of seven layers (red line) obtained with VELEST from travel time inversion of P and S wave arrival times and its comparison plot with the preliminary velocity of Kumar et al. 2009 (green line) and Kamble et al., 1974 (blue line)(after Parija et al 2016).
                                                      Fig.B: The detailed parameters for Kumar et al., 2009.
                                                        Fig.C: Present Velocity Model (after Parija et al., 2016).

                                                        List of Figures

                                                        • Fig.1: Graphical representation of propagation of body waves.
                                                        • Fig.2: Graphical representation of propagation of surface waves.
                                                        • Fig.3: Seismometer
                                                        • Fig.4: Depth v/s velocity graph for p-waves (brown) and s-waves (purple).
                                                        • Fig.5: Regional geologic map of the Himalayan orogen. Main sources are from Liu (1988), Frank et al. (1995), Fuchs and Linner (1995); Yin and Harrison (2000), Ding et al. (2001), DiPietro and Pogue (2004). All map symbols are defined below the map(after Yin, 2005).(Blue rectangle shows the study region).
                                                        • Fig.6: Satellite Image of BBS and SMA stations of Uttarakhand and Himanchal Pradesh (yellow-used in this study).
                                                        • Fig.7: Initial steps to view waveform (©Command Prompt).
                                                        • Fig.8: To give details of the input year, month, date, time and interval (©Command Prompt).
                                                        • Fig.9: Giving plotting and filter details (©Command Prompt)
                                                        • Fig.10: Waveform Window along with an event (©SEISAN 10.3).
                                                        • Fig.11: Waveform window after picking amplitudes and marking p and s-waves (©SEISAN 10.3).
                                                        • Fig.12 : cmd window showing event location (blue), magnitude (green), depth (white) (©Command Prompt).
                                                        • Fig.13: GUI of ISOLA Source Code (©ISOLA).
                                                        • Fig.14: The Crustal model definition form (©ISOLA).
                                                        • Fig.15: The Event Info Window (©ISOLA).
                                                        • Fig.16: Selection of stations on map (©ISOLA).
                                                        • Fig.17: Waveform of BHTW station’s NS, EW and Z component after the Instrument correction and origin alignment (©ISOLA).
                                                        • Fig.18: Seismic Source Definition (a) The Single Source definition form (b) The Source Preparation (©ISOLA).
                                                        • Fig.19: The Green Function window (©ISOLA).
                                                        • Fig.20: cmd window appear to run the inversion (©Command Prompt).
                                                        • Fig.21: Invert Window (©ISOLA).
                                                        • Fig.22: Comparison of Observed and Synthetic seismograms of the stations used for inversion (Event date: 06-06-2018).
                                                        • Fig.23: Comparison of Observed and Synthetic seismograms of the stations used for inversion (Event date: 14-06-2018).
                                                        • Fig.24: The Correlation vs. DC% plot (Event date: 06-06-2018).
                                                        • Fig.25: The Correlation vs. DC% plot (Event date: 14-06-2018).
                                                        • Fig.26: The Correlation vs. Depth plot (Event date: 06-06-2018).
                                                        • Fig.27: The Correlation vs. Depth plot (Event date: 14-06-2018).
                                                        • Fig. A: The minimum 1D velocity model of seven layers (red line) obtained with VELEST from travel time inversion of P and S wave arrival times and its comparison plot with the preliminary velocity of Kumar et al. 2009 (green line) and Kamble et al., 1974 (blue line)(after Parija et al 2016).
                                                        • Fig. B: The detailed parameters for Kumar et al., 2009.
                                                        • Fig. C: Present Velocity Model (after Parija et al., 2016).

                                                        List of Tables

                                                        • Table 1: Seven BBS Stations used in the study.
                                                        • Table 2: Type of event on the basis of time-lag.
                                                        • Table 3: Event Information (Source: USGS).
                                                        • Table 4: Moment Tensor Solution (Calculated using ISOLA).
                                                        • Table 5: Geometric Parameters of the Fault (Calculated using ISOLA).        
                                                        Written, reviewed, revised, proofed and published with