Earthquake Source, Seismic Velocity Structure and Attenuation of the Jammu and Kashmir Himalaya

Powali, Debarchan (2019) Earthquake Source, Seismic Velocity Structure and Attenuation of the Jammu and Kashmir Himalaya. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Debarchan Powali (12RS019)) 12RS019.pdf - Submitted Version Restricted to Repository staff only Download (19MB)

Abstract

The Cenozoic continent–continent collision between the Indian plate and the Eurasian plate gave rise to the youngest mountain belt, the Himalayan mountains, and the largest orogenic plateau, The Tibetan Plateau. The Himalayan mountains form a ~2400 km long arc, which extends from Namcha Barwa in the east, to Nanga Parbat in the west. The collision is ongoing at present with ~50 mm yr⁻¹ convergence between India and Eurasia, of which ~18–20 mm yr⁻¹ is accommodated within the ~200 km wide Himalayan belt. This convergence results in underthrusting of the northern edge of the Indian plate beneath the Himalayan mountains and the Tibetan Plateau. The gently north-dipping Main Himalayan Thrust (MHT), which marks the top of the downgoing Indian plate, forms a basal detachment beneath the Himalayan mountains, and facilitates the underthrusting. Modeling of velocities, measured using Global Positioning System (GPS), across the Himalayan arc have shown that the shallow segment of the MHT is frictionally locked, while the deeper segment creeps aseismically. The transition from locked to creep on the MHT is termed as the unlocking zone or the locking line, which is marked by a belt of micro-seismicity and intermittent moderate-to-strong earthquakes (e.g. 1991 Uttarkashi, 1999 Chamoli, 2013 Kishtwar earthquakes). This zone is also conjectured to be the point of initiation of mega-thrust earthquakes (Mw > 7.5), which occasionally relieves the accumulated stress on the locked segment of the MHT. Over the past two centuries several mega-thrust earthquakes have ruptured the locked segment of the MHT, either partially (e.g. 1833 Nepal, 1905 Kangra, 2015 Gorkha earthquakes) or fully (e.g. 1934 Nepal, 1950 Assam, 2005 Kashmir earthquakes). There are several segments of the Himalayan arc which have not produced a mega-thrust earthquake in the past few centuries and are termed as 'seismic gaps'. GPS velocities reveal that these regions are accumulating elastic strain to be released in future major-to-great earthquakes. Most significant among these gaps are the Kashmir Himalayan seismic gap, straddling Jammu and Kashmir and Himachal Himalaya; central Himalayan seismic gap, in western Nepal; and the Sikkim-Bhutan Himalayan seismic gap. In order to quantify the seismic hazard associated with future mega-thrust earthquakes in these seismic gaps, it is important to understand the crustal structure, study the source properties of past major earthquakes to unravel the seismotectonics, and characterize the attenuation properties of the crust, to quantify the ground shaking from future earthquakes. Geophysical studies over the past few decades have unravelled the broad structure of the Tibetan Plateau and the central Nepal Himalaya. Such studies have also been conducted in the Indian Himalaya through 1-D seismological arrays across the Sikkim Himalaya, NW Himalaya, Uttaranchal Himalaya, and Garhwal-Kumaon Himalaya. These studies have provided 1-D crustal structure, Moho geometry and the geometry of the deeper segment of the MHT. However, little knowledge of the deep structure exist of the regions close to the two syntaxis of the Himalaya, e.g the Jammu and Kashmir Himalaya (west) and the Arunachal Himalaya (east). In order to address this gap in our knowledge in the Jammu and Kashmir Himalaya, we designed a broadband seismological field experiment across the state of Jammu and Kashmir. We envisaged to establish a 2-D array of seismograph systems across the various litho-tectonic units. Through multiple international collaborative funding/projects between IISER Kolkata, University of Cambridge UK and Shri Mata Vaishno Devi University (SMVDU), Katra, we have been able to deploy 24 seismograph systems starting from July 2013. The deployment was done in three phases and 18 stations of this network is presently operational. The details of the deployment and the performance of the stations have been documented in Chapter 2 of this thesis. From southwest to northeast our stations have been deployed across the Shiwalik Himalaya, Lesser Himalaya, Higher Himalaya and the Tethyan Himalaya between latitudes of 32.50°N and 34.25°N, and longitudes of 73.75°E and 76.25°E. The Shiwalik Himalayan stations span from the foothills of the Jammu-Kishtwar region to the Pir Panjal Ranges, in the west, and Himachal Himalaya, in the east. On the Lesser and Higher Himalaya we deployed stations upto the Kishtwar window to the north, and in the Pir Panjal Ranges to the east. For the Tethyan Himalaya we located our stations north of the Kashmir Valley on the Zanskar Ranges. Data from this network have been used to address the following: (i) crustal structure, geometry of the Moho and the MHT beneath the Jammu and Kashmir Himalaya; (ii) hypocentral location of small-to-moderate earthquakes and source mechanism of moderate-to-strong earthquakes; and (iii) seismic attenuation characteristics of the crust. In this region of our interest a major earthquake occurred in the recent past, the 2005 Kashmir earthquake (Mw 7.6). This earthquake broke the Muzzafarabad and Tanda Faults close to the western Himalayan syntaxis and produced a well documented surface rupture. A number of studies had been done on the mainshock rupture, but no comprehensive study is available using the aftershocks. While our seismograph network was being established in the Jammu and Kashmir Himalaya, I began my PhD work with source studies of the 2005 Kashmir mainshock and aftershocks, to develop a seismo-tectonic model (Chapter 3 of this thesis). I used teleseismic data from three global seismograph arrays to study the spatio-temporal evolution of the mainshock rupture using back-projection. My results confirm that the mainshock rupture was bilateral with an average rupture velocity of ~2 km s⁻¹. Source time function show a compact rupture with majority of the energy released within the first ~20 s. I used the rupture velocity from the back projection, to model the mainshock focal mechanism as a propagating line source and found that the earthquake occurred on an oblique thrust fault dipping ~35° to the NE. From the modeled seismic moment, average slip and width of the fault, computed from hypocentral depth and dip, I show that ~25 km of the mainshock rupture was blind, and extended beyond the western syntaxis of the MBT on the Indus-Koistan Seismic Zone (IKSZ). I modeled the aftershocks using centroid moment tensor inversion to unravel a back-thrust close to the hypocentral zone of the mainshock. All other aftershocks occurred to the NW of the mainshock surface rupture above the blind segment of the fault. I calculated the Coulomb failure stress due to the mainshock on optimally oriented aftershock planes to show that the moderate-to-strong aftershocks within a month of the mainshock, were triggered by the mainshock rupture. Combining all these information, I have proposed a seismo-tectonic model for the NW Himalayan syntaxis. I then used P-wave receiver function analysis on data from the broadband seismological field experiment in Jammu and Kashmir Himalaya to study the crustal structure (Chapter 4 of this thesis). Data from a total of 18 station across the Shiwalik Himalaya (11 stations), Lesser Himalaya (2 stations), Higher Himalaya (2 stations), Kishtwar Window (1 stations) and Tethyan Himalaya (2 stations) have been used for this study. Receiver functions have been computed at Gaussian width 2.5 to study the details of the crustal structure. I performed a depth-Vp/Vs domain stacking of the receiver functions in narrow bins of back-azimuth and distance range to obtain the average crustal thickness beneath each station, and also unravel the lateral variation in the crustal structure. Across the Jammu-Kishtwar Himalaya, I computed common conversion point (CCP) stack profile to highlight the geometry of the Moho, the MHT and the intra-crustal phases. Results from this study show that the underthrust Indian crust beneath the Jammu and Kashmir Himalaya has an average thickness of ~40 km and dips northward at ~7–9°. This is overlain by a northward thickening Himalayan wedge which is ~8–10 km thick in the Shiwalik Himalaya to ~25–30 km thick in the Tethyan Himalaya. The Moho is highlighted by the large positive impedance contrast boundary at a depth of ~45 km beneath the Shiwalik Himalaya deepening northward to ~65 km beneath the Higher Himalaya. Across the MKT, in the Shiwalik Himalaya, and beneath the Kishtwar window, the Moho flexes northward and deepens by ~10 km each time. Beneath the Lesser Himalaya the Moho has an up-warp of ~5 km. Beneath the Pir Panjal Ranges the Moho is remarkably flat at ~56 km. North of the Kashmir Valley the Moho dips steeply underneath the Zanskar Ranges from ~56 km to ~62 km. On the CCP the MHT is highlighted by the low velocity layer (LVL) at a depth of ~8 km beneath the Shiwalik Himalaya to ~25 km beneath the Higher Himalaya. The average dip on the MHT is ~9° and has a steeply NE dipping frontal ramp beneath the Kishtwar window. The MKT, MBT and MCT are marked by LVLs in the CCP and splays updip from the MHT. A comparison of the CCP by overlay plotting the source mechanism of the 2013 Kishtwar earthquake (Mw 5.7) and hypocentral location of small-to-moderate earthquake show that the seismicity is concentrated on or above the MHT, in the unlocking zone. Finally, I have used the local earthquake coda waves to estimate the temporal decay of amplitude as a function of lapse time at a range of frequencies from 1 Hz to 12 Hz. Using the slope of this decay, I have computed the quality factor (Q) at each frequency (f). Linear fitting of the logarithm of Q as a function of logarithm of f gives the Q at 1 Hz (referred to as Q₀) and the frequency dependence of Q (referred to as η). I have used a total of 1478 source-receiver paths for the Q₀ and η calculation, sampling the entire northwestern Himalaya. My results reveal maximum and minimum values of Q₀ to be ~69 and ~680, respectively, with an error weighted average of ~198. For η the maximum value is ~1.83 and the minimum value is ~0.41, with an error weighted average of ~1.02. Low average Q₀ and high η is indicative of tectonically active regions and can be explained by the large concentration of small-to-moderate earthquakes in the NW Himalaya. Future studies to regionalise the Q₀ and η across the Jammu and Kashmir Himalaya will be undertaken.

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