SHEAR WAVE VELOCITY AND ATTENUATION STRUCTURE BENEATH KUMAON-GARHWAL HIMALAYA: IT’S COMPARISON WITH ACTIVE TECTONICS LIKE GULF OF CALIFORNIA, MEXICO

Abstract

Damage produced during an earthquake, significantly influenced by the interaction of seismic wavefield with shallow subsurface structures, as well as by the attenuation characteristics of the subsurface material. In this thesis, the shear-wave velocity (𝑉𝑠) and attenuation (𝑄𝑠−1) structures have been derived for the Kumaon-Garhwal (KG) Himalayas, an area that is complex in terms of topography, tectonics, and seismicity. There are two parts to the thesis: the first investigates the comprehensive 2D near-surface 𝑉𝑠 structure beneath the KG region and the second investigates the three-dimensional (3D) shear-wave 𝑄𝑠 structure beneath the eastern Kumaon Himalaya, India and the south-central Gulf of California, Mexico. Major seismic events have occurred in and around the studied profiles, suggesting that the region is susceptible to future earthquakes. Therefore, this study is essential for preliminary seismic risk assessment, urban planning and the delineation of special study areas where additional geophysical and geo-engineering investigations are required before major developments. Moreover, to better understand the geodynamics of the two regions, the 3D shear-wave 𝑄𝑠 structure beneath the Kumaon Himalaya has been compared with the 𝑄𝑠 structure for the south-central Gulf of California, Mexico. First part of my work investigates the site-specific velocity-depth structure to develop the first 2D near-surface 𝑉𝑠 model for the KG region. In this section, the ambient noise survey has been conducted along the two profiles i.e., (1) the Rishikesh-Karnaprayag profile and (2) the Tanakpur-Dharchula profile. Field measurements for incessant microtremor recordings were accomplished at 454 locations in the studied region. The HVSR data at these sites were constrained by the 243 in-situ dispersion measurements (two-sided phase velocity curves) acquired along the profiles. The joint inversion results explored the 𝑉𝑠 characteristics down to ~1300 m depth. The reliability of modeled HVSR curves have been examined by comparing the observed HVSR data with theoretical data, generated using the modal summation approach. Furthermore, several numerical experiments have been performed to investigate the effect of input parameters on theoretical HVSR curves. Later, the interpolation of all 454 velocity-depth models with respect to position and elevation yielded the 2D shear-wave 𝑉𝑠 structures for the studied profiles. Analysis of 𝑉𝑠 structures revealed high 𝑉𝑠 values for the major thrust planes such as the Himalayan Frontal Thrust (HFT), Main Boundary Thrust (MBT), Ramgarh Thrust (RT), South Almora Thrust (SAT), Tons Thrust (TT), North Almora Thrust (NAT), Berinag Thrust (BT) and Askot Thrust (AT). The .... model for secondary structures such as the Singtali Thrust, Kaliyasaur fault, Rudraprayag anticline, and Almora Dudatoli syncline inferred significant velocity contrast compared to the host rocks. Most of the Siwalik Himalaya, the Almora nappe, and the Pithoragarh section showed an even distribution of .... values upto ~1300 m depth. Small variations in .... values correlate well with local structural features. The joint inversion results in the Garhwal Himalayan region have been validated by additional in-situ litholog investigations. The .... model at litholog sites effectively mapped the vertical rock boundaries within the upper 30 m depth. The 1D shear-wave .... values (260-680 m/s) for the upper 30 m, categorized the studied region into zone B and zone C type of soil in accordance with EUROCODE 8 provisions. Overall, this section provides a comprehensive 2D near-surface .... model for the KG Himalaya that could support future earthquake studies in the region. Second part of my work investigates the shear-wave attenuation (.....1) structure beneath the eastern Kumaon Himalaya, India and the south-central Gulf of California, Mexico. Although the two regions are geographically apart, both are young, highly active and have the potential to cause large earthquakes. For the Kumaon Himalayas, the strong motion accelerograms, recorded on a local network, have been utilized to compute the S-wave spectrum at ~ 0.097 to 8.0 Hz. The spectrums were then inverted by application of damped least square inversion to estimate the 3D shear-wave .... structure and source strength of the considered earthquakes. The obtained .... structures suggested that .... depth variations are remarkably formed in the studied area. A low .... anomaly has been imaged in the central part of the region that correlates well with the duplex structure present beneath the eastern Kumaon Himalaya. The structure extends upto a depth of ~18.0 km, showing significant variations in crustal structure with depth. Around the Main Central Thrust (MCT) and Vaikrita Thrust (VT), an abrupt increase in .... values have been mapped. The obtained .... values are regressed as ....(..)=(158 } 18) ..(0.84 } 0.12) for frequency range ~0.097-8.00 Hz. Additionally, the inverted source spectrum has been utilized to compute the source parameters based on Brune fs ..2 model. The estimated values of seismic moment (..0), stress drop (ƒ¢..), and source radius (..) correlate well with the forward computation of source parameters. The spatial distribution of earthquake hypocenters with their observed magnitude and stress drops on the heterogeneous .... structure remarkedly explains the inconsistency in the steady accumulation of stress by brittle shallow crust (. 20.0 km). For the south-central Gulf of California, Mexico, I have inverted 25 well-located earthquakes recorded at three broadband stations of the regional network RESBAN operated by CICESE (Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California) and three Ocean Bottom Seismographs (OBS) of the Sea of Cortez Ocean Bottom Array (SCOOBA) experiment. The 𝑄𝑠 values are obtained at various frequencies in ~ 0.16-8.0 Hz range. The fact that the crust is highly heterogeneous beneath the GoC, Mexico is also supported by the inversion results. For shallow depths (0-4.0 km), the low 𝑄𝑠 (~10) at 0.5 Hz correlates well with the sediments deposited in the Guyamas basin. As the sediment thickness decreased towards the Carmen basin, the inversion imaged an increase in 𝑄𝑠 values (320 and 800 at 1.0 and 2.0 Hz). The presence of igneous sills in the upper layer sediments explains the abrupt increase in 𝑄𝑠 values. For 4.0-12.0 km depth, there is an increase in 𝑄𝑠 values for the high-density continental crust, while the oceanic region shows a decrease in 𝑄𝑠 values. Thermal flow from the mantle accounts for the decrease in 𝑄𝑠 values in the bottom layer.