R. K. Chadha

Sedimentary Thickness Variations in the Indo‐Gangetic Foredeep from Inversion of Receiver Functions

Abstract We utilize receiver functions from ten broadband seismic stations deployed along a north–south profile traversing the Indo‐Gangetic plains in northern India to investigate the sedimentary thickness variations in this hitherto less‐studied foreland basin south of the Himalaya. Parameterization of the velocity structure adopting the neighborhood algorithm approach shows that the data can be satisfactorily modeled for low‐velocity sediments having shear‐wave velocities in the range of 0.72–2.5u2009u2009km/s with thicknesses varying from 0.5 to 3.7xa0km beneath the individual stations. The velocity–depth functions obtained in this study are important for earthquake‐hazard assessment of the densely populated urban centers spread over this region, in terms of predicting strong ground motions due to large earthquakes in the Himalaya.

Fractal and b-Value Mapping in Eastern Himalaya and Southern Tibet

Abstract Fractal (correlation) dimension and b -value are determined from ∼1300 well-located earthquakes recorded at 32 seismic stations in eastern Himalaya and southern Tibet during 1993–2003. The spatial correlation of 0.9 is indicative of more clustered events in the region, while a b -value of 1.02 implies a highly active seismic region. A detailed study of the frequency–magnitude distribution and fractal dimension as a function of depth is also made. The results suggest structural variability at different depth levels in the Tibet–Himalaya collision zone that reflects highly heterogeneous and high-grade metamorphism in the region.

Lg Attenuation Characteristics across the Indian Shield

Abstract New measurements of Lg Q are obtained using a reliable two-station method to understand the attenuation characteristics across the Indian subcontinent. We use data from 514 events with magnitude≥3.5 and depth less than 40xa0km collected during 2006–2008 from 17 stations deployed across India. We measure 1-Hz Lg Q ( Q 0 ) values between many pairs of stations. Finally, 34 high-quality interstation paths were selected from 336 possible pairs that are (1)xa0aligned approximately with the source and (2)xa0separated enough to permit the use of the standard two-station method for Lg Q measurement. We found some spatial variation in Lg wave attenuation across India. Low- Q regions are present in western India. We attribute this to the recent tectonic activities in this region. The central Indian platform shows a moderate to high Q 0 value. The southernmost part of India exhibits higher values, which may indicate that the crust in this part is more stable. The obtained values correlate with the regional crustal heterogeneities and tectonic trends. As a whole, the estimated interstation Q 0 values compare well with the areas of moderate seismicity across the Earth.

Analysis of Coseismic Water-Level Changes in the Wells in the Koyna–Warna Region, Western India

Abstract We test the hypothesis that coseismic water-level changes in wells are proportional to coseismic volumetric strain by analyzing available data from the Koyna–Warna region of western India. A total of 18 cases of water-level changes have been reported at ten wells corresponding to six earthquakes of M ≥4.3 that occurred in the region from 1997 to 2005. Out of these, clear unambiguous steplike coseismic water-level changes have been observed in ten cases at five confined wells. We used basic poroelastic theory to simulate volumetric strain and corresponding water-level changes and find that all cases show consistency in sign between reported coseismic water-level changes and simulated volumetric strain. All confined wells with high strain sensitivity that are located near earthquake epicenters show good agreement in magnitude between simulated and reported volumetric strain, thereby supporting this hypothesis.

Transient Changes in Well-Water Level in Bore Wells in Western India Due to the 2004 Mw 9.3 Sumatra Earthquake

Large water level changes were observed in six bore wells in the Koyna region in western India following the Great Sumatra earthquake of M wxa09.3 on 2004 December 26. This earthquake occurred at 00:58:50 coordinated universal time (UTC) off the coast of Sumatra located about 3000xa0km from the Koyna region. The arrival time of the P wave recorded at the Koyna seismic station was 01:04:24.45 (UTC). The anomalous water level changes were observed at 01:15 (UTC) at all of these wells where the sampling rate was 15xa0min. No abnormal changes were noticed in the preceding sampling time at 01:00 (UTC), clearly indicating that these changes occurred due to the passage of seismic waves generated by the Sumatra earthquake. However, due to the large sampling interval, it is difficult to correlate the water level changes either to shear or surface waves. No local earthquake was recorded by the network during this period. The anomalous water level changes were of the order of 2–65xa0cm and show either a spikelike or a steplike change. These types of changes are attributed to the dynamic strain induced by the passage of seismic waves, most probably long-period surface waves.

A Matlab based GUI Application in Hydro seismicity of the Koyna – Warna Region, India

Hydroseismicity was proposed to explain the occurrence of intraplate earthquakes in regions. Koyna - Warna region one of the reservoir triggered seismicity zone in India, which is an example of intraplate earthquake region. The seismicity is continuing after 45 years of impoundment of the Koyna reservoir in 1961 and 25 years of impoundment of Warna reservoir in 1985. The excess of previous maximum water level, the annual rise of water level in the reservoirs and filling rate of these reservoirs are some of the important factors in increasing the pore pressure. In this study, we used the daily changes in the water levels in both the reservoirs and calculated the pore pressure from starting of the impoundment of the reservoir up to 2008. We also studied the pore pressure changes with depth for various values of hydraulic diffusivity. An automatic formulation has been developed in Matlab for computing and plotting the pore pressure history.