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Detection of seismic quiescences before 1991 Uttarkashi (MW 6.8) and 1999 Chamoli MW (6.6) earthquakes and its implications for stress change sensor

Chingtham Prasanta, Sharma Babita

Acta Geophysica

2022

Vol. 70, no 2

509--523

The statistically point process model known as epidemic-type aftershock sequence (ETAS) model is employed for systematically investigating the seismic quiescence or seismic anomalies around the focal regions of large/strong earthquakes for NW Himalaya. For this propose, the model predicted the expected occurrence rates of earthquakes by estimating the model parameters from the earthquake occurrences times using maximum likelihood method, has been used. Then the exhibited relative quiescence due to decreasing occurrence rates from the modeled ones can be identified by inspecting the abnormally downward deviated plot from the extended cumulative curve of the Residual Point Process (RPP) events. Examination of such RPP events in the whole time interval exhibits significant 1.5 years and 2.0 years of relative seismic quiescence before the strong 1991 Uttarkashi (MW 6.8) and 1999 Chamoli (MW 6.6) earthquakes, respectively. Considering the optimally oriented planes of Uttarkashi earthquake, the Coulomb stress changes (ΔCFS) have been investigated to check the rate of seismicity around the focal region of Chamoli earthquake. It has been found that ΔCFS of Uttarkashi earthquake exhibits stress shadow in or near the source zone of Chamoli earthquake and eventually decreases seismicity rates due to seismic quiescence in the source zone. On the other hand, the detected quiescence and activation relative to the predicted seismicity rate are consistent with the obtained Coulomb stress to depict the associated anomalies being sensitive enough to detect a slight stress change in the study region. Henceforth, the increased or decreased seismic activity due to seismic activation or quiescence is found to be consistent with the patterns of the Coulomb’s stress changes calculated on the ruptured fault planes of Uttarkashi earthquake. Hence, this ETAS model based on statistical technique can thus be incorporated with other sensitive geophysical instruments for identifying seismically quiet period not only in the seismic gaps, but also in its neighborhoods along the Himalayan range for mitigating seismic hazards due to impending great earthquakes.

Spatial distribution of high frequency spectral decay factor kappa (κ) for Delhi, India

Mittal Himanshu, Sharma Babita, Sandhu Manisha, Kumar Dinesh

Acta Geophysica

2021

Vol. 69, no. 6

2113--2127

The recorded strong motion data in the Delhi region provide an excellent opportunity to study high-frequency decay parameter, kappa (κ) for the National Capital (Delhi) region and to further understand its implications to study the site effects characterized by different stations within the vicinity of the study region. The kappa values are estimated at 30 locations from 99 accelerograms of 19 earthquakes recorded in the Delhi region and are found to vary from place to place depending upon the controlling parameters, primarily the site characterization. The estimated average values of ‘κ’ lie in the range 0.0118–0.0537 s for the various locations of the region depending upon the source, path, and site characteristics of earthquakes considered in the present study. The distance dependence is found insignificant, while there is a scatter in the variation of κ values with that of magnitude which indicates that κ is more related to the site characteristic for the entire Delhi region which in turn reveals the fact of the basic criterion of the κ parameter. To affirm the total attenuation on the instruments, the site effects demonstrate the behavior of amplification to the geological exposure. It has been found that the various sites under consideration for the study area amplify between 0.6 and 7.0 Hz predominant frequency ( fpeak) and agree with the geological arrangements of the region. Based on the present study, the most vulnerable areas are the northeastern region of Delhi which lies in proximity to the food plains of Yamuna river and alluvial deposits of younger origins of the foreland basin along with the southwestern part of Delhi capital which is comprised of the water-saturated alluvial deposits. The estimated ‘κ’ values are found to be correlated with those of the estimated site amplification and are useful in strong ground motions simulation for the proper evaluation of seismic hazard to build a seismic risk resilient society.