Homogeneous earthquake catalogue for Northeast region of India using robust statistical approaches

Regular seismic hazard assessment requires essentially an updated and refined homogenous earthquake catalogue for the study region. Here, we have compiled the earthquake data for Northeast region of India in a chronological order from International Seismological Centre and Global Centroid Moment Tensor databases during the period 1 January 1900 to 31 April 2016. For this purpose, the regression techniques such as least square (SR), inverse least square (ISR), orthogonal (OR) and generalized orthogonal (GOR) which is the best one, out of that are employed for converting different types of magnitude scales, such as surface-wave magnitude (MS), body-wave magnitude (mb) and local magnitude (ML) into a single homogenized moment magnitude, MW. The homogenized catalogue is then treated with 'runs test' to estimate p-value of 0.8421 which suggest no spurious reporting on the catalogue. The prepared catalogue has also been declustered using standard procedure. Furthermore, the magnitude of completeness for space and time with 90% confidence level has been achieved. The seismicity parameters, namely magnitude of completeness MC, a-value and b-value are found to be 4.6, 7.50 and 0.95(±0.023), respectively. The observed low b-value implies that the study region is tectonically very active with the presence of asperity

Statistical analysis of aftershock sequences related with two major Nepal earthquakes: April 25, 2015, MW 7.8, and May 12, 2015, MW 7.2

Present study describes the statistical properties of aftershock sequences related with two major Nepal earthquakes (April 25, 2015, MW 7.8, and May 12, 2015, MW 7.2) and their correlations with the tectonics of Nepal Himalaya. The established empirical scaling laws such as the Gutenberg–Richter (GR) relation, the modified Omori law, and the fractal dimension for both the aftershock sequences of Nepal earthquakes have been investigated to assess the spatio-temporal characteristics of these sequences. For this purpose, the homogenized earthquake catalog in moment magnitude, MW is compiled from International Seismological Center (ISC) and Global Centroid Moment Tensor (GCMT) databases during the period from April 25 to October 31, 2015. The magnitude of completeness, MC, a and b-values of Gutenberg–Richter relationship for the first aftershock sequence are found to be 3.0, 4.74, 0.75 (±0.03) respectively whereas the MC, a and b-values of the same relationship for the second aftershock sequence are calculated to be 3.3, 5.46, 0.90 (±0.04) respectively. The observed low b-values for both the sequences, as compared to the global mean of 1.0 indicate the presence of high differential stress accumulations within the fractured rock mass of Nepal Himalaya. The calculated p-values of 1.01 ± 0.05 and 0.95 ± 0.04 respectively for both the aftershock sequences also imply that the aftershock sequence of first main-shock exhibits relatively faster temporal decay pattern than the aftershock sequence of second main-shock. The fractal dimensions, DC values of 1.84 ± 0.05 and 1.91 ± 0.05 respectively for both the aftershock sequences of Nepal earthquakes also reveal the clustering pattern of earthquakes and signifies that the aftershocks are scattered all around the two dimensional space of fractured fault systems of the Nepal region. The low b-value and low DC observed in the temporal variations of b-value and DC before the investigated earthquake (MW 7.2) suggest the presence of high-stress concentrations in the thrusting regimes of the Nepal region before the failure of faults. Moreover, the decrease of b-value with the corresponding decrease of DC observed in their temporal variations can primarily act as an indicator for possible prediction of major earthquakes in the study region

Characteristics of earthquake swarm activity observed in the Palghar region of Indian Peninsula from January 2019 to October 2020

The present study emphasizes the seismic characteristics of earthquake swarm activity of the Palghar region that occurred in the hard strata of Deccan traps of the Indian peninsula. This activity is well monitored by National Centre for Seismology (NCS) from December 2018 to October 2020. The related data inferred that initially before the swarm activation there was a barricade to stop the entry of fluids into the permeable zone. Later the fluids are injected into the fault zone at a feasible place may be near the surface at the time of swarm initiation. Further the associated fluids percolated through several channels in many directions forming a multi-dimensional system of cracks and fissures at the level of swarm progression through two distinctive clusters during the time of present study along normal fault orientations. During this process, the heterogeneous material within the subsurface layers might have experienced a decrease in the effective stress due to the increase in the pore pressure which is the resultant of the percolation of fluids within the permeable zone. The outcome of the results shows a comparable strategic increase in the seismicity after the soil saturation during and after the rainfall in the affected zone. Also, the study of the seismicity accompanying fluid injections indicates that the fluid triggering seismicity is demonstrated by the increased slope of the magnitude frequency distribution in the form of spatial b-value which has been found reliable for a catalogue for Palghar swarm activity. The coulomb stress change is also examined to find out the consistency of stress patterns with the earthquake occurrences in the Palghar region which gives the characteristics of swarm sequence. The study emphasized the fundamental and geo-mechanical behavior of fluid migration through multi-directional cracks and fissures as the cause for the evolution and further growth of the Palghar swarm activity

Forecasting seismicity rate in the north-west Himalaya using rate and state dependent friction law

<p>In this study, rate and state Coulomb stress transfer model is adopted to forecast the seismicity rate of earthquakes (<i>M_W ≥</i> 5) in the north-west Himalaya region within the testing period 2011–2013. Coulomb stress changes (ΔCFF), considered to be the most critical parameter in the model, exhibit stress increase in the whole study region, excluding the Chaman fault of the Kirthar range where significant stress shadow has been observed. The estimated background seismicity rate varies in the range 0.0–0.7 in the region, which is preoccupied by low aftershock duration of <50 years. Furthermore, a low <i>b-</i>value that varies between 0.54 and 0.83 is observed in Kirthar ranges, Karakoram fault and Pamir-Hindukush region. However, areas like Hazara syntaxis of the northern Pakistan and northern Pamir of the Eurasian plate exhibit higher <i>b-</i>values in the range 1.23–1.74. Considering constant constitutive properties of the faults (i.e. <i>A</i>σ = 0.05 MPa), our forecast model for variable ΔCFF and heterogeneous <i>b-</i>value successfully captures the observed seismicity rate of earthquakes. Results have been verified using statistical <i>S</i>-test. However, the model fails to capture the observed seismicity rate during the period when reconstructed for average <i>b-</i>value to be 0.86 and no change in ΔCFF (ΔCFF = 0).</p