b-value characterization of earthquakes in the Eastern Himalayan and Indo-Burman Plate Boundary Systems

Abstract:

We study the frequency-magnitude relationship for earthquakes in the Eastern Himalayan and Indo-Burman plate boundary system using the Gutenberg-Richter relationship given as: log (N) = a - bM (where N is the number of event with magnitude >= M and a, b are constants). In our analysis we use earthquakes of magnitude (m_b) > 3, recorded between 2001 and 2014, by the Northeast Telemetered Network. Our analysis is done as a two-step process: (a) estimation of the moment magnitude (M_w) of the earthquake by fitting the P-wave source spectra using a circular fault model; and (b) linear regression to the distribution of frequency of earthquakes within given ranges of moment magnitude using the Gutenberg-Richter relationship. The magnitude of completeness for the set of earthquakes used in our study was found to be (M_w) 4. Accordingly, the range of earthquake magnitude was chosen to be in increments of 0.5 between (M_w) 4.0 and 6.0. We study the b-value for both interval and cumulative frequency of earthquakes. Our estimated b-value for the entire dataset is 0.54 and 0.77, respectively. We observe a distinctive variation in the b-value for plate boundary and intra-plate earthquakes. We sub-divided the region in three zones and estimated the b-values: (i) Eastern-Himalayan zone with b-values of 0.46 (interval) and 0.71 (cumulative); (ii) Indo-Burma convergence zone with b-values of 0.30 (interval) and 0.58 (cumulative); and (iii) northeastern India (including the Shillong Plateau and Bengal Basin) with b-values of 0.31 (interval) and 0.59 (cumulative). The global average b-value is known to be ~1. b-values lower than the global average indicate that the cumulative energy released by earthquakes is lower than the accumulated energy through plate convergence. This can also be empirically related to stress accumulation in a region. Using this understanding, we infer that the intra-plate region of northeast India and the Indo-Burman plate boundary is accumulating elastic strain to be released in future large earthquakes. We also explore possible relationship between b-value and earthquake stress drop as a function of depth.