Earthquake recurrence in NW and central Himalaya

Abstract

In this study we compare Himalayan seismic moment release estimates derived from strain rate observations with those derived from large historical earthquakes, and to this end we use a reassessed catalogue of historical earthquakes from western and central Himalaya since the beginning of the 16th century. We have computed seismic moment rates within six contiguous segments along the Himalayan arc and compared these, using Kostrov’s formula, with moment rates computed from recent global strain rate estimates and regional studies. While the ratios between strain-based moment-rate estimates and those inferred from observed seismicity vary significantly between the segments, we find on the average consistently larger strain-based values by about a factor of two, based on seismicity from the last 515years. The moment-rate ratio is, however, significantly reduced when shorter catalogues are used, to 1.28 for the last 215years and to 1.05 for the last 115years, which is an almost perfect match. The possible inclusion of afterslip in the model would further improve the 515-year match. This is indicating that a significant part of the difference, possibly most of it, is likely to be caused by incompleteness of the longer earthquake catalogue, possibly combined with underestimated magnitudes. The difference between geodetic and seismic estimates for the more complete part of the catalogue is smaller than previously reported along the western Himalayan frontal thrust. In fact, the only region where a significant moment-rate difference is found in our study is in SE Himachal Pradesh. In terms of seismic hazard it is found that the moment rate reduction of about a factor of two, when going from 115 to 515years, leads to a reduction in the 475-year PGA of about 26%. It is also found that using 50years of USGS seismicity data between 1963 and 2012 leads to a 40% lower hazard as compared to using moment release for the last 115years.