LAUSR

Abstract We investigate the spatial variations of Pg attenuation structure in Nepal Himalaya. We implement the “Two-Station method” on 2325 waveforms obtained from 435 events recorded at 151 stations deployed across Nepal Himalaya to measure the Pg-wave quality factor (Q). These station-station measurements are inverted further for investigating the spatial variations of Pg attenuation using a back projection algorithm. The model contains variations across the region, with much of the region being dominated by high attenuation. It is observed that the areas around the existing faults and lineaments exhibit very low Q values. A few exceptional high Q pockets are observed to be limited predominantly in the north western side of our study area. We hypothesize that intrinsic attenuation plays a major role in causing the high apparent Pg attenuation in the crust of Nepal Himalaya. This may be mainly caused by highly pressurized fluids trapped within a thin low velocity layer (LVL) at shallow depths. Moreover it is interesting to observe that all the past earthquakes of M w > 7 since 1934, fall into the regions of low Q values but not the lowest. It may be anticipated that the very low Q areas are too weak to sustain the accumulation of high energy release during a large magnitude earthquake. Therefore the observed low Q structures across the study area may imply a weak crust and subsequently a favorable zone for future rupture.