Source Time Function and Near Source Ground Motion of the 2015 Gorhka, Nepal, Earthquake

Abstract:

During the he Mw7.8 Gorkha earthquake, the strong ground motion record at KATNP was obtained in the Kathmandu basin as a hanging wall site. The observed ground motions show mach smaller PGA and response spectra level than the expected from the ground motion prediction equations. Although the large slip is mapped beneath the Kathmandu area by the source inversion result, it is very important issue to understand the reason that the ground motion is not so large.

I compared source time functions and its Fourier amplitude spectra (source spectra) among the similar size earthquakes. From the finite source fault database of USGS from 2000, I have selected the crustal and inter-plate earthquakes whose seismic moment is ranging from twice to half of that of the Gorkha earthquake. I used the database of source time functions by USGS to avoid the inter-event systematic difference by the analysis method. Totally 17 events were selected. In the time domain, the source time function of the Gorkha earthquake seems to show smoother and smaller peak moment rate value among those of events.

The average and the standard deviation (α) of the source spectra of those events are calculated from the source time functions. The source spectra of the Gorkha earthquake show smaller value than the average-σ in the frequency range between 0.05 and about 1Hz. Although the source time functions from teleseismic records in the frequency range larger than about 0.5Hz is not so reliable and we need the information of more higher frequency range to discuss the near fault strong motion accelerations, the fact that the source spectra of the Gorkha earthquake shows systematically smaller than the similar size events would explain the weak generation of high frequency ground motions in the source area. This kind of analysis of the source time function database is useful to understand the near-source ground motion characteristics even there are not strong motion records.