The 2015 Gorkha, Nepal, Earthquake Sequence: II. Broadband Simulation of Ground Motion in Kathmandu

摘要

The relatively low damage in the Kathmandu Valley caused by the 2015 M-w 7.8 Gorkha earthquake has attracted much attention. To gain a deeper understanding of this phenomenon, we conduct broadband ground-motion simulations for both the mainshock and the M-w 7.2 Dolakha aftershock through a hybrid method that combines deterministic 3D synthetics at relatively low frequencies (< 0.3 Hz) and semi-stochastic synthetics at higher frequencies (> 0.3 Hz). Because they are summarized in a companion paper (Wei et al., 2018), the 3D deterministic synthetics were generated by embedding a finite-fault rupture model in a 3D velocity model that is characterized by a simplified basin structure for the Kathmandu Valley. We tested different weighting schemes using a finite slip model and backprojection results to weight the high-frequency sources. Our simulations were guided by fitting the observations from five strong-motion stations in Kathmandu Valley and the intensity and mortality distributions. Site effects were handled by amplitude spectra ratio derived from the vertical component of a hard-rock station (KTP). Our broadband ground-motion simulations show that (1) the stress parameter (3.8 MPa) of the mainshock was much lower in comparison to the M-w 7.2 aftershock (23 MPa) that suggests the rupture process of the mainshock was relatively deficient in radiating high-frequency energy and different fault friction property between the mainshock and the aftershock; (2) the soft deposits in the Kathmandu Valley experienced a pervasive nonlinear site response during the mainshock and the M-w 7.2 aftershock, which also contributed to the reduction of high-frequency motions; and (3) the high-frequency ground motions during the mainshock were primarily radiated from the down-dip rupture. Hence, we suggest considering the difference in the distribution of high-frequency radiation and fault slip in the broadband ground-motion simulations for scenario and historical earthquakes.