ALBERT

Description: We analyze ground motions produced by the 25 April 2015 Gorkha, Nepal, earthquake ( M w  7.9) and five of its larger aftershocks (5.3≤ M w ≤7.2) which were recorded in the distance range 100–900 km by two recently installed strong-motion networks in the Indo-Gangetic Plains (IGP). In comparison with the largest M w  7.2 aftershock, we find that the peak ground accelerations (PGAs) of the M w  7.9 mainshock are somewhat higher, but peak ground velocities (PGVs) and peak ground displacements (PGDs), respectively, are roughly equal or even smaller. Furthermore, PGAs of the 26 April M w  6.7 aftershock are similar to those of the M w  7.2 aftershock, but PGVs and PGDs are much smaller. PGAs in the IGP during the sequence are associated with frequencies f of ~4 Hz, whereas PGVs and PGDs occur at f ~0.1 Hz. This implies that the source spectrum of the M w  7.9 mainshock at f ~0.1 Hz was similar to that of the M w  7.2 aftershock, whereas the spectral levels of the M w  7.2 and 6.7 aftershocks were nearly equal at f 4 Hz. This is confirmed by spectral ratios at the IGP sites: the ratios of the M w  7.9 mainshock to the M w  7.2 aftershock are about 1 in the intermediate frequency range of 0.04–0.3 Hz, whereas those of the M w  7.2–6.7 aftershocks are 1 at f 〉1 Hz and 10 at f ~0.1 Hz. This is also in agreement with reported teleseismic moment-rate spectra of these three events. Existing ground-motion prediction equations (GMPEs), based on worldwide data, do very poorly in predicting observed peak motions during the Gorkha sequence. Hence, we develop a new GMPE for the region which, in spite of the sparse dataset and peculiarities of some of the events, should provide a more reliable estimation of seismic intensities than those presently available. Electronic Supplement: Tables of station locations and peak ground acceleration (PGA), peak ground velocity (PGV), and peak ground displacement (PGD) values; and figures of spectral ratios, band-pass filtered PGA, PGV, and PGD values, residuals, and comparison of PGAs for alternative model.