Strong Ground Motion in the Indo‐Gangetic Plains during the 2015 Gorkha, Nepal, Earthquake Sequence and Its Prediction during Future Earthquakes

Abstract

Abstract 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.