S43D-2837
Aftershock Records in the Kathmandu Valley of the 2015 Gorkha, Nepal, Earthquake

Thursday, 17 December 2015
Poster Hall (Moscone South)
Michiko Shigefuji1, Nobuo Takai2, Tsutomu Sasatani2, Subeg Bijukchhen2, Masayoshi Ichiyanagi1,2, Sudhir Rajaure3 and Megh Raj Dhital3, (1)Institute of Seismology and Volcanology, Hokkaido University, Sapporo, Japan, (2)Hokkaido University, Sapporo, Japan, (3)Tribuvan University, Kathmandu, Nepal
Abstract:
The devastating earthquake, named the Gorkha Earthquake, was followed by a series of aftershocks: more than 350 of them greater than M 4 and four aftershock greater than M 6. The rupture of main shock originating 80 km NW of capital Kathmandu propagated towards east. The ensuing aftershock activities are concentrated in the eastern part of the rupture area.

The aftershock of Mw 6.6 occurred about half an hour later at epicentre near to that of the main shock. The other three large aftershocks however, were originated in the eastern extreme of the rupture zone. The aftershock of Mw 7.3 that occurred on 12th May 2015 brought about more damages to infrastructures already vulnerable due to the main shock.

To understand the site effect of the Kathmandu valley structure, we installed continuous recording accelerometers in four different parts of the valley. Four stations were installed along a west-to-east profile of the valley at KTP (Kirtipur; hill top), TVU (Kirtipur; hill side), PTN (Patan) and THM (Thimi).

The surface S-wave velocity of the KTP site was over 700 cm s-1, but for each of the other three sites it was less than 200 cm s-1. These velocities are consistent with the geological formations; KTP is above hard rock, and TVU, PTN and THM are over the lake sediment of the valley. It is normal for the amplitude of earthquake motion to be larger in areas lying above sedimentary soil than in areas above hard rock, and these motions can be amplified further by certain deep underground structures. To know deep underground structure using with aftershock records, we installed more four instruments in the Kathmandu basin after main shock.

We analysed the strong-motion data of these five aftershocks recorded in the eight strong-motion accelerometers. The station of KTP is considered as reference site to compare the effect of sediments on the earthquake waves.

The large aftershocks all have highest Peak Ground Velocity (PGV) at TVU and the station of KTP showed the least PGV as expected. The records at the sediment site are rich in long period waves. The spectral ratio of the sediment sites show significant amplification in the 0.2-1 Hz.

For further work we will analyse the site effect of the Kathmandu basin by using these aftershocks and construct a 3-D velocity structure of the basin.

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