Characteristics of Strong Ground Motion in the Kathmandu Valley During the 2015 Gorkha, Nepal Earthquake

Abstract eng:
On 25 April 2015, a large interplate earthquake Mw 7.8 occurred in the Himalayan Range of Nepal. The focal area estimated was about 200 km long and 150 km wide, with a large slip area under the Kathmandu Valley where our strong motion observation stations were installed. Kathmandu is the capital of Nepal and is located in the Kathmandu Valley, which is formed by soft lake sediments of PlioPleistocene origin. Large earthquakes in the past have caused significant damage as the seismic waves were amplified in the soft sediments. Four stations were installed along a west-to-east profile of the valley at KTP (Kirtipur), TVU (Kirtipur), PTN (Patan) and THM (Thimi); KTP is a rock site and the others are sedimentary sites. The strong ground motions were observed during this large damaging earthquake. The maximum horizontal peak ground acceleration at the rock site was 271 cm/s2, and the maximum horizontal peak ground velocity at the sedimentary sites reached 112 cm/s. We found while the peak accelerations were smaller than the predicted values, the peak velocities were approximately the same as the predicted values. The horizontal components were rotated to N207E and N117E directions. The velocity waveforms at KTP showed about 6 s triangular pulses on the N207E and the UD components; however the N117E component was not a triangular pulse but a single-cycle sinusoidal wave. This distinguishing 6 s triangular pulses can be predicted by the regression model of “FlingStep Pulse”. The derived displacements at KTP are characterized by a monotonic step on the N207E and UD components. The vector sum of these displacement waveforms is 147 cm. Inside the Kathmandu Valley, the 163 cm vector-sum deformation and the same moving direction as the USGS fault normal direction (N205E) were obtained from GPS data. Our results derived from the KTP acceleration records are consistent with these observations. The vertical ground velocities observed at the sedimentary sites have also the same pulse motions as observed on the rock site. On the contrary, the horizontal ground velocities (also accelerations) at the sedimentary sites have a long duration with conspicuous long-period oscillations; these are effect of the horizontal valley response. The horizontal valley response is characterized by large amplification and prolongation of oscillations. However, the predominant period and envelope shape of their oscillation differ from site to site; these features demonstrate that the long-period valley response of the Kathmandu Valley is considerably complicated. Considering geological formations in and around the Kathmandu Valley, we may expect that the maximum horizontal peak ground acceleration is observed at the sedimentary site. However, the peak ground acceleration was recorded at the rock site KTP; this acceleration was given by isolated large ground acceleration. Finally we make a preliminary examination of the nature of the isolated large ground acceleration.

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