Site Classification in Central Asia

Session: Alpine-Himalayan Alpide Shallow Earthquakes and the Current and the Future Hazard Assessments [Poster]
Type: Poster
Date: 4/30/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The horizontal-to-vertical spectral ratio (HVSR) of the small and moderate shallow crustal earthquake ground motions was employed as a first order approximation of the site response in the Tien Shan of central Asia which is one of the world’s largest, youngest and most active intracontinental orogens. We deployed a database including 2207 records with hypocentral distances up to 500 km, from 468 earthquakes having moment magnitudes between 3.0 and 5.5 captured by 25 broadband stations from 5 different networks, located in Afghanistan, Tajikistan and Kyrgyzstan. We fitted a Gaussian-shape pulse function to evaluate peak frequencies and amplitudes of the obtained HVSRs. 17 stations (68%) have no peak, 4 stations (16%) have only one peak and the rest have more than one peak frequency. Then, we classified the stations based on the fundamental-mode natural frequency according to the Japan engineering design practice into the NEHRP site classes. Furthermore, we approximated the time-averaged shear-wave velocity in the uppermost 30 m using the fundamental model peak frequency and its corresponding amplitude. Finally, we compare the HVSRs obtained from P-waves, S-waves, coda and pre-event noise. All peak frequencies including the fundamental mode estimated from different seismic phases are in good agreement; whereas generally the amplitude of the P-wave window is the lowest, the amplitudes of S-wave and noise windows are similar to the whole record and the amplitudes of early and late coda windows are the highest. We also observed that in several stations, the results from the noise window is not stable because there are some anomalous high amplitudes and peaks in their HVSRs window indicating the existence of some artifacts such as wind and traffic. This observation suggests the HVSR technique using the ambient-noise vibration should be performed with care and visual inspection of records.

Presenting Author: Farhad Sedaghati

Authors