Quantify Long-Period Vertical Site Response at Basins Using Teleseismic Large Earthquakes
Abstract
Long-period oscillations in thick soft basins can be damaging for tall buildings. Motivated by the strong amplification at 5-s period in the Kathmandu Basin from the 2015 Mw 7.8 Nepal earthquake, we quantify the site response from vertically incident SH waves from large teleseismic earthquakes in basins using the spectral ratio of broadband records recorded within basin compared to those recorded at nearby rock sites. Using broadband seismic records from the Hi-CLIMB seismic array deployed from 2002-2004, we obtain spectral ratios in the period range of 0.5 - 20 s that are similar for 2 teleseismic M7+ events at different azimuths. Both show strong amplification factors of 10-30 in a narrow period band of 2.5 - 4 s. The corresponding oscillations are clear in the time series of broadband records in the Kathmandu Basin. This observation can be explained by the thick lake bed sediments previously indicated by field observations and microtremor analysis. To verify the approach, we also perform a systematical study in the Los Angeles Basin, where widely-distributed broadband records are available for many years and where the subsurface structure has been well studied. Compared to a bedrock site in Pasadena, we observe amplification of 3 - 5 over a broad range of frequencies (from 1 - 10 s) in downtown Los Angeles sites without a narrow-band amplification peak frequency, comparable to that predicted from the SCEC community velocity model. Given the frequent M7+ large earthquakes that occur globally every year, such an approach can provide a direct measure of site-specific amplification from vertical incident waves in basins around the world, and would help constrain the shallow layered structure for full three-dimensional wave propagation simulations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.S21D..04Y
- Keywords:
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- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY;
- 7255 Surface waves and free oscillations;
- SEISMOLOGY;
- 7290 Computational seismology;
- SEISMOLOGY