Ground motion amplification in the Kanto Basin from future Itoigawa-Shizuoka earthquakes near Tokyo using virtual earthquakes.

Marine A. Denolle, Pierre Boué, Naoshi Hirata, Shigeki Nakagawa, & Gregory C. Beroza

Submitted August 15, 2016, SCEC Contribution #6851, 2016 SCEC Annual Meeting Poster #273

The Tokyo Metropolitan area is subject to high seismic risk due to the nearby triple junction and because it is sitting atop the deep Kanto sedimentary basin. In addition, the Itoigawa-Shizuoka Tectonic Line (ISTL) is one of the crustal faults that comprises a zone of active plate boundary, can host oblique-strike slip earthquakes with magnitudes greater than 7, with up to 30% of probability in 30 years (The Headquarter of Earthquake Research Promotion, 2015), and is only ~ 150-200km away from Tokyo. Currently at a mature stage in its seismic cycle, predicting strong ground motion from earthquakes on it is important for seismic hazard assessment. Previous studies using the ambient field have suggested that ISTL sources might have particularly strong amplification in the Kanto Basin (Denolle et al., 2014). We take advantage of the dense seismic network MeSO-net (MEtropolitan Seismic Observation network) and of the locations of the Hi-net (High Sensitivity Seismograph network) stations near the ISTL to build virtual earthquakes from the ambient seismic field and predict strong ground motion in the 1 – 10 seconds period range.

First, we construct the Earth’s impulse responses from deconvolution of the ambient seismic field recorded between Hi-net stations (as sources) and MeSO-net stations (as receivers) with a method that preserves the relative amplitudes of the response. Then, we correct for ray bending by optimally rotating the 9 component Green tensor to minimize the coupling between transverse and non-transverse components. Sharp edges around the basin are responsible for scattering of fundamental-to-first overtone surface-wave mode, with magnified amplitude and extended shaking duration. We apply the virtual earthquake approach that turns each station-source near the fault into a double-couple point source by correcting the Green tensor for source depth and mechanism. The depth and mechanism correction is a source of uncertainty in peak ground motion and in shaking duration, which we quantify using simple velocity models. To verify that our relative amplitudes are reliable and to calibrate them to absolute levels, we use recordings of the M6.7 Northern Nagano earthquake that occurred on November 22, 2014 at the northern end of the fault segment to calibrate absolute amplitudes. Finally, we estimate the long period ground motions for a suite of of M7+ scenario ruptures on the ISTL and explore the role of waveguides, basin resonance, and basin edges in strong ground motion. 

Key Words
ground motion, ambient seismic noise

Citation
Denolle, M. A., Boué, P., Hirata, N., Nakagawa, S., & Beroza, G. C. (2016, 08). Ground motion amplification in the Kanto Basin from future Itoigawa-Shizuoka earthquakes near Tokyo using virtual earthquakes.. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Ground Motion Prediction (GMP)