Near-Source Strong Pulses During Two Large MJMA 6.5 and MJMA 7.3 Events in the 2016 Kumamoto, Japan, Earthquakes
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Extremely large ground motions with strong pulses were observed near source faults during two large MJMA 6.5 and MJMA 7.3 events of the 2016 Kumamoto earthquakes in Japan. To investigate the mechanisms for generation of near-source strong pulses during both events, we first performed strong ground motion simulations in a broadband frequency range between 0.2 and 10 Hz using the empirical Green’s function method. For both the M6.5 and M7.3 events, strong motion generation area (SMGA) source models were prepared to simulate the ground motions, which were able to reproduce well the characteristics of the observed ground motions in and around the source areas. We also conducted ground motion simulations based on hypothetical simple source models to study the effect of rupture directivity on near-source strong ground motions. The principal findings regarding rupture directivity effects on near-source strong ground motions are as follows: (1) During the M6.5 event, the forward and upward rupture directivities from two strike-slip SMGAs caused two distinct strong pulses in the fault normal and parallel components, respectively. (2) During the M7.3 event, the upward rupture directivity along the fault dip direction from the strike-slip SMGA, including a small normal-slip, caused a strong pulse in both the fault parallel and normal components.
KeywordsThe 2016 Kumamoto earthquakes rupture directivity effect strong motion generation area empirical Green’s function method
Strong motion data from K-NET, KiK-net, and F-net were provided by the NIED. We also used moment tensor solutions routinely determined by F-net. The JMA unified earthquake catalog was produced by the JMA in cooperation with the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Most of the figures were drawn using Generic Mapping Tools (Wessel and Smith, 1998). We are grateful to Yoshimitsu Fukushima, Luis A. Dalguer, Catherine Berge-Thierry, Fabrice Hollender, Philippe Renault, Dogan Seber, Changjiang Wu, and Marion Bard who organized the 2nd international workshop on Best Practice in Physics-based Fault Rupture Models for Seismic Hazard Assessment of Nuclear Installations (Best-PSHANI), May 14–16, 2018, Cadarache-Château, France. The presentations and discussions during this conference stimulated us to focus and clarify this study. The careful reviews and comments by two anonymous reviewers and the guest editor Yoshimitsu Fukushima were quite helpful in improving the manuscript. This study was based on the 2016 research project “Examination for uncertainty of strong ground motion prediction for inland crustal earthquakes” by The Secretariat of the Nuclear Regulation Authority (NRA), Japan.
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