1:45 PM - 3:15 PM
[SSS06-P12] Rupture directivity of moderate-sized aftershock of the 2016 Kumamoto Earthquake estimated by dense observation
Keywords:Rupture directivity
For understanding earthquake faulting mechanism, we need to know how rupture propagates on an earthquake fault plane. Detailed rupture propagating process for large earthquakes have been estimated in various regions in all of the world. However, there are not many studies about this process for small earthquake than magnitude of 5 because of both smaller amplitude and shorter process duration than those for large earthquakes. In recent years, studies have attempted to estimate the direction of rupture propagation for small- and medium-scale inland earthquakes in Japan by dense seismic observations. However, there are only a few cases in which the direction has been studied in detail in the region where strike-slip earthquakes occur. In this study, we estimate the directions of rupture propagating for earthquakes in strike slip stress regime using aftershock data of the 2016 Kumamoto earthquake (M7.3).
It is known that apparent rupture duration in case of unilaterally faulting depends upon the fault geometry relative to the station. Therefore, we measured pulse widths of direct P-waves observed at various stations and attempted to estimate the direction of rupture propagation from spatial distribution of the pulse widths. We used seismograms observed at seismic stations including 110 sites temporally deployed in the Hinagu Fault zone (i.e. one of major faults of the Kumamoto earthquake). The earthquake of which we estimate the direction is an events with MJMA4.7 June 26, 2022 at around center of the seismic network. As reference event for measuring rupture direction, we adopted events of the MJMA3.8 on May 31, 2022, and the MJMA1.4 on June 27, 2022. Those occurred within 0.5 km of the target earthquake. The pulse widths of the target earthquake with respect to the reference earthquakes are obtained.
The spatial distribution of the relative pulse width showed shorter width found in SSW from the epicenter, which suggested that rupture propagated toward SSW. The estimated direction is generally parallel to the strike direction of one of the nodal planes of focal mechanism of this target earthquake and is also consistent with the strike of the Hinagu fault. It is found that the present method can identify the earthquake fault plane from two nodal planes and the rupture directivity. On the other hand, we also found some part of the pulse width distribution could not be explained simple unilateral fault slip. This is an important issue for further investigation.
It is known that apparent rupture duration in case of unilaterally faulting depends upon the fault geometry relative to the station. Therefore, we measured pulse widths of direct P-waves observed at various stations and attempted to estimate the direction of rupture propagation from spatial distribution of the pulse widths. We used seismograms observed at seismic stations including 110 sites temporally deployed in the Hinagu Fault zone (i.e. one of major faults of the Kumamoto earthquake). The earthquake of which we estimate the direction is an events with MJMA4.7 June 26, 2022 at around center of the seismic network. As reference event for measuring rupture direction, we adopted events of the MJMA3.8 on May 31, 2022, and the MJMA1.4 on June 27, 2022. Those occurred within 0.5 km of the target earthquake. The pulse widths of the target earthquake with respect to the reference earthquakes are obtained.
The spatial distribution of the relative pulse width showed shorter width found in SSW from the epicenter, which suggested that rupture propagated toward SSW. The estimated direction is generally parallel to the strike direction of one of the nodal planes of focal mechanism of this target earthquake and is also consistent with the strike of the Hinagu fault. It is found that the present method can identify the earthquake fault plane from two nodal planes and the rupture directivity. On the other hand, we also found some part of the pulse width distribution could not be explained simple unilateral fault slip. This is an important issue for further investigation.