9:00 AM - 9:15 AM
[SSS10-01] Generation mechanism of lower crustal microearthquakes: The case of deep earthquake cluster beneath western Hiroshima prefecture
Keywords:lower crustal earthquake, generation mechanism, moment tensor, non-double-couple component
Inland earthquakes usually occur in the upper crust, which is shallower than about 15 km, but also in the lower crust. For example, in southern California, microseismicity that extends from the lower crust to the upper mantle has been detected below the fault zone by dense seismic array observations (Inbal et al., 2016). In Japan, an earthquake cluster of about 10 km in diameter is known to occur at a depth of 30 km in the western part of Hiroshima prefecture. Although lower crustal earthquakes are thought to be related to the occurrence of large inland earthquakes, their generation mechanism is poorly understood. In this study, we focused on the earthquake cluster beneath the western Hiroshima prefecture and estimated their moment tensor solutions.
We analyzed about 70 earthquakes of Mj > 1.5 that have occurred since 2004. In order to discuss the existence of non-double-couple components, we applied the iterative relative moment tensor inversion (Imanishi and Uchide, in preparation), which is an extension of the inversion method originally proposed by Dahm (1996). Our method iteratively applies the relative moment tensor inversion to source clusters improving each moment tensor as well as their relative accuracy, which overcomes the problem of the original method that errors in the mechanism of reference events lead to biased solutions for other events.
The results show that each earthquake can generally be explained by a double couple focal mechanism. However, they show a variety of faulting types, which is different from the feature in the upper crust in this region that strike-slip earthquakes with E-W compression predominate. It is likely that the high pore fluid pressure allows for the rupture of pre-existing cracks with various fault geometries. We also found that each earthquake contains a small non-double-couple component that exceeds the estimation error. Although we did not find any distinct features in the spatial distribution of the non-double-couple components, there is a temporal change with a cycle of 4-5 years, suggesting a periodic process of crack opening and closing. These results suggest that fluid is periodically injected into the cluster, which causes the lower crustal earthquakes.
Acknowledgements. We are grateful to Japan Meteorological Agency for the hypocenter list. Seismograph stations used in this study include permanent stations operated by the National Research Institute for Science and Disaster Resilience (Hi-net), Japan Meteorological Agency, Earthquake Research Institute at the University of Tokyo, as well as Geological Survey of Japan, AIST.
We analyzed about 70 earthquakes of Mj > 1.5 that have occurred since 2004. In order to discuss the existence of non-double-couple components, we applied the iterative relative moment tensor inversion (Imanishi and Uchide, in preparation), which is an extension of the inversion method originally proposed by Dahm (1996). Our method iteratively applies the relative moment tensor inversion to source clusters improving each moment tensor as well as their relative accuracy, which overcomes the problem of the original method that errors in the mechanism of reference events lead to biased solutions for other events.
The results show that each earthquake can generally be explained by a double couple focal mechanism. However, they show a variety of faulting types, which is different from the feature in the upper crust in this region that strike-slip earthquakes with E-W compression predominate. It is likely that the high pore fluid pressure allows for the rupture of pre-existing cracks with various fault geometries. We also found that each earthquake contains a small non-double-couple component that exceeds the estimation error. Although we did not find any distinct features in the spatial distribution of the non-double-couple components, there is a temporal change with a cycle of 4-5 years, suggesting a periodic process of crack opening and closing. These results suggest that fluid is periodically injected into the cluster, which causes the lower crustal earthquakes.
Acknowledgements. We are grateful to Japan Meteorological Agency for the hypocenter list. Seismograph stations used in this study include permanent stations operated by the National Research Institute for Science and Disaster Resilience (Hi-net), Japan Meteorological Agency, Earthquake Research Institute at the University of Tokyo, as well as Geological Survey of Japan, AIST.