9:45 AM - 10:00 AM
[SCG52-04] Lower crustal microseismicity in Japanese Islands
Keywords:lower crustal earthquake, Japanese Islands, microearthquakes
Seismic activity in inland area is generally confined to the upper crust (0-15 km). On the other hand, it is considered that brittle failures do not occur in the lower crust, because plastic deformation dominates there. However, geological investigations showed the existence of pseudotachylytes in exhumed lower crustal sections (e.g., Takagi et al. 2000), which is direct evidence for seismic faulting. Furthermore, dense seismic array observations revealed that microseismicity is occurring down to the upper mantle at an actively deforming fault zone in southern California (Inbal et al., 2016). These observations suggest that brittle failures can occur even at the lower crustal depth. 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 identified the earthquakes occurring in the lower crust of the Japanese Islands based on the Japan Meteorological Agency earthquake catalog and investigated their characteristics by seismic waveform analysis. The main results are as follows.
(1) Most of the identified lower crustal earthquakes are smaller than magnitude 3, and the b-value is not much different from the earthquakes in the upper crust.
(2) Seismograms of lower crustal earthquakes contain high frequency components, which is similar to those in the upper crust.
(3) Lower crustal earthquakes were found in various places, but they do not overlap with the focal region of deep low-frequency earthquakes.
(4) There are three active regions: the focal area of the 2011 M7.0 Iwaki earthquake, Kii Peninsula, and southern part of Hiroshima- and Okayama-prefecture.
(5) There is a seismic gap between the lower crustal earthquake activity and the upper crustal earthquake activity.
(6) Focal mechanisms of the lower crustal earthquakes show various faulting types and variably oriented nodal planes, which may suggest fluid intrusion into the source area.
In the presentation on the day, we will summarize the features of lower crustal earthquakes in Japanese Islands based on further data analysis and discuss a relation with the generation mechanism of large inland earthquakes.
(1) Most of the identified lower crustal earthquakes are smaller than magnitude 3, and the b-value is not much different from the earthquakes in the upper crust.
(2) Seismograms of lower crustal earthquakes contain high frequency components, which is similar to those in the upper crust.
(3) Lower crustal earthquakes were found in various places, but they do not overlap with the focal region of deep low-frequency earthquakes.
(4) There are three active regions: the focal area of the 2011 M7.0 Iwaki earthquake, Kii Peninsula, and southern part of Hiroshima- and Okayama-prefecture.
(5) There is a seismic gap between the lower crustal earthquake activity and the upper crustal earthquake activity.
(6) Focal mechanisms of the lower crustal earthquakes show various faulting types and variably oriented nodal planes, which may suggest fluid intrusion into the source area.
In the presentation on the day, we will summarize the features of lower crustal earthquakes in Japanese Islands based on further data analysis and discuss a relation with the generation mechanism of large inland earthquakes.