Abstract
Abstract We present the crustal structure around the fault zone pertaining to the 1938 Kutcharo earthquake (M 6.0), northern Japan, to consider why large earthquakes have occurred around calderas. The study was based on gravity anomalies and magnetotelluric and direct-current (DC) electrical-resistivity survey data. The density structure obtained from gravity anomalies indicated that the fault plane corresponded to the main depression boundary of the Kutcharo caldera. The resistivity section, based on audio-frequency magnetotelluric surveys, indicated that the estimated fault plane was located along the boundary of resistivity blocks, which also corresponded to the depression boundary. A detailed resistivity section in the ruptured zone revealed by a DC electrical-resistivity survey showed a discontinuity of layers, implying cumulative fault displacements. These results indicate that the 1938 earthquake was an abrupt slip along the main depression boundary of the Kutcharo caldera. The most likely hypothesis pertains to fluid intrusion along the depression boundary. However, additional seismic and geodetic studies are required to identify other feasible earthquake mechanisms.
Highlights
Large earthquake events (M > 5) have often been reported around large volcanic calderas, such as the Long Valley caldera in the United States (e.g., Hill et al, 2003; Prejean et al, 2003), the Onikobe caldera in north-eastern Japan (Umino et al, 1998), and the Rabaul caldera in Papua New Guinea (Mori and McKee, 1987)
The resistivity section, based on audio-frequency magnetotelluric surveys, indicated that the estimated fault plane was located along the boundary of resistivity blocks, which corresponded to the depression boundary
Density and resistivity structures were investigated around the seismogenic zone of the 1938 Kutcharo earthquake (M 6.0) to identify the relationships between the earthquake and the caldera structure
Summary
Large earthquake events (M > 5) have often been reported around large volcanic calderas, such as the Long Valley caldera in the United States (e.g., Hill et al, 2003; Prejean et al, 2003), the Onikobe caldera in north-eastern Japan (Umino et al, 1998), and the Rabaul caldera in Papua New Guinea (Mori and McKee, 1987). Using 2-D forward processing (Talwani et al, 1959), we modelled the density structure along the A–A profile to identify the density distribution around the ruptured zones (Fig. 2), which cross the estimated fault of the 1938 earthquake. Features of the final density model (Fig. 3(b)) and their validity are as follows: 1) The boundary depth between units A-1 and B did not vary significantly around the rupture zones of the 1938 earthquake, which is validated by the borehole data (545, 697, and 507 m in boreholes TS-3, TS-6, and TS-1, respectively). 4) Because the density models that followed the constraints explained in 1) and 2) did not completely fit the observed gravity data, the calculated gravity was slightly lower than the observed gravity around borehole TS-6 (Fig. 3(a)) This inconsistency may be alleviated when the compaction of units A and B is further investigated or a 3-D model is developed. This inconsistency may be alleviated when the compaction of units A and B is further investigated or a 3-D model is developed. 5) No borehole data were available for the unit boundaries at the north-east of borehole TS-1, so the depths were not determined
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