Abstract
We examined the spatial relationship between seismicity and upper crustal structure in the Wakayama region, northwestern Kii Peninsula, Japan, by investigating microearthquake focal mechanisms and the local stress field. The focal mechanisms of most events studied fall into three categories: (1) normal faulting with N–S-oriented T-axes mainly occurring at shallow depths, (2) reverse faulting with E–W-oriented P-axes dominating at intermediate depths, and (3) strike-slip faulting with N–S-oriented T-axes and E–W-oriented P-axes mainly seen at greater depths. The stress field varies with depth: the shallow part is characterized by a strike-slip-type stress regime with N–S tension and E–W compression, while the deep part is characterized by an E–W compressional stress regime consistent with reverse faulting. The depth-dependent stress regime can be explained by thermal stress caused by a heat source, as expected from geophysical observations. Geologic faults, acting as weak planes, might contribute to generate shallow normal fault-type and deeper strike-slip fault-type microearthquakes.Graphical
Highlights
The distribution of earthquake clusters in the shallow crust is not homogeneous and is probably controlled by the heterogeneous structure of the upper crust (e.g., Katao and Ando 1996)
Our results indicate that most focal mechanisms are categorized into three groups, and the depths in each group are distinct: (1) N–S tensional events driven by normal faulting occur mainly in the shallower part of the area; (2) E–W compressional-type events, driven by reverse faulting, occur mainly in the middle of the depth range; and (3) N–S tensional and E–W compressional events occur mainly at the greatest depths (Fig. 8)
The present results show that the stress field in the shallower part of Wakayama region is dominated by N–S tension and E–W compression, generating strike-slip faulting, while the deeper part is an E–W compressional reverse-fault regime (Fig. 9b–h)
Summary
The distribution of earthquake clusters in the shallow crust is not homogeneous and is probably controlled by the heterogeneous structure of the upper crust (e.g., Katao and Ando 1996). We estimated the stress field of the study area using our focal mechanisms and a stress inversion method proposed by Michael (1984, 1987) This technique makes the following assumptions: (1) the direction of tangential traction on the fault plane is parallel to the slip direction, (2) the stress field is uniform in the analysis area during the time window studied, (3) the magnitude of the tangential traction is the same on all fault planes considered, and (4) earthquakes occur along pre-existing weak planes having various strikes and dips. These results indicate that the stress field varies with depth: the shallower part of the stress field shows N–S tension and E–W compression, consistent with strike-slip faulting, while the deeper part shows. The obtained stress ratio R of close to 1.0 means that the magnitude of σ2 is almost the same as σ3, which is consistent with the observation that both reverse and strikeslip events have P-axes oriented E–W
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