Focal mechanisms and stress field in the Atotsugawa fault area, central Honshu, Japan

Abstract

We have determined 151 high quality focal mechanism solutions for earthquakes that occurred between January 2005 and December 2006 in and around the Atotsugawa fault area in central Honshu, Japan. We used P-wave first motion polarity data observed by a dense temporary seismic observation conducted in the area by the Japanese University Group. The types of obtained focal mechanism solutions are predominantly strike-slip, however, some earthquakes exhibit reverse-and normal-fault type focal mechanisms. Without regard to faulting type, the averaged directions of compressional (P) and extensional (T ) axes are rather uniform, N70 • W and N16 • E, respectively. We found that not a small number of normal-fault type earthquakes occurred in a small cluster near the central part of Atotsugawa fault, and in a very short period from the end of March to the beginning of April in 2005. In order to estimate stress field around the fault, we applied a stress tensor inversion method to the focal mechanism solutions. Both the maximum principal stress (σ 1 ) and the minimum principal stress (σ 3 ) axes are almost horizontal and trend N72 • W to N77 • W and N14 • E to N20 • E, respectively. The direction of σ 1 and the fault trace form an angle of 43 • -48 • . It is clear that the σ 1 axis is neither perpendicular nor parallel to the Atotsugawa fault, indicating strong coupling of fault. The stress tensor inversion also suggests local extensional stress field at a deep (>8 km) central part of Atotsugawa fault. We present a hypothesis that the extensional stress is caused locally in a transition zone from the fluid-rich aseismic creep zone to the seismogenic zone that is interpreted as an asperity ruptured by the 1858 Hietsu Earthquake (M = 7.0).

Depth-dependent stress field in and around the Atotsugawa fault, central Japan, deduced from microearthquake focal mechanisms: Evidence for localized aseismic deformation in the downward extension of the fault

“…All events are plotted by open circles in a color triangle diagram (Figure 8a, bottom right), which suggests that most of the other events are a mixture of reverse and strike‐slip components. As was the case with the results obtained by Wada et al [2003], Ito et al [2007], and Katsumata et al [2010], our results reveal that many earthquakes with large reverse components are also occurring. In order to investigate the spatial pattern of focal mechanisms, we plotted their faulting types by means of colored circles in the cross section (Figure 8b).…”

“…They divided the region containing the Toyama plain as well as the Atotsugawa fault system into 9 subareas, in which each area had a size of 18 and 23 km in the NS and EW directions, respectively. Although the subareas for the stress tensor computation are different from each other, the S 1 directions determined in this study agree well with those determined by Katsumata et al [2010]. This suggests that the S 1 direction is stable over the region and its estimate is not influenced by the definition of subareas.…”

“…There are also a few events with a reverse‐faulting or normal‐faulting component. Together with a temporary seismic observation, focal mechanisms of earthquakes smaller than M 3 could be determined by using the P wave first motions [ Wada et al , 2003; Ito et al , 2007; Katsumata et al , 2010]. The result shows that earthquakes with reverse‐faulting and normal‐faulting components are certainly occurring around the Atotsugawa fault system, implying a spatial variation of the local stress field.…”

“… Katsumata et al [2010] found 10 normal‐faulting microearthquakes that occurred in a small cluster near the central part of the Atotsugawa fault and at depths greater than 8 km. The majority of those events occurred within one month after the occurrence of the M j 4.1 earthquake of 24 March 2005, where M j is the magnitude reported by the JMA.…”

“…Except for the present research, the study by Katsumata et al [2010] is the only one that has carried out a stress tensor inversion in and around the Atotsugawa fault system. They divided the region containing the Toyama plain as well as the Atotsugawa fault system into 9 subareas, in which each area had a size of 18 and 23 km in the NS and EW directions, respectively.…”