Elsevier

Earth and Planetary Science Letters

Volume 382, 15 November 2013, Pages 111-116
Earth and Planetary Science Letters

Aftershocks near the updip end of the 2011 Tohoku-Oki earthquake

https://doi.org/10.1016/j.epsl.2013.09.007Get rights and content

Highlights

  • Near-trench aftershock observation of the 2011 Tohoku-Oki earthquake using OBS.

  • Aftershocks occurred in both overriding and subducting plates.

  • Focal mechanisms indicate that trench-normal tension is pre-dominant in both plates.

  • Low seismicity along plate interface is consistent with near-complete stress drop.

  • Some compressional events suggest a strengthening of the shallowest megathrust.

Abstract

The March 2011 Mw 9.0 Tohoku-Oki earthquake activated shallow aftershocks with normal faulting focal mechanisms near the trench axis. To investigate stress state in the shallow subduction zone near the trench axis, we used ocean bottom seismographs (OBSs) to record earthquakes from August to October 2011. We estimated hypocenter locations and focal mechanisms of the recorded earthquakes by using a grid-search method in a 2-D velocity model based on an active seismic survey. The results show that most of the earthquakes occurred within both the overriding and subducting plates, with very few on the plate interface and none in the most seaward 45 km of the overriding plate (aseismic wedge). The low seismicity along the plate interface is consistent with the nearly complete stress drop on the megathrust fault in the Mw 9.0 earthquake. Focal mechanisms indicate that trench-normal tension is pre-dominant in both plates and extends to a depth of about 30 km at least. On the other hand, several trench-normal compressional earthquakes were located at the landward end of the aseismic wedge. These earthquakes suggest horizontal compression of the overriding prism that may be caused by a strengthening of the shallowest part of the megathrust.

Introduction

The 2011 Tohoku-Oki earthquake (Mw 9.0) ruptured a roughly 200 km wide and 500 km long segment of the subduction fault along the Japan Trench (e.g., Yagi and Fukahata, 2011) (Fig. 1). The most significant feature of the 2011 Tohoku-Oki earthquake is a huge coseismic slip (>50 m) near the trench axis that breached the trench. The rupture initiated about 110 km west of the trench axis at a depth of 21 km (Chu et al., 2011) and propagated both updip and downdip. The near-trench coseismic slip over 50 m has been estimated by several studies based on seismic data (e.g., Yagi and Fukahata, 2011), tsunami data (e.g., Fujii et al., 2011), and seafloor geodetic data (e.g., Ito et al., 2011). Changes in seafloor bathymetry and subseafloor seismic structure due to the earthquake show that the coseismic rupture reached the trench axis (Fujiwara et al., 2011, Kodaira et al., 2012).

After the 2011 Mw 9.0 Tohoku-Oki earthquake, shallow aftershocks with normal faulting focal mechanisms have been activated in both overriding plate and incoming/subducting Pacific plate near the trench axis (Asano et al., 2011). The aftershock activity could be due to a tensional stress caused by the 2011 Tohoku-Oki earthquake. However, the insufficient accuracy in hypocenter locations, especially in depth, determined from land observations makes it difficult to compare the aftershock activity with crustal structures obtained by active seismic studies (e.g., Kodaira et al., 2012).

Ocean bottom seismograph (OBS) observations enable accurate hypocenter locations for offshore earthquakes. Extensive OBS observations began soon after the mainshock, and several campaigns have taken place (Shinohara et al., 2011, Shinohara et al., 2012). Moreover, OBSs deployed before the 2011 Tohoku-Oki earthquake show that interplate earthquakes were activated seaward of the mainshock hypocenter after the M 7.3 largest foreshock but stopped after the mainshock (Suzuki et al., 2012). All these OBS observations were conducted with about 25 km instrument spacing, which is comparable to that of the land observation networks in Japan. However, this spacing is still too wide to locate very shallow (∼10 km) earthquakes, some of which have been detected by land observations (Asano et al., 2011). Here, we present aftershock distributions and focal mechanisms based on OBSs more densely deployed near the trench axis.

Section snippets

Observations and analysis

The OBS aftershock observations have been conducted repeatedly to cover the whole rupture area of the 2011 Tohoku-Oki earthquake with approximately 25 km spacing between the OBSs (Shinohara et al., 2011, Shinohara et al., 2012). By early August, we also completed the deployment of additional 9 OBSs on the trench inner-slope area, near the trench axis off Miyagi, with approximately 10 km station spacing (Fig. 1). The OBS network used in this study includes these densely spaced instruments and 17

Results

We have picked arrival time data for about 1500 earthquakes from August 6 to October 4, 2011 and located 991 earthquakes in the 3-D search space each with at least 10 P-wave arrival time data and location errors less than 3 km (Fig. 2). The magnitudes of these earthquakes range from 0.9 to 5.0. Averaged traveltime residuals for each OBS ranging between −0.20 and 0.25 s for the first grid-search decreased to those ranging between −0.03 and 0.03 s for the second search. Most of the events

Discussion

Our near-trench OBS observations show that between the epicenter of the 2011 Tohoku-Oki earthquake and trench axis aftershocks occurred in both the overriding and subducting plates but almost none along the plate interface (Fig. 2b), consistent with estimates based on both onshore (Asano et al., 2011) and offshore (Suzuki et al., 2012) remote observations. Earthquake activity along the shallow plate interface with thrust focal mechanisms was observed approximately 50 km or more landward of the

Conclusion

In our OBS observation period, almost no earthquakes occurred along the plate interface where large coseismic slip occurred during the 2011 Tohoku-Oki earthquake. Nearly complete stress drop due to the large coseismic slip may be the reason. The earthquakes beneath the trench inner-slope near the trench axis occurred both within the overriding and subducting plates. There is a 45 km wide aseismic wedge at the toe of the overriding prism. As reported from the borehole breakout of the IODP JFAST

Acknowledgments

This study is partly supported by the Special Coordination Funds for the Promotion of Science and Technology (MEXT, Japan) titled as the integrated research for the 2011 off the Pacific coast of Tohoku Earthquake. We thank Kelin Wang, Roy Hyndman, Honn Kao, and two anonymous reviewers for their comments and suggestions to improve the manuscript. Natural Resources Canada, Geological Survey of Canada supports K.O. as a visiting scientist.

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  • Cited by (0)

    1

    Now at: International Research Institute of Disaster Science, Tohoku University, Sendai 980-8579, Japan.

    2

    Now at: National Research Institute for Earth Science and Disaster Prevention, Tsukuba 305-0006, Japan.

    3

    Now at: Graduate School of Science, Kobe University, Kobe 657-8501, Japan.

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