Unusual low-angle normal fault earthquakes after the 2011 Tohoku-oki megathrust earthquake
© Yagi et al. 2015
Received: 16 March 2015
Accepted: 11 June 2015
Published: 26 June 2015
A few low-angle normal fault earthquakes at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis, were detected immediately after the 2011 Tohoku-oki earthquake. After that, however, no such normal fault events have been observed until the occurrence of the 2014 M W 6.6 Fukushima-oki earthquake. Here we analyze the teleseismic body waveforms of the 2014 Fukushima-oki earthquake. We first compare the observed teleseismic body waves of the 2014 Fukushima-oki earthquake with those of the largest previous low-angle normal fault aftershock (M W 6.6), which occurred on 12 March 2011, and then estimate the centroid depth and moment tensor solution of the 2014 Fukushima-oki earthquake. The teleseismic body waves and moment tensor solution of the 2014 Fukushima-oki earthquake are similar to those of the 2011 normal fault aftershock, which suggests that the 2014 Fukushima-oki earthquake occurred at a similar depth and had a similar mechanism to that of the 2011 aftershock. We detected five low-angle normal fault aftershocks at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis, and confirmed that all of them except for the 2014 Fukushima-oki earthquake occurred within 17 days after the mainshock. The occurrence of these low-angle normal fault events is likely to reflect the reversal of shear stress due to overshooting of slip during the 2011 Tohoku-oki earthquake. We speculate that a fast but heterogeneous recovery of stress state at the plate interface may explain why these events preferentially occurred immediately after the megathrust event, while one of them occurred with a significant delay. In order to better understand the characteristics of stress state in the crust, we have to carefully observe the ongoing seismic activity around this region.
Keywords2011 Tohoku-oki earthquake Low-angle normal fault earthquakes Slip overshoot
On 11 July 2014, the Fukushima-oki, Japan, earthquake occurred in the large slip area of the 2011 Tohoku-oki earthquake (M W 9.1). The earthquake hypocentral parameters and magnitude determined by the Japan Meteorological Agency (JMA) are as follows: origin time = 4:22 12 July 2014 (Japan Standard Time); epicenter location = (37.05° N, 142.32° E); depth = 33 km; M JMA = 7.0. The fault parameters of one of the conjugate fault planes determined by the Global CMT (GCMT) project (Dziewonski et al. 1981; Ekström et al. 2012) are dip = 27°, strike = 185°, and slip angle = −107°, with a depth of 12 km, which suggests that the 2014 Fukushima-oki earthquake was a low-dip-angle normal fault event, at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis (http://www.globalcmt.org; last access on 13 February 2015).
In general, the teleseismic body waves contain information on the depth range of the rupture area, since the shape of the theoretical waveforms varies with depth, as a consequence of the different timing of the reflected phases (e.g., pP and sP). In this study, we compare the observed teleseismic body waves of the 2014 Fukushima-oki earthquake with those of the largest low-angle normal fault aftershock and then estimate the centroid depth and moment tensor solution of the 2014 Fukushima-oki earthquake. We also extract low-angle normal fault earthquake activity at approximately the depth of the plate interface and discuss the occurrence mechanism of the 2014 Fukushima-oki earthquake.
Teleseismic body waveforms
Moment tensor solution
As can be seen in Fig. 4, the smallest variance area corresponds to a centroid depth from 6 to 30 km. The focal mechanism solution of the small variance area shows that one of the two possible fault motions is characterized by low-angle normal faulting. As shown in Fig. 3, the rupture propagated mainly eastward from the hypocenter, and the high-angle fault plane is inconsistent with the characteristic of the observed waveforms. The strike and dip of the low-angle normal faulting are about 170° and 21°, respectively, which is consistent with the geometry of the plate interface. These results show that observed waveforms can be well explained by back slip motion along the plate interface. A result of finite fault inversion also supports this interpretation (Additional file 1: Figure S1). The total seismic moment (M 0) is 9.6 × 1018 Nm (M W = 6.6), which is in agreement with the GCMT solution: 7.8 × 1018 Nm (http://www.globalcmt.org; last access on 13 February 2015).
Seismicity of low-angle normal fault earthquakes
We investigated the aftershock activity of the 2011 Tohoku-oki earthquake by focusing on the low-angle normal fault earthquakes that are likely to have occurred at the plate interface.
Previous studies (e.g., Asano et al. 2011) analyzed seismicity immediately after the 2011 Tohoku-oki earthquake and reported the occurrence of many normal fault aftershocks. Most of them occurred near the trench and in the outer-rise of the Pacific plate, while some occurred along the coastal area of Japan. Just a few low-angle normal fault events were detected, and they have been interpreted as interplate aftershocks that indicate the reversal of shear stress due to overshooting of slip during the 2011 Tohoku-oki earthquake (Ide et al. 2011; Yagi and Fukahata 2011). However, it should be emphasized that low-angle normal fault earthquakes at approximately the depth of the plate interface are unusual in subduction zones.
Yagi and Fukahata (2011) have demonstrated that the largest low-angle normal fault event on 12 March 2011 (Fig. 1) occurred at approximately the depth of the plate interface with a strike nearly parallel to the trench axis. Based on the inversion analysis of waveform data, they have also confirmed that the fault slip certainly occurred along the low-angle fault plane not along the conjugate high-angle plane based on the inversion analysis of waveform data. The close resemblance of the observed waveforms between the 2014 Fukushima-oki and the 12 March 2011 earthquakes strongly suggests that the 2014 Fukushima-oki earthquake is also a low-angle normal fault event at approximately the depth of the plate interface with a strike nearly parallel to the trench axis. The moment tensor inversion (Fig. 4) supports this idea, although the centroid depth of the 2014 Fukushima-oki earthquake cannot be as sharply determined as for the 2012 event because the 2014 event has a shorter duration of the source time function. Both events occurred in the large stress drop area of the 2011 Tohoku-oki earthquake, which is also consistent with the idea that the slip overshoot during the 2011 Tohoku-oki earthquake has triggered the 2014 Fukushima-oki earthquake as well as the 12 March 2011 earthquake.
Some studies (e.g., Ide et al. 2011; Hasegawa et al. 2011; Yagi and Fukahata 2011) have advocated that the fault rupture during the 2011 Tohoku-oki earthquake released most of the tectonically accumulated stress. This has recently been shown to correspond with an immediate increase of the b-value parameter (i.e., the slope of the frequency-magnitude distribution of earthquakes) after the 2011 Tohoku-oki earthquake, along the mainshock rupture region (Tormann et al. 2015). However, the large post-seismic slip in the down-dip region (e.g., Ozawa et al. 2011) and viscoelastic relaxation in the asthenosphere (Watanabe et al. 2014; Yamagiwa et al. 2015) have contributed to the recovery of stress in the large slip area of the 2011 Tohoku-oki earthquake. A fast recovery has been inferred recently from the return of b-values to pre-mainshock levels by September 2014 (Tormann et al. 2015). Stress inversion analysis using focal mechanism data (Hardebeck 2012) seems to point out a similar fast recovery to pre-Tohoku stress levels. The rapid recovery of stress state should restrain the activity of low-angle normal fault earthquakes at approximately the depth of the plate interface. In fact, we have not found similar events since the end of March 2011 until 11 July 2014 (Fig. 5b). Nevertheless, the 2014 Fukushima-oki earthquake has happened. What does the occurrence of this earthquake mean from a geophysical point of view?
We hypothesize that such occurrences are related to the strong spatial heterogeneity of stress state in the crust. In the case of the 2011 Tohoku-oki earthquake, it is almost impossible to detect spatial heterogeneity of slip with a length scale of a few tens of kilometers from inversion analyses of geodetic and seismic data. Hence, due to several possible reasons (e.g., strong patches, dynamic effects), strong spatial heterogeneity of stress could have been produced by the 2011 Tohoku-oki earthquake. In addition, the stress recovery process can contribute to enhance the spatial heterogeneity of stress; indeed, the result of Tormann et al. (2015) implies that there are small patches along the megathrust where the stress recovery could be significantly delayed. Such patches may initiate rare normal-fault events like the 2014 Fukushima-oki earthquake. The decrease of fault strength due to an increase of pore fluid pressure might have further encouraged the occurrence of such “anomalous” events. Note that fluids are abundant at or close to the plate interface (e.g., Matsubara and Obara 2011). In order to better understand the characteristics of stress state, in particular the evolution of spatial heterogeneity in a short and long term, we have to carefully observe the ongoing seismic activity around this region.
Waveform data of the African Array, Berkeley Digital Seismograph Network, Canadian National Seismograph Network, Caribbean Network, GEOFON, Geoscience Australia, GEOSCOPE, Global Seismographic Network, Global Telemetered Seismograph Network, Lamont-Doherty Cooperative Seismographic Network, Leo Brady Network, United States National Seismic Network, and USArray Transportable Array were accessed through IRIS. Figures were generated with Generic Mapping Tools (Wessel and Smith 1998). We acknowledge the comments by anonymous reviewers in improving the manuscript. This study was supported by Grant-in-Aid for Scientific Research 24540450 of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) to YY and YF.
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