Imaging the source regions of normal faulting sequences induced
by the 2011 M9.0 Tohoku-Oki earthquake
Aitaro Kato,1Toshihiro Igarashi,1Kazushige Obara,1Shinichi Sakai,1Tetsuya Takeda,2 Atsushi Saiga,3Takashi Iidaka,1Takaya Iwasaki,1Naoshi Hirata,1Kazuhiko Goto,4 Hiroki Miyamachi,4Takeshi Matsushima,5Atsuki Kubo,6Hiroshi Katao,7
Yoshiko Yamanaka,8Toshiko Terakawa,8Haruhisa Nakamichi,8Takashi Okuda,8 Shinichiro Horikawa,8 Noriko Tsumura,9Norihito Umino,10 Tomomi Okada,10 Masahiro Kosuga,11Hiroaki Takahashi,12 and Takuji Yamada12
Received 27 October 2012; revised 14 December 2012; accepted 18 December 2012; published 29 January 2013.
[1] Intense swarm-like seismicity associated with shallow
normal faulting was induced in Ibaraki and Fukushima prefectures, Japan, following the 2011 Tohoku-Oki earthquake. This seismicity shows a systematic spatiotemporal evolution, but little is known of the heterogeneity in crustal structure in this region, or its influence on the evolution of the seismicity. Here, we elucidate a high-resolution model of crustal structure in this region and determine precise hypocenter locations. Hypocenters in Ibaraki Prefecture reveal a planar earthquake alignment dipping SW at ~45, whereas those in Fukushima Prefecture show a more complex distribution, consisting of conjugate sets of aligned small earthquakes. On the north of the hypocenter of the largest earthquake in the sequence (the M7.0 Iwaki earthquake), we imaged a high-velocity body at shallow depths that lacks aftershock seismicity. Based on fault source models, the large-slip region of the Iwaki earthquake is situated along a zone that roughly coincides with this high-velocity body. We delineated a separate low-velocity anomaly directly beneath the hypocenter of the Iwaki earthquake, indicating crustalfluids in this region. We hypothesize that strong crust underwent structural failure due to the infiltration of crustal
fluids into the seismogenic zone from deeper levels, causing the Iwaki earthquake.
Citation: Kato, A., et al. (2013), Imaging the source regions of normal faulting sequences induced by the 2011 M9.0 Tohoku-Oki, Geophys. Res. Lett., 40, 273–278, doi:10.1002/grl.50104.
1. Introduction
[2] The 11 March 2011 Tohoku-Oki earthquake
(magni-tude (M) 9.0) ruptured a mega-thrust fault off the eastern shore of northern Honshu, Japan, where the Pacific plate is subducting beneath an overriding continental plate at a con-vergence rate of ~8.5 cm/year [e.g., Loveless and Meade, 2010]. The large extensional stress perturbations associated with the Tohoku-Oki earthquake [e.g., Ozawa et al., 2011] resulted in widespread seismicity across much of NE Japan [e.g., Toda et al., 2011]. A remarkable aspect of this induced seismicity was a strong increase in swarm-like shallow normal faulting along the Pacific coast of NE Japan, most notably in Ibaraki and Fukushima prefectures (Figure 1). The progression of this induced seismicity shows a system-atic spatiotemporal evolution. Specifically, an initial M5.7 event occurred in the northern part of Ibaraki Prefecture about 8 minutes after the Tohoku-Oki mainshock rupture (Figure 1a), which subsequently led to the development of swarm-like activity with a M6.1 earthquake on 19 March, slightly to the north of the M5.7 event. After these initial events, the activity then jumped northeastward, to the south-ern end of Fukushima Prefecture, close to its southsouth-ern coast-line, accompanied by a M6.0 earthquake on 23 March. Finally, in the area between thefirst two clusters (swarms) of earthquakes, the largest earthquake in the sequence (the M7.0 Iwaki earthquake) took place on 11 April 2011 [Hikima, 2012; Fukushima et al., 2013]. The Iwaki earth-quake was accompanied by a series of surface ruptures up to ~15 km long along two faults, characterized by steeply dipping fault planes [Mizoguchi et al., 2012].
[3] Some previous studies have proposed that these
sequences of shallow normal faulting were caused by an increase in extensional differential stress, possibly due in turn to the southwesterly extension of the large slip zone of the Tohoku-Oki mainshock rupture that occurs along the plate boundary (inset in Figure 1b) [e.g., Kato et al., 2011a; Imanishi et al., 2012; Kato and Igarashi, 2012]. Sim-ilar processes have been proposed in relation to the 2010 Maule earthquake in Chile [Farías et al., 2011; Ryder
All Supporting Information may be found in the online version of this article.
1
Earthquake Research Institute, University of Tokyo, Tokyo, Japan.
2
National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan.
3
Tono Research Institute of Earthquake Science, Association for the Development of Earthquake Prediction, Mizunami, Japan.
4
Graduate School of Science and Engineering, Kagoshima University, Kagoshima, Japan.
5
Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, Japan.
6
Kochi Earthquake Observatory, Faculty of Science, Kochi University, Kochi, Japan.
7
Disaster Prevention Research Institute, Kyoto University, Uji, Japan.
8
Earthquake and Volcano Research Center, Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan.
9
Faculty of Science, Chiba University, Inage, Japan.
10
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai, Japan.
11
Graduate School of Science and Technology, Hirosaki University, Hirosaki, Japan.
12
Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Corresponding author: A. Kato, Earthquake Research Institute, Univer-sity of Tokyo, Tokyo, Japan. ([email protected])
©2013. American Geophysical Union. All Rights Reserved. 0094-8276/13/10.1002/grl.50104
et al., 2012]. In addition to changes in regional stress patterns in the crust, it has been argued that local heterogene-ities in crustal structure play an important role in controlling the spatiotemporal evolution of seismicity and associated faulting behavior [e.g., Michael and Eberhart-Phillips, 1991; Chiarabba and Amato, 2003; Chiarabba et al., 2009; Kato et al., 2009, 2010; Zhao et al., 2011]. However, there are few constraints on heterogeneous crustal structure in the source region or the relationship between the heterogeneity and induced seismicity.
[4] While online seismic networks of the National
Research Institute for Earth Science and Disaster Prevention (NIED) and the Japan Meteorological Agency (JMA) were in operation throughout the Japanese islands, the average
spacing of the stations (20–30 km) was too large to allow for a high-resolution analysis of crustal heterogeneities in the source region. Therefore, to assess the high-resolution velocity structure, we deployed a dense network of tempo-rary seismic stations that became operational on 28 March 2011 (Figure 1b). Seismic tomography analysis combined with such a dense network is a powerful tool for performing high-resolution imaging of crustal structures, as well as for determining precise earthquake locations. These seismic data allow us to evaluate and discuss the relationships between heterogeneities in crustal structure and the spatio-temporal evolution of seismicity, as well as assess the possible role of crustalfluids in these processes.
140.2˚ 140.4˚ 140.6˚ 140.8˚ 36.6˚ 36.8˚ 37˚ 37.2˚ Y = 0 km Y = -6 km Y = -12km Y = -18km Y = 12km Y = 6 km 2011/04/11(M7.0) 0 10 20 30 40 Depth (km) M2 M4 M6 Stations Active faults 0 5 10 km
Large slipzone 0 5 10 0 M7.0 2011/03/11 - 03/21 2011/03/22 - 04/11 2011/04/11 - 04/30 (a) (b) Fukushima Ibaraki X = 0 km M5.7 initial event M6.0 M6.1 km 2011/04/12(M6.4) 2011/03/23(M6.0) 2011/03/19(M6.1) 2011/03/11(M5.7) 36.6˚ 36.8˚ 37˚ 37.2˚ 140.4˚ 140.6˚ 140.8˚ 141˚ 140.4˚ 140.6˚ 140.8˚ 141˚ 140.4˚ 140.6˚ 140.8˚ 141˚
Figure 1. (a) Spatiotemporal evolution of induced seismicity in the northern part of Ibaraki Prefecture and the southern part of Fukushima Prefecture. The earthquakes plotted are listed in the JMA catalog and occurred at depths shallower than 10 km, with M≥ 1.0. (b) Map of seismic stations and earthquakes used in the tomography analysis, with earthquakes shown as circles with radii scaled to earthquake magnitude and colored according to depth. The grid used in the tomographic analysis is plotted with gray crosses. The open squares indicate the locations of temporary offline (64 closely spaced squares) and permanent online seismic stations. The moment tensors (in red and white) of large events (M≥ 6.0) were determined by NIED. The red lines delineate the surface traces of major active faults. Inset map shows the location of the study area with respect to prefectures in Japan and the large-slip zone of the 2011 Tohoku-Oki mainshock, from Kato and Igarashi [2012].
2. Data and Methods
[5] The seismic network deployed for this study consists
of an array of 64 closely spaced seismometers (average station spacing of<4 km; Figure 1b) with a 1 or 2 Hz natural frequency, which continuously recorded three-component signals at a sampling rate of 100 or 200 Hz. The arrival times of both P- and S-waves for a total of 1348 earthquakes of M≥ 2.5, which are listed in the JMA catalog (for the period between 28 March and 31 October 2011), were manually picked from the waveforms observed by both the temporary and permanent seismic stations. To model the velocity struc-ture and to determine precise hypocenters in the target region, the double-difference tomography method [Zhang and Thurber, 2003] was applied to the P- and S-wave data for each earthquake. Within the target area, the grid spacing for the tomography is 3 km, both horizontally and vertically (Figure 1b). A series of tests was carried out to evaluate the resolution of thefinal velocity structure models (Supporting Information, Figs. S1 and S2), and based on those results, we deduced that the P-wave velocity (Vp) structure within
the target area is quite well resolved. The root mean square
(RMS) travel time residual reduced from 0.19 s to 0.09 s af-ter 20 iaf-terations.
[6] We also relocated a number of additional small
magni-tude (M< 2.5) earthquakes that were not used in the tomo-graphic analysis. We first applied an automated picking algorithm to the waveforms of small earthquakes recorded by our dense seismic network from July to October 2011. We considered the source locations of the 1348 seismic events with M≥ 2.5 relocated by tomographic analysis to represent ‘master events’ for these additional 7458 smaller earthquakes, such that the detailed evaluation of hypocenter distributions in the present study uses a collective total of 8,806 earthquakes (Figure 2 and Supporting Information, Figure S3). For details, please see the Supporting Information, Text S1.
3. Results
[7] Depth sections of the relocated hypocenters in Ibaraki
reveal aligned earthquakes that define a plane dipping SW at ~45 (see Y< –3 km in Figures 2 and S3). The down-dip extent of the SW-dipping alignment is about 10 km. Minor
Y = -18 km Y = -15 km Y = -12 km Y = -9 km Y = -6 km Y = -3 km Y = 0 km Y = 3 km Y = 6 km Y = 9 km Y = 12 km Y = 15 km Depth(km) Depth(km) Depth(km) E25N W25S Depth(km) 4/11(M7.0) 4/12(M6.4) 3/19(M6.1) 3/23(M6.0) Cross-section(km) -4 -2 0 2 4 δVp/Vp (%) 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 -15 -10 -5 0 5 10 -15 -10 -5 0 5 10 15 20 25 -15 -10 -5 0 5 10 15 20 25 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20
Figure 2. Vertical depth sections of Vpvelocity perturbations and nearby earthquakes. The cross-sections are constructed
along lines drawn from W25S to E25N (see Figure 1b). Relocated earthquakes (superimposed gray circles) correspond to those distributed within 1.5 km (laterally) of each vertical cross-section. The masked areas marked by gray color on these vertical depth sections correspond to regions where model resolution is relatively low (as defined in the Supporting Informa-tion, Fig. S1). The red arrows at the top of each section correspond to the locations of surface ruptures. The red and white moment tensor solutions for the largest earthquakes are shown using a lower hemisphere projection rotated into the plane of each section.
NE-dipping alignments are also visible on some sections (e.g., C10 and C11 in Fig. S3). In contrast, the relocated hypocenters in the southern part of Fukushima Prefecture show complex distributions, consisting of numerous small planar alignments of earthquakes that have various orienta-tions (Y> –6 km in Figure 2). Although in the western region many of these planar alignments of earthquake hypo-centers dip to the SW at ~45in sections C5, C6 and C8 in Fig. S3, some additional alignments are also visible on these plots, including WNW-, NW-, and SE-dipping alignments (C1–C4, C7, and C9 in Fig. S3). Some of these planar align-ments appear to represent conjugate sets of planes that dip in opposite directions (C1, C3, and C4 in Fig. S3). Further-more, the distributions of earthquake epicenters reveal a remarkable change between Ibaraki and Fukushima prefec-tures. For instance, below Ibaraki Prefecture, the epicenters are tightly clustered along a NNW–SSE trending corridor about 7 km wide, whereas in Fukushima Prefecture the epicen-ters are more broadly (~40 km wide) distributed (Figure 1).
[8] Aftershocks associated with the M7.0 Iwaki
earth-quake appear to be roughly aligned along planes dipping towards the SW at angles of approximately 45(gray dashed lines in the depth sections in Figure 2), defining somewhat vague trends in comparison with the clearly aligned relo-cated hypocenters observed in Ibaraki Prefecture (i.e., on the depth sections in Figure 2 where Y< –3 km). The down-dip extent of the aftershock areas associated with the Iwaki earthquake is limited (<10 km). Notably, the surface extensions of the inferred fault planes defined by aftershock events roughly coincide with the locations of observed surface ruptures (arrows in Figure 2). Curiously, however, there are no aftershocks recorded at depths shallower than 4 km that might have been used to link these known surface ruptures to the inferred fault planes that dip at 45to the SW. Because the dip angles of these surface ruptures are reported
to be nearly vertical (as observed in outcrops) [e.g., Mizoguchi et al., 2012], we suppose that the dip angle of these fault planes must change in a listric fashion from 45 at depth to near vertical angles as they approach the surface. Indeed, these depth variations of fault dips are supported by an analysis of coseismic radar interferograms [Fukushima et al., 2013]. A similar pattern of seismicity has been reported for the 1995 Grevena Ms 6.6 normal faulting earthquake in northern Greece [Chiarabba and Selvaggi, 1997].
[9] The depth sections and maps of P-wave velocities reveal
low-velocity anomalies at depths of ≥5 km within the study area, directly beneath earthquake clusters (Figures 2 and 3). In particular, a relatively large volume, low-velocity body appears to be present directly beneath the hypocenter of the Iwaki earth-quake at depths of ~9 km (–3 km < Y < 3 km, in Figure 2). In contrast, on the northern side of the mainshock hypocenter of the Iwaki earthquake, a comparatively high-velocity body is imaged at shallower depths of 3 and 6 km (areas labeled HV and encircled by white dashed lines in Figures 3a and 3b). Within this high-velocity body, seismicity is lower than in the surrounding regions. Some of these same features of the Vp
model are also present in the Vsmodel (Supporting Information,
Fig. S4).
4. Discussion and Conclusions
[10] Most of the observable planar alignments of
earth-quake in depth sections throughout the study area exhibit dip angles of 40–50to the SW (Figures 2 and S3). These dip angles are not relatively optimal for normal faulting [Sibson, 2000]. This might suggest that the normal faulting earthquakes occurred along weak pre-existing faults. The progression and distribution of seismicity shows some important differences between northern Ibaraki and southern Fukushima prefectures. For instance, in Ibaraki Prefecture, several fault planes dip
0 5 10 km HV 04/11(M7.0) (a) (b) (c) -4 -2 0 2 4 δVp/Vp (%) Depth 6 km Depth 3 km Depth 9 km 03/23(M6.0) 03/19(M6.1) First (M5.7) 5.5 5.6 5.7 5.8 5.9
Average Vp (depth 3 and 6 km) (km/s)
04/11(M7.0) (d) 140.6˚ 140.8˚ 140.6˚ 140.8˚ 140.6˚ 140.8˚ 140.6˚ 140.8˚ 36.8˚ 37˚
Figure 3. Map views of observed Vp perturbations at three representative depths: (a) 3 km, (b) 6 km, and (c) 9 km.
Relocated earthquakes (superimposed dots) correspond to those distributed (vertically) within 1.5 km of each depth section. The white dashed lines denote the location of a high-velocity body (HV) that is discussed in the main text. (d) Average Vpat depths between 3 and 6 km. The small dots indicate the locations of epicenters of the JMA catalog that
occurred before the M7.0 Iwaki earthquake, but subsequent to the Tohoku-Oki rupture. The red star demarcates the epicenter of the largest event: the M7.0 Iwaki earthquake. The white stars indicate the locations of epicenters of other large earth-quakes. The red lines demarcate the traces of major active faults.
towards the SW, concentrated within a relatively narrow region, whereas the region beneath Fukushima Prefecture contains numerous, widely distributed fault planes with a range of dip directions. The occurrence of this complex array of fault ruptures in Fukushima Prefecture, including those linked with the M7.0 Iwaki earthquake, is consistent with observations of multi-segmented surface deformation revealed by analysis of coseismic radar interferograms [e.g., Fukushima et al., 2013].
[11] On the northern side of the hypocenter of the Iwaki
earthquake, a high-velocity body with a lack of aftershock seismicity was imaged at shallow depths of 3 and 6 km (Figures 2 and 3). In addition, a fault source model of the Iwaki earthquake, based on geodetic and strong motion wave-form analysis [e.g., Fukushima et al., 2013; Hikima, 2012], shows that large-slip patches along the associated SW-dipping faults are also located on the northern side of the mainshock hypocenter at depths shallower than 6 km. Therefore, the large slips that took place in association with the Iwaki earthquake must have occurred along the steeply dipping portions of these faults, and the large-slip patches are roughly located within the high-velocity body imaged in this study.
[12] Many previous tomographic studies have indicated a
tendency for coseismic slip events to be concentrated within confined high-velocity bodies [e.g., Michael and Eberhart-Phillips, 1991; Chiarabba and Amato, 2003; Kato et al., 2010], even in the case of the 2011 M9.0 Tohoku-Oki megathrust earthquake [Zhao et al., 2011]. It is generally believed that such high-velocity bodies coincide with the more brittle and competent parts of the Earth’s crust. The competent part of the crust in the studied area might with-stand further increases in stress caused by the two adjacent clusters of seismic events in neighboring crustal domains where seismic velocities are lower than in the high-velocity body (Figure 3d). Then, after a while (a month in this case), the subsequent rupture of another earthquake (i.e., the M7.0 Iwaki earthquake) would take place within the high-velocity body,finally releasing the stress that had built-up between the two adjacent, previously formed seismic clusters in low-velocity regions.
[13] Some of the low-velocity anomalies of the present
study are imaged beneath earthquake alignments, especially beneath the hypocenter of the Iwaki earthquake (Figures 2 and 3). Other recent studies employing dense networks of seismic stations have revealed that such low-velocity anomalies lie directly beneath the seismogenic zone, or at least within its lower reaches [e.g., Nakajima and Hasegawa, 2008; Kato et al., 2009; Kato et al., 2011b]. These types of anomalies may indicate the presence of crustal fluids. Indeed, we identified several migrations of hypocenters as a function of time during each swarm sequence (Supporting Information, Fig. S5), reflecting diffu-sive processes of localizedfluids and aseismic slip [e.g., Ro-land and McGuire, 2009]. The low-velocity anomalies im-aged in this study thus imply the potential involvement of crustalfluids in triggering the induced seismicity. Regional (larger-scale) tomography studies conducted in NE Japan [Tong et al., 2012] indicate thatfluids within the lower crust are infiltrating the overlying seismogenic zone from a deeper fluid reservoir located at a depth of ~20 km. Consequently, it seems likely that this type offluid migration could result in a marked reduction in the shear strength of the overlying crust, ultimately triggering ruptures along pre-existing faults in the region [e.g., Miller et al., 2004; Kato et al., 2011b].
[14] Given these considerations, we hypothesize that
far-field extensional deformation associated with the Tohoku-Oki mainshock triggered the sequences of normal faulting in rela-tively weak regions of crust in the Ibaraki and Fukushima pre-fectures, during the initial stages of deformation that took place prior to the Iwaki earthquake (Figure 3d). Eventually, the com-paratively strong (i.e., high-velocity) parts of the crust were ruptured, probably in association with the infiltration of deeply sourced crustal fluids into the overlying seismogenic zone [Tong et al., 2012], and ultimately resulting in the largest earth-quake in the sequence (M7.0 Iwaki earthearth-quake). These crustal heterogeneities likely played an important role in controlling the observed systematic spatiotemporal evolution of the in-duced seismicity across the region.
[15] Acknowledgments. We are grateful to Claudio Chiarabba and Roland Bürgmann for constructive and useful comments. We thank H. Zhang for allowing us to use the tomoDD code and acknowledge NIED and JMA for allowing us to use the waveform data. This study was partially supported by the Observation and Research Program for the Prediction of Earthquakes and Volcanic Eruptions under MEXT.
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