Preceding seismic activity and slow slip events in the source area of the 2011 Mw 9.0 Tohoku-Oki earthquake: a review
© Hasegawa and Yoshida; licensee Springer. 2015
Received: 24 February 2015
Accepted: 8 May 2015
Published: 3 June 2015
The 2011 Tohoku-Oki earthquake ruptured a large area of the megathrust east of NE Japan. The earthquake’s magnitude was 9.0, substantially larger than predicted. It is important to know what occurred in the source area prior to this great megathrust earthquake to improve understanding of the nucleation processes of large earthquakes and risk assessments in subduction zones. Seafloor observation data revealed the existence of two extremely large slip patches: one just updip of the mainshock hypocenter and the other 80–100 km to the north near the trench axis. For 70–90 years before 2003, M > 6 events and slips of M > c. 7 events on the megathrust occurred in the areas surrounding these two large slip patches. Seismic activity had increased since at least 2003 in the downdip portion of the source area of the Tohoku-Oki earthquake. In addition, long-term accelerated slow slip occurred in this downdip portion of the source area in the decades before the Tohoku-Oki earthquake. About 1 month before the earthquake, a slow slip event (SSE) took place at relatively shallow depths between the two large slip patches, accompanied by foreshock activity. Both the slow slip and foreshocks propagated from north to south toward the southern large slip patch. Two days before the earthquake, an M 7.3 foreshock and an associated postseismic slip began at relatively deep depths in the megathrust between the two large slip patches. In addition, a slow slip type event seems to have occurred approximately half a day after the M 7.3 foreshock near the mainshock hypocenter. This slow slip event and the foreshock activity again propagated from north to south toward the mainshock hypocenter. These long- and short-term preceding seismic and aseismic slip gradually reduced the interplate coupling, increased shear stresses at the two large slip patches (i.e., two strong asperity patches), and finally led to the rupture of the great Tohoku-Oki earthquake.
The Mw 9.0 Tohoku-Oki earthquake on March 11, 2011, the largest earthquake in the modern history of Japan, occurred along the plate interface east of Tohoku, NE Japan. The rupture area was approximately 500 km long and 200 km wide (e.g., Japan Meteorological Agency, http://www.jma.go.jp/jma/indexe.html; Geospatial Information Authority of Japan, http://www.gsi.go.jp/ENGLISH/index.html), extending for about two thirds of the length of the megathrust zone east of the NE Japan arc. The earthquake caused severe damage in NE Japan. In particular, the massive tsunami it generated caused many casualties along the Pacific coast, resulting in nearly 20,000 dead or missing individuals.
The 2011 Tohoku-Oki megathrust earthquake was an unprecedented great earthquake because of the amount and quality of observation data. It occurred in the most densely instrumented subduction zone in the world. Nationwide dense seismic and GPS networks, with a station separation of c. 20 km, were deployed on land in the Japanese islands after the 1995 M 7.3 Kobe earthquake. Seafloor geodetic monitoring systems with GPS-acoustic ranging were deployed on the landward slope of the major trenches. Particularly, prior to the Tohoku-Oki earthquake, intensified observations of the seafloor through seismic and geodetic monitoring, including ocean bottom pressure (OBP) gauges, had been made in the area off Miyagi Prefecture, where an extremely large slip occurred in the Tohoku-Oki earthquake. This is because of the high probability of earthquake occurrence in this area as assessed by the Government’s Headquarters for Earthquake Research Promotion (Earthquake Research Committee, Headquarters for Earthquake Research Promotion, http://www.jishin.go.jp/main/index-e.html), although the forecast itself, based mainly on records of previous large earthquakes, dramatically underestimated the magnitude of the 2011 event. The maximum forecast magnitude for this area was 8.2, much smaller than the Mw 9.0 event that actually occurred.
The abundant data collected can improve our understanding of this megathrust earthquake, including the rupture process, the large size of the generated tsunami, and why such a large magnitude (Mw 9.0) earthquake occurred in this subduction zone. Numerous papers have been published based on various analyses of these data. Several papers [1–7], including a review paper , report anomalous earthquake activities and/or slow slip events (SSEs) in the source area preceding the Tohoku-Oki earthquake. Some of the observational evidence in these reports was obtained from the intensified observations of the seafloor geodetic monitoring in the source area. It is particularly important to know the details of what occurred overall in the source area prior to this great megathrust earthquake, to improve our understanding of the stress accumulation and nucleation processes of large earthquakes and for seismic hazard assessments and mitigation in subduction zones. In this review, we focus on the activity of earthquakes and SSEs in the source area of the 2011 Tohoku-Oki earthquake in the pre-mainshock period and their relation to the mainshock rupture.
Coseismic slip distribution: two large slip patches
Many researchers have studied the coseismic slip distribution and rupture process of the M 9.0 Tohoku-Oki megathrust earthquake by analyzing seismic, geodetic, and tsunami data. Slip distributions, estimated soon after this earthquake, can be classified into two groups: one having a large slip near the trench axis [9–13] and the other having a large slip near the mainshock hypocenter [14–18]. Subsequently, data from instruments installed at the sea bottom right above the source area were published, and these seafloor geodetic data provided clear evidence for a large slip in the near-trench area [19–21]. For example, GPS-acoustic ranging data show that the horizontal displacement at a site c. 50 km landward of the trench axis amounts to 31 m toward the ESE, in contrast to the maximum value of 5.3 m from onshore GPS data along the Pacific coast. Information on crustal deformation was also provided by OBP gauges installed directly above the source area. Slip inversions including these seafloor data indicate an anomalously large slip in the near-trench area far off-Miyagi Prefecture, with the maximum slip exceeding c. 50 m.
The large near-trench slip seems to be consistent with the spatial distribution of aftershocks that occurred at the plate interface. After the Tohoku-Oki earthquake, interplate earthquakes did not occur in the mainshock large slip area but only in the surrounding areas . Repeating earthquakes and smaller magnitude interplate earthquakes also did not occur in the large slip area after the Tohoku-Oki earthquake . The stress field estimated from focal mechanisms also seems to support the large near-trench slip model. The stress field in the upper plate was completely transformed after the Tohoku-Oki earthquake. The minimum compressive stress axis became oriented in the direction of the large near-trench slip area, even near the trench axis, which is consistent with the static stress change by the large near-trench slip. This provides evidence for the large near-trench slip at shallow depths during the mainshock rupture .
Different inversion procedures and data are used among the slip models shown in Fig. 1 to estimate coseismic slip distribution. For example, the model shown in Fig. 1d did not involve a large upheaval at the station (TJT1) located above the large slip area c. 20 km landward of the trench axis. In this area, the easternmost portion of the upper plate is an accretionary prism, composed of extremely low rigidity materials, with a width of 25–30 km [27,28]. It is probable that an inelastic response was involved to some extent in the extremely large upheaval at that station, and so the data may not be appropriate for inclusion in the slip inversion, as noted by Romano et al. . The dip angle of the assumed fault plane at the shallowest portion is also different among the slip models in Fig. 1, which strongly affects the estimated slip amount for this area. Thus, it is difficult to determine whether or not the largest slip actually occurred at the shallowest portion of the plate interface.
Large earthquakes that occurred in the footwall after the Tohoku-Oki earthquake also support the group 2 slip models. Five Mw > 7 earthquakes occurred around the source area in the Pacific plate after the Tohoku-Oki earthquake. One of these events, on December 7, 2012, was a doublet that occurred in the trench-outer rise region. An Mw 7.2 thrust event first ruptured at depths of 50–70 km, followed by an Mw 7.1–7.2 normal-faulting event about 27 km to the SSW at depths of 10–30 km . The centroid locations of the doublet and the other four Mw > 7 events are shown in Fig. 2c. The orange and blue lines attached to the centroid locations are the orientations of the P- and T-axes. A comparison of the slip distribution with the locations of the Mw > 7 events clearly shows that these events occurred at locations where the shear stress increased favorably, triggering these large events by the static stress change of the mainshock rupture. Four Mw > 7 normal or strike-slip faulting events are located to the ESE of the southern primary large slip patch, with the T-axis oriented trench-perpendicular, and an Mw 7.1 thrust event near the coast line located to the WNW of the southern large slip patch with the P-axis oriented trench-perpendicular, which indicates that the increased stress due to the static stress change is large at these locations. The exception is the Mw 7.2 thrust event (50–70 km deep) that occurred as the first event of the doublet. This corresponds to the compressional deep trench-outer rise event  of the double-planed shallow seismic zone in the forearc region . This event was located slightly further from the plate interface along which the mainshock slip occurred, and so the amplitude of the static stress change was relatively small at this event location. This event may have occurred because of the increased plate bending stress due to the mainshock rupture.
As described above, the slip distribution commonly visible in the group 2 slip models (Fig. 1b–d) is supported by the spatial distribution of the focal mechanisms of earthquakes that occurred in the Pacific plate after the Tohoku-Oki earthquake. On the other hand, the group 1 slip model (Fig. 1a) cannot explain the observed spatial distribution of the focal mechanisms and so will be excluded from the discussion hereafter.
Romano et al.  also estimated the coseismic slip distribution of the Tohoku-Oki earthquake by a joint inversion of onshore GPS, seafloor geodetic, and tsunami data, but their slip model is not included in this review. A three-dimensionally heterogeneous structure model was used in their inversion, and the depth-dependent rigidity caused a slightly different spatial pattern of coseismic slip compared with that in Fig. 1d of Romano et al. , which was obtained under the assumption of homogeneous elastic constants. We cannot directly compare the slip model of Romano et al.  with slip distributions of preceding earthquakes and SSEs, since they were estimated under the assumption of homogeneous elastic constants. Therefore, we did not use the slip model of Romano et al.  but used slip models obtained using homogeneous elastic constants, as shown in Fig. 1. Moreover, if we convert the slip distribution in Romano et al.  to a moment release distribution, we would expect to obtain a similar spatial pattern to that in the slip model of Romano et al. , because the trend of large slips at shallower depths, compared with Romano et al. , is caused mainly by the depth-dependent rigidity.
Activity of earthquakes and SSEs preceding the Tohoku-Oki earthquake
Seismic activity before 2003
Earthquake and SSE activity from 2003 to January 2011
From 2003, after the relatively calm period of c. 64 years, seismic activity in the source area of the Tohoku-Oki earthquake increased remarkably. In the c. 8 years preceding the Tohoku-Oki earthquake, six c. M 7 events occurred in the source area, in contrast to the lack of such events in the preceding c. 22 years. SSEs also began to occur on the plate interface after about 2003. These activities continued until the occurrence of the Tohoku-Oki earthquake.
The area of the postseismic slip estimated by Ozawa et al. , which is shown in Fig. 4b, is also almost identical to the slip area of the long-term transient estimated by Mavrommatis et al. , as shown in Fig. 6a. This indicates that the anomalously large postseismic slips of the c. M 7 earthquakes reported by Suito et al.  are explained by the fact that this long-term transient on the plate interface was also included in their inversions of postseismic slips of the c. M 7 earthquakes. Not only coseismic and postseismic slips of the c. M 7 events but also an accelerated aseismic slip occurred for >10 years before the Tohoku-Oki earthquake in the downdip portion (or SW portion) of the source area. This long-term precursory aseismic slip eroded the locked region composed of the two large slip patches.
Foreshock and SSE activity just before the mainshock
About 1 month before the Tohoku-Oki earthquake, foreshock and SSE activity began in close proximity to the two large slip patches.
On March 9, 2011, two days before the earthquake, an M 7.3 foreshock occurred in the vicinity of the hypocenter of the Tohoku-Oki earthquake. The slip area of this M 7.3 event, estimated from onshore GPS and OBP gauge data , is shown by a pink ellipse in Fig. 4c. The figure shows that it was again located at the plate interface between the two large slip patches but relatively deep compared with the SSE that had begun about 1 month earlier (the rectangle of red broken lines in Fig. 4c). This M 7.3 foreshock was accompanied by aftershock activity and postseismic slip. Hypocenters of the aftershocks are shown by yellow circles in Fig. 4c. The postseismic slip, again estimated from onshore GPS and OBP gauge data , is shown by an orange ellipse in Fig. 4c. The postseismic slip broke an area just updip of the coseismic slip area and propagated from north to south toward the hypocenter of the Tohoku-Oki earthquake. The estimation using the waveform correlation method by Kato et al.  showed that aftershock activity of the M 7.3 foreshock also propagated toward the hypocenter of the Tohoku-Oki earthquake at a speed of c. 10 km/day (Fig. 7). The postseismic slip lasted for more than 2 days, reaching c. Mw 6.8 , and finally leading to the rupture of the Tohoku-Oki earthquake.
The history of the seismic and quasi-static slips along the plate interface described above and the slip distribution of the Tohoku-Oki earthquake, as summarized in Fig. 4, suggest the existence of persistent locked areas that probably correspond to strong asperity patches. Seismic and aseismic slips on the megathrust surrounding these two strong asperity patches (i.e., the two large slip patches) increased shear stress and finally quasi-static slips propagated toward the initial rupture point of the Tohoku-Oki earthquake within the southern strong patch, triggering the great M 9.0 megathrust earthquake. This is what actually occurred in the source area prior to this great megathrust earthquake, which is important in relation to understanding the nucleation processes of great megathrust earthquakes.
The preceding slip history in the source area on the megathrust and the coseimic slip distribution of the Tohoku-Oki earthquake suggest the existence of a persistent locked area. The slip amount of 50–65 m indicates that this area had been locked for several hundred years. The core of this persistent locked area is thought to be composed of two strong asperity patches existing in the shallow portion of the megathrust zone. It is important to understand what causes such strong asperity patches on the plate interface.
A potential problem with this interpretation is the spatial resolution of the obtained P-wave velocity image. Arrival time data used in the tomographic inversions were obtained at routine seismic stations on land. A dense temporary OBS network was deployed after the Tohoku-Oki earthquake above the large slip area. However, the result from Zhao et al.  cannot be confirmed from the tomographic image obtained using data from this network . This is because the spatial extent of the OBS network was too small for confirmation; only the southern asperity patch was covered by the network. Further investigations of seismic tomography with higher spatial resolutions are required, based on a dense OBS network covering a broader area that includes the two strong asperity patches.
We have reviewed the activity of earthquakes and SSEs in the source area prior to the Tohoku-Oki earthquake. Seafloor observation data from directly above the large slip area of this event played an important role in constraining the coseismic slip distribution of the mainshock, showing the existence of two extremely large slip patches: one just updip of the mainshock hypocenter, 60–80 km landward of the trench axis, and the other 80–100 km to the north, near the trench axis. For c. 90 years before the Tohoku-Oki earthquake, M > 6 earthquakes occurred away from the two large slip patches. For c. 70 years, until 2003, slips of c. M > 7 earthquakes on the megathrust occurred in the areas surrounding the two large slip patches. Seismic and SSE activity increased after 2003. Six c. M 7 events occurred for 8 years from 2003 to 2010 in contrast to a complete absence of c. M 7 events for the preceding c. 22 years. These events occurred in the downdip portion of the source area of the Tohoku-Oki earthquake. Additionally, in this downdip portion of the source area, an increasing slip rate on the megathrust and/or updip migration of deep aseismic slip was observed in the decades before the Tohoku-Oki earthquake. These long-term precursory seismic and aseismic slips reduced the interplate coupling, partly eroding the locked region composed of the two large slip patches. About 1 month before the earthquake, an SSE took place at a relatively shallow depth between the two large slip patches. Associated with this SSE, foreshocks also began. Both slow slip and foreshock activity propagated from north to south toward the southern asperity patch. Two days before the earthquake, an M 7.3 foreshock occurred in the vicinity of the hypocenter of the Tohoku-Oki earthquake, again in the area between the two asperity patches but relatively deep compared with the SSE that had begun about 1 month earlier. This foreshock was accompanied by aftershock activity and postseismic slip located in the updip area between the two strong asperity patches. In addition, a slow slip event began c. 1.5 days before the Tohoku-Oki mainshock. This slow slip event and foreshock activity again propagated from north to south toward the mainshock hypocenter, the initial rupture point of the Tohoku-Oki earthquake, leading to the final rupture of the great megathrust earthquake.
The authors are grateful to Kenji Satake for providing the opportunity to write this review paper. Thoughtful comments by Kenji Satake, Paul Segall, and an anonymous reviewer were helpful in improving this manuscript.
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