Large intraslab earthquake (2011 April 7, M 7.1) after the 2011 off the Pacific coast of Tohoku Earthquake (M 9.0): Coseismic fault model based on the dense GPS network data
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 9 June 2011
Accepted: 24 July 2011
Published: 21 February 2012
We propose a source fault model for the 2011, April 7, earthquake (M 7.1) deduced from a dense GPS network. The coseismic displacements obtained by GPS data analysis clearly show the spatial pattern specific to intraslab earthquakes, not only in the horizontal components but also the vertical ones. A rectangular fault with uniform slip was estimated by a non-linear inversion approach. The results indicate that the simple rectangular fault model can explain the overall features of the observations. The amount of moment released is equivalent to Mw 7.17. The hypocenter depth of the main shock estimated by the Japan Meteorological Agency is slightly deeper than the neutral plane between down-dip compression (DC) and down-dip extension (DE) stress zones of the double-planed seismic zone. This suggests that the depth of the neutral plane was deepened by the huge slip of the 2011, M 9.0, Tohoku Earthquake, and the rupture of the thrust M 7.1 earthquake was initiated at that depth, although more investigations are required to confirm this idea. The estimated fault plane has an angle of ∼60 degrees from the surface of the subducting Pacific plate. It is consistent with the hypothesis that intraslab earthquakes are thought to be a reactivation of the pre-existing hydrated weak zones produced in the bending process of oceanic plates around outer-rise regions.
A huge earthquake of Mw 9.0 occurred beneath the Pacific Ocean in northeastern Japan on March 11, 2011. The event was named by the Japan Meteorological Agency (JMA) as the 2011 off the Pacific coast of Tohoku Earthquake (hereafter the 2011 Tohoku Earthquake). This earthquake generated a gigantic tsunami, which caused a devastating disaster including the loss of more than 15,300 lives as of June 6, 2011. Around the focal area, large earthquakes (M 7 ∼ 8), named as “Miyagi-oki earthquakes”, have been recognized to occur repetitively with a recurrence time of ∼37 years (The Headquarters for Earthquake Research Promotion, 2011). Further discussion on the spatial distribution of interplate coupling during the interseismic period (e.g., Suwa et al., 2006; Hashimoto et al., 2009; Wallace et al., 2009), and also its relationship to the coseismic slip distribution of the 2011 Tohoku Earthquake, is needed. Iinuma et al. (2011) estimated the coseismic slip distribution based on the dense cGPS network and concluded that the Miyagi-oki asperities, which did not slip in the 2005 Miyagi-oki earthquake (M 7.2), were ruptured during the 2011 Tohoku Earthquake.
In this short paper, we propose an M 7.1 earthquake fault model based on the onshore GPS displacement data obtained by a dense GPS network consisting of the site operated by Tohoku University and the Geospatial Information Authority of Japan (GSI). We also discuss the relationship between the aftershock distribution and the estimated fault plane.
2. GPS Data and Processing
3.1 Coseismic displacement field
The displacement field strongly suggests that the M 7.1 event was not a typical interplate earthquake, because the horizontal displacements must be eastward at GPS sites near the coastline, and the vertical displacement would show subsidence if this was an interpolate event. These observations indicate this earthquake was an intraslab event.
3.2 Coseismic fault model and aftershock distribution
Estimated fault parameters of the optimal rectangular fault. Longitude, latitude, and depth denote the location of the upper-left corner of a rectangular fault plane, looking down from the eastward side.
Figure 3 shows the map-view of the estimated fault plane (red rectangles) and the aftershock distribution (blue circles). The coseismic slip reached up to 2.4 m, an equivalent seismic moment of 7.2 × 1019 Nm(Mw 7.17 with an assumed rigidity of 50 GPa). The calculated static stress-drop of this event is approximately 4.0 MPa. Our fault model explains the GPS data well and reproduces the complex spatial pattern of deformation around the coastline. Several GPS sites along the coastline made important contributions to constraining the fault plane. One example are the westward displacements observed at KNK and MYAT, which constrain the location of the southern edge of the fault plane. In a similar way, the vertical displacements at KNK, EN3, OHSU, and NATR are also critical for the fault geometry determination. These GPS sites show uplift signals, while the uplift signals decrease rapidly with the epicentral distance. This suggests that the dip angle of the coseismic fault could be higher, even though there may be some trade off between dip angle, depth of the fault, and slip amount. Assuming this earthquake is an intraslab one, the estimated model in this study explains the observed GPS displacements well. Based on these geodetic results, we conclude that this earthquake is not an interplate earthquake but is an intraslab earthquake.
Figure 4 shows the relationship between the coseismic slip distribution due to the 2011 Tohoku Earthquake by Iinuma et al. (2011) and the location of the M 7.1 earthquake. It is clear that the M 7.1 fault plane is located beneath the large coseismic slip of the 2011 Tohoku Earthquake.
The estimated M 7.1 fault plane is just located along the aftershock distribution (Fig. 4). The accuracy of the hypocenter depths of the aftershocks might have systematic and/or random errors because the focal area is out of the seismic network. The fault geometry based on the geodetic technique also has some uncertainty for similar reasons. In any case, comprehensive characteristics between the fault plane location and the aftershock distribution show a similarity.
The M 7.1 event was followed by its aftershocks along the down dip edge of the estimated fault plane. Within the subducting slab, a neutral stress plane of the double-plane deep seismic zone (Hasegawa et al., 1978) exists between the upper-plane down-dip compressional (DC) stress regime and the lower-plane down-dip extension (DE) stress regime. Kita et al. (2010) estimated the neutral plane depth from the upper plate interface beneath the Tohoku region based on a stress tensor inversion analysis. They concluded that the neutral plane is located about 22 km beneath the upper plate interface. The JMA hypocenter of the M 7.1 event is located slightly deeper than the neutral plane (Fig. 4). This means that the rupture of the M 7. 1 event was initiated near the neutral plane, although much of the slip in the event occurred within the down-dip compressional region above. It is surprising that such a large reverse-fault-type intraslab earthquake initiated within the neutral region. At present, we cannot conclude whether the rupture of the April 7 earthquake initiated from the neutral plane or not. Reasons why this reverse intraslab event was initiated within the neutral regions could be (1) the hypocenter depth determined by the JMA is deeper than the true depth, and/or (2) the depth of the neutral stress plane between DC and DE stress became much deeper due to the large slip of the M 9 2011 Tohoku Earthquake along the plate interface. The second scenario may be possible if the down-dip compressional stress was increased by the large slip of the M 9 earthquake along the plate interface, since the focal area of the M 7.1 event is located just beneath the down-dip edge of the large slip area along the plate interface (see Fig. 4). There is, however, scope for further investigation into the relationship between the neutral stress plane and the starting point of the M 7.1 fault slip.
The bending of oceanic plates around trench outer-rise areas in subduction zones generates an extensional stress field, which causes normal faulting in the uppermost part of oceanic plate (Masson, 1991). Recent studies suggest a hypothesis that pre-existing weak, and preferentially hy-drated, faults, generated by the normal faulting around the trench outer-rise areas before subduction, are responsible for intraslab earthquakes (e.g., Jiao et al., 2000; Ranero et al, 2005; Hino et al, 2009). Normal faults generally have a high dip angle of roughly ∼60 degree measured from the plate interface. The angle between the coseismic fault plane estimated in this study and the subducting plate interface is also ∼60 degrees (Fig. 4). It suggests that this M 7.1 intraslab event is a reactivation of a pre-existing outer-rise normal fault, but with a reverse sense of motion due to the changed stress regime after the slab subduction.
We propose a coseismic fault model for the 2011, April 7, earthquake (M 7.1) deduced from a dense GPS network. The GPS coseismic displacements clearly indicate intraslab earthquake characteristics. The rectangular fault model was estimated by a non-linear inversion approach. The results indicate that a simple rectangular fault model can explain the observations very well, especially at the coastal GPS sites. The amount of moment release was equivalent to Mw 7.17, with a coseismic slip of 2.4 m. The geometry of the estimated fault plane is consistent with the aftershock alignments. The hypocenter depth of the event is around the neutral plane between the DC and DE zones. The location of the event may be controlled by a temporal change of the location of the neutral stress plane, which we suggest has been deepened due to stress changes related to the large slip of the 2011 Tohoku Earthquake along the plate interface, although more investigations are required to confirm this idea. The estimated fault plane has an angle of ∼60 degree from the subducting plate interface. This can be interpreted by the hypothesis that intraslab earthquakes are caused by a reactivation of the pre-existing hydrated weak zones produced in the bending process of oceanic plates at the outer-rise zone.
We thank the Miyagi Prefectural government for providing us helicopter flights to collect the GPS data from the Kinkasan and Enoshima islands. We thank to the DOCOMO Engineering Inc. for permitting us to use the equipment of the WIDESTAR II High-speed Mobile Satellite Communications Service for GPS data collection. GPS data were provided by GSI and NAO and by a research project conducted by JNES to establish the evaluation techniques of seismogenic faults. We thank Dr. Laura Wallace and Dr. Masayuki Murase for comments that significantly improved the manuscript. We thank Dr. Taku Urabe and Dr. Takeshi Matsushima for helping us to use the VSAT telemetry system. We also thank the staffs of JMA for allowing us to use seismological data. The seismic data analyzed in this study were provided by the seismic networks of the JMA, NIED (Hi-net), and national universities. We used Generic Mapping Tools (Wessel and Smith, 1998) to make figures.
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