A 3-D electrical resistivity model beneath the focal zone of the 2008 Iwate-Miyagi Nairiku earthquake (M 7.2)
© Ichihara et al.; licensee Springer. 2014
Received: 11 December 2013
Accepted: 26 May 2014
Published: 10 June 2014
The 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) was a shallow inland earthquake that occurred in the volcanic front of the northeastern Japan arc. To understand why the earthquake occurred beneath an active volcanic area, in which ductile crust generally impedes fault rupture, we conducted magnetotelluric surveys at 14 stations around the epicentral area 2 months after the earthquake. Based on 56 sets of magnetotelluric impedances measured by the present and previous surveys, we estimated the three-dimensional (3-D) electrical resistivity distribution. The inverted 3-D resistivity model showed a shallow conductive zone beneath the Kitakami Lowland and a few conductive patches beneath active volcanic areas. The shallow conductive zone is interpreted as Tertiary sedimentary rocks. The deeper conductive patches probably relate to volcanic activities and possibly indicate high-temperature anomalies. Aftershocks were distributed mainly in the resistive zone, interpreted as a brittle zone, and not in these conductive areas, interpreted as ductile zones. The size of the brittle zone seems large enough for a fault rupture area capable of generating an M 7-class earthquake, despite the areas distributed among the ductile zones. This interpretation implies that 3-D elastic heterogeneity, due to regional geology and volcanic activities, controls the size of the fault rupture zone. Additionally, the elastic heterogeneities could result in local stress concentration around the earthquake area and cause faulting.
The magnetotelluric (MT) method reveals the distribution of electrical resistivity and has been used to clarify the geology, high-temperature anomalies, and fluid distribution around earthquake zones (e.g., Mitsuhata et al. 2001; Ogawa et al. 2001; Sarma et al. 2004; Unsworth and Bedrosian, 2004; Ichihara et al. 2008, 2009, 2011; Wannamaker et al. 2009; Yoshimura et al. 2009). Mishina (2009) conducted MT surveys along three survey lines across the northern edge, central part, and southern edge of the aftershock area (Figure 1) and estimated resistivity distributions based on two-dimensional (2-D) inversions. The models showed low-resistivity anomalies around the earthquake area that imply crustal fluid flows. They also showed significant differences among the resistivity profiles, which indicate strong three-dimensionality. However, 2-D inversion of a strong three-dimensional (3-D) resistivity structure often results in inaccurate models (e.g., Siripunvaraporn et al. 2005b). Additionally, MT data were not measured around the epicenter of the main shock. In this study, we conducted wide-band MT measurements around the epicentral area and updated the resistivity models based on a 3-D inversion. Then, we interpreted the geological and thermal heterogeneity based on the estimated resistivity model and discussed the relationship between the earthquake and these heterogeneities.
Magnetotelluric measurements and impedances
Wide-band MT surveys were conducted at 14 sites along a profile passing through the epicenter of the earthquake in August 2008 (Figure 1). We recorded two horizontal components of electric field and three components of magnetic field using MTU2000 systems (Phoenix Geophysics, Ltd., Toronto, Canada). The electric and magnetic fields were measured using Pb-PbCl2 electrodes and induction coils, respectively. The recorded time series were converted into frequency-domain MT impedance tensors between 320 and 0.00034 Hz by using the SSMT200 system (Phoenix Geophysics, Ltd.). The remote reference technique (Gamble et al. 1979) was applied in the estimation of MT impedances using horizontal magnetic field data from Sawauchi station (Figure 1), which yielded high-quality MT responses.
The RMS misfits of the sensitivity test models (see text for details)
3 Ω m
10 Ω m
30 Ω m
100 Ω m
300 Ω m
We next constrained the reliable resistivity ranges of the C-2, C-3a, C-4, and C-5 conductors based on the following additional sensitivity tests (Toh et al. 2006). In these tests, we replaced the conductors in the inverted model with 100, 30, and 10 Ω m. The replaced areas are enclosed by dashed lines in Figures 4 and 5, except for the blocks that showed lower resistivity than the replacing resistivities. The RMS misfits of these models are shown in Table 1. To examine whether the filled test models were significantly different from the original inverted models, we adopted the F test. Based on the F test with a 95% confidence level, C-2, C-3a, C-4, and C-5 with resistivities higher than 30, 30, 30, and 100 Ω m, respectively, were significantly worse compared with the original inverted model, which indicated that the resistivity of the conductors should be lower than these resistivities.
Although C-1, C-2, C-3b, C-4, and C-5 were also found in the previous study based on the 2-D inversion method (Mishina 2009), their shapes and distribution depths are different in the present model. The C-2 and C-3b conductors are in shallower areas in the 3-D model than in the 2-D models. This inconsistency is probably due to inaccuracy in the 2-D inversion, because large |β| values (>10°) above C-2 and C-3b (Figure 2) indicate a strong 3-D effect in the MT impedances. Additionally, a conductor beneath Mt. Yakeishi in the 2-D model does not occur in the 3-D model. The likely reason for this difference in the models is that the 2-D inversion may have detected C-2, which is distributed alongside but not below the 2-D survey line (Figure 4), because 2-D inversion often shows conductors distributed off the profile (e.g., Siripunvaraporn et al. 2005b).
The C-1 conductor reflects Tertiary sedimentary rocks because these rocks show low resistivity (1 to 10 Ω m) in the NE Japan area (Takakura 1995; Ichihara et al. 2011) and are distributed from the surface to a depth of 3,000 m (maximum) beneath the Kitakami Lowland, according to geological and seismic surveys (e.g., Kato et al. 2006). However, the C-1 conductor is not shown in blank areas of the MT stations (between lines Y and I and east of line N), although seismic surveys found thick sediment in these areas (Kato et al. 2006). In order to assess the impact of the surface conductive sediment in the blank areas to the present MT data, we filled these areas (purple dashed line in Figure 4, depth 0.5 to 3.0 km) in the inverted model with conductor (5 Ω m) and calculated MT impedances (‘test C-1’ in Figure 3). The calculated impedances are slightly changed from these of the inverted model except for the long-period impedances in the eastern part of C-1 area where deep conductors such as C-4 and C-5 also affect the long-period MT responses as we discuss later. This indicates that the present MT data are hard to detect C-1 in the blank areas, and thus, conductors are possibly distributed. On the other hand, the resistive zone including R-1 beneath C-1 and the aftershock area is reliable regardless of the shape of C-1 conductor because it slightly affects the MT impedances above the R-1 while the MT responses are significantly changed when R-1 is covered with 30 Ω m (sites Y170 and I820 in Figure 3). The R-1 are interpreted as granites because these rocks are distributed beneath the Tertiary sedimentary rocks and are a basement rock of the NE Japan arc (e.g., Sato 1994).
The C-4 conductor is possibly required to explain out-of-quadrant phases in the YX component at sites Y200, Y210, and Y220 because the anomalous large phases are not explained when the C-4 conductor is filled with 300 Ω m (Figure 3). Similarly, out-of-quadrant phases observed at site I823 are not explained when C-5 is filled with 300 Ω m (Figure 3). On the other hand, strong channeling of telluric current due to the shallow conductor beneath the Kitakami Lowland (C-1) is also a candidate for the anomalous large phases because a shallow conductor complex sometimes induces out-of-quadrant phases (e.g., Ichihara and Mogi 2009; Ichihara et al. 2013). Indeed, the above hypothesis model that the conductor is inserted in the blank areas of the MT sites (Figure 4) increases YX phase in the anomalous phase areas (Figure 3). However, the true resistivity distribution around the Kitakami Lowland is difficult to obtain based on the present data because the shallow resistivity distribution is not constrained in the blank area of MT stations, as we discussed previously.
The aftershocks are dominantly distributed in the resistive zone but are slightly within the C-1 and C-2 conductors. Because these are interpreted as granitic and Tertiary sedimentary rocks and high-temperature areas, respectively, the aftershocks occurred in brittle areas but rarely in ductile areas. This indicates that the seismicity depended highly on three-dimensional elastic heterogeneity. As mentioned in the ‘Introduction,’ the magnitude and rupture area of the 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) are anomalously large for an earthquake occurring in a volcanic area where ductile zones are generally distributed. However, this study has indicated that the ductile zones related to volcanic activities are patchily distributed and that the size of the brittle area is large enough for M 7-class earthquakes to occur. These elastic heterogeneities may also have been responsible for the earthquake occurrence in a different way because elastic heterogeneities may result in local stress concentration zones and can cause faulting (e.g., Ichihara et al. 2008, 2013; Iio et al. 2002). These interpretations imply that the MT method can detect elastic heterogeneities that may control the occurrence and magnitude of the large inland earthquakes. Therefore, three-dimensional resistivity modeling based on MT surveying is important for understanding earthquake occurrences.
We conducted magnetotelluric surveys at 14 stations around the focal area of the 2008 Iwate-Miyagi Nairiku earthquake (M 7.2). Based on the MT impedances along four profiles by the present and previous studies, a preliminary 3-D resistivity model was obtained using WSINV3D code. The resistivity model showed a shallow conductive zone (C-1) and a few distinct conductive areas around the focal area (C-2, C-3a, C-4, and C-5). C-1 was interpreted as Tertiary sediment based on its geological distribution. C-2 and C-3a possibly indicate high-temperature zones related to volcanic activities beneath Mt. Kurikoma and Onikobe Caldera. Aftershocks were distributed mainly in the resistive zone and not in the aforementioned conductive zones, which implies that elastic heterogeneity due to volcanic activity and geology may control the magnitude and occurrence frequency of such earthquakes. However, this study could not constrain the precise resistivity distribution in the blank areas of MT stations. Thus, dense surveys between the existing profiles of MT stations are required for more detailed interpretations.
We thank the landowners in the study region for their permission to establish observation sites on their land. H. Shinohara of Akita University provided assistance with the data acquisition. The Nittetsu Mining Consultants Co., Ltd. provided us with their continuous geomagnetic records at the Sawauchi station as remote references. The Geographical Survey Institute provided the MT impedance at the Esashi station. Prof. Okada, Graduate School of Science, Tohoku University, provided the seismicity data. Generic Mapping Tools software (Wessel and Smith 1998) was used to draw some of the figures. The manuscript was improved by thoughtful comments from two anonymous reviewers. This research was supported by the Grant-in-Aid 20900001 (Kaken-hi) and the Multidisciplinary Research Project for High Strain Rate Zones of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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