Ambient noise tomography in the Naruko/Onikobe volcanic area, NE Japan: implications for geofluids and seismic activity
© Tamura and Okada. 2016
Received: 24 August 2015
Accepted: 22 December 2015
Published: 16 January 2016
To understand the earthquake generation in volcanic areas, it is important to investigate the presence of geofluids in the uppermost crust. We applied ambient noise tomography to the Naruko/Onikobe volcanic area and constructed a detailed 3-D S-wave velocity (V s) model using continuous records from a dense seismic network and surrounding stations. The low-velocity zones were found beneath Naruko Volcano, Onikobe Caldera, and Mt. Kurikoma. The low-velocity zone beneath Onikobe Caldera may correspond to a magma reservoir, which is also characterized by surrounding S-wave reflectors. The molten magma originates from the upwelling flows in the mantle wedge. We also conducted the relocation of aftershocks of the 2008 Iwate–Miyagi Nairiku earthquake by double-difference tomography based on the obtained velocity model. Beneath Mt. Kurikoma, aftershock distribution delineates one of the unfavorably oriented fault planes of the main shock, which implies that the low-velocity zone around the fault plane is related to the presence of overpressurized fluid.
This region is influenced by the Tohoku backbone range strain concentration zone along the volcanic front (Miura et al. 2004), and compressional inversion tectonics are predominant (Sibson 2009). This is explained by the relative motion of the plate subduction in the EW direction and the crustal softening due to geofluids such as molten magma and geothermal water in the crust (Hasegawa et al. 2009). Many normal faults were formed under the extensional stress regime during the Miocene when the Japan Sea opened, and they act as reverse faults under the current compressional stress regime (Okada et al. 2012). Consequently, several large shallow earthquakes with reverse-type focal mechanisms have occurred in and around the study area [e.g., the 2003 northern Miyagi earthquake (M 6.4) and the 2008 Iwate–Miyagi Nairiku earthquake (M 7.2)].
The 2008 Iwate–Miyagi Nairiku earthquake occurred in the central part of NE Japan on June 14, 2008. The earthquake was typical of inland (intraplate) earthquakes in NE Japan, and its magnitude was 7.2. The focal mechanism was reverse-type with a NW–SE-oriented P-axis. A large slip occurred along the westward dipping plane (e.g., Ohta et al. 2008), although conjugate faulting also occurred along the eastward dipping plane (e.g., Takada et al. 2009). The spatial extent of the aftershock area was approximately 60 km in the strike direction. The aftershock area extended further south of the large slipped area (e.g., Iinuma et al. 2009), and the study area corresponds to the southwest end of the aftershock area. Before the 2008 earthquake, preceding seismic events occurred on the same fault plane in the vicinity of the mainshock of the 2008 earthquake, including an M 4.5 earthquake on April 19, 1999, and an M 4.9 earthquake on February 11, 2000 (Okada et al., 2012).
Several seismic tomography studies have been carried out in and around the focal area, and distinct low-velocity zones (LVZ) with high P-wave velocity (V p)/V s have been recognized in the lower crust beneath volcanoes (Nakajima and Hasegawa 2003; Okada et al. 2010; Okada et al. 2012; Okada et al. 2014). They are considered to be molten magma originating from the upwelling flows in the mantle wedge. In addition, some LVZs with high V p/V s in the upper crust are thought to be overpressurized fluid (Okada et al. 2012). At present, however, the structures of the volcanoes and calderas remain unclear because seismic body-wave tomography cannot resolve structures in the uppermost crust. Detailed information on volcanic structures is therefore necessary to understand the relationship between probable geofluids and the generation of earthquakes.
With the development of seismic interferometry, which provides the Green’s function of wave propagation between a pair of stations from the correlation of ambient noise recordings, its application to tomography has developed greatly in recent years. The tomography technique is referred to as ambient noise tomography (ANT) and has been applied at various scales depending on the frequency ranges of microseisms and sensors. The microseismic band containing ambient noise ranges approximately from 0.05 to 1.0 Hz, at which the primary and secondary microseisms are generated at the sea floor with different mechanisms (Longuest-Higgins 1950; Hasselmann 1963). Because surface waves at these frequencies are sensitive to a few tens of kilometers in depth, ANT has been applied to many regions to study crustal structures (e.g., Shapiro et al. 2005; Moschetti et al. 2007; Lin et al. 2008; Li et al. 2009; Yang et al. 2011; Nicolson et al. 2012). At local scales, ANT has proven to be an effective tool for imaging volcanic structures (e.g., Masterlark et al. 2010; Jay et al. 2012; Nagaoka et al. 2012; Jaxybulatov et al. 2014; Mordret et al. 2015; Shomali and Shirzad 2015).
In this study, we perform ANT using continuous noise records observed by a dense seismic network and surrounding seismic stations. We present the results and discuss the presence of geofluids and their relationship to the aftershock distribution of the 2008 earthquake.
Data and methods
We used continuous velocity records from October 2010 to March 2012 observed by 14 permanent stations and 50 dense temporary stations (Okada et al. 2014). Figure 1 shows the station distribution in the study area. The permanent stations include 1-Hz short-period sensors operated by Hi-net/NIED and Tohoku University. The temporary stations, which we call the Naruko array, comprise short-period sensors with a natural frequency of 2 Hz and are operated by Tohoku University. The average interval of the Naruko array is a few kilometers. The sampling frequency of all the sensors used in this study is 100 Hz. We only used the vertical components of seismographs to extract Rayleigh waves, which have predominant sensitivity to SV-wave velocity. In our processing, ambient noise tomography was performed as follows: (1) preprocessing, (2) noise cross-correlation, (3) dispersion curve measurement, (4) tomographic inversion, and (5) construction of a 3-D V s model.
Numbers of ray paths selected by the data selection criteria at each frequency
Number of paths
Results and discussion
Hori et al. (2004) discovered many reflected S-waves SxS at velocity boundaries within the crust in seismograms of local earthquakes. They estimated locations of those boundaries based on arrival time differences between the SxS and direct S-waves with an accuracy of less than 100 m. These reflectors can be interpreted as thin fluid-filled cracks (Matsumoto and Hasegawa 1996; Umino et al. 2002) that would be produced under high pressure and filled with geofluids.
Figure 9b shows the V s model and the aftershock distribution of Okada et al. (2012) determined by double-difference tomography. In comparison with this model, the model in this study is able to resolve smaller velocity anomalies. As a result, the distribution of aftershock relocation is improved; some aftershocks at very shallow depths shift reasonably deeper (line C-C’), and others are more closely aligned on the fault trend (line D-D’).
A 3-D V s model determined by ANT revealed detailed velocity structures beneath volcanoes in the Naruko/Onikobe volcanic area. The LVZs in the uppermost crust are better resolved than in previous seismic tomography studies. Combined with the distribution of S-wave reflectors and aftershocks of the 2008 earthquake, the LVZs are characterized as molten magma or overpressurized fluid. The aseismic LVZ surrounded by S-wave reflectors beneath Onikobe Caldera may correspond to a magma reservoir. The molten magma, which is recognized in the lower crust as high V p/V s LVZs by previous seismic tomography studies, originates from upwelling flows in the mantle wedge. Around the LVZ beneath Mt. Kurikoma, aftershock distribution delineates a fault plane, which corresponds to one of the misoriented fault planes of the mainshock of the 2008 earthquake. As suggested in previous studies, the presence of overpressurized fluid is the plausible cause for the misoriented faulting, and therefore it is likely that the LVZ contains overpressurized fluid. These LVZs imply crustal softening and stress concentration in the Tohoku backbone range, and the presence of the geofluids characterizes different volcanic structures and complex faulting in the Naruko/Onikobe volcanic area.
This study was supported by a grant from the Scientific Research Program on Innovative Areas, “Geofluids: Nature and Dynamics of Fluids in Subduction Zones” at the Tokyo Institute of Technology (21109002). We acknowledge JMA and NIED for providing the data. We are grateful to T. Matsuzawa, J. Nakajima, N. Uchida, M. Yamamoto, T. Sato, S. Hori, K. Tachibana, T. Kono, S. Hirahara, T. Nakayama, S. Suzuki, T. Demachi, T. Kaida, and Y. Chiba for the operation of the Naruko array.
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