Seismic activity near the Moriyoshi-zan volcano in Akita Prefecture, northeastern Japan: implications for geofluid migration and a midcrustal geofluid reservoir
© Kosuga; licensee Springer. 2014
Received: 22 November 2013
Accepted: 15 July 2014
Published: 25 July 2014
The 2011 off the Pacific coast of Tohoku (Tohoku-oki) earthquake caused increased seismicity in many inland areas in Japan. A seismic cluster north of the Moriyoshi-zan volcano in Akita prefecture, Tohoku District, is of interest in light of the contribution of geofluids to seismic activity. We observed a seismic cluster characterized by the migration of seismicity and reflected/scattered phases. We relocated hypocenters of the cluster using data from temporal observations and the hypoDD location technique, which significantly increased the hypocentral accuracy. We interpreted a complex spatiotemporal variation of seismicity in the cluster as the migration of pore fluid pressure from multiple pressure sources. The hydraulic diffusivity of the cluster was in the range of 0.01 to 0.7 m2/s and increased with time, implying that the migration of hypocenters accelerated after a pathway for fluids was formed by fracturing of the wall rock during the initial stage of seismic activity. A prominent feature of the seismograms is a reflected/scattered phase observed at stations around the volcano. We regard the phase as S-to-S scattered waves and estimated the location of the scatterers using a back-projection method. The scatterers are inferred to be located about 5 km northwest of the Moriyoshi-zan volcano, at an approximate depth of 13 km. The Moriyoshi-zan area is one of the source areas of deep low-frequency earthquakes that have been interpreted as events generated by the migration of geofluids. The depth of the scatterers is close to the upper limit of the depth at which low-frequency earthquakes occur. Thus, we interpret the observed scatterers to be a reservoir of geofluid that came from the uppermost mantle accompanying contemporaneous low-frequency earthquakes.
KeywordsThe 2011 off the Pacific coast of Tohoku earthquake Triggered seismicity Hypocenter migration Scattering, Geofluid
The 2011 off the Pacific coast of Tohoku earthquake (MW 9.0) induced a tsunami that struck along the coast from the Tohoku to Kanto Districts on Honshu Island, Japan, causing indescribable, severe damage. This megathrust earthquake, with a fault length of up to 500 km along the Japan Trench, brought significant seismicity changes in areas far from the source fault. Hirose et al. (2011) comprehensively investigated the change in seismic activity generated by this earthquake and observed increased activity in many areas from Hokkaido to Kyushu, in particular, in and around the Kanto District. They noted that areas of triggered seismicity tend to be distributed along the volcanic front, and that the start time of seismic activity in different areas is variable. Many researchers have investigated the characteristics of triggered seismic activity in the areas around the Kanto District (Ishibe et al. 2011; Yukutake et al. 2011a), along the eastern coast from Ibaraki to Fukushima Prefectures (Kato et al. 2011; Imanishi et al. 2012), and the Tohoku region (Okada et al. 2011; Kosuga et al. 2012; Terakawa et al. 2013; Okada et al. 2014). The observed seismicity may be triggered by the dynamic effect of the seismic wave of the mainshock (Miyazawa 2011, 2012), by the static effect of crustal deformation caused by megathrust faulting (Kato et al. 2011; Kosuga et al. 2012), and by weakening of inland faults due to the associated increase in pore fluid pressure (Okada et al. 2011; Terakawa et al. 2013).
In this study, we first relocate the hypocenters in the Moriyoshi-zan area using the combined arrival time data of the permanent and the temporary stations and by employing the hypoDD location technique (Waldhauser and Ellsworth 2000). We then investigate the migration of hypocenters in detail using the relocated hypocenters, considering the accuracy of the hypocenter locations. We interpret the spatiotemporal variation of the hypocenters based on the migration of fluid pressure. The other main purpose of this paper is to examine the source location of scattered waves from the clusters around the volcano. We apply a back-projection method to obtain an image of the strong scattering zone using the average residual root mean square (RMS) envelopes, that is, the difference between the observed and theoretical envelopes. Considering the observations of earthquake migration and the location of scatterers, we discuss the influence of geofluids on seismicity in the Moriyoshi-zan area.
The Moriyoshi-zan is a Quaternary volcano with a height of 1,454 m located about 20 km west of two active volcanoes, Akita-Yakeyama and Hachimantai, that form the volcanic front running N to S in the central part of the Tohoku region (Figure 1b). According to Nakagawa (1983), the volcanic activity of the Moriyoshi-zan volcano can be divided into two stages. In the early stage, which occurred during the middle Pleistocene, pyroclastics with a small amount of lava flow erupted to form a cone-shaped stratovolcano, followed by the formation of an ellipse-shaped caldera. In the later stage, lava flows effused, and lava domes extruded near the caldera wall. No volcanic activity is observed at present.
Temporary observation in the Moriyoshi-zan area
Results and discussion
Migration of seismic activity
where r is the distance from the pressure source point, t is the time after the pressure increase, and D is the hydraulic diffusivity (Shapiro et al. 1997). In the case of earthquakes triggered by pore pressure diffusion, r is the distance to the front, and the majority of earthquakes are distributed within this distance (Shapiro et al. 1997). For all the events in Figure 7a, in particular in the later stage, a D value of 0.01 m2/s is in good agreement with the observed migration rate. However, during the initial stage, a larger D value of 0.05 to 0.1 m2/s is considered more appropriate.As shown in Figure 6, the spatiotemporal variation of seismic activity is very complex, comprising repeated formation of small clusters over brief time periods. Thus, we tried to explain the complex spatiotemporal distribution of seismicity by sequences of seismic migration from multiple pressure sources with variable hydraulic diffusivity. We used the following procedure to estimate four unknown parameters for each pressure source. The parameters are the location of pressure source, time of pressure increase, and hydraulic diffusivity. First, we set a period of curve fitting by specifying a pair of starting and ending events. Because Equation 1 constrains the seismic migration front, we divided the hypocentral data in the period into several time bins and used the event with the greatest distance in each bin for the curve fitting. Next, we performed a grid search to seek the unknown parameters. We set 3-D grids beneath the initial hypocenter, which may be a natural hypothesis to consider the upward migration of geofluids. Best parameters were selected as those with the smallest RMS residual between the observed and estimated distances. We conducted the above procedure by varying the period and obtained a location, time, and residual for every period. Considering the hypocenter accuracy, we performed the fitting if the distance change in the period exceeds 300 m, which is approximately twice the size of the absolute error ellipsoid of the hypoDD location. Finally, we selected the sequences in increasing order of RMS residuals, avoiding sequences whose periods overlap with those previously selected. We checked the range of distance changes for the selected sequences and found that the range was much larger than 300 m in most cases.
Comparison with previous studies
In this paper, we examine the triggered seismic activity after the Tohoku-oki earthquake in the Moriyoshi-zan area. Okada et al. (2011) also examined seismic activity in the wider Tohoku region and found that some areas of increased seismicity are located above the edge of the low-velocity zone in the lower crust. This led them to suggest that the observed activity is due to heterogeneity in the lower crust and the presence of overpressurized fluid (Miller et al. 2004). According to Okada et al. (2014), the earthquake cluster studied in this paper is also located above the edge of the low-velocity zone beneath the Moriyoshi-zan volcano. Kosuga et al. (2012) reported the coseismic rotation of the stress field in the northern Tohoku region by the Tohoku-oki earthquake. Since the calculated coseismic stress change in the region was smaller than 1 MPa (Yoshida et al. 2012), Terakawa et al. (2013) considered that the pore fluids play an important role in the activation of local seismicity and coseismic stress rotation in the central Akita region. In this study, we provide additional observational data for considering the role of geofluids using hydraulic diffusivity and the locations of scatterers as candidates for geofluid reservoirs.
The remotely triggered seismicity after the 2002 Denali fault earthquake (MW = 7.9) has been extensively examined both in volcanic and non-volcanic areas. Short-term (hours to days) increases in seismicity were observed in Yellowstone (Husen et al. 2004), Mt. Rainier (Prejean et al. 2004), the Geysers and Coso geothermal fields (Prejean et al. 2004), and Mammoth Mountain, Long Valley Caldera (Johnston et al. 2004). Several researchers have proposed a variety of physical models that might generate remotely triggered earthquakes in the volcanic areas. The models involve changes in crack conductivity and pore fluid pressure in the hydrothermal system (e.g., Brodsky et al. 2003), changes in fluid pressure by the oscillation of bubbles (e.g., Linde et al. 1994; Brodsky et al. 1998), changes in the state of magma bodies (e.g., Linde et al. 1994), and changes in the friction across a fault surface (e.g., Gomberg and Davis 1996; Voisin, 2002). Since the earthquake swarms in Long Valley Caldera and Mount Rainier seem to represent a delayed response to the Denali fault earthquake, Prejean et al. (2004) suggested that earthquakes may be triggered by more than one physical process. Since the Moriyoshi-zan is a Quaternary volcano with no volcanic activity at present, some of the above models relating to magma and gas cannot be applied to the area. The resultant models may be related to geofluid or fault properties. Our observation of hypocenter migration and S-to-S scattered waves suggest the contribution of geofluids.
We interpreted the hypocenter migration to be the result of the propagation of pore fluid pressure. The hydraulic diffusivity D was 0.01 m2/s across the entire investigation period and 0.05 to 0.1 m2/s for the initial stage of seismic activity (Figure 7a). For shorter time intervals, D values varied from 0.01 to 0.7 m2/s and were mostly <0.4 m2/s (Figure 8). D values were also estimated by previous studies using natural, induced, and injection-triggered earthquakes. The D values are 0.4 to 7 m2/s on the basis of reservoir-induced seismicity (Simpson et al. 1988), 0.5 m2/s from water injection-induced seismicity (Shapiro et al. 1997), and 0.27 m2/s (Parotidis et al. 2003) and 0.5 to 1.0 m2/s (Yukutake et al. 2011b) from the spatiotemporal distribution of swarm activity. The D values estimated for the Moriyoshi-zan area are considered comparable to those in other regions.
Seismograms from the events in the Moriyoshi-zan area exhibit prominent S-to-S scattered waves from the scatterers located approximately 5 km northwest of the Moriyoshi-zan volcano and at a depth of 13 km (Figure 13). Hori and Hasegawa (1991) observed a phase similar to the X-phase in seismograms from an earthquake swarm in 1982 near the volcano. They interpreted the phase as an S-to-S reflected wave and estimated the location of the reflector using travel time data. They estimated several reflectors (dashed lines in Figure 13) from different sets of event-station pairs. The reflector labeled ‘a’ in Figure 13 is very close to the scatterers estimated in this study. Hence, we propose that the observed scatterer/reflector probably represents the same geofluid reservoir. It is interesting to note that the scatterers found in this study are slightly shallower than the reflector estimated by Hori and Hasegawa (1991). This depth discrepancy may be caused by the difference in the locations of earthquakes and stations and by the different methods and velocity structures used in the two analyses.
The Moriyoshi-zan area is one of the source areas of DLF earthquakes that occur beneath active volcanoes in northeastern Japan (Hasegawa and Yamamoto 1994; Kamaya and Katsumata 2004; Takahashi and Miyamura 2009). The stars in Figure 13 denote the hypocenters of DLF earthquakes in the JMA catalog. The JMA started flagging low-frequency events in their catalog after 1998. The location of the events is approximately 15 km WSW of the Moriyoshi-zan volcano at a depth range of 20 to 40 km. The observed source area has a vertically elongated shape, which is typical of DLF events in northeastern Japan (Takahashi and Miyamura 2009). The DLF earthquakes that occur well below the elastic-plastic boundary are interpreted as events generated by the activity of geofluids (e.g., Hasegawa and Yamamoto 1994). The upper depth limit of low-frequency earthquakes is close to the depth of geofluid reservoirs, though the location is different in the map view. Based on this assumption, we can imagine the path of a geofluid as follows; the geofluid moves up accompanying DLF earthquakes and accumulates in the middle crust, forming geofluid reservoirs; then, the geofluid moves further up towards the seismogenic zone, if the conditions of fluid migration are fulfilled. Hasegawa et al. (2005, 2012) has already proposed a similar idea involving the contribution of geofluid to the generation of large inland earthquakes. Our investigation in this paper supports this idea by providing a detailed study of the activation of local seismicity after the Tohoku-oki earthquake and the distribution of scatterers estimated by back-projection.
If the seismicity was triggered by geofluids, we need to explain why there is no seismicity and no scattering between the reservoir and the pressure sources in Figure 13. As mentioned in the section ‘Hypocenter location’, the aseismic zone forms a conical shape centered at the volcano. The fluid pathway is located in this aseismic zone; hence, seismicity was not observed. The behavior of scattering is dependent on both the wavelength and the size of the scatterers. In the back-projection analysis, we used envelopes with a predominant frequency of 16 Hz, for which the wavelength is on the order of 100 m. Considering the duration of the X-phase was up to 1 s, the reservoir may be assumed to be an aggregate of scatterers of this size. Furthermore, the size of the pathway from the reservoir may be considerably smaller and invisible at frequencies equal to or lower than 16 Hz. However, direct evidence is lacking for the existence of geofluid in the postulated reservoir. A plausible approach to the problem is a close examination of scattered phases and their temporal variation. In Figure 10, we aligned seismograms on the catalog P-wave picks, which is not adequate for that purpose. The alignment and stacking of waveforms of similar earthquakes based on the CC picks may be useful, as Rowe et al. (2004) and DeShon et al. (2007) have shown. Thus, increasing the spatial resolution of hypocenters and pressure sources as well as the scattering structure near the geofluid reservoir would be the next step of this study, using waveforms of smaller earthquakes we have not analyzed in this paper.
We examined the triggered seismic activity in the Moriyoshi-zan area after the Tohoku-oki earthquake based on hypocenters relocated using the combined data of permanent and temporary observations and the hypoDD location technique. We interpreted the spatiotemporal variation in seismic activity as the result of the migration of fluid pressure. We estimated a hydraulic diffusivity of 0.01 to 0.7 m2/s, which is comparable to other regions. For shorter time periods, we found that the diffusivity increased with time. We observed prominent S-to-S scattered waves and estimated the locations of scatterers using the back-projection method. These scatterers were located approximately 5 km northwest of the Moriyoshi-zan volcano, at a depth of 13 km. This depth is close to the upper limit of the DLF earthquakes, which are thought to be related to geofluids. Thus, we regard the scatterers as reservoirs of geofluids that rise from the uppermost mantle accompanying low-frequency earthquakes and as the sources of overpressure fluids that caused the migration of seismicity.
I used hypocentral parameters and phase picks from the JMA catalog, which was prepared by the JMA and the Ministry of Education, Culture, Sports, Science, and Technology in Japan. The catalog hypocenters were determined using data from JMA, NIED, Hirosaki University, Tohoku University, Hokkaido University, University of Tokyo, and Aomori Prefecture. I thank the National Research Institute for Earth Science and Disaster Prevention (NIED) and Tohoku University for providing waveform data. I thank Kazuma Masukawa, Masaaki Chiba, and Katsuhito Sato of Hirosaki University for their contribution to the observations and the acquisition of data. Constructive comments from two anonymous reviewers were helpful in revising the manuscript. All figures were drawn using the Generic Mapping Tools (GMT) developed by Wessel and Smith (1998). This work was supported by JSPS KAKENHI Grant Number 21109002.
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