Spatial distribution of crack structure in the focal area of a volcanic earthquake swarm at the Hakone volcano, Japan
© 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. 2013
Received: 16 February 2012
Accepted: 10 May 2012
Published: 19 February 2013
We have performed shear wave splitting analyses for seismograms recorded at stations located just above, and outside, the focal area of the earthquake swarm at the Hakone volcano, Japan, in August 2009. Average values of the direction of faster split shear wave polarization (Φ) at two stations above the focal area correspond to each focal alignment of the earthquake swarm. In contrast, average values of Φ at three stations outside the focal area correspond to the direction of the maximum horizontal compressional stress. We found that the average values of the time lag between the two split shear waves inside the focal area are relatively high compared with those outside the focal area. These facts suggest that cracks with a high density aligned parallel to the faults of the earthquake swarm in the focal area. Crustal fluid was selectively injected into this pre-existing cracked media accompanied by effective normal stress reduction in the cracks, resulting in the earthquake swarm.
It is generally believed that earthquake swarms are caused by the migration of crustal fluid (e.g., Nur, 1974). Earthquake swarms sometimes occur in geothermal, or volcanic, areas, e.g., in the Long Valley Caldera of California (Savage and Cockerham, 1984) and the Yellowstone volcanic field of the western United States (Farrell et al., 2009). Hill (1977) proposed a model in which volcanic earthquake swarms are caused by faults arranged according to the local stress field, with magmatic/fluid intrusions in brittle rock. Sibson (1996) suggested a model of swarm activity in which earthquakes are triggered by fluid migration through a highly permeable structure. It is therefore important to investigate crack distribution related to the swarm to understand its occurrence in volcanic areas.
Many intense earthquake swarms have been reported in the Hakone caldera, Japan. There were major swarms in 2001, 2006, and 2009 (Tanada et al., 2002, 2007; Yukutake et al., 2011). Mannen (2008) put forth a model in which increased pore pressure in cracks, caused by hydrothermal injection, generated the Hakone volcano swarms. The 2009 swarm activity was from 4 to 12 August. Yukutake et al. (2011) located the hypocenters of 1156 events during this period with the Double-Difference method. Hypocenters of the 2009 swarm were distributed on two vertical planes. The activity migrated from the depths, and the north, toward shallow zones and the south. This hypocenter migration also suggests a relationship between the swarm and crustal fluid (Yukutake et al., 2011).
Shear wave splitting is a phenomenon in which an S-wave propagating through anisotropic media splits into two quasi S-waves. Each quasi S-wave polarizes into orthogonal directions with different velocities. In the case of shear wave splitting caused by the preferred orientation of cracks, the quasi S-wave polarized parallel to crack orientation travels faster than the other quasi S-wave. The orientation of faster S-wave polarization is therefore parallel to the direction of maximum horizontal compressional stress (e.g., Crampin, 1987; Kaneshima et al., 1987). In contrast, the anisotropic behavior near faults is different from that around the fault just after the earthquake occurrence. The orientation of faster S-wave polarization is parallel to the fault strike (e.g., Zhang and Schwartz, 1994). Tadokoro et al. (1999) concluded that the fault-parallel direction of faster S-wave polarization is caused by new fractures generated by shear faulting during the mainshock. Shear-wave splitting analysis is the favored method to detect local seismic structure, as well as to estimate the direction of the tectonic stress field.
We performed shear-wave splitting analysis for seismograms recorded at stations located just above and outside the focal area of the earthquake swarm at Hakone volcano in August 2009, to depict crack distribution and address the relationship between crack structure and swarm occurrence.
2. Data and Method
The number of data and results of S-wave splitting analysis.
Number of events
143 ± 3
86 ± 2
148 ± 2
86 ± 3
113 ± 2
86 ± 2
105 ± 3
49 ± 4
130 ± 4
43 ± 7
143 ± 3
52 ± 5
156 ± 7
54 ± 12
3. Results and Discussion
The average and standard error of Φ at stations just above the focal area (KZR and T.OSS) are shown in Table 1 and Fig. 1. The angle Φ is measured clockwise from the north. Since we used almost the same events at KZR and T.OSS, the Φ difference at the two stations obviously reflects a difference of anisotropy in the paths. Yukutake et al. (2011) detected focal alignment by the Double-Difference method and concluded that the alignments correspond to small faults. Directions of the northern and southern focal alignments were N110°E and N155°E, respectively. Average Φ at T.OSS (105 ± 3°) was almost parallel to the direction of the northern focal alignment (N110°E).
In addition, we found a variation of Φ with azimuthal angle at KZR. We divide the events of the KZR analysis into the following two groups: (1) events belonging to the focal alignment just below KZR, with an azimuth parallel to the focal alignment below this station (N22°W-N8°E) (Group-I); and (2) other events (Group-II) (Fig. 1(b)). Averages of Φ for Groups I and II are 148 ± 2° and 113 ± 2°, respectively; these values are significantly different. Because the observed shear-wave splitting reflects anisotropy of the ray path, the difference of Φ between Group-I and Group-II shows a different origin of anisotropy on each ray path. Most of the ray paths from Group-I pass through the focal region beneath KZR. Thus, results from Group-I events indicate anisotropy in the region, along with the focal alignment. Group-II manifests anisotropy outside the focal region (the region between the northern focal alignment and KZR). The average Φ for Group-I (148 ± 2°) was almost parallel to the direction of southern focal alignment (N155°E). The fact that Φ averages at T.OSS and for Group-I at KZR nearly parallel the direction of each focal alignment beneath the stations suggests that cracks aligned parallel to the faults of the earthquake swarm were distributed inside the swarm focal region. The crack structure indicates a fine structure in the small faults reported by Yukutake et al. (2011).
Averages, and a standard error of Φ, at three stations outside the focal area are also shown in Table 1 and Fig. 1. The Φ averages at KIN and KZY parallel to the direction of the maximum horizontal compressional stress (N139°E) in the study area (Yukutake et al., 2007). The rose diagram at KOM in Fig. 1 has two peaks. One of the two peaks is at 135°, parallel to the direction of maximum horizontal compressional stress. This parallelism indicates that cracks generated by the tectonic stress field prevailed outside the focal area of the swarm. Honda and Tanada (2006) also reported several peaks of the Φ direction at KOM from analysis for the period 2002 and 2005, similar to the present result. It is believed that the sparse distribution of Φ reflects the universal crack distribution just below KOM.
Yukutake et al. (2011) concluded that the 2009 swarm activity was caused by water migration in the source region. The highly-fractured medium can be the cause of the crustal fluid migration into the source region: the preexisting cracks whose orientations are parallel to the epicenter alignment with high density caused the crustal fluid to inject selectively into the cracked media, resulting in pore pressure increase (normal stress reduction) and the 2009 swarm. Daniel et al. (2011) reported that effective stress decreased nearly 8 MPa under constant shear stressing-rate conditions caused by pore pressure increase, during the 2003–2004 Ubaye seismic swarm activity in France. Nodal planes of the focal mechanism revealed by Yukutake et al. (2011) are parallel to strikes of the northern and southern focal alignments. These findings support our idea.
We found a spatial variation of crack orientation and density related to the 2009 earthquake swarm at Hakone volcano. Crack orientations inside the focal area were parallel to each fault strike of the swarm. In contrast, the orientation outside the focal area corresponded with the direction of the original maximum horizontal compressional stress. Crack density inside the focal area is extremely great, 3.5–5.9%. We conclude that crustal fluid selectively injected into preexisting cracked media of high crack density, accompanied by effective normal stress reduction in the cracks, resulting in the earthquake swarm. S-wave splitting analysis in the focal area of a volcanic earthquake swarm is a useful means to investigate the relation between crack distribution and swarm occurrence.
The comments from Professor Martha Kane Savage and an anonymous reviewer are instructive in improving our manuscript. We used the Generic Mapping Tool (Wessel and Smith, 1995) to draw the maps. This research was partly supported by grants-in-aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 21740328).
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