Seismicity and crustal structure in the vicinity of the southern Itoigawa-Shizuoka Tectonic Line
© 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 2010
Received: 13 January 2009
Accepted: 20 November 2009
Published: 4 March 2010
The Erratum to this article has been published in Earth, Planets and Space 2015 62:620040433
The southern part of the Itoigawa-Shizuoka Tectonic Line represents the boundary between the rapidly uplifting Akaishi range and the Kofu basin. The tectonic relation between the different parts of the fault system and the surrounding geological units is yet to be fully explained. In order to reveal seismic activity that might be related to the southern segments of the fault system, a seismic array observation was conducted in the autumn of 2005. The arrival times of 434 local earthquakes, two vibroseis shots and eleven explosive shots were used in a joint inversion to determine earthquake locations and a 3-D velocity structure of the crust. The relocated events are in good correlation with the estimated deeper extension of the active faults in the area. The seismic velocity model jointly obtained coincides with the geological structure in the area of study. In particular, a thick moderately low Vp zone was found beneath the Akaishi range and a high velocity zone beneath the Kofu basin. The moderately low velocity was attributed to the relatively young sedimentary rocks that form the accretionary prism units of the SW Japan arc and the high velocity to the older igneous rocks constituting the Izu-Bonin arc crust.
In the southern ISTL there have been no detailed studies that enable us to come to solid conclusions about the crustal structure in the area and the deeper extension of the faults. Imanishi et al. (2006) have solved the focal mechanism solutions of 83 events in the area and proved that they are in accordance with the regional deformation estimated by geological studies and by the slip sense of the ISTL faults. Nakamichi et al. (2007) have done a detailed P-wave velocity (Vp), S-wave velocity (Vs) and Vp/ Vs tomography study beneath Mount Fuji and some parts of their tomogram cover the east side of the ISTL fault system. Recently, Ikeda et al. (2009) conducted a shallow seismic reflection profile and gravity measurements along the ISTL in the Kofu basin. From their observations they have proposed that the southern ISTL forms a low angle thrust fault that extends into the lower crust forming an active nappe structure. On a regional scale (i.e., hundreds of km in scale), the structure of the crust and the upper mantle in central Japan has been revealed by tomography studies (Nakajima and Hasegawa, 2007; Kamiya and Kobayashi, 2007). The grid spacing in their studies, however, is limited by the spacing of the online seismic network stations in the area and is 0.2° and 0.1° in horizontal direction and 15 km and 8 km in depth respectively. Furthermore, the resolution of these studies does not provide adequate details on the structure of the crust along the ISTL fault system. There is also great controversy regarding the slip rate and recurrence interval of the various fault segments of the southern ISTL. Previously conducted trench excavations across the Ichinose fault, revealed dip slip rates as low as 0.3 to 0.5 mm/yr and a recurrence interval greater than 5,000 yr (Toda et al., 2000). Later trench excavations resulted in a recurrence interval of approximately 3,200 yr and combined with geomorphological observations set the total deformation rate up to 3.5–3.7 mm/yr (Miura et al., 2002). More recent seismic reflection profiling and gravity measurements across these faults suggest a slip rate as high as 7.5–11 mm/yr (Ikeda et al., 2009).
In order to reveal the crustal structure along the southern parts of the ISTL, it is essential to observe and accurately locate the local seismicity in the area. The present study uses seismic data observed by both the temporary and online station networks in order to image a local detailed velocity model for the southern ISTL region. The near-surface velocity model correlated well with the surface geological structures that have been published (Geological Survey of Japan, 2003) and their deep extensions were delineated. Also investigated is the relationship between the regional seismicity and the deep structure of the ISTL fault system. Finally, we proposed a model for the tectonic evolution of the southern ISTL.
2. Seismic Array Observation
The first linear array in the present study was deployed across the Wakamiya and Aoyagi faults and run for a 2-month period from 25 August 2003 to 16 October 2003 (Fig. 3(b)). The array consisted of 49 stations with a spacing of 1∼1.5 km and nine additional stations distributed in the surrounding area. We used 1 Hz-seismometers of which waveforms had been continuously recorded by removable HDD recorders that were run on alkaline batteries (Shinohara et al., 1997). The recorders utilized a Global Positioning System (GPS) receiver to maintain the accuracy of the internal clock of the recorder. This configuration enabled us to have a one month continuous recording at 200 Hz sampling rate before the batteries depleted.
In 2005 we moved further south and deployed 60 stations in two parallel linear arrays from 15 Sep. 2005 to 23 Dec. 2005. We covered the Hakushu fault area with a linear array of 25 stations and the Onajika-toge and Shimotsuburai faults with 30 stations at an average spacing of 1.5 km. In addition, five stations were deployed in between the linear arrays. Parallel to the linear array stations, the Geological Survey of Japan (GSJ, AIST) deployed eight offline stations scattered in the southern ISTL area that took continuous recordings from 8 Sep. 2005 to 30 Nov 2005 (Imanishi et al., 2006). In total 235 stations were used for the present study.
3. Data Analysis
The reliability of the tomography inversion results was evaluated by means of a checkerboard resolution test (CRT) (Humphreys and Clayton, 1988). We assigned ±5% velocity perturbations of the initial velocity model and 50 ms and 100 ms random noise for the P and S wave arrival time respectively, and calculated synthetic data using the source-receiver distribution as for the real data. The synthetic dataset was inverted with the initial unperturbated velocity model using the same inversion parameters. The checkerboard patterns were satisfactorily reproduced in a horizontal range of -30 ≤ × < 30 km and up to a depth of 20 km (Figs. 7 and 8). This area correlates with the area of our dense seismic array stations.
4. Results and Discussion
4.1 Correlation of the 3D velocity model and geology
The correlation of Vp with specific rocks cannot be interpreted uniquely. On a regional scale however, specific geologic formations can be recognized by their Vp trace. To the west of the geological ISTL the Vp ranges from 5.4 to 5.8 km/s. These values are within the range of values expected for the low grade metamorphic and sedimentary rocks comprising the Shimanto belt of the Cretaceous to Tertiary accretionary prism of the southwest Japanese Arc (Taira, 2001). Beneath the Kofu basin and between the geo logical ISTL and the tectonically active ISTL, the Vp ranges from 6.2∼6.8 km/s at deep depths, which greatly differs from the velocity range of the rocks of the Shimanto belt. From observations of the outcropping middle crust of the Izu-Bonin arc in the Tanzawa Mountains, we know that it mainly consists of felsic plutonic rocks (tonalite) (Taira et al., 1998; Kawate and Arima, 1988). The average Vp values to the east of the geological ISTL and below the Kofu basin fit the laboratory values for rocks of that composition (Christensen and Mooney, 1995). This evidence suggests that the rocks in the area between the geological and neotectonically active ISTL have the same or similar composition as of the Tanzawa tonalite and probably originate from the Izu-Bonin arc middle crust.
Focal mechanism solutions by Imanishi et al. (2006) are also depicted, suggesting a complex stress regime including both strike slip and reverse faulting. However, in general, as is seen in Fig. 8(a), near the ISTL the reverse fault type is dominant while a strike-slip fault type is dominant near the MTL (Imanishi et al., 2006). The last major event on the Hakushu fault is estimated to have occurred sometime between 6,650 and 7,000 years ago and the fault has recurrence interval of approximately 5,000 years (Toda et al., 2000). This is clear evidence that there is still strain accumulation at the deeper parts of the southern ISTL fault system although the main tectonic process that created these faults ended in the Miocene period.
4.3 Tectonic evolution of the southern ISTL Based on a high-resolution reflection/refraction survey, Ikeda et al. (2009) proposed that the southern ISTL forms an active nappe structure with a high slip rate. The neotectonically active strand of the ISTL (Shimotsuburai-Ichinose faults) forms a west-dipping low-angle (∼15°) fault that separates the Kofu basin fill on the east from the highly deformed Miocene rocks of the Koma block to the west. They have suggested that the active strand of the ISTL with the low-dip angle is likely to merge downdip to the high-angle geological ISTL further to the west. However, the deep geometries of these two faults were not well constrained (figure 6 in Ikeda et al. (2009)), because the reflection/refraction survey area was spatially limited.
We interpret that both geological and active fault ISTL are byproducts of the same major tectonic event that is the arc-arc collision in the Miocene, but express different stages in the collision. The geological ISTL could be a result of the middle Miocene accretion of the Koma terrane. In contrast, the neotectonically active ISTL could be associated with the underthrusting of the accreted parts by the middle crust of the Izu-Bonin arc in the middle Miocene to Pleistocene. Trench excavations across the Ichinose fault suggest that there is an increase of the rupturing in the recent several ten thousand years (Miura et al., 2002). In addition, the activity on the active strand of the ISTL dates back only to 2–3 Ma (Ikeda et al., 2009). These results imply that the active strand of the ISTL could be a reactivated fault within the accreted Middle-Late Miocene Izu-Bonin volcanic arc complexes, possibly the boundary between the Koma and Misaka blocks.
There have been still some discrepancies in interpretations about the tectonic evolutions associated with the ISTL between the previous study (Ikeda et al., 2009) and the preset study, further geophysical and geological studies in the area are required to provide us with information about the detailed whole structures associated with this fault system.
In order to reveal the tectonic process that controls the earthquake genesis mechanism in the Akaishi range region, two seismic station array observations were conducted across the southern segments of the ISTL active fault system. We revealed the detailed crustal structures, applying the tomography inversion analysis to arrival time data from 434 events observed by the temporary stations. The 3D velocity model obtained shows a thick moderately low Vp zone beneath the Akaishi range and a high velocity zone beneath the Kofu basin. The relocated hypocenters in the area of the Y = 8 km cross section have lined up along this velocity boundary. This boundary coincides with the deeper extension of the Hakushu fault of the ISTL fault system, which has not produced a major event for more than 6,500 years. The observed seismicity suggests that there is still strain build up at the deeper parts of the fault system.
The 3D velocity model is in accordance with the geological structures. The moderately low Vp zone fits with the surface expression of the low grade metamorphic rocks that constitute the Chichibu-Shimanto belts of the southwest accretionary prism of the Japanese arc, and the high Vp zone coincides with the igneous rocks that form the Izu-Bonin arc crust. Thus we can illustrate the downward continuation of the surface geological units and comprehend the tectonic evolution of the southern ISTL. The middle Miocene to late Pliocene collision of the Izu-Bonin arc with the outer zone rocks of the Japanese arc has produced an extensive thrust fault system that is identified by its surface expression as the southern segments of the ISTL fault system, and governs the predominantly reverse fault character of the earthquake mechanisms that are observed in the area. In the area of the Hakushu fault segment, we were able to image the ISTL as a velocity boundary in the crust and also correlate the local seismicity to that boundary. It forms a 35°-40° reverse fault that extends deep into the middle crust. In addition, the present study provides us with a clue to investigate the relation of the geological ISTL and its active strand, represented by the Onajika-toge fault and Ichinose fault respectively. The geological ISTL is interpreted to be the inactive Miocene boundary between the Cretaceous-Tertiary accretionary prism of the southwest Japanese arc and the Koma block of the Izu-Bonin volcanic arc. In contrast, the neotectonically active ISTL could be attributed to a reactivation of the boundary fault between the Koma and the Misaka blocks.
We are most grateful to GSJ, AIST members for the kind provision of the focal mechanism solutions and to H. Zhang for the use of the TomoDD tomography code. We thank the Japanese Meteorological Agency (JMA) and the National Research Institute for Earth Science and Disaster Prevention and the staff at the Earthquake Information Center of the Earthquake Research Institute of the University of Tokyo for providing the waveform data collected at the online stations. We also thank the GEOSYS and JGI seismic crew for helping with the data acquisition. Most of the figures in this paper were created using the GMT program (Wessel and Smith, 1998). This study is supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology under a special project entitled “A high priority investigation in the ISTL fault zone”.
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