Dense Seismic Array Observations Along the Hanno- Misaka Line, Kanto Mountains, Central Japan

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1 Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2.,**3 pp.,+1,,1 * +, + +, + Dense Seismic Array Observations Along the Hanno- Misaka Line, Kanto Mountains, Central Japan Eiji Kurashimo *, Hiroshi Sato, Susumu Abe +, + +, + Earthquake Research Institute, University of Tokyo JGI, Inc. and Naoko Kato Abstract In central Japan, the Philippine Sea plate (PHS) subducts beneath the Tokyo Metropolitan area in the Kanto region. In the western Kanto region, the Izu-Bonin arc within the PHS has been colliding from the south with the Honshu arc, forming a complex structure called the Izu collision zone (ICZ). The geometry of the subducting PHS and the overlying crustal structure of the ICZ are important to constrain the process of earthquake occurrence. To obtain detailed images of PHS beneath the northern part of the ICZ, dense seismic array observations were carried out along a 0* - km-long line between Hanno and Fuefuki trending ENE-WSW. Seventy-six three-component portable seismographs were deployed on the survey line at /** to 1/* m intervals, and waveforms were recorded during a four-month period from October,**2. High-quality data were obtained for the analysis using receiver function and seismic interferometry to obtain images of PHS. We relocated,0- local earthquakes with arrival time corrections at each station. The relocated hypocenter distribution shows that a double seismic zone associated with the westward subducting Pacific plate is located at a depth of +**,** km beneath the survey line. The seismicity associated with the northwestward subducting PHS can be traced to a depth of about -* km beneath the Tanzawa Mountains. Key words: Philippine Sea plate, Izu collision zone, dense seismic array observation + 1,**1 /,**3 Wu et al.,,**1 1 -* 1* * ekura@eri.u-tokyo.ac.jp ++- **-,

Fig. +. Location map of the,**2 dense seismic array observation. The study area is indicated by solid square (insert map). The white lines indicate surface trace of the faults.

2 Fig. +. Location map of the,**2 dense seismic array observation. The study area is indicated by solid square (insert map). The white lines indicate surface trace of the faults. The broken lines indicate the prefectural boundaries. Blue diamond symbols indicate temporary seismic stations (this study). Dark green and purple lines denote the deep seismic reflection survey line (Sato et al.,,**/ ; Sato et al.,,**0).,**/ Fig. + -*.* km,**0,**2,**-,**2 Fig. + +,,, km Sato et al.,,**/ Fig

3 Table +. Specification of observation systems., Digital-nG /2 0* km +, Nominal +** ngpeak+*.** Hz *., G Fig. + +,* db +*. Hz -/ km,/ km,**2 +* +1,**3, * km MEMS /** 1/* m 10 +*. Hz MEMS - Fig., Fig. - Fig., +,+ /** m km M *** m Ta- P ble + S Table, GPS Fig km M +/. /*. 1*. cm 0kms MEMS P S Digital-- +, 0 kms kms +, +* - 0/ mm +1/ mm +.. kg +1. -, AD P S Fig. - P S 219 j j

4 Table,. Specification of observation sites. 220

5 Table,. (Continued) S */. VpVs,2* +, - Fig...-.,2* P S P +,,-/+ S +/, 230 +,, 0*.,**2 +* +3,**3, 2., 0,0 P S Fig.. 0*+ Hypomh Hirata and Matsu ura, +321,/ km Fig. / + P,/ km VpVs +1-. S 221

Fig.,. Example of observed seismograms (Origin time:,**2/ ++ / +,+ : /3 : 3.,-, Latitude: -/. 2*0/ N Longitude: +-3. +.0/ E, +,+. 0- km deep, M JMA+. 1 ).

6 Fig.,. Example of observed seismograms (Origin time:,**2/ ++ / +,+ : /3 : 3.,-, Latitude: -/. 2*0/ N Longitude: / E, +,+. 0- km deep, M JMA+. 1 ). Horizontal axis is the source-receiver o# set. Automatic gain control with /-s window is applied. (TOP) UD component. (Middle) NS component. (Bottom) EW component. 222

Fig. -. Example of observed seismograms (Origin time:,**2/ +, /,-. : /. :,-. 0,, Latitude: -/. 0+1-N Longitude: +-2. 1+*2 E, +.. +3 km deep, M JMA+./). Horizontal axis is the source-receiver o# set.

7 Fig. -. Example of observed seismograms (Origin time:,**2/ +, /,-. : /. :,-. 0,, Latitude: -/. 0+1-N Longitude: *2 E, km deep, M JMA+./). Horizontal axis is the source-receiver o# set. Automatic gain control with /-s window is applied. (TOP) UD component. Reduction velocity is 0 km/s. (Middle) NS component. + /, Reduction velocity is -.. km/s ( 0*. km/s/ - ). (Bottom) EW component. Reduction velocity is -.. km/s. The timescale of the horizontal component record section is compressed by a factor of + /, - to allow easier comparison between the P and S wave record sections. Arrows show late arrivals. 223

8 Fig... Distribution of hypocenters determined by the JMA during the seismic array observation (from October +3,,**2 to February 2,,**3). During the seismic array observation, the JMA located 0*+ earthquakes in this area. The circle symbols are scaled to earthquake magnitude. The plus symbols indicate additional seismic stations used in this study for hypocenter determination. O-C P **++. S *.*2/ Fig. 0 O-C O-C O-C 1** m O-C O-C * O-C P *.**. S *.**, Fig. 1 /* km */. km Fig. /. P-wave velocity structure model used for hypocenter determination. The value of Vp/Vs is assumed to be

9 Fig. 0. Station corrections. Value of the station corrections are shown using a color scale. (a) P-wave. (b) S-wave. 225

10 Fig. 1. Map showing the location of hypocenter distribution determined in this study. The circle symbols are scaled to earthquake magnitude. Solid and broken arrows indicate the double seismic zone associated with the westward subducting Pacific plate and the seismicity associated with the northwestward subducting Philippine Sea plate, respectively. The plus symbols indicate additional seismic stations used in this study for hypocenter determination. km +** km PS - -* km - / +,,, km -*,**3.* km 0* km PS 226

11 ,**3 Hirata, N. and M. Matsu ura, +321, Maximum-likelihood estimation of hypocenter with origin time eliminated using nonlinear inversion technique, Phys. Earth Planet, Inter.,.1 /* 0+.,** /0. Sato, H., N. Hirata, K. Koketsu, D. Okaya, T. Iwasaki, T. Ito, K. Kasahara, T. Ikawa, S. Abe, T. Kawanaka, M. Matsubara, R. Kobayashi and S. Harder,,**/, Earthquake source fault beneath Tokyo, Science, -*3,.0,.0..,** ,.,**2 0* Wu, F., D. Okaya, H. Sato, and N. Hirata,,**1, Interaction between two subducting plates under Tokyo and its possible e# ects on seismic hazards, Geophys. Res. Letts., -.,L +2-*+,doi: +*. +*,3 /,**1GL-*10-. (Received January 2,,*+*) (Accepted March,,,*+*) 227

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