Bull Earthq Res Inst Univ Tokyo Vol 2,**3 pp,33-*/ * +, + + + + +, + + Focal Mechanism Solutions of Micro- and Small Earthquakes Occurred in the Western Kanagawa Area Situated in the Izu Collision Zone + + + + + Toshikazu Tanada *, Youhei Yukutake, Ryou Honda, Masataka Harada, Hiroshi Ito, Satoru Nagai, Hidekazu Sugihara +, and Akio Yoshida, + + Hot springs Research Institute of Kanagawa Prefecture Department of Geosciences, National Taiwan University, R O C Taiwan Abstract Focal mechanism solutions of earthquakes in the western Kanagawa area are compiled and characteristics of fault types in the Izu collision zone are discussed Reverse, strike-slip, and oblique fault mechanisms are seen in the Izu collision zone Although only about +* per cent of micro- and small earthquakes, are associated with strike-slip fault mechanisms, the largest and the second largest earthquakes (M 0* and M /0 ) are of that type It was found that the direction of the P axis is somewhat di# erent in the western and eastern Tanzawa areas : in western Tanzawa the average direction of the P axis is NW-SE, while it is NNW-SSE in eastern Tanzawa We think this di# erence in the P axis direction reflects di# erences in tectonic conditions in those areas In the Ashigara plain area, mechanism solutions are mostly reverse and oblique faults with the P axis direction between E-W and NNW-SSE Key words: focal mechanism, Kanagawa Izu collision zone, Metropolitan Seismic Observation Network (MeSO-net) +,*+*,**1 /,**2 + 2 /,*+* MeSO-net MeSO-net P * e-mail: tanada@onkenodawarakanagawajp,/* **-+ /20 299
Fig + Location map of seismograph station Station and seismicity (,***-,**3) map in and around Kanagawa prefecture ; Hot Springs Res Inst of Kanagawa Pref, ; Earthquake Res Inst of Tokyo Univ, ; National Res Inst for Earth Science and Disaster Prevention Gray circles show seismicity from,*** to,**3 (M + ) Dotted lines show prefectural borders MeSO-net,**,, P T P +30* +33* P,**2,**0,***,**0 2 +322 Okada +33, +33, +333 PT + P, T 0* P 0* P T 3 -* +33*, 0, 3* P +,**,**1 +323,** - + Hardebeck and Shearer P 300
Fig, Classification of focal mechanism by areas of the western Kanagawa area The areas indicate the Western Tanzawa area (TW), the Eastern Tanzawa area (TE), the Ashigara Plain area (AP) and the Hakone Volcano area (HV) The area division map refers Fig - Focal mechanism was estimated by Yukutake et al (,**0) -a P,**,**1 +331 +*,**3 +, -,, P a - b + +, - 3 P - P / 0-3 P,**,**0,**0,**,**1 P 301
Fig - The P axis distribution in the western Kanagawa area (a) Modified map of Fig in Yukutake et al (,**0) (b) Result by Japan Meteorological Agency for earthquakes more than Magnitude * TW; Western Tanzawa area, TE ; Eastern Tanzawa area, AP; Ashigara Plain area and HV; Hakone Volcano area,**0, P / / 0 +322 3 / +331 ++ + M * ++ // +330-0 M +,*** /, M *,**1 1, M,**1 +* + M 3 / +323 +330 +331 ++ + M * ++ / M + P +32, + P +32 b a - b / P P, b,**0,**,**1,**0 P,**,**1 +32* +331 +*,**0 / 0 P b - b +* /, +32-2 2, 0* 0*, +32 +32 a / 0 P 302
Fig Focal mechanism distribution in the western Kanagawa area (a) Result by Japan Meteorological Agency for earthquakes of Magnitude * classes (b) Result by Japan Meteorological Agency for earthquakes more than Magnitude /* P +33* 2 / /- +33* 2 / /- / +3-- 1 P +322 Ishida and Kikuchi, +33, ; +33- P MeSO-net Ishida and Kikuchi, +33, ; +33-; +33- Ishida and Kikuchi +33, +33- P P /,**31 +0 M, +33- Hi-net / a MeSO-net b - b / b P P MeSO-net,**0 0 MeSO-net 303
0 +** km MeSO-net Fig / Focal mechanism of the earthquake which occurred at */ : +0, July +0,,**3 (M,) (a) The source mechanism which is obtained from initial P wave motions recorded at stations in Fig + (b) Same as (a) Except for illustrated initial P wave motions at MeSO-net stations (Gray and open circles with fat outline) / P Fig 0 Waveforms (UD) observed at MeSO-net stations The star shows the epicenter and black circles are MeSO-net stations 304
, 0 0*, /0 P Seno et al, +330 P P MeSO-net Ishida, M and Kikuchi, M, +33,, A possible foreshock of a future large earthquake near Odawara, Central Japan Geophys Res Lett, +3, +03/ +032 +33, -*,- + + -+0 +333,3 +, -*,*+* MeSO-net 2 1+ 22,*+* 2 +,1 +* +32 +32-2 2 -+ +03 +1, +323 +322 3 / + +1/ +13 +32 a, +32-2 2 -+ +1- +2- +32 b +32- ++ +32 -, 2* 3+ Okada, Y, +33,, Internal deformation due to shear and tensile faults in a half-space, Bull Seism Soc Am, 2,, +*+2 +** +33- +*, / /0 Seno, T, T Sakurai and S Stein, +330, Can the Okhotsk plate be discriminated from the North American plate?, J Geophys Res +*+, ++-*/ ++-+/,** P +/*,**1 /1 +,0 +-,,**2 +3 * 0/ 02,**2 +33*,**1 +- +21 +3,**0-2 0310 (Received February +1,,*+*) (Accepted March +0,,*+*) +330 +330-0 M / 0 /0 +1/ +2 +33- +*, --1-* Hardebeck, JL and PM Shearer,,**,, A new method for determining first-motion focal mechanisms, Bull Seism Soc Am, 3,,,,0,,10 +322 /2 No 3 /-1 /1 +33- +*, -2+ -3, 305