Hasegawa  Akira

Hori  Shuichiro

Umino  Norihito

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Hori, Shuichiro

Umino, Norihito

Hasegawa, Akira

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Bright S-Wave Reflections ExtensivelyDistributed Beneath the Backbone Range of theNE Japan Arc (Extended Abstract)著者 Hori  Shuichiro, Umino  Norihito, Hasegawa Akira雑誌名 The science reports of the Tohoku University.Fifth series, Tohoku geophysical journal巻 36号 4ページ 526-530発行年 2003-05URL http://hdl.handle.net/10097/45442TOhoku Geophys. Journ. (Sci. Rep. TOhoku Univ., Ser. 5), Vol. 36, No.  4, pp. 526-530, 2003Brig ht  S-  Wave Reflectors Extensively Distributed Beneath   the Backbone Range of the NE Japan Arc           (Extended  A  bstract) SHUICHIRO  HORT, NORIHITO  UMINO and AKIRA HASEGAWA Research Center for Prediction of Earthquakes and  'Volcanic Eruptions,          Graduate School of Science, Tohoku University,                Sendai  980-8578, Miyagi, Japan                 (Received December 20, 2002)   Distinct phases reflected from mid-crustal S-wave reflectors (S x S phases) are often observed in seismograms of nearby shallow earthquakes at stations of the Tohoku University seismic network. Figure 1 shows examples of these reflected phases.    We estimated locations of these S-wave reflectors (bright spots) by using arrival time differences between the reflected and direct waves. The S-wave reflectors are distributed in the whole area of NE Japan, that is, not only beneath volcanic area but also beneath active faults. Depths to the reflectors beneath volcanic areas are shallower than those beneath active faults. Deeper cut-off depth of the reflectors has a positive correlation to that of shallow microearthquakes. It seems that the cut-off depths of both the reflectors and earthquakes are principally governed by the geotherms in the crust (Hasegawa et  al., 2000).    Prominent S-wave reflectors are detected beneath the Senya fault which caused 1896 M7.2 Rikuu earthquake (Fig. 3). Some of them are located in the fault plane of the M7.2 earthquake which is estimated from a distribution of recent microearthquakes. Other horizontal reflectors are also detected in the deeper extension of the  fault they may be SH P  S.SH 0-1T ilki',;',141:11.1,'.1,11,.',."^,,,:,..:..,,,....,  ' ,I1' , •  f I,     ,,,...:,,.. /  ,,t ii, •  il..4.ft' , ,,.11  1 , ...,.4...SxS•,,,..,,, ....            Reflector  a                   •                     •  Reflector b •       '• •             ' •  •  •  •  •      SxS Reflector  c         1 Sec1S.  tS.  Fig.  1. Examples of seismograms showing SxS phases. Those SxS phases are    usually predominant in SH component seismograms. Amplitudes, predominant   frequencies and durations of SxS phases vary from place to place. Locations a,    b and c are shown in Fig. 3.527 140E  141E  142E y  s.^ (#1°   )`sliC31984/1 - 1999/7A Active volcano   Active fault40N 0  0 1 [MI  339N Fig.  2. Locations of vertical cross sections showing depth distribution of the    reflectors (NS, AA'). Triangles, circles and short lines denote locations of active    volcanoes, low-frequency microearthquakes and active faults, respectively. H:  0-40  km  1,70E13 279 38Na part of detachment fault. In the deeper extension of the fault of 1998 M5.0 Sendai earthquake, many reflectors are also detected (Umino et  al., 2002).  S--wave reflectors are also detected beneath the focal area of 1962 M6.5 northern Miyagi earthquake (Fig. 4). It seems that they connect the focal area of M6.5 event with hypocenters of low frequency microearthquakes occurring near the Moho discontinuity.    This close spatial relationship between S-wave reflectors (bright spots), earthquake faults and low frequency microearthquakes suggests that the bright spots are important for better understanding of deep slip process in seismogenic inland fault systems. 528Vp E  21   31 Senva  fault                                           }IP              • •]-.10:::                                        : . •                                                          : •  ME,  S  Sanyo  fault. . N ConradVs  20   3040 VpNs E  2[   3040kfr. o  Senva  fault  , N                                          ,-,...      -. „: . ' . . - • • ---  '-------------__  -  —  —  .  -.  -  -------  Air ,,,  .7.147e.Ak....   x  Conrad / Reflector 6  3  0 .!  a  -3  g 21.1351_60 1.75  1 , 1.70 1.56Fig. 3. Vertical cross sections of P and S wave velocity and Vp/Vs ratio along line    NS (Nakajima et  al., 2001). Vertical exaggeration is 2. Black and red circles    denote locations of shallow earthquakes and low frequency  microcarthquakes,    respectively. White lines show the temperature of the crust estimated from P    wave velocity perturbations (Hasegawa et  al., 2000). Active volcanoes and   Senya fault are shown by red triangles and thick line, respectively. Deeper cut-   off depths of reflectors and shallow earthquakes become locally shallow beneath    the volcanic areas. This leads us to infer that the cut-off depth is prescribed by    temperature there and the crustal temperature is considerably inhomogeneous in    horizontal direction. Arrows a, b and c in the upper  panne' denote locations of    reflectors which caused S x S phases shown in the seismograms in Fig. 1VpVs A  140.5E  0    artoopi.wv.10 E  20  E.  ar3040  A  140.5  E 0  iwook,111, 10 f  20VpNs30 40 010SG  .c 20  O.  a3040141.0 E             .4" A  140.5  EA' ConraMoho 141.0E  A'  7rEonrad mfa  141.0 EMohoA' / Reflector 6   ae 3  .2 -3 61.851.801.751.70 1.65Fig. 4. Vertical cross sections of P and S wave velocity and Vp/Vs ratio along line    AA' (Nakajima et  at., 2001).  Fault planes of 1962 M6.5 N-Miyagi EQ (Kono et  al., 1993) and 1996 Onikobe EQ (M5.9 &  5.7) are shown by gray rectangles (Umino    et  al., 1998). Red circles denote locations of low frequency microearthquakes.    S-wave reflectors are distributed just above the high Vp/Vs region which spreads    widely beneath the focal areas of 1962 and 1996 events.529530                              References Hasegawa, A., Yamamoto, A., Umino,  N., Miura, S., Horiuchi, S., Zhao, D., Sato, H.,  2000  : Seismic       activity and deformation process of the overriding plate in the northeastern Japan subduc-      tion zone.  Tectonophysics 319, 225-239. Kano, T., Nida, K., Matsumoto, S., Horiuchi, S., Okada, T., Kaihara, K., Hasegawa, A., Hori, S.,       Umino, N., Suzuki, M.,  1993  : Microearthquakes activity in the  focal area of the 1962       northern Miyagi earthquake (1962, M6.5) (in Japanese with English abstract), J. Seismol.        Soc. Jpn., 46, 85-93. Nakajima, J., Matsuzawa, T., Hasegawa, A., Zhao, D.,  2001  :  Three-dimensional structure of Vp, Vs       and Vp/Vs beneath the northeastern Japan  arc  : Implications for arc magmatism and      Fluids. J. Geophys. Res., 106, 21843-21857. Umino, N., Matsuzawa, T.,  Hari, S., Nakamura, A., Yamamoto, A., Hasegawa, A., Yoshida, T.,       1998,  1996  :  Onikobe Earthquakes and their relation to crustal structure (in Japanese with       English abstract). J. Seismol. Soc. Jpn., 51, 253-264. Umino, N., H, Ujikawa and A. Hasegawa,  2002  : Distinct S-wave reflectors (bright spot) detected       beneath the Nagamachi-Rifu fault, NE Japan. Earth Planets Space., 54, 1021-1026.

Bright S-Wave Reflections Extensively Distributed Beneath the Backbone Range of the NE Japan Arc (Extended Abstract)

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Bright S-Wave Reflections Extensively Distributed Beneath the Backbone Range of the NE Japan Arc (Extended Abstract)

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