High-density Seismic Array Observations Across the Northern Focal Area of the,**2 Iwate-Miyagi Nairiku Earthquake

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1 Bull Earthq Res Inst Univ Tokyo Vol 20,*++ pp + +*,**2 + +, * , -, High-density Seismic Array Observations Across the Northern Focal Area of the,**2 Iwate-Miyagi Nairiku Earthquake + +, + + Eiji Kurashimo *, Hiroshi Sato, Susumu Abe, Takaya Iwasaki, Takashi Iidaka, Naoko Kato, Mamoru Saka, Shin Koshiya, Masaru Noda, Masato Serizawa, Fumiaki Sato, - - -, + Fumiko Watahiki, Taro Mogi, Rui Hanada, Taku Kawanaka, Shinichi Sakai, Aitaro Kato, Toshihiko Kanazawa +, - and Naoshi Hirata Earthquake Research Institute, the University of Tokyo JGI, Inc Faculty of Engineering, Iwate University Abstract On June +,,**2, the,**2 Iwate-Miyagi Nairiku Earthquake (M JMA 1,) occurred in the central part of northern Honshu, Japan The northern part of the focal area is near the Dedana fault, which is south of the western marginal faults of the Kitakami lowlands Seismic reflection/refraction experiments were conducted across the Dedana fault in,**0 and,**1 Deeper extensions of the west-dipping listric faults were well demonstrated on the seismic reflection section Revealing the relationship between an active fault system and aftershock distribution is important to constrain the process of earthquake occurrence We conducted high-density seismic array observations across the northern focal area to investigate the relationships between aftershock distribution and crustal structure We obtained high signal-to-noise ratio data Prominent late arrivals, probably reflected waves from the deeper part of the crust, can be recognized on the record sections The aftershock observations are summarized in this paper, and a detailed analysis showing the precise aftershock distribution and seismic velocity structure in and around the northern focal area of the,**2 Iwate Miyagi Nairiku earthquake will be published in a separate paper Key words: the Iwate-Miyagi Nairiku Earthquake in,**2, active fault system, aftershock observations +,** km,*,**2 M 1,,**2 / km +/ km JMA 0,**2 * ekura@eriu-tokyoacjp ++- **-,

2 ++/ MS,*** Sato, +33 Kato et al,,**0;, 2 LS2,** SD,**2 +,/ Hz +33+ LS2,** SD +,/ Hz + LS2,** SD LS2,** SD Kato et al, 2 MS,***,**0,**0,**1,/* Hz +/* m,11,**2;,**2 + km,* MS,*** - LS2,** SD +* km MS,*** + / MS,*** -,2, / Hz GS-++ D +* Hz SM-1 MS,*** + 0 ch MS,***, -,* km + MS,*** GS-++ D MS,*** GS-++ D SM-1 - GS-++ D, Hz L-,, D MEMS Table + - Fig,, /3*,**0,**1,**2 ;,**2 * km SEG-Y Barry et al, +31/ Fig +,**21 +2 ** +*, 1 +, +2 ** 2 LS2,** SD mkine,**0,- MS,*** mg 2

,**2 Fig + Location map of the,**2 Iwate Miyagi Nairiku earthquake The study area is indicated by a solid square (inset map) The red and blue lines indicate the seismic array using vertical-component

magnitude Fig - ++2 km M,+ Fig 22 km M * JMA 0 kms P S Fig - P S-P Fig 22 km JMA S-P + 3 Fig, Distribution of hypocenters located by the JMA during the period of seismic array observation (from +2 :

3 ,**2 Fig + Location map of the,**2 Iwate Miyagi Nairiku earthquake The study area is indicated by a solid square (inset map) The red and blue lines indicate the seismic array using vertical-component seismometers Blue circles indicate seismic stations with three component seismometers Aftershock hypocenters located by the JMA (,**2/ 0 / + 1 / + ) are shown as open circles scaled to the earthquake magnitude Fig km M,+ Fig 22 km M * JMA 0 kms P S Fig - P S-P Fig 22 km JMA S-P + 3 Fig, Distribution of hypocenters located by the JMA during the period of seismic array observation (from +2 : ** h on July to +2 : ** h on July +,, Japan standard time) During the period of seismic array observation, JMA located /3* events The circles are scaled to the earthquake magnitude Focal depths of the earthquakes are indicated with a color scale Crosses indicate the location of temporary seismic stations

4 Table + Specification of observation sites 4

5 ,**2 Table + (Continued) 5

6 Table + (Continued) 6

7 ,**2 Table + (Continued) 7

8 Table + (Continued) 8

,**2 Fig - An example of observed seismograms (Origin time:,**2/ 1 //,+ : *2 : +2 20*, Latitude: -3 ++32-N Longitude: +* 2-/-- E, Depth : ++ 2 km, M

phase are indicated by an arrow Fig An example of observed seismograms (Origin time:,**2/ 1 // +- : + : -0 0* Latitude: -3 +*3** N Longitude: +* 322

9 ,**2 Fig - An example of observed seismograms (Origin time:,**2/ 1 //,+ : *2 : +2 20*, Latitude: N Longitude: +* 2-/-- E, Depth : ++ 2 km, M JMA, + ) recorded on the EW-line (vertical component) The horizontal axis is the source-receiver o# set Reduction velocity is 0km/s Remarkable later phase are indicated by an arrow Fig An example of observed seismograms (Origin time:,**2/ 1 // +- : + : -0 0* Latitude: -3 +*3** N Longitude: +* 322 E, Depth : 2 2 km, M JMA *) recorded on the EW-line (vertical component) The horizontal axis is the source-receiver o# set Reduction velocity is 0km/s 9

10 ,** ,*,**2,**0,**1,**2 +, +,0 Barry, K M, D A Cavers and CW Kneale, +31/, Recommended standards for digital tape formats, geophysics, P S *, - -/, Kato N, H Sato and N Umino, Fault reactivaion and active tectonics on the fore-arc side of the back-arc rift system, NE Japan,,**0, Journal of Structural Geology,,2,,*++,*,, +33+ PS -1 p,**2,*,**2 1,**2 A ++ *+,**0, /3 +*1 ++0,**2,**2,** Sato, H, +33, The relationship between late Cenozoic tectonic events and stress field and basin development in northeast Japan Journal of Geophysical Research 33,,,,0,,,1,**2,**2 : +,1 +-* (Received November 2,,*+*) (Accepted November +1,,*+*) 10

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