TY - JOUR
T1 - Deep crustal structure around the Atotsugawa fault system, Central Japan
T2 - A weak zone below the seismogenic zone and its role in earthquake generation
AU - Nakajima, Junichi
AU - Kato, Aitaro
AU - Iwasaki, Takaya
AU - Ohmi, Shiro
AU - Okada, Tomomi
AU - Takeda, Tetsuya
N1 - Funding Information:
Acknowledgments. We thank D. Zhao for providing us with tomographic codes, and A. Hasegawa and T. Matsuzawa for fruitful discussions. We used the unified earthquake catalog compiled by the Japan Meteorological Agency (JMA) and seismic data from the National Research Institute for Earth Science and Disaster Prevention, Hokkaido University, Hirosaki University, Tohoku University, University of Tokyo, Nagoya University, Kyoto University, Kochi University, Kyushu University, Kagoshima University, National Institute of Advanced Industrial Science and Technology, the Tokyo Metropolitan Government, the Shizuoka Prefectural Government, the Kanagawa Prefectural Government, the City of Yokohama, the Japan Marine Science and Technology Center, and the JMA. All of the figures in this paper were plotted using GMT (Wessel and Smith, 1998). Constructive reviews by Martin Reyners and an anonymous reviewer improved the manuscript. This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions.
PY - 2010
Y1 - 2010
N2 - We carried out travel-time tomography around the Atotsugawa fault in central Japan, using arrival-time data obtained from a dense temporary seismograph network in joint observations by the Japanese University group. The observed velocities beneath the fault are 10-13% lower than those of the assumed host rocks (pyroxene amphibolite and hornblende-pyroxene gabbro) in the lower crust. Because the seismogenic layer is thickest in the central part of the fault, reaching a depth of ∼15 km, we infer that the low-velocity anomaly is caused by aqueous fluids. Fluid fractions in the lower crust are estimated to be 2-3% and ∼10%, assuming pyroxene amphibolite and hornblende-pyroxene gabbro, respectively. A distinct low-velocity anomaly is imaged in the central part of the Atotsugawa fault at a depth of 10 km, where seismic activity is very low at the upper 7 km and creeplike movement is observed at the surface. This anomaly is horizontally isolated but vertically connected to the lowvelocity anomaly in the lower crust. We interpreted that abundant aqueous fluids supplied from the lower crust are responsible for this anomaly. High pore fluid pressure may enhance the stability of frictional slip, resulting in aseismic or episodic slip along the fault.
AB - We carried out travel-time tomography around the Atotsugawa fault in central Japan, using arrival-time data obtained from a dense temporary seismograph network in joint observations by the Japanese University group. The observed velocities beneath the fault are 10-13% lower than those of the assumed host rocks (pyroxene amphibolite and hornblende-pyroxene gabbro) in the lower crust. Because the seismogenic layer is thickest in the central part of the fault, reaching a depth of ∼15 km, we infer that the low-velocity anomaly is caused by aqueous fluids. Fluid fractions in the lower crust are estimated to be 2-3% and ∼10%, assuming pyroxene amphibolite and hornblende-pyroxene gabbro, respectively. A distinct low-velocity anomaly is imaged in the central part of the Atotsugawa fault at a depth of 10 km, where seismic activity is very low at the upper 7 km and creeplike movement is observed at the surface. This anomaly is horizontally isolated but vertically connected to the lowvelocity anomaly in the lower crust. We interpreted that abundant aqueous fluids supplied from the lower crust are responsible for this anomaly. High pore fluid pressure may enhance the stability of frictional slip, resulting in aseismic or episodic slip along the fault.
KW - Aqueous fluids
KW - Fault creeping
KW - Lower crust
KW - Seismic tomography
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U2 - 10.5047/eps.2010.06.007
DO - 10.5047/eps.2010.06.007
M3 - Article
AN - SCOPUS:77957668126
VL - 62
SP - 555
EP - 566
JO - Earth, Planets and Space
JF - Earth, Planets and Space
SN - 1343-8832
IS - 7
ER -