TY - JOUR
T1 - The weakened lower crust beneath the Nobi fault system, Japan
T2 - Implications for stress accumulation to the seismogenic zone
AU - The Research Group for the Joint Seismic Observations at the Nobi Area
AU - Nakajima, Junichi
AU - Kato, Aitaro
AU - Iwasaki, Takaya
N1 - Funding Information:
We used arrival time data within the unified catalogue produced by the JMA. Stations of the Kiban network used in this study are maintained and operated by the National Research Institute for Earth Science and Disaster Prevention (NIED), University of Tokyo, Nagoya University, Kyoto University, National Institute of Advanced Industrial Science and Technology (AIST), and the Japan Meteorological Agency (JMA). Constructive and careful reviews by Dr. L. Chiaraluce and two anonymous reviewers improved the quality of the manuscript. All of the figures were plotted using GMT ( Wessel and Smith, 1998 ). This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan as part of its Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions.
Publisher Copyright:
© 2015 Elsevier B.V.
PY - 2015/8/1
Y1 - 2015/8/1
N2 - The 1891 M8 Nobi earthquake, which occurred along the Nobi fault system in central Japan, is the largest crustal earthquake in Japanese historical records. Here, we present a new estimate of the 3-D seismic velocity structures around the Nobi fault system using a large number of arrival time data obtained from both temporary and permanent seismic stations. The results show that the middle and lower crust in the northern part of the Nobi fault system has lower seismic velocities that are 4-9% lower than those of the surrounding area. This low-velocity crust most likely represents a zone containing 2-3 vol % of pore fluids that reduce the strength of the middle and lower crust. This inference suggests that deformation in this weakened crust, caused by a regional stress regime regulated by the eastward movement and collision of the Amurian plate with the North American plate, is dominated by anelastic processes. In addition, the seismogenic layer in the northern part of the fault system is ~ 5 km thinner than in the southern part, suggesting that stress is efficiently concentrated within the seismogenic layer in the northern part. This finding explains why the seismic rupture for the Nobi earthquake nucleated at the northern end of the fault system. Our results suggest that a weakened zone in the middle and lower crust is an important control on stress loading process within the seismogenic layer and thus the seismogenesis of crustal earthquakes.
AB - The 1891 M8 Nobi earthquake, which occurred along the Nobi fault system in central Japan, is the largest crustal earthquake in Japanese historical records. Here, we present a new estimate of the 3-D seismic velocity structures around the Nobi fault system using a large number of arrival time data obtained from both temporary and permanent seismic stations. The results show that the middle and lower crust in the northern part of the Nobi fault system has lower seismic velocities that are 4-9% lower than those of the surrounding area. This low-velocity crust most likely represents a zone containing 2-3 vol % of pore fluids that reduce the strength of the middle and lower crust. This inference suggests that deformation in this weakened crust, caused by a regional stress regime regulated by the eastward movement and collision of the Amurian plate with the North American plate, is dominated by anelastic processes. In addition, the seismogenic layer in the northern part of the fault system is ~ 5 km thinner than in the southern part, suggesting that stress is efficiently concentrated within the seismogenic layer in the northern part. This finding explains why the seismic rupture for the Nobi earthquake nucleated at the northern end of the fault system. Our results suggest that a weakened zone in the middle and lower crust is an important control on stress loading process within the seismogenic layer and thus the seismogenesis of crustal earthquakes.
KW - Amurian plate
KW - Collision
KW - Fluids
KW - Lower crust
KW - Nobi earthquake
KW - Philippine Sea slab
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U2 - 10.1016/j.tecto.2015.05.021
DO - 10.1016/j.tecto.2015.05.021
M3 - Article
AN - SCOPUS:84938747045
VL - 655
SP - 147
EP - 160
JO - Tectonophysics
JF - Tectonophysics
SN - 0040-1951
ER -