HR: 0800h
AN: S41A-0923    [Abstracts]
TI: Fault Zone Structure Extending to Seismogenic Zone in Depth Direction Inferred from the Analysis of Fault Zone Waves
AU: * Mamada, Y
EM: y-mamada@rcep.dpri.kyoto-u.ac.jp
AF: Research Center for Earthquake Prediction, DPRI, Kyoto Univ., Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Kuwahara, Y
EM: y-kuwahara@aist.go.jp
AF: Geological Survey of Japan, AIST, Higashi 1-1-1, central 7 Tsukuba, Ibaraki, 305-8567 Japan
AU: Nishigami, K
EM: nishigam@rcep.dpri.kyoto-u.ac.jp
AF: Research Center for Earthquake Prediction, DPRI, Kyoto Univ., Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Ito, H
EM: hisao.itou@aist.go.jp
AF: Geological Survey of Japan, AIST, Higashi 1-1-1, central 7 Tsukuba, Ibaraki, 305-8567 Japan
AB: Fault zone characterized as low-velocity zone of seismic waves generates fault zone trapped waves and head waves. These waves are useful to detect the small scale structure of fault zone (the width of the zone from a few tens of meters to hundreds of meters). By use of these waves, structure of several fault zones has been imaged. Some of studies image fault zone as continuous vertical layer at depth, the others as vertical layer localized only at shallow (up to 2 km) depth. In this study we infer the fault zone structure, considering the possibility of the fault zone discontinuity (in other words, fault zone has termination at some position) at depth. We used earthquake data recorded at 500-m-long linear seismometer array across the Mozumi-Sukenobu fault, central Japan, at a depth of 300 m in the underground survey tunnel. Focusing on initial P-waves arrival within the array, two patterns are found: (1) initial P-waves from the two boundaries (between fault zone and host rock) propagating toward center of fault zone, as a result, the latest travel time of initial phase is found within the fault zone (2) initial P-waves propagating from the one boundary to the other boundary. Pattern (1) appears in the most of events occurring at distance more than 15 km east of the array. We investigated the relation between the source location and the initial P-waves arrival pattern within the array by numerical simulation for two models: (a) model with continuous fault zone at depth and (b) the model with discontinuity of fault zone. Results show that pattern (1) appears in the case of sources locating within the fault zone or within the extension of fault zone. Pattern (1) also appears in the case of sources outside of the extension of fault zone when the fault zone has discontinuity. Observation and simulation results indicate two possibilities; all the events showing arrival pattern (1) are located within the fault zone when fault zone is continuous or fault zone has termination between the array and 15 km east of the array along the fault. Next we evaluated intensity of trapped wave excitation by the ratio of Rin to Rout where Rin=Etin/Esin and Rout=Etout/Esout for all events. Etin, Esin, Etout and Esout show energy (square of amplitude) of trapped waves and S waves within and outside the fault zone, respectively. This intensity increases as trapped waves excitation is larger. Estimated intensity indicates systematic variation for spatial distribution of events. Intensity less than 2 (smaller trapped waves excitation) is found for the events shallower than about 8 km and up to 15 km east of the array along the fault. The events up to 15 km east of the array and deeper than about 8 km indicate the intensity greater than 2 (larger trapped waves excitation). And the latter event group is likely to be located at the center of fault zone and the former one at the border of fault zone. The events more than 15 km east of the array also show the intensity greater than 2 independent of the focal depth. Observed initial P-waves arrival pattern and intensity can be explained by the model that fault zone has discontinuity around 15 km east of the array along the fault and extends down to a depth of about 8 km (seismogenic zone) at least. This result is contrast to some of recent studies showing that the fault low-velocity zone is located at only shallow depths, less than about 2 km.
DE: 9320 Asia
DE: 8010 Fractures and faults
DE: 7218 Lithosphere and upper mantle
DE: 1734 Seismology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting