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| Titel |
Simulated precursory large aseismic slip at the deeper extension of the seismic region along the Nankai Trough, SW Japan |
| VerfasserIn |
Makiko Ohtani, Nobuki Kame, Masao Nakatani |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250131030
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| Publikation (Nr.) |
 EGU/EGU2016-11380.pdf |
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| Zusammenfassung |
| At the subduction zone along the Nankai Trough, SW Japan, large earthquakes around M8
had occurred repeatedly. Their intervals (around 100—200 years) have been identified
precisely from old historical documents combined with geological surveys (Sangawa, 2011).
The most recent events occurred in 1944 (the Showa To-Nankai EQ.) and 1946 (the Showa
Nankai EQ.) when modern satellite geodetic networks had not been developed
yet.
The existence of short-term aseismic processes before the 1944 and 1946 events has
been inferred from the leveling or interview records. Two-times level difference
measurements showed the displacement of north down before the 1944 event (Mogi, 1986),
and the water level of some wells were reported to have dropped before the 1946
event (Sato, 1982). These phenomena were observed within several days before the
earthquakes, and each could have been caused by 2 m slip on the plate interface at
the deeper extension of the seismic region before each event (Linde and Sacks,
2002).
In this study, we simulate the cycle of large earthquakes in a quasi-dynamic 2D model to
investigate aseismic slip acceleration in the deeper extension of seismic fault. We consider a
flat plate interface with a shallow dipping angle of 15˚ for the depth 0—60 km mimicking
the Nankai Trough. Following Nakatani and Scholz (2006) and Yoshida et al. (2013), we
introduce an intrinsic cut-off time for healing into the state evolution law of the
rate-and-state friction. The intrinsic time leads to a corresponding cut-off velocity (V cx)
beyond which velocity strengthening occurs. We assume that V cx is depth dependent
(1—10−9m/s). We show that this depth variation in V cx can possibly produce large aseismic
slip.
In our simulation, the bottom part of the fault below the deeper extension exhibits a
constant slip rate loaded by a subducting plate velocity (4.5 cm/year). This bottom slip drives
the adjacent deeper locked part and aseismic slip starts to accelerate. Because of the
introduction of low V cx there, the slip cannot monotonously accelerate to seismic slip at the
same depth. Instead, the aseismic slip propagates to the shallower part where the slip
accelerates following the increasingly higher V cx at the depth, and finally reaches to seismic
slip at the shallow part with the large V cx. The seismic slip starting at shallow part then
propagates bilaterally to the shallower and deeper parts, and develops into a large
earthquake. For example, the point at 20 km depth starts to slip aseismically 5.4
days before the earthquake and 54% of the slip occurs as the precursory aseismic
slip. The deeper part produces the longer-lasting aseismic slip with the smaller
velocity.
This simulated aseismic slip may correspond to the several-days precursors of the
1944/1946 events. Our results also suggest that the observed short-term aseismic slip
acceleration is a part of the longer-term aseismic slip that has started at deeper parts, which
may be detected at the next To-nankai/Nankai earthquakes with the help of recently installed
modern observation networks (Do-Net, Hi-Net, and GEONET) around the Nankai region. |
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