Spatial distribution of atypical aftershocks of the 1995 Hyogo-ken Nanbu earthquake

Yamanaka, Hiroshi; Hiramatsu, Yuji; Katao, Hiroshi

doi:10.1186/bf03352441

Cited by 9 publications

(5 citation statements)

References 20 publications

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“…Such a rapid fault healing is, however, likely to occur. Borehole breakout images taken in September 1996 [ Tsukahara et al , 2001] and P ‐axes of the focal mechanisms of the aftershocks [ Yamanaka et al , 1999] that the direction of the local σ H along the Nojima fault has been perpendicular to the fault strike. The direction differs from the regional tectonic stress.…”

Section: Resultsmentioning

confidence: 99%

Evidence for rapid fault healing derived from temporal changes in S wave splitting

Tadokoro

Ando

2002

Geophysical Research Letters

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[1] Tadokoro et al. [1999] had made an S wave splitting analysis on the Nojima fault zone that ruptured during the 1995 Hyogo-ken Nanbu earthquake for a period 9 -12 months after the mainshock. The direction of faster shear wave polarization j had been parallel to the fault strike, suggesting fractures of shear fault origin. In this study, we made an additional S wave splitting analysis for a period 33 -45 months after the mainshock and detected the rotation of j, that is, the direction j was parallel to the regional maximum horizontal compressional stress which is oblique to the fault strike. This temporal change suggests that all the fractures of shear fault origin had completely healed and cracks of tectonic stress origin recovered between the two periods. This is strong evidence for rapid fault healing which completed within 33 months after the mainshock.

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Section: Resultsmentioning

confidence: 99%

Evidence for rapid fault healing derived from temporal changes in S wave splitting

Tadokoro

Ando

2002

Geophysical Research Letters

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“…On the contrary, for the 1995 Kobe Earthquake, aftershocks along the mainshock rupture zone showed various directions of the P axes and variety in the mechanism types. The reverse fault types and the strike-slip types co-existed in narrow areas (Katao et al, 1997;Yamanaka et al, 2002). This difference might be due to differences in the degree of heterogeneity of the initial stress and strength distributions in the source regions of the two earthquakes.…”

Section: Discussionmentioning

confidence: 99%

“…10. Following Yamanaka et al (2002), we classified the focal mechanisms into four types: normal fault type with the plunge of the P axis > 60…”

Section: Focal Mechanisms Of Aftershocksmentioning

confidence: 99%

High resolution 3-D velocity structure in the source region of the 2000 Western Tottori Earthquake in southwestern Honshu, Japan using very dense aftershock observations

Shibutani

Katao

2005

Earth Planet Sp

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We carried out high density aftershock observations a week after the 2000 Western Tottori Earthquake for 40 days. We deployed 72 seismic stations in and around the aftershock area. The average spacing of the stations in the aftershock area was 4-5 km. We determined accurate hypocenters and focal mechanisms for ∼1,000 aftershocks and obtained a high resolution 3-D velocity structure in the source region. High P and S wave velocity anomalies (> 4%) near the southeasternmost aftershock area at 2 km depth correlated with Jurassic to Late Cretaceous plutonic and high pressure metamorphic rocks. The depth distribution of the P and S wave velocities along the mainshock fault showed that high velocity anomalies were located at the shallow southeastern edge and the deeper central part of the aftershock area. The ratio between P and S wave velocities in the high velocity anomalies was a little higher (∼1.75) than the average value (∼1.70) in the upper crust. These results indicate that the high velocity anomalies could correspond to the plutonic or metamorphic rocks. The distributions of the high velocity anomalies and large slips of the mainshock were complementary. These suggest that the high velocity anomalies could be stronger than the surrounding materials and might behave as barriers to the mainshock rupture.

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“…For the 1995 Hyogo-ken Nanbu earthquake, about 17% of the aftershocks have source mechanisms different from the regional stress field, which are called "atypical" aftershocks (Yamanaka et al, 2002). In our case, the source mechanisms except SS-A may correspond to atypical events.…”

Section: Discussionmentioning

confidence: 70%

A numerical analysis of seismic waves for an anisotropic fault zone

Nakamura

Takenaka

2006

Earth Planet Sp

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In this study we examine the effects of anisotropy on the seismic wavefield in a fault zone from computation of the synthetic seismograms for a simple fault zone model and a variety of seismic wave sources. The fault zone is modeled by a homogeneous vertical layer with transverse isotropy, induced by cracks, sandwiched between isotropic half-spaces (host rocks). The symmetry axis of the transverse isotropy is horizontal and perpendicular to the fault zone strike. We calculate the synthetic seismograms for this anisotropic fault zone model using a semianalytical method, the propagator matrix method. The synthetic seismograms show a later phase arriving after the main shear-wave in the horizontal component perpendicular to the fault zone strike at most stations near the fault zone. It is the slower shear-wave (q S 2 ) and its reverberation. The amplitude of this phase and the time delay from the main shear-wave arrival are proportional to the degree of anisotropy, which suggests that observing such phase in field measurements may imply the presence of an anisotropic fault zone. We also perform the shear-wave splitting measurements by applying the cross-correlation method to the synthetic seismograms for various sources. For a strike-slip source, the synthetic seismograms show that the wavefield is more affected by the velocity structure than by the degree of anisotropy, which makes it difficult to estimate the anisotropic (shearwave splitting) parameters. For normal and dip-slip fault sources with the strike parallel to or striking against the fault zone, the effects of anisotropy is so dominant that the anisotropic fault zone can be detected. These results suggest that the determination of the anisotropic properties in the fault zone would require an appropriate station deployment and the source type information.

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Spatial distribution of atypical aftershocks of the 1995 Hyogo-ken Nanbu earthquake

Cited by 9 publications

References 20 publications

Evidence for rapid fault healing derived from temporal changes in S wave splitting

Evidence for rapid fault healing derived from temporal changes in S wave splitting

High resolution 3-D velocity structure in the source region of the 2000 Western Tottori Earthquake in southwestern Honshu, Japan using very dense aftershock observations

A numerical analysis of seismic waves for an anisotropic fault zone

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