Yasuto Kuwahara

Characteristic features of local breakdown near a crack-tip in the transition zone from nucleation to unstable rupture during stick-slip shear failure

Abstract Characteristic features of local breakdown near a propagating crack-tip during slip failure nucleation and its transition process to unstable rupture under mode II conditions are experimentally studied using a rock sample with a simulated fault, the size of which is large compared with the size of the breakdown zone. In addition, distinctive features in the nucleation zone are compared with those in the zone of steady, dynamic rupture propagation. It is found that the local breakdown stress drop, the local dynamic stress drop, the local stress increase immediately before the slip-weakening process, the dimensionless parameter S defined by Das and Aki, and the shear fracture energy are functions of crack length in the transition zone from nucleation to unstable rupture; i.e., they all increase with crack growth in the nucleation zone. This is contrasted with the observations in the zone of steady dynamic rupture propagation, where these physical parameters do not depend on crack length. An increase in the magnitudes of these parameters with crack growth in the nucleation zone results in increasing resistance to crack extension with crack growth in the nucleation zone. The reason for the increase in crack growth resistance in this zone is that a slip failure nucleus is formed at a point (or zone) where the local strength and the shear fracture energy are at a minimum. This basic physical information can provide the key to earthquake prediction, since the nucleation itself is a precursory phenomenon. The ratio of the size of the breakdown zone to the crack length decreases abruptly beyond the critical crack length. Rupture velocity v accelerates with crack growth, and reaches the shear wave velocity or super-shear, and v is found to be expressed empirically as a function of crack length in the nucleation zone. A positive correlation is found between S and v in the nucleation zone (v increases with increasing S). However, no such correlation was observed in the zone of steady, dynamic rupture propagation, where v seems to have reached the shear wave velocity of super-shear for S = 0.5. This is consistent with Das and Akis numerical result. The interrelationships between shear stress, slip displacement, slip velocity and acceleration near the crack-tip during the breakdown in the nucleation zone are revealed, and they are compared with those in the zone of dynamic rupture propagation. The present experiments suggest that the relationship between shear stress and slip displacement is more fundamental during the breakdown process than the relationship between shear stress and slip velocity. A model is presented to describe the breakdown process in the nucleation zone

Broadband seismic observation at the Sakurajima Volcano, Japan

We installed a portable broadband seismometer (Streckeisen STS-2) at the Sakurajima volcano, which has been very active in the recent years. The recorded seismograms show a wide variety (both in temporal and spectral contents) of seismic events, from explosions to tremors, and exhibit the importance of such broadband seismometry at volcanos. We present examples of seismograms to show the potential of broadband seismic observation in monitoring volcanic activities.

Source Parameters and Rupture Velocities of Microearthquakes in Western Nagano, Japan, Determined Using Stopping Phases

We use an inversion method based on stopping phases (Imanishi and Takeo, 2002) to estimate the source dimension, ellipticity, and rupture velocity of microearthquakes and investigate the scaling relationships between source parameters. We studied 25 earthquakes, ranging in size from M 1.3 to M 2.7, that occurred between May and August 1999 at the western Nagano prefecture, Japan, which is characterized by a high rate of shallow earthquakes. The data consist of seismograms recorded in an 800-m borehole and at 46 surface and 2 shallow borehole seismic stations whose spacing is a few kilometers. These data were recorded with a sampling frequency of 10 kHz. In particular, the 800-m-borehole data provide a wide frequency bandwidth with greatly reduced ground noise and coda wave amplitudes compared with surface recordings. High-frequency stopping phases appear in the body waves in Hilbert transform pairs and are readily detected on seismograms recorded in the 800-m borehole. After correcting both borehole and surface data for attenuation, we also measure the rise time, which is defined as the interval from the arrival time of the direct wave to the timing of the maximum amplitude in the displacement pulse. The differential time of the stopping phases and the rise times were used to obtain source parameters. We found that several microearthquakes propagated unilaterally, suggesting that all microearthquakes cannot be modeled as a simple circular crack model. Static stress drops range from approximately 0.1 to 2 MPa and do not vary with seismic moment. It seems that the breakdown in stress drop scaling seen in previous studies using surface data is simply an artifact of attenuation in the crust. The average value of rupture velocity does not depend on earthquake size and is similar to those reported for moderate and large earthquakes. It is likely that earthquakes are self-similar over a wide range of earthquake size and that the dynamics of small and large earthquakes are similar.

Slow initial phase generated by microearthquakes occurring in the western Nagano Prefecture, Japan ‐The source effect‐

Near-source observations of microearthquakes occurring in the Western Nagano prefecture show that durations of recorded slows initial phases were almost constant independently of focal distance. Although a closer inspection revealed that the durations of the slow initial phases slightly increased with focal distance, they were not proportional to focal distance. Thus, it is concluded that the slow initial phases do not mainly reflect the whole path Q. If the slow initial phases were attributed to the strong anelastic attenuation in the surface layer combined with that of the whole path Q, the rise time of the P-wave velocity pulse at the source for a M2.4 event would be estimated as about 3 ms and the source radius of the M2.4 event would be estimated as about 10 m. This value of the source radius is too small for M2.4. This suggests that the slow initial phase does not arise from the strong anelastic attenuation in the surface layer. It is likely that the slow initial phase does not mainly reflect a path effect but a source effect.

The seismicity, fault structures, and stress field in the seismic gap adjacent to the 2004 Mid-Niigata earthquake inferred from seismological observations

We investigate the seismicity, fault structures, and stress field in the southern seismic gap adjacent to the source region of the 2004 Mid-Niigata earthquake based on precise earthquake locations and focal mechanism solutions. The double-difference earthquake locations reveal detailed fault structures including five discrete clusters. A stress tensor inversion method indicates that, at the southern ends of the Muikamachi and the Tokamachi faults, the stress state changes from thrust faulting to a strike-slip faulting regime. We show two possibilities for the deep-fault geometry of the southern part of the Muikamachi fault. Our preferred fault geometries of the Muikamachi fault differ from that of the mainshock, especially for the geometry of the dip direction. It is likely that the Muikamachi fault is divided into two segments whose boundary corresponds to the southern end of the source region of the 2004 event, and that the rupture area was terminated due to this fault segment boundary. The Coulomb failure stresses that are induced by the mainshock and the largest aftershock increase at the southern part of the Muikamachi faults. Furthermore, the angle between the maximum principal stress and the preferred fault planes implies that the shear stress resolving along the planes are large. This evidence suggests a high seismic risk in the seismic gap.

Changes in groundwater level associated with the 2003 Tokachi-oki earthquake

Groundwater level and flow rate at 44 wells are continuously observed by the Geological Survey of Japan and the Shizuoka and Gifu Prefectural Governments for monitoring seismic and volcanic activities. The 2003 Tokachi-oki earthquake (M8.0) occurred off the south coast of Hokkaido Island, Japan on September 26, 2003. The epicentral distance to the nearest observation well is about 250 km and that to the farthest is about 1200 km. At the 22 wells, we detected changes in groundwater level or flow rate in relation to the earthquake. Most of the changes are coseismic step-like changes and/or short-period oscillations. In the nearest two observation wells, long-period oscillations with the periods of 39 and 53 minutes were also observed for several days after the earthquake, which is likely due to tsunami. In comparison between distributions of changes in groundwater level and theoretical coseismic strain by the fault model, it is clear that step-like increases were found in the contraction area of the coseismic strain. The relationship between amounts of the observed step-like groundwater-level changes and theoretical ones, calculated by the fault model using strain sensitivities of groundwater level indicates that the groundwater levels in the several wells responded to the coseismic strain.

Off-fault aftershocks of the 2005 West Off Fukuoka Prefecture Earthquake: Reactivation of a structural boundary?

After the occurrence of the 20 March 2005 West Off Fukuoka Prefecture Earthquake (Mj 7.0), off-fault aftershocks occurred in and around Hakata bay adjacent to the main fault of the earthquake. The locations of the activity seem to coincide with the Ishido-Uminonakamichi fault (I-U fault), which is a structural boundary and not considered as an active fault. In order to elucidate relations between these off-fault aftershocks and the I-U fault, we determine the double-difference earthquake locations and focal mechanism solutions. Results of the present study show that the off-fault aftershocks are aligned in a sense of left-lateral faulting striking 120° from the north and in that of their conjugate right-lateral fault structures striking 30? from the north. Comparing the trends of these aligned structures with the orientation of the I-U fault, we conclude that earthquakes in and around the Hakata bay occurred along these aligned structures, not along a fault plane inferred from the surface trace of the I-U fault. The analysis of Coulomb failure stress due to the mainshock also supports this conclusion. Stress tensor inversion shows that the stress field in and around the Hakata bay is characterized by a strike-slip faulting. The angles between the fault trends and the maximum principal stress suggest that the aligned structures found in this study are favorably oriented, while the fault plane inferred from the surface trace of the I-U fault is unfavorably oriented.

Crustal heterogeneity as inferred from seismic coda wave decomposition by small-aperture array observation

Abstract Seismic coda waves have complex waveforms and are considered to be caused by 3-dimensional structural heterogeneities. We carried out 3-component small-aperture array observations of seismic coda waves from natural earthquakes in Tsukuba, central Japan, to decompose complex coda waveforms and to clarify the nature of crustal heterogeneity. Epicentral distances in the present study are less than 200 km. The observational results are summarized as follows: 1) The remarkable feature of the coda from natural earthquakes is the difference between the P-coda and the S-coda especially in the vertical component. The P-coda consists mainly of coherent P-wave phases which have almost the same slowness vector as the initial P-wave motion, whereas the S-coda is decomposed into incoherent phases with random propagation directions. 2) Coherency of waveforms among the array sensors of the vertical component rapidly becomes low from the high value of coherency of the direct S-wave. 3) In the two horizontal components, radial and transverse ones, both the P- and S-coda are decomposed into incoherent random phases. 4) Coherency in the early S-coda of the horizontal components gradually becomes low compared to the vertical component. These features indicate that S to P converted waves at horizontal boundaries of the structure are dominant in the P-coda of the vertical component, while the P-coda of the horizontal components and the S-coda consist of scattered waves from the lateral heterogeneity. This interpretation leads to a model of crustal heterogeneity in which the degree of the lateral heterogeneity is smaller than the degree of vertical heterogeneity of layered structures.

Deep structure of the Nagamachi-Rifu fault deduced from small aperture seismic array observations

A seismic reflection survey using both explosives and vibrators was conducted in June 2001 around the Nagamachi-Rifu fault, northeastern Japan. We carried out observations of four small aperture seismic arrays in the area to reveal detailed structures of the fault. Array analysis was applied to waveform data from 15 explosives to obtain P-wave scatterer distributions in the area. The obtained P-wave scatterer distribution correlates in space with microearthquake activities and heterogeneous structures such as S-wave reflectors, a structure of caldera, and Mohorovicic discontinuity. We could also image that a sub-horizontal layer with a length of about 10 km exists in the deep extension of the Nagamachi-Rifu fault beneath the seismogenic layer.

Groundwater level changes related to the ground shaking of the Noto Hanto Earthquake in 2007

The Geological Survey of Japan, AIST has been monitoring groundwater using the observation network of about 40 stations in and around the Kinki and Tokai districts for earthquake prediction research. The Noto Hanto Earthquake in 2007 (MJMA 6.9) occurred in the northwestern part of the Noto Hanto Peninsula, Japan on March 25, 2007. The epicentral distance to the nearest and furthest observation well is about 100 km and 400 km, respectively. Changes in groundwater level related to the Noto Hanto Earthquake were detected at many stations. Most of these were changes in the long-term trend and/or short-term oscillation; there were a few steplike changes. The coseismic static volumetric strain changes calculated from the fault model ranged between 10−9 and 10−10 at most of the observation stations. These changes were therefore considered to be groundwater changes caused by the ground shaking because groundwater level sensitivity to crustal volumetric strain is a few centimeters to 10−8 strain at the most. We compared the coseismic groundwater level changes for the Noto Hanto Earthquake with those for five recent large earthquakes and obtained rough characteristics of the effect of the ground shaking on the groundwater level at each of the observation stations.