Kazutoshi Imanishi

Source Scaling Relationships of Microearthquakes at Parkfield, CA, Determined Using the SAFOD Pilot Hole Seismic Array

We estimate the source parameters of 34 microearthquakes at Parkfield, CA, ranging in size from M -0.2 to M 2.1, by analyzing seismograms recorded by the 32-level, 3-component seismic array installed in the SAFOD Pilot Hole. We succeeded in obtaining stable spectral ratios by stacking the ratios calculated from the moving windows taken along the record following the direct waves. These spectral ratios were modeled to determine seismic moments and corner frequencies assuming an omega-squared model. Static stress drops and apparent stresses of microearthquakes at Parkfield display moment-independent scaling in agreement with scaling laws reported for moderate and large earthquakes. It is likely that the dynamics of microearthquakes at Parkfield is macroscopically similar to that of larger tectonic earthquakes.

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.

Stress drop variations among small earthquakes before the 2011 Tohoku‐oki, Japan, earthquake and implications for the main shock

It is important to assess the likely rupture characteristics of future megathrust earthquakes. One approach is to study the spatiotemporal variation of geophysical properties in active subduction zones. We explore this idea by examining stress drops of 1563 small earthquakes (Mw 3.0–4.5) shallower than 80 km in the Tohoku-oki region before the 2011 Tohoku-oki earthquake. Although individual stress drop estimates exhibit considerable scatter, we find a strong increase in stress drop with depth between 30 km and 60 km, whereas stress drops for shallower and deeper events, respectively, are nearly constant. We also identify lateral variations in stress drop along strike. Higher-than-average stress drops are found in East Aomori-oki and Miyagi-oki, whereas Sanriku-oki is a moderate stress drop area. The high stress drop zone in Miyagi-oki is located just south of the large slip area of the 2011 Tohoku-oki earthquake, and possibly acted as a barrier to further rupture propagation during the event. In addition, the frequency dependence of the seismic radiation observed during the main shock, with proportionally higher frequencies coming from the deeper parts of the fault, mimics the depth dependence we see in small earthquakes in the same region. These results imply that smaller pre-main shock earthquakes can provide insight into the fault properties and consequent rupture processes of future megathrust earthquakes.

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.

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.

Determination of focal mechanisms of nonvolcanic tremor using S wave polarization data corrected for the effects of anisotropy

We propose a method to determine the focal mechanisms of nonvolcanic tremors using anisotropy-corrected S polarizations measured directly from noisy tremor signals. The successful retrieval of polarization angles in noisy signals owes much to the observation that, in general, tremors propagate slowly and therefore do not change their location immediately. This feature of tremors enables us to use longer time windows to compute polarization angles (e.g., 1 min or longer), resulting in a stack of particle motions. We applied the method to a tremor episode that occurred beneath the Kii Peninsula in April 2013. Although the majority of inferred focal mechanisms suggested that tremors manifest shear slip on the plate interface, some events with strike-slip components were also found to have occurred. The proposed method provides us with a tool for investigating smaller-scale spatial distributions and temporal variations of focal mechanisms throughout tremor zones.

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.

Imaging of crustal heterogeneous structures using a slowness‐weighted back‐projection with effects of scattering modes: 2. Application to the Nagamachi‐Rifu fault, Japan, area

[1] This is the second paper in a two-part series on a newly developed imaging approach for small-scale heterogeneities (<1 km) in the crust with effects of scattering modes. In the present paper, we estimated a detail three-dimensional spatial distribution of small-scale heterogeneities around the Nagamachi-Rifu fault, northeastern Japan, in a frequency range of 2-16 Hz, using the imaging approach presented in the first paper. We used seismograms recorded by dense three-component seismic arrays that were deployed in this region for 15 explosion sources. As one of our important results, there are concentrations for only P-S but not P-P scatterers near the surface trace of the fault. P-S scatterers in a frequency range of 8-16 Hz are localized near the surface trace of the fault, implying the possibility of strong heterogeneities with a size of 0.08 km there. In the Shirasawa caldera region, the characteristics of seismic scatterers seem to convert from large P-S to large P-P relative scattering coefficients with its transition depth range of 5-8 km. This feature implies that the materials composed of seismic scatterers may show a systematic variation with depth. Finally, the strength of scattering coefficients is rather weak in the coseismic area of the 15 September 1998 earthquake with a magnitude of 5.2, the largest recent event in this area. This result suggests that this coseismic area is rather homogeneous and can hold local stress larger than in the surrounding portions of the fault system.

Non-self-similar source property for micro-foreshocks of the 2014 Mw 6.2 Northern Nagano, central Japan, earthquake

Foreshocks are considered a part of the preparation process for large earthquakes and, as such, can provide important constraints on earthquake generation. We examine the characteristics of the four-day-long foreshock sequence of the 2014 Northern Nagano earthquake (Mw 6.2) using seismograms recorded by a dense seismic observation network including data from a deep borehole closest to the foreshock region. The improved earthquake catalog shows a slow-speed migration of the sequence toward the mainshock hypocenter, implying possible slow-slip transients which should have caused stress loading at the mainshock hypocenter. Analysis of source spectra reveals that foreshocks for Mw < 1.5 display a weaker dependence of corner frequency on magnitude than expected for self-similar scaling, suggesting that these micro-foreshocks can be regarded as small low-frequency earthquakes. So far, we might have missed the expected weak low-frequency pre-seismic signals, because they emerge only below a certain magnitude and are generally embedded in background noise.

Stress field in the source region after the 2007 Mw 6.6 Niigataken Chuetsu-oki earthquake deduced from aftershock focal mechanisms: Implication for a pre-mainshock stress field

Stress fields in the source region before and after the 2007 Mw 6.6 Niigataken Chuetsu-oki earthquake were investigated using aftershock focal mechanism solutions. We have attempted to determine aftershock focal mechanisms using P-wave polarity data as well as body wave amplitudes because this earthquake occurred offshore, where observation station coverage is poor. This approach enabled us to obtain 76 well-determined aftershock focal mechanisms. Although the stress field in the studied area is known to be generally characterized by a reverse-faulting regime, the application of a stress tensor inversion method to the aftershock focal mechanisms revealed that the stress field spatially varied on a scale smaller than the fault dimension of the mainshock, with a mixture of strike-slip and reverse faulting regimes. The post-mainshock stress field estimated by the stress tensor inversion was compared with the theoretical stress field computed using an observed non-uniform slip distribution of the mainshock and variously assumed pre-shock stress fields. The results of this comparison suggest that the pre-mainshock principal stress in the WNW-ESE direction was dominant and that the magnitudes of the other two principal stresses were similar.