Shiro Ohmi

Temporal variations of crustal structure in the source region of the 2007 Noto Hanto Earthquake, central Japan, with passive image interferometry

The passive image interferometry technique is applied to the continuous seismic waveform data obtained around the source region of the 2007 Noto Hanto Earthquake, central Japan. We computed the autocorrelation function (ACF) of band-pass filtered seismic noise portion recorded with each short-period seismometer at several seismic stations for 1 day each. In some stations, comparison of each 1-day ACF shows temporal evolutions of the ACF, which are interpreted as the change of seismic velocity structure in the volume considered. Sudden changes of ACF are detected to be associated with the occurrence of the main shock in one station of the four stations analyzed. Gradual changes of ACFs in the preceding 2 weeks of the main shock are also recognized in two stations, which would be of great importance for understanding the stress state before the occurrence of earthquakes.

Three-dimensional P- and S-wave velocity structure below Unzen volcano

Abstract Unzen volcano, located in southwestern Japan, erupted on November 17, 1990 after 198 years of dormancy and has been ejecting lava since May 20, 1991. In this paper, we present three-dimensional P- and S-wave velocity variations below Unzen volcano using 22,473 P- and 14,349 S-wave arrival times from 3457 local earthquakes recorded on a local network of twelve seismic stations. The model was parameterized by 24,000 approximately 2.0 km cubic blocks, targeting a volume of 120 × 100 × 20 km. A prominent low-velocity anomaly (greater than 4% slowness perturbation) starting from 2.5 to 5.0 km depth beneath the volcano is observed on both the P- and S-wave inversions. Below Unzen volcano slightly lower amplitude low-velocity anomalies are further observed to a depth of 12.5 km for P- and 7.5 km for S-waves. Shallow low-velocity anomalies are observed below the Chijiwa bay area (northwest of Unzen), and deeper anomalies are seen below Shimabara bay (southwest of Shimabara peninsula). This NW-SE-trending feature represents an elongated system of dykes which supply the shallow magma reservoirs.

Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake derived from a combined analysis of temporary online observations and permanent observations

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

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.

Temporal changes of subsurface velocities during strong shaking as seen from seismic interferometry

The deconvolved waveforms reflect the subsurface velocity structure, and their horizontal and vertical components correspond to Sand P-wave, respectively, traveling from the borehole to the ground surface. The strong motion records with smaller values of peak acceleration do not include significant non-linear effects, so the deconvolved waveforms of the observed accelerations can be well simulated by the program SHAKE91.

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 2000 Western Tottori Earthquake—Seismic activity revealed by the regional seismic networks—

On October 6, 2000, the 2000 Western Tottori Earthquake (Mjma 7.3) occurred in the western Tottori prefecture area, southwestern Japan. It initiated at a depth of 12 km at the bottom of the seismogenic zone, which was derived from aftershock distribution. The aftershocks extend over a 35 km length in a north-northwest direction. Spatial and temporal distribution of the aftershocks exhibits local characteristics in the fault region. The northern part consists of earthquake clusters while the southern part consists of a rather simple lineament of aftershocks, and the spreading and decaying rate of the aftershocks is slower in the northern part. This contrast is possibly due to the heterogeneity of the fault system and probably affected the rupture process of the mainshock. Two swarm sequences occurred in the surrounding region after the mainshock. One initiated 48 hours after the mainshock 25 km southwest of the main aftershock distribution. The other started 20 hours after the mainshock northeast of the mainshock on the southeast flank of Daisen volcano. These activities are probably induced seismicity due to stress changes in the focal region. Pre-seismic swarm activities occurred in the focal region from 1989 and deep low-frequency earthquakes were observed since 1999. It is important to understand the relationship between these possible precursory phenomena and the occurrence of the mainshock.

Estimation of Q-Values in the Seismogenic and Aseismic Layers in the Kinki Region, Japan, by Elimination of the Geometrical Spreading Effect Using Ray Approximation

The Q -value is one of the parameters controlling attenuation of seismic waves with distance. Attenuation relations in crust used in the earthquake engineering applications usually consider models with uniform Q and geometrical spreading. In this work we try to estimate a nonuniform Q -value based on the ray geometrical spreading in a nonuniform velocity model. We estimate Q -values in the seismogenic and aseismic zones of the Kinki region (Japan) using Hi-net data. The Hi-net network consists of high-sensitivity seismometers in 100-200 m boreholes. We assumed a two-layer model of Q ( f ) (seismogenic and aseismic zones), with uniform Q in each layer, and we applied a method for the separation of source, path, and site effects. Path lengths in the layers were calculated using raytracing. A geometrical spreading term was calculated for a realistic 1D velocity model (consisting of three layers over the Moho). Inversion was performed in two steps. (1) The Q value in the seismogenic layer was estimated using shallow earthquake data (depth < 20 km), assuming a one-layer Q model. (2) Data from subduction zone earthquakes covering the aseismic zone (with depths 20-70 km) and two-layer Q model (0-20 and 20-70 km) were used to calculate Q in the aseismic zone, where the Q -value for the upper layer was constrained by results of step 1. The total number of records used was 628. Only direct S -wave data were used to calculate Fourier amplitude spectra in the high-frequency range 1-10 Hz. Validation of the method and inversion results were made by inversion of synthesized data. We discuss in detail several possible sources of errors of the estimation of Q -values. The results of inversion showed a higher Q in the upper layer, Q ( f ) = 180 f 0.7 for the seismogenic layer, than that in the lower, Q ( f ) = 90 f 0.8 for the aseismic zone. This result supports the model of the crust containing a brittle seismogenic layer and a ductile aseismic zone. We proposed amplitude versus distance attenuation model for Kinki region, Japan, based on estimated Q -values and geometrical spreading. Manuscript received 8 October 2002.

Deep low-frequency earthquakes near the downward extension of the seismogenic fault of the 2000 Western Tottori earthquake

Deep low-frequency (DLF) events have often been discussed in association with fluid (magma) activity around volcanoes, in this paper, however, we will show another example of DLF activity beneath active faults. On October 6, 2000, a Mw=6.7 crustal earthquake occurred in the western Tottori, southwest Japan. Beneath the focal region of the earthquake, DLF earthquakes had been observed prior to the mainshock and the activity increased after the mainshock. The events are distributed around 30 km depth and seem to be located around the downward extension of the seismogenic fault. Three types of DLF events have been observed and their seismological features indicate fluid activity around the focal region of the DLF events. If the downward extension of the seismogenic fault in the lower crust exists, fluid activity indicated by the DLF events possibly affect the aseismic slip process of the deep portion of the fault and likely control the occurrence of the earthquakes on the seismogenic portion of the fault.

Volcanic deformation at Unzen, Japan, visualized by a time-differential stereoscopy

Abstract Using a pair of old and new photographs taken at the same location, a method named as a time-differential stereoscopy is successfully applied to detect visually the deformation at Unzen volcano, Japan. On September 6, 1992, gradual movement of a lava lobe was visible in a stereo image as relief surrounded by immobile areas. In this case, accelerated movement of a lava block was detected immediately before the time of its collapse, which resulted in the occurrence of a pyroclastic flow. In November 1994, growth of a lava spine was also measured with time-differential stereoscopy. A possible fluctuation of the growth rate might have correlated with an abnormal oscillation of tilt, with a period of about 40 h. This convenient photographic method is recommended for various occasions to detect volcanic and any other deformations as a supplement to precise instrumental measurements.