Kin’ya Nishigami

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.

Scaling law between corner frequency and seismic moment of microearthquakes: Is the breakdown of the cube law a nature of earthquakes?

[1]xa0The corner frequency, fC, and the seismic moment, M0, of microearthquakes (local magnitude from −1.3 to 1.3) are estimated from the records of sensors located at an 1800-m-deep borehole at the Nojima fault, central Japan. The digital waveforms of each event are recorded with two different sampling rates, 10 kHz and 100 Hz. The source parameters estimated from high-sampling data satisfy a relationship of M0 ∝ fC−3, corresponding to a constant stress drop law, while those calculated from low-sampling data follow a relationship of M0 ∝ fC−4. A lack of high frequency components in the 100 Hz sampling data seem to be obvious, indicating that the value of fC and M0 are incorrectly estimated and the relationship of M0 ∝ fC−4 is an artifact caused by the limited frequency band of these data. The scaling relationship of M0 ∝ fC−3 is, thus, also valid for microearthquakes, and the breakdown of the constant stress drop law, reported in the literature, is not well founded.

Spatial Variations in Fault-Zone Structure along the Nojima Fault, Central Japan, as Inferred from Borehole Observations of Fault-Zone Trapped Waves

Abstract We investigated the fault-zone structures (fault-zone width, shear-wave velocity, and Q s) of the Nojima fault, Japan, using the Love-wave-type fault-zone trapped waves (LTWs) recorded at two borehole stations—TOS2 and HRB—located along the fault. TOS2 (depth: 1673xa0m) is located at an end of the fault, while HRB (depth: 600 or 720xa0m) is located in the middle section of the fault where the greatest surface displacement of 2xa0m was recorded during the 1995 Kobe earthquake. The distance between the two stations is about 4xa0km. We found 22 records that exhibit typical LTW. We assumed the fault-zone structure to be a 2D uniform low-velocity wave guide and estimated the averaged fault-zone structure from the hypocenter to receiver by modeling the LTWs. Because we can infer the low-velocity fault zone to a depth of 8xa0km by observing the duration of LTWs, we obtain the average fault-zone structure from the borehole stations to a depth of approximately 8xa0km. The width, shear-wave velocity, and Q s of the fault zone beneath HRB are 100xa0m, 2.9 km/sec, and 60, respectively. On the other hand, the average width of the fault zone beneath TOS2 is 220xa0m, which is larger than that beneath HRB. We also conducted 3D finite-difference modeling of LTWs to confirm the spatial variations in the fault-zone structure to a depth of 8xa0km. The numerical simulation suggests that the Nojima fault has uniform elastic and attenuation properties along the fault zone to a depth of 8xa0km, while the width of the fault zone increases with the distance from the HRB to the south.

Initial rupture process of microearthquakes recorded by high sampling borehole seismographs at the Nojima fault, central Japan

Abstract We analyze high sampling waveforms of the initial part of P-wave recorded at the 1800-m-deep borehole seismographs at the Nojima fault from December 1999 to May 2000 to clarify the initial rupture process of microearthquakes. We select 12 events with high S/N, whose magnitudes range from −0.3 to 2.2 and hypocentral distances from 1 to 11xa0km. We adopt the two different source models by Sato and Hirasawa (1973) and by Sato and Kanamori (1999) . The former (model by Sato and Hirasawa (SH model)) generates only a ramp-like onset of velocity pulse. The later (model by Sato and Kanamori (SK model)) is able to generate a weak initial phase that is controlled by a trigger factor and the length of pre-existing crack. We perform the waveform inversion to estimate the optimum source parameters of each model. Waveforms of 5 of the 12 events are clearly reproduced by both SH model and SK model with a large trigger factor and a small length of pre-existing crack. The others are explained by not SH model but only SK model with a small trigger factor and a large length of the pre-existing crack, indicating that the weak initial phase is a nucleation phase and reflects the source process. These seven events satisfy roughly a relation that a large event has a large length of the pre-existing crack; the final crack length is proportional to the length of the pre-existing crack.

Spatial variation in coda Q and stressing rate around the Atotsugawa fault zone in a high strain rate zone, central Japan

We investigated a detailed spatial distribution of coda Q around the Atotsugawa fault zone in a high strain rate zone, central Japan, using waveform data from dense seismic observations. Low coda Q at lower frequencies is localized along the fault zone, showing a good spatial correlation with a low velocity zone in the lower crust. On the other hand, we find no characteristic spatial pattern of coda Q at higher frequencies. The spatial correlation between the low coda Q at the lower frequencies, and the low velocity zone, suggests that ductile deformations below the brittle-ductile transition zone in the crust contribute to the variation in coda Q at lower frequencies. We estimated a spatial variation in the stressing rate of 15–18 kPa/year in the crust from that of coda Q in the analyzed region. This value is greater than that estimated from GPS data. We conclude, therefore, that a high deformation rate below the brittle-ductile transition zone causes the high stressing rate, which results in the high strain rate along the fault zone observed by GPS.