Tsutomu Kiguchi

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.

Stress state along the Anninghe-Zemuhe fault zone, southwestern China, estimated from an array of stress orientation measurements with a new method

Measurements of in-situ stress orientations at relatively shallow depths were conducted at 11 sites along the Anninghe-Zemuhe fault zone, southwestern China, with a newly developed method. The Anninghe fault in the fault zone has been considered to have a potential for a large earthquake with a magnitude of about 7.5, while the Zemuhe fault shows very little such potential. The present study has mainly two objectives: one is to show new data of the spatial distribution of the stress orientations around the fault zone, which is obtained directly from the measurements; the other is to examine the possibility to detect a stress pattern in relation to the past activities of earthquakes along the fault zone. The observed principal stress orientations are distributed between the NNW-SSE and NW-SE directions, which is consistent with the earthquake focal mechanism solutions near the fault zone. On the other hand, it is unclear whether the observed stress pattern is related to past activities of the fault zone, since the data fluctuation was too large to determine this. We note that the stress orientations are favorable for slip on the Anninghe fault, while being unfavorable on the Zemuhe fault, from the present experimental data.

A New Method for Stress Orientation Measurements by Using Borehole Deformation and its Applications to the Active Faults

A new sensor tool for borehole deformation using a laser displacement sensor has been developed for stress orientation measurement at shallow depths. The new method for estimation of stress field using this tool is based on following principles: 1) The borehole drilled in a stress field is quasistatically deformed due to viscous property of rock around a borehole. 2) The orientation of maximum horizontal compressive stress (SHmax) is determined from a minor axis of an ellipse fitted to a deformed shape of borehole cross section. One of the advantages of this method to estimate stress orientation is a remarkably lower cost than existing methods, such as stress release and hydraulic fracturing methods. Radial displacements of the borehole wall under creep deformation are measured with the laser displacement sensor rotating continuously around a borehole axis. A total resolution of 0.7μm is achieved by stacking of several thousands data. A field performance test confirms that the new tool can detect the creep deformation, about ten μm increasing of the borehole radius in a measurement of half a day just after drilling. After the performance test, we applied this new method to active faults in order to evaluate stress state of the faults. In the measurement for the Kego fault in Fukuoka Prefecture, western Japan, the average orientation of SHmax is about N60°E for the two measurement point around the southern edge of the fault. The estimated orientation is found to be unfavorable for a strike slip on the Kego fault. In the application to the Atotsugawa fault, central Japan, the orientation of SHmax is estimated to be ENE-WSW by the measurements in a mine cavity near the fault. This orientation is consistent with the trend of the directions of P-axes of the focal mechanism solutions of ultra-micro earthquakes along the fault.