Yoshihiro Hiramatsu

Scaling relationship between the duration and the amplitude of non‐volcanic deep low‐frequency tremors

[1] We investigate a duration-amplitude relation of non-volcanic deep low-frequency (DLF) tremors in the Tokai region, southwest Japan, to constrain the source process of the tremors. We apply two models to the distribution, one is an exponential model as a scale bound distribution and the other a power law model as a scale invariant distribution. The exponential model shows a better fit to the duration-amplitude distribution of the tremors than a power law model, implying that the DLF tremors are caused by a scale-bound source process. The source process of the DLF tremors, therefore, differs from those for earthquakes. We suggest that the non-volcanic DLF tremor is possibly caused by a fixed source dimension with variable excess pressure of fluid or variable stress drop.

The 1998 Miyako fireball’s trajectory determined from shock wave records of a dense seismic array

A high velocity passage of a meteoroid through the atmosphere generates a shock wave with a conical front. When the shock front arrives at the surface, it causes high frequency ground motions that are registered on the seismograms. We can use seismological data to determine the trajectory of the meteoroid in the atmosphere. A strong shock wave from the 1998 Miyako fireball is recorded by more than 20 stations in a dense array of seismographs installed in the northeastern region of Honshu Island, Japan. We determine the velocity and the trajectory of the fireball in the upper atmosphere using the arrival times of the shock wave at the stations.

Temporal changes in coda Q −1 and b value due to the static stress change associated with the 1995 Hyogo‐ken Nanbu earthquake

The seismicity in the Tamba region, northeast of the Hyogo-ken Nanbu earthquake in Japan (January 17, 1995; MJMA 7.2), increased significantly following this earthquake. This increase suggests that the static stress change due to a large earthquake causes a change in the crustal condition or dynamics. In order to reveal the changes quantitatively, we investigate the temporal variation in coda Q−l and b value in the Tamba region. We analyze the waveform data of many shallow microearthquakes (M 1.5–3.0) in the region recorded in a period from 1987 to 1996. Coda Q−1 is estimated in 10 frequency bands in a range of 1.5–24 Hz based on the single isotropic scattering model. At frequencies between 1.5 and 4.0 Hz the temporal variation in coda Q−1 shows significant correlation with the occurrence of the Hyogo-ken Nanbu earthquake; coda Q−l increases after the event. A variation in b value whose sign is opposite to that of coda Q−1 is recognized. The fracture dimensions of microearthquakes that contribute to the variation in b value are estimated to be 400 m. This scale length is consistent with the characteristic scale length of scatterer, 300–600 m, which contributes effectively to a temporal variation in coda Q−1. The crustal activity in the Tamba region is possibly controlled by the heterogeneity with dominant scale of 102 m. The stress sensitivity of the coda Q−1 change is estimated to be 10 (MPa)−1. This value is an order of magnitude larger than the stress sensitivity of seismic velocity reported before.

Thick and anisotropic D″ layer beneath Antarctic Ocean

[1] Waveform modeling and travel times analyses of S, ScS and SKS phases recorded at the broad-band permanent station SYO in the Antarctic are used to determine the shear wave velocity structure and transverse isotropy in the D layer beneath the Antarctic Ocean. The SH wave structure has a discontinuity with the velocity increase of 2.0% at 2550 km. The SV structure is similar to PREM model. The magnitude of the anisotropy is highest at the top of D layer and lowest at the core-mantle boundary. The D layer beneath the Antarctic Ocean is significantly thicker than those beneath Alaska and the Caribbean Sea. We attribute this anisotropic D layer to paleo-slab materials. The subduction in and around the Antarctic Ocean has started ∼180 Ma and is the one of the oldest in the world. It has provided a large amount of the slab materials in the lowermost mantle. Citation: Usui, Y., Y. Hiramatsu, M. Furumoto, and M. Kanao (2005), Thick and anisotropic D layer beneath Antarctic Ocean.

Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

We investigate a detailed spatial variation in shear wave splitting in the zone of inland high strain rate, called the Niigata-Kobe Tectonic Zone (NKTZ), central Japan. Most observations show stress induced anisotropy, that is, the orientation of the faster polarized shear wave is parallel to the axis of the maximum horizontal compressional strain rate estimated from GPS data. Others show structure induced anisotropy, that is, the orientation is parallel to the strike of active faults. For the stress induced anisotropy, time delays normalized by the path length in the anisotropic upper crust is proportional to the differential strain rate. We estimate a spatial variation in stressing rate of the upper crust beneath the high strain rate zone based on a response of the normalized time delay to a step-wise stress change caused by a moderate-sized earthquake. The variation in the stressing rate of 3 kPa/year estimated from shear wave splitting is coincident with that from GPS data. We conclude, together with other seismological features in the NKTZ reported previously, that the high strain rate in the NKTZ is attributed to the high deformation rate below the brittle-ductile transition zone in the crust.

Seismic anisotropy near source region in subduction zones around Japan

Abstract Broad-band seismographs have been distributed widely over the Japanese islands and have provided us with high-quality digital waveform data in recent years. We have investigated splitting of ScS waves from deep earthquakes in the Kuril and Izu-Bonin subduction zones recorded on the STS seismograms. We used ScS waves from 13 events observed in Japan and recognized shear-wave splitting which is mainly caused by anisotropy in the lithospheric slab. The anisotropy was confirmed around the source region by comparing ScS-wave splitting from deep earthquakes in different regions and at different depths with S-wave splitting from deep events just beneath central Japan. We have found that the anisotropic regions exist within the subducting slab, although they are distributed locally in and around the source, possibly as patches with a diameter of 100 km or less. Discrepancy in the direction of the fast polarized shear waves between two nearby events is observed in the Izu-Bonin subduction zone: the splitting of the shallower event (289 km) shows its fast polarization direction to be parallel to the fossil motion of the Pacific plate, but that of the deeper event (361 km) immediately beneath is parallel to the current motion. These observations suggest that the change in shear-wave splitting with depth comes from the modification of preferred orientation of minerals depending on depth. We propose in this study that the reorientation of minerals (olivine) occurs owing to the change in physical conditions associated with the phase transformation of olivine (α phase) to modified spinel (β phase).

Seismic wave velocity changes and stress build‐up in the crust of the Kanto‐Tokai Region

We show there are large temporal P-wave velocity changes in the crust of the Kanto-Tokai region in Japan from repeated observations of explosions. We find that the variation is the response of the crustal rocks to two components of stress change, the tidal stress and the secular stress accumulation. The inferred annual rate of fractional velocity change (Δν/ν/year) caused by the stress accumulation in the upper crust is of the order of 10−3/year. The results indicate that the build-up of stress in the crust can be monitored by precise observations of seismic wave velocities.

On Presence of Seismic Anisotropy in the Asthenosphere beneath Continents and its Dependence on Plate Velocity: Significance of Reference Frame Selection

We examine the possibility of seismic anisotropy in the asthenosphere due to present plate motion using SKS splitting results. The fast directions of anisotropy correlate weakly with the directions of the absolute plate motion (APM) for all APM models. Weak correlation indicates the possibility of asthenospheric anisotropy as well as frozen anisotropy in the lithosphere. Detection of strain rate dependence of anisotropy is helpful to further conclusion of the problem. The selection of reference frame is important to describe shear deformation in the asthenosphere beneath continent due to plate motion. The behavior of hot spots to the mesosphere, fixed or drifted by mantle return flow, is a key of the selection of the reference frame. For the NNR-NUVEL1 model, APM correlated anisotropy appears only at plate velocity faster than 1.4 cm/yr. It suggests the new possibility of the formation of asthenospheric anisotropy in addition to frozen anisotropy in the lithosphere. A critical plate velocity for the formation of anisotropy can be caused by the dislocation-diffusion transition as a function of strain rate on a deformation mechanism map of the upper mantle olivine.

Fault model of the 2007 Noto Hanto earthquake estimated from coseismic deformation obtained by the distribution of littoral organisms and GPS: Implication for neotectonics in the northwestern Noto Peninsula

We investigate the coseismic vertical crustal movement along the northern and western coast of the Noto Peninsula caused by the Noto Hanto earthquake on March 25, 2007, from the distribution of supra-, mid- and infra-littoral organisms. The highest uplift of 44 cm is observed at Akakami and the maximum subsidence of 8 cm at Fukami. We construct a rectangular fault model with a uniform slip in elastic half-space using both the coseismic vertical displacement estimated from the distribution of these organisms and the coseismic crustal deformation obtained by GPS. The model shows a reverse fault with a right-lateral slip of 1.3 m in a 18.6 km×14.5 km area. The seismic moment is 1.0×1019 N m (MW 6.6) using a rigidity of 30 GPa. The geometry of the source fault is consistent with the distribution of aftershocks and active faults, and the fault is restricted to the central area of the aftershock area. Relationships among the fault, the distribution of aftershocks, active faults, and geological blocks around the source area suggest that geological structures restrict the fault size of the earthquake. By considering an inclined altitudinal distribution of marine terraces and the coseismic vertical crustal deformation detected in this study, we estimate that the recurrence of earthquakes during the past 120 kyr would produce a vertical crustal deformation of ≈12 m and the background tectonic uplift would reach ≈28 m.

Postseismic displacements following the 2007 Noto peninsula earthquake detected by dense GPS observation

We have been conducting dense GPS observation in and around the epicentral region of the 2007 Noto peninsula earthquake since March 25, 2007, in order to detect postseismic displacements. Continuous observation has been underway at 12 sites to fill the gap of GEONET. Preliminary analysis of data up to early May shows that initial postseismic displacement rapidly decayed within 20 days after the occurrence of the mainshock. Horizontal displacements do not exceed 20 mm even at sites above the aftershock zone for this period. We also found a maximum uplift of about 20 mm there. Inversion of postseismic displacements with the variable slip model suggests a nearly right-lateral afterslip of less than 5 cm on the shallow portion of the source fault. Fitting a theoretical function to a time series of coordinate changes also suggests that the observed postseismic displacements might have been generated by afterslip.