Ryou Honda

Ground motion and rupture process of the 2004 Mid Niigata Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

The 2004 Mid Niigata Prefecture earthquake (37.289°N, 138.870°E, 13.1 km, MJMA 6.8; JMA), also known as the 2004 Niigata Prefecture Chuetsu earthquake, was a thrust type earthquake that occurred on October 23, 2004 at 17:56 (JST). Strong ground motions of PGA 800-1700 cm/s2 and PGV 60-130 cm/s were observed at stations located immediately above the source region. We deduced the rupture process of this earthquake with a multi-time-window linear waveform inversion procedure. We used near-fault strong ground motion data observed at nine K-NET and KiK-net stations within 50 km from the epicenter. In order to obtain appropriate Green’s functions for the waveform inversion, we constructed two velocity structure models for stations on the hanging wall and one structure model for stations on the footwall. The estimated total slip distribution contains three asperities: (a) around the hypocenter, (b) in the upper-middle section of the fault plane, and (c) southwest of the hypocenter. The maximum slip is 3.8 m at the hypocenter and the total seismic moment is 1.2 × 1019 Nm, which corresponds to Mw=6.7. The moment rate functions in asperities (a) and (c) have a short rise time, while those in asperity (b) have a longer rise time.

A magma‐hydrothermal system beneath Hakone volcano, central Japan, revealed by highly resolved velocity structures

High-resolution images of subsurface structures are necessary to understand the transport processes of crustal fluids from deep magma sources and their relationship to earthquake swarms in active volcanic regions. Based on a seismic tomography approach, we have developed a new model for the magma-hydrothermal system beneath Hakone volcano, central Japan, where shallow earthquake swarms and crustal deformation associated with inflation of an open-crack source are often observed. By applying travel-time data for local earthquakes to a tomographic inversion, we obtained highly resolved seismic velocity structures that show a region of low P-wave velocity (Vp), low S-wave velocity (Vs), and high Vp/Vs ratios at depths of 10–20 km beneath the volcano, corresponding to the location of the open-crack source. We suggest that the high Vp/Vs ratios represent a deep magma chamber with a high concentration of melt and/or fluids. Deep low-frequency earthquakes, located just beneath this high Vp/Vs zone, may indicate that magmatic fluids are supplied from below. Above the high Vp/Vs zone, a region of low Vp, low Vs, and low Vp/Vs ratios exists at depths of 3–10 km, suggesting the presence of crack-filled water or CO2 supplied from the inferred deep magma chamber. Many earthquake swarms occur in this low Vp/Vs zone, indicating that crustal fluids play an important role in generating the swarms. Similar relationships between magma reservoirs, overlying hydrothermal systems, and swarm activity have been reported from other volcanic areas and thus may be a ubiquitous feature beneath active volcanoes.

Array Back-Projection Imaging of the 2007 Niigataken Chuetsu-oki Earthquake Striking the World’s Largest Nuclear Power Plant

The 2007 Niigataken Chuetsu-oki earthquake occurred near the Kashiwazaki–Kariwa nuclear power plant in Japan, the largest in the world. The strong motions were recorded by seven seismometers installed at the foundation slab (base-mat) of the plant and exceeded the design level of the ground motion for the plant. The strong motion observed by the seismographs in and around the plant show high coherency with three significant pulses. In order to understand the cause of these pulses, the rupture process of the earthquake was estimated using these seismograms. The seismograph network was taken into account as a dense array and semblance-enhanced waveform stacking was performed. By projecting the power of the stacked waveforms onto the fault plane, the asperities that generated significant pulses were successfully separated. The first and third pulses were generated at the hypocenter and the southwest edge of the rupture zone, respectively. The rupture propagated toward the southwest and terminated offshore from the power plant. The overall pattern of the imaged asperities coincides well with the slip distribution determined by conventional waveform inversions.

Rupture process of the 2005 West Off Fukuoka Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

We have investigated the rupture process of the 2005 West Off Fukuoka Prefecture earthquake by the multitime- window linear waveform inversion method using the strong ground motion data recorded at 11 K-NET and KiK-net stations. From the waveforms of the P-wave portion, it is indicated that the energy release in the first few seconds was markedly lower than the subsequent part, and this causes difficulty in identifying onset of the S-wave. To decide an appropriate time window for the waveform inversion, we estimate the S-wave onset using aftershock records. The inverted slip distribution shows a single asperity of 8 km × 6 km and its center located 8 km to the southeast and 6 km above the hypocenter. The asperity explains most of the large-amplitude signals in the observed waveforms. The turning point from the initial low-energy-release rupture to the main high-energyrelease rupture is estimated from the spatial variation of the observed initial rupture phase. It is found 3.3 s after the initiation of the rupture at about 4 km to the southeast of the hypocenter. Stress drop during the initial rupture is estimated to be in the same order of those of moderate size aftershocks, which indicates that the initial rupture is an ordinary dynamic rupture.

Stress-induced spatiotemporal variations in anisotropic structures beneath Hakone volcano, Japan, detected by S wave splitting: A tool for volcanic activity monitoring

Hakone volcano, located at the northern tip of the Izu-Mariana volcanic arc, Japan, has a large caldera structure containing numerous volcanic hot springs. Earthquake swarms have occurred repeatedly within the caldera. The largest seismic swarm since the commencement of modern seismic observations (in 1968) occurred in 2001. We investigated the anisotropic structure of Hakone volcano based on S wave splitting analysis and found spatiotemporal changes in the splitting parameters accompanying the seismic swarm activity. Depth-dependent anisotropic structures are clearly observed. A highly anisotropic layer with a thickness of ~1.5 km is located beneath the Koziri (KZR) and Kozukayama (KZY) stations. The anisotropic intensity in the region reaches a maximum of 6–7% at a depth of 1 km and decreases markedly to less than 1% at a depth of 2 km. The anisotropic intensity beneath Komagatake station (KOM) decreases gradually from a maximum of 6% at the surface to 0% at a depth of 5 km but is still greater than 2.5% at a depth of 3 km. At KZY, the anisotropic intensity along a travel path of which the back azimuth was the south decreased noticeably after the 2001 seismic swarm activity. During the swarm activity, tilt meters and GPS recorded the crustal deformation. The observed decrease in anisotropic intensity is presumed to be caused by the closing of microcracks by stress changes accompanying crustal deformation near the travel path.

Seismotectonics in the Tanzawa Mountains area in the Izu-Honshu collision zone of central Japan, as revealed by precisely determined hypocenters and focal mechanisms

We investigate the detailed distribution of hypocenters and focal mechanisms beneath the Tanzawa Mountains, central Japan, where the Izu-Bonin arc has collided into the central part of the Honshu arc. Remarkable differences are found to exist between the hypocenter distributions in the western and eastern parts. The hypocenters of earthquakes in the eastern part tend to be distributed in a horizontal zone, whereas those in the western part are distributed in a volume. The focal mechanisms in the eastern part are right-lateral reverse faulting mechanisms, and one of the nodal planes is consistent with the geometry of the Philippine Sea (PHS) plate in the region. These results suggest that most earthquakes in the eastern part occur along the upper surface of the subducting PHS plate. In contrast, the focal mechanisms in the western part, especially deep in the western part, exhibit a different feature. The stress states in these two regions are found to be significantly different. The maximum and minimum principal stress axes in the eastern part are slightly inclined, whereas those in the western part are oriented in approximately the vertical and horizontal directions, respectively. The stress field in the eastern part may be caused by a slab pull force induced from the deeper part of the subducted plate.

Strong ground motion recorded by high-rate sampling GPS at the closest site to the 2003 Tokachi-oki earthquake

The Mw 8.1 earthquake occurred on September 25, 2003, off the southeast coast of Hokkaido, Japan. Since 2000 we have conducted high-rate sampling GPS measurements and precise gravity surveys in Erimo Peninsula, the closest site to the source region of the 2003 event. Strong ground motion recorded by GPS at the point of Erimo Peninsula, located just above the second asperity of the earthquake, shows two major pulses as large as about 56 cm on the EW component. Displacements obtained from the integration of accelerograms very close to our GPS site are consistent with each other, showing the absolute displacement field generated by the magnitude 8-class earthquake. Synthetic seismogram from a similar fault model by Yamanaka and Kikuchi (2003) would predict the amplitude of the second pulse to be about one half of that observed. Synthetic NS component from the GSI fault model (2003) is not consistent with our observations both on amplitude and polarity. The amplitude of ground motions detected by our GPS observation is more than one order larger than the noise level of the GPS survey, so this discrepancy is not due to insufficient GPS observation. We rather think that this suggests that our observations closest to the earthquake would give an insight into the detail of the source processes of the earthquake, which cannot be resolved from observations away from the source region. Static deformation at the point of Erimo Peninsula is consistent with the GSI fault model but not with the Yamanaka and Kikuchi model. The static analysis of our GPS measurement evidently describes the continuous post-seismic deformation as well as the co-seismic displacement in the source region until November.

Spatial distribution of crack structure in the focal area of a volcanic earthquake swarm at the Hakone volcano, Japan

We have performed shear wave splitting analyses for seismograms recorded at stations located just above, and outside, the focal area of the earthquake swarm at the Hakone volcano, Japan, in August 2009. Average values of the direction of faster split shear wave polarization (Φ) at two stations above the focal area correspond to each focal alignment of the earthquake swarm. In contrast, average values of Φ at three stations outside the focal area correspond to the direction of the maximum horizontal compressional stress. We found that the average values of the time lag between the two split shear waves inside the focal area are relatively high compared with those outside the focal area. These facts suggest that cracks with a high density aligned parallel to the faults of the earthquake swarm in the focal area. Crustal fluid was selectively injected into this pre-existing cracked media accompanied by effective normal stress reduction in the cracks, resulting in the earthquake swarm.

Rupture process of the largest aftershock of the M 9 Tohoku-oki earthquake obtained from a back-projection approach using the MeSO-net data

The largest aftershock (Mw 7.8) of the giant M 9.0 Tohoku-oki earthquake occurred near the coast of Ibaraki Prefecture about thirty minutes after the main shock. We have imaged the rupture process of the Mw 7.8 earthquake by back-projection of waveform data from the Metropolitan Seismic Observation network (MeSO-net). Original acceleration seismograms were integrated. They were then band-pass filtered in the frequency range of 0.1–1.0 Hz. We assumed a fault plane on the plate boundary with a dimension of 115 km ×175 km, and this was divided into 112 subfaults. Travel times from each of the subfaults to observation sites were calculated by using a 3-D velocity structure model. Applying the restrictions that the rupture velocity is smaller than 4 km/s and the rupture duration on each subfault is less than 25 s, we obtained a rupture propagation image by projecting the power of the stacked waveforms. Propagation of the rupture toward north and east was suppressed by the existence of those areas that had radiated a large seismic energy at the main shock occurrence, or at the occurrence of the M 7.0 earthquake in 2008. The westward propagation of the rupture stopped at the area where the Philippine Sea plate lies over the Pacific plate.

Effect of complex fault geometry and slip style on near-fault strong motions and static displacement

Although there are many studies that deal with complex slip distribution or rupture propagation on an earthquake fault, they usually regard a fault system as a fault of simple geometry. Actual fault systems have highly heterogeneous slip distribution and very complicated shapes, as is often observed through field surveys of surface breaks. In this study, we synthesize seismograms including static displacement near a fault using the discrete wavenumber method in order to estimate the effects of the above types of fault complexity in a quantitative manner. We introduce a complex slip distribution based on the Nojima Fault associated with the 1995 Hyogo-ken Nanbu earthquake. As a result, we show that strong motions at a frequency of lower than 1.0 Hz are strongly affected by the complexity of the fault geometry, at a scale of not more than several km, rather than the rupture propagation style. Distributions of static displacement fluctuate, depending on the fault geometry characterized by the length of each fault segment. Such small-scale variations in fault geometry (≤1 km) have been mostly ignored prior to this work. Our results also suggest that details of fault segmentation and bending can be determined by dense observations (e.g., GPS or geological surveys) of static displacement near a fault system, indicating the importance of simultaneous studies on static and dynamic near-fault motions.