Yoshiko Yamanaka

Source process of the recurrent Tokachi-oki earthquake on September 26, 2003, inferred from teleseismic body waves

On September 26, 2003, a large earthquake with a magnitude of 8.0 occurred along the Kuril trench off Tokachi, Hokkaido, Japan. We investigated the source process by using teleseismic P- and SH-wave data. The main source parameters are as follows: the seismic moment 1.0 × 1021 Nm (Mw= 8.0); (strike, dip, rake) = (230°, 20°, 109°); the depth of initial break point 25 km; source duration 40 sec; and the maximum slip 5.8 m. We estimated the fault area to be 90 × 70 km2, the average slip 2.6 m, and the stress drop 5.0 MPa. This earthquake was an interplate earthquake associated with the subduction of the Pacific plate. The rupture propagated northward from a shallow to a deep region. In this area, a great earthquake (magnitude 8.2) occurred in 1952. We also made limited inversion of nearfield records of the 1952 event and found that the 2003 asperity was also ruptured in 1952. Our result suggests that the 2003 Tokachi-oki earthquake was a recurrent event of the 1952 Tokachi-oki earthquake.

Weak Interplate Coupling by Seamounts and Repeating M ~ 7 Earthquakes

Subducting seamounts are thought to increase the normal stress between subducting and overriding plates. However, recent seismic surveys and laboratory experiments suggest that interplate coupling is weak. A seismic survey in the Japan Trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude M ∼ 7. Both observed seismicity and the pattern of rupture propagation during the 1982 M 7.0 event imply that interplate coupling was weak over the seamount. A large rupture area with small slip occurred in front of the seamount. Its northern bound could be determined by a trace of multiple subducted seamounts. Whereas a subducted seamount itself may not define the rupture area, its width may be influenced by that of the seamount.

Three-dimensional P-wave velocity structure beneath the Indonesian region

Abstract We present the P-wave seismic tomography image of the mantle to a depth of 1200 km beneath the Indonesian region. The artb inversion method is applied to a dataset of 118,203 P-wave travel times of local and teleseismic events taken from ISC bulletins. Although the resolution is sufficient for detailed discussion in only a limited part of the study region, the results clarify the general tectonic framework in this region and indicate a possible remnant seismic slab in the lower mantle. Structures beneath the Philippine Islands and the Molucca Sea region are well resolved and high-velocity zones corresponding to the slabs of the Molucca Sea and Philippine Sea plates are well delineated. Seismic zones beneath the Manila, Negros and Cotabato trenches are characterized by high-velocity anomalies, although shallow structures were not resolved. The Molucca Sea collision zone and volcanic zones of the Sangihe and Philippine arcs are dominated by low-velocity anomalies. The Philippine Sea slab subducts beneath the Philippine Islands at least to a depth of 200 km and may reach depths of 450 km. The southern end of the slab extends at least to about 6°N near southern Mindanao. In the south, the two opposing subducting slabs of the Molucca Sea plate are clearly defined by the two opposing high-velocity zones. The eastward dipping slab can be traced about 400 km beneath the Halmahera arc and may extend as far north as about 5°N. Unfortunately, resolution is not sufficient to reveal detailed structures at the boundary region between the Halmahera and Philippine Sea slabs. The westward dipping slab may subduct to the lower mantle although its extent at depth is not well resolved. This slab trends N-S from about 10°N in the Philippine Islands to northern Sulawesi. A NE-SW-trending high-velocity zone is found in the lower mantle beneath the Molucca Sea region. This high-velocity zone may represent a remnant of the former subduction zone which formed the Sulawesi arc during the Miocene. The blocks along the Sunda and Banda arcs are less well resolved than those in the Philippine Islands and the Molucca Sea region. Nevertheless, overall structures can be inferred. The bowl-shaped distribution of the seismicity of the Banda arc is clearly defined by a horseshoe-shaped high-velocity zone. The tomographic image shows that the Indian oceanic slab subducts to a depth deeper than 300 km i.e., deeper than its seismicity, beneath Andaman Islands and Sumatra and may be discontinuous in northern Sumatra. Along southern Sumatra, Java and the islands to the east, the slab appears to be continuous and can be traced down to at least a depth of the deepest seismicity, where it appears to penetrate into the lower mantle.

Source rupture process of the Papua New Guinea earthquake of July 17, 1998 inferred from teleseismic body waves

A large earthquake (Ms 7.1) occurred off northwest coast of Papua New Guinea (PNG), and a massive tsunami attacked villages to cause a devastating damage. In an attempt to ascertain the tsunami source, we investigate the source rupture process using teleseismic data at IRIS network as well as local data at Jayapura, Irian Jaya, station. The source parameters obtained are: (strike, dip, slip) = (301°, 86°, 91°); the seismic moment = 4.3 × 1019 Nm (Mw = 7.0); the duration of main rupture = 19 s; the centroid depth = 20 ± 5 km; the extent of rupture along the fault strike = 40 km; the average dislocation = 1.8 m; the stress drop = 7.3 MPa. The tsunami magnitude Mt determined from tide-gage data at long distance is 7.5, significantly larger than Ms, so that the PNG earthquake is characterized as a tsunami earthquake. Tsunami earthquakes might have been caused by slow rupture, submarine landslide, and high-angle dip-slip. Our teleseisimic analysis precludes the first two candidates and favors the third one as a source of the present earthquake, although it does not necessarily exclude the possibility of an aseismic landslide induced by the main shock or its aftershocks.

A third volcanic chain in Kamchatka : thermal anomaly at transform/convergence plate boundary

The Kamchatka volcanic arc, which is located at the northern edge of the Kurile arc, consists of three volcanic chains, all parallel to the trench axis. In contrast, most subduction zones have only two subparallel volcanic chains. The third chain in Kamchatka, which is farthest from the trench, is characterized by the occurrence of voluminous plateau lavas; volcanoes in the two chains closer to the trench are stratovolcanoes typical in arc magmatism. The third chain magmatism is also unusual in that lavas show concentrations of incompatible elements intermediate between those in the two trenchward chains. Both the unusual occurrence of the third volcanic chain and the unusual lava chemistry could be caused by partial melting of K-amphibole bearing peridotites in the downdragged hydrous layer at the base of the mantle wedge under anomalously high-temperature conditions associated with the characteristic tectonic setting of transform/convergence transition in the region.

Arc stresses determined by slabs: Implications for mechanisms of back-arc spreading

There is an anti-correlation in stress field between the back-arc area and the shallow portion of the slab. Generally slabs showing down-dip compression have tensional back-arcs, and the reverse. We explain this anti-correlation by a force balance in the fore-arc wedge between the slab pull, collision force in the upper plate, and the ridge push. Back-arc spreading can occur as a passive response to the down-dip compressional slab. The Mariana, Kyushu and Aegean arcs are the exceptions to the anti-correlation; in these arcs, the back-arc is tensional although the slab is in down-dip tension. This may be because the fore-arc is driven by the mantle drag toward the trench, resulting in compression balanced with the slab pull. The flow in the mantle would cause back-arc spreading in this case.

Simulation of strong ground motions caused by the 2004 off the Kii peninsula earthquakes

Strong ground motions caused by the Mj 7.4 2004 earthquake that occurred in the Nankai Trough to the southeast of the Kii Peninsula, Japan are simulated by a three-dimensional (3D) finite-difference method (FDM) using a fault-rupture model obtained by inversion of teleseismic seismograms and a 3D subsurface structure model for central Japan. Through simulations of the foreshock (Mj 7.1), the structural model is refined by comparison with observations, and the modified model is used to simulate the mainshock. The simulation provides a reasonable reproduction of the ground motions caused by the mainshock, including site amplification effects in the sedimentary basins of Osaka and Noubi. However, the current simulation model has limitations in producing the large and extended ground motion due to long-period Love waves in the Kanto Plain, as the model does not account for the sharp frequency selectivity for Love waves in the surficial structure of the Bouso Peninsula. It therefore appears necessary to develop a better model for longer-period waves.

The March 25, 1998 Antarctic Earthquake: Great earthquake caused by postglacial rebound

A large Mw = 8.1 earthquake occurred off southeast coast of Antarctica near the Balleny Island region on March 25, 1998. We inverted teleseismic body-wave records to determine the rupture pattern using an iterative deconvolution method. The source parameters obtained are: the centroid depth=20km, (strike, dip, rake)= (282, 83, −1), the seismic moment M0 = 1.6 × 1021 Nm (Mw = 8.1), the length L = 200 km, and the average slip D = 4.4 m. This earthquake occurred in the mid-plate but there has been no reports of such large earthquakes in this region. Furthermore, the source mechanism cannot be related to the plate motion inferred from the nearby transform faults. Therefore this earthquake is not a usual tectonic event. Here we show that the compressional axis of our source mechanism coincides with the horizontal crustal motion predicted by the Earth’s response to present-day and past ice mass changes in Antarctica. Our result suggests that the 1998 Antarctica earthquake is caused by the postglacial rebound in the Antarctica.

Short-term spatiotemporal variations in the aftershock sequence of the 2004 mid-Niigata prefecture earthquake

We deployed 56 temporary seismic stations within approximately a month after the occurrence of the 2004 mid-Niigata prefecture earthquake. Using manually-picked arrival data obtained from the temporary and surrounding permanent seismic stations, 1056 aftershocks have been relocated. Based on the spatiotemporal variations in the relocated aftershocks, the cluster activities associated with the mainshock and some large aftershock events are identified. The aftershocks associated with the mainshock, the largest occurred on the two steep west-dipping planes at an angle of 60° and approximately 5 km away. In contrast, the aftershocks following the event on Oct. 27 are aligned on east-dipping plane at a low angle of 25°. It is further observed that the aftershock area extended in both northeastward and southwestward directions at a later stage. The triggered seismicity around the northeast edge was more significant than that around the southwest edge. This difference could be understood by the discrepancy in the shear stress level accumulated at the dynamic shear rupture due to the mainshock.

Continuous long-term array analysis of seismic records observed during the 2011 Shinmoedake eruption activity of Kirishima volcano, southwest Japan

We deployed a seismic array at a site 5 km east of Shinmoedake volcano, in the Kirishima volcanic complex of southwest Japan, five days after the sub-Plinian eruption on 26 January, 2011. The array record between February and September 2011 included explosion earthquakes and episodes of weak continuous tremor during eruption periods. We estimated slownesses and back azimuths of seismic waves on a sliding 1-min window using the semblance method. The slownesses of the weak continuous tremor clustered within the range 0.2–0.8 s/km, consistent with a mix of body and surface waves. A probabilistic approach based on a grid search was used to estimate the source locations of the explosion earthquakes and weak continuous tremor. The sources of the explosion earthquakes were beneath the crater at depths of −0.5–1 km above sea level, while the source of the weak continuous tremor was beneath the northern part of Shinmoedake at depths between 1 km below sea level and 1 km above sea level. This latter region corresponds to a shallow low-resistivity layer, suggesting that hydrothermal processes are more plausible than magmatic processes as the generating mechanism of the weak continuous tremor.