Taku Urabe

A detailed subduction structure in the Kuril trench deduced from ocean bottom seismographic refraction studies

Abstract In 1983, we conducted an ocean bottom seismograph (OBS) experiment in the southernmost part of the Kuril trench, beneath which the Pacific Plate is subducting under Hokkaido Island, Japan. The aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling, using explosives and airguns. The total length of the profiles was 560 km, along which ten OBSs were deployed. The observed data were of good quality, which enabled us to obtain a detailed velocity structure of the active margin down to 20–30 km. We constructed a velocity structure model by ray-tracing and amplitude modeling. In the oceanic basin, the crust has a typical oceanic structure characterized by three layers with P-wave velocities of 1.8, 3.8–6.5 and 6.5–7.0 km/s. The velocity gradient in layer 3 increases downward from 0.075 to 0.10 s −1 . The Moho discontinuity was well constrained by clearly observed P n and P m P phases. The total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin. The P-wave velocity is 7.9 km/s beneath the Moho discontinuity, which increases downward with a rather small velocity gradient, 0.015–0.03 s −1 . Beneath the continental slope, we found relatively low velocity (2.5 to 5.5–5.8 km/s) material. The oceanic Moho discontinuity associated with the subducting plate was traced down to a depth of 25 km. Our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure. This result is in remarkable contrast with the velocity structure in the active margin of the Ryukyu trench area (about 1000 km south of the present experimental area), where we found a 12 km-thick, prominent low-velocity wedge is situated 50–150 km landward from the trench axis. On the seaward side of the wedge, the surface of the igneous basement undulates severely. The wedge was probably formed by materials of oceanic origin. Such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern Pacific.

Vulcanian eruptions with dominant single force components observed during the Asama 2004 volcanic activity in Japan

On September 1, 2004, Mt. Asama in central Japan erupted for the first time in 21 years. Between this moderate eruption and mid-November of the same year, 4 additional moderate eruptions occurred. We installed 8 broadband seismic stations in addition to the short period seismic network around the volcano and succeeded in recording the near-field seismic signals associated with the summit eruptions. The results of the waveform inversions clearly show that the force system exerted at the source region is dominated by vertical single force components. The source depths of the single force are shallower than 200 m from the bottom of the summit crater, and the order of magnitude of the single force is 1010–1011N. The source time history of each vertical single force component consists of two downward forces and one upward force. The initial downward force probably corresponds to the sudden removal of a lid capping the pressurized conduit. The drag force due to viscous magma moving upward in the conduit can explain the upward force. The correlation between the single force amplitudes and the amounts of volcanic deposits emitted from the summit crater are not necessarily positive, suggesting that the amount of deposits remaining within the summit crater may have played an important role in the excitation of the single force.

Crustal structure across the middle Ryukyu trench obtained from ocean bottom seismographic data

Abstract In 1988 an extensive geophysical investigation was performed at the middle Ryukyu island arc, S. Japan. An aim of the investigation is to obtain a detailed crustal structure at the trench-arc-back-arc system by various geophysical surveys. In this paper we present fine crustal models obtained from ocean bottom seismographic (OBS) data along two profiles. One profile was acquired from the northwest edge of the Philippine sea plate to the Okinawa Island (profile 2-A), and the other profile, which is perpendicular to profile 2-A, was located on the Ryukyu arc (profile 3). A crustal model of profile 2-A is characterized by a sedimentary wedge and subducting oceanic crust. The sedimentary wedge consists of four layers, whose velocities are 1.8, 2.8–2.9, 3.5 and 4.7–5.0 km/s at the top of each layer. The maximum thickness of the wedge is 9 km at 50 km landward from the trench. Underlying the sedimentary wedge, the model shows subducting oceanic layer 2 and 3 (5.3 and 6.8 km/s). A crustal model of profile 3 indicates three layers of sediment, whose velocities are 1.7–2.0, 2.4–2.8, and 3.7 km/s, above a 5.6–6.0 km/s layer. A low Q value layer at the bottom of the sedimentary layer is required to explain an observed amplitude reduction versus offsets. Three deep crustal reflection phases are observed along profile 3. These phases are interpreted as the reflection phases from the oceanic crust subducting beneath the Okinawa island. The depth of the subducting oceanic moho is deduced to be 26 km at 120 km landward from the trench axis by the reflection phases.

Aftershock seismicity and fault structure of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA7.0) derived from urgent joint observations

On March 20, 2005, a large MJMA7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2–16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of MJMA5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault.