Koichiro Obana

Aftershock distribution of the 26 December 2004 Sumatra-Andaman earthquake from ocean bottom seismographic observation

We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.

Urgent aftershock observation of the 2004 off the Kii Peninsula earthquake using ocean bottom seismometers

The 2004 off the Kii Peninsula earthquake occurred on September 5, 2004. Knowing the precise aftershock distribution is important for understanding the mechanism of this earthquake. However, the hypocenter of the main shock was located more than 100 km offshore from the nearest station of the land observation network. In the three days after the main shock, we started ocean bottom seismometer (OBS) observation in order to determine the precise distribution of the aftershocks. We assumed a seismic velocity structure for the hypocenter calculation, based on the results of previous seismic refraction study. The station corrections were incorporated to locate the hypocenter precisely. The hypocenters located within an area covered by five OBSs show relatively small errors. It is found that the OBS-located hypocenters are located about 5.5 km east-southeast from those by JMA and the depth range of the aftershocks is about 5–25 km just beneath the Nankai trough axis. The aftershock hypocenters can be grouped into two clusters at different depths of about 10 km and about 20 km. It is inferred that the main shock also has a depth of 5–25 km. Since this extent of the main shock was larger than one of the oceanic crust of the Philippine Sea plate, the fault plane of the main shock extended at the upper most mantle of the Philippine Sea plate. Although we cannot assign the actual fault plane of the main shock form our observation results, it is clarified that intra-plate earthquakes occurred near the trench region. Our OBS result supports that the main shock was the earthquake not at the plate boundary but within the bending Philippine Sea plate near the trough axis.

Precise aftershock distribution of the 2007 Chuetsu-oki Earthquake obtained by using an ocean bottom seismometer network

The Chuetsu-Oki Earthquake occurred on July 16, 2007. To understand the mechanism of earthquake generation, it is important to obtain a detailed seismic activity. Since the source region of the 2007 Chuetsu-oki Earthquake lies mainly offshore of Chuetsu region, a central part of Niigata Prefecture, it is difficult to estimate the geometry of faults using only the land seismic network data. A precise aftershock distribution is essential to determine the fault geometry of the mainshock. To obtain the detailed aftershock distribution of the 2007 Chuetsu-oki Earthquake, 32 Ocean Bottom Seismometers (OBSs) were deployed from July 25 to August 28 in and around the source region of the mainshock. In addition, a seismic survey using airguns and OBSs was carried out during the observation to obtain a seismic velocity structure below the observation area for precise hypocenter determination. Seven hundred and four aftershocks were recorded with high spatial resolution during the observation period using OBSs, temporally installed land seismic stations, and telemetered seismic land stations and were located using the double-difference method. Most of the aftershocks occurred in a depth range of 6–15 km, which corresponds to the 6-km/s layer. From the depth distribution of the hypocenters, the aftershocks occurred along a plane dipping to the southeast in the whole aftershock region. The dip angle of this plane is approximately 40°. This single plane with a dip to the southeast is considered to represent the fault plane of the mainshock. The regions where few aftershocks occurred are related to the asperities where large slip is estimated from the data of the mainshock. The OBS observation is indispensable to determine the precise depths of events which occur in offshore regions even close to a coast.

Micro-seismicity around the seaward updip limit of the 1946 Nankai Earthquake dislocation area

We observed micro-seismicity around the seaward updip limit of the 1946 Nankai earthquake dislocation area by using pop-up type ocean bottom seismographs deployed for three months in 1998. At the subduction zone, we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely. We constructed 3-D P- and S-wave velocity structure models based on the results of airgun-OBS seismic surveys along the Nankai Trough. We located 83 earthquakes using these 3-D models. These hypocenters show two groups of seismicity. One seismicity group locates in oceanic layer 3 within a subducting seamount. These earthquakes may reflect the deformation of the seamount caused by its subduction. The other seismicity group locates near the updip limit of the thermally modeled locked zone. These hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust.

Precise aftershock distribution of the 2011 off the Pacific coast of Tohoku Earthquake revealed by an ocean-bottom seismometer network

The 2011 off the Pacific coast of Tohoku Earthquake occurred at the plate boundary between the Pacific plate and the landward plate on March 11, 2011, and had a magnitude of 9. Many aftershocks occurred following the mainshock. Obtaining a precise aftershock distribution is important for understanding the mechanism of earthquake generation. In order to study the aftershock activity of this event, we carried out extensive sea-floor aftershock observations using more than 100 ocean-bottom seismometers just after the mainshock. A precise aftershock distribution for approximately three months over the whole source area was obtained from the observations. The aftershocks form a plane dipping landward over the whole area, nevertheless the epicenter distribution is not uniform. Comparing seismic velocity structures, there is no aftershock along the plate boundary where a large slip during the mainshock is estimated. Activity of aftershocks in the landward plate in the source region was high and normal fault-type, and strike-slip-type, mechanisms are dominant. Within the subducting oceanic plate, most earthquakes have also a normal fault-type, or strike-slip-type, mechanism. The stress fields in and around the source region change as a result of the mainshock.

Structural heterogeneities around the megathrust zone of the 2011 Tohoku earthquake from tomographic inversion of onshore and offshore seismic observations

We performed a three-dimensional seismic tomography around the coseismic slip area of the 2011 Tohoku earthquake. By combining data from the aftershock period collected by ocean bottom seismographs (OBSs), OBS data from previous studies off the Miyagi coast, and land seismic data, we modeled the detailed seismic velocity structure along the plate boundary from the Japan Trench to near the coastline. Our results indicate that VP, VS, and VP/VS along the plate boundary change drastically about 60 km landward from the trench axis. Trenchward of this boundary, velocities are consistent with a fluid-rich environment (low VP and VS and high VP/VS) associated with the occurrence of sediment compaction and opal-to-quartz metamorphism. This area also corresponds to the contact between the slab and upper crust or between the slab and the sediment layer. A comparison of our results with numerical simulations and geological studies suggests that thermal pressurization might have occurred near the trench axis during the 2011 Tohoku earthquake. To the west of this boundary, where VP, VS, and VP/VS have moderate values, our model showed small-scale heterogeneous structures in the subducted oceanic crust near the hypocenter of the Tohoku earthquake with regions of low and high VP/VS on the updip and downdip sides of the hypocenter, respectively. We interpret the former as corresponding to the strong coupling patch and the latter as the localized fluid-rich zone. Small-scale heterogeneities thus may affect the nucleation and rupture processes of large earthquakes.

Seismic imaging and velocity structure around the JFAST drill site in the Japan Trench: low V p, high V p/ V s in the transparent frontal prism

Seismic image and velocity models were obtained from a newly conducted seismic survey around the Integrated Ocean Drilling Program (IODP) Japan Trench Fast Drilling Project (JFAST) drill site in the Japan Trench. Pre-stack depth migration (PSDM) analysis was applied to the multichannel seismic reflection data to produce an accurate depth seismic profile together with a P wave velocity model along a line that crosses the JFAST site location. The seismic profile images the subduction zone at a regional scale. The frontal prism where the drill site is located corresponds to a typically seismically transparent (or chaotic) zone with several landward-dipping semi-continuous reflections. The boundary between the Cretaceous backstop and the frontal prism is marked by a prominent landward-dipping reflection. The P wave velocity model derived from the PSDM analysis shows low velocity in the frontal prism and velocity reversal across the backstop interface. The PSDM velocity model around the drill site is similar to the P wave velocity model calculated from the ocean bottom seismograph (OBS) data and agrees with the P wave velocities measured from the core experiments. The average V p/V s in the hanging wall sediments around the drill site, as derived from OBS data, is significantly larger than that obtained from core sample measurements.

Seismicity at the Eastern End of the 1944 Tonankai Earthquake Rupture Area

The rupture areas of the large interplate thrust earthquakes along the Nankai trough, offshore southwestern Japan, are divided into several segments. The 1944 Tonankai earthquake ruptured one of the segments off of the Kii Peninsula. In 2005, we conducted an ocean-bottom seismograph experiment at the eastern end of the rupture area of the 1944 earthquake. Little seismic activity was observed on the plate interface; most earthquake activity was within the subducting and overriding plates. Aftershocks of the 2004 off Kii Peninsula earthquake, which was an intraplate earthquake in the subducting Philippine Sea plate, were mainly located in the subducting oceanic crust and uppermost mantle. However, several earthquakes at the eastern end of the rupture area of the 1944 earthquake were in the sedimentary wedge, with a focal mechanism indicating deformation by a subducting seamount. The earthquakes and faults in the sedimentary wedge show deformation related to the irregular topography of the subducting oceanic crust to the east of the rupture area of the 1944 Tonankai earthquake. In contrast, few earthquakes were observed in the sedimentary wedge in the rupture area of the 1944 earthquake. Difference in topography of the subducting oceanic crust in the two areas might have caused spatial variations of interplate coupling, which, in turn, caused the observed differences in the deformation of the sedimentary wedge. The spatial variation of interplate coupling may affect rupture propagation during large interplate earthquakes and cause the segmentation of large interplate earthquakes along the Nankai trough.

Along‐arc variation in seismic velocity structure related to variable growth of arc crust in northern Izu‐Bonin intraoceanic arc

The Izu-Bonin arc is an intraoceanic island arc where the Pacific plate subducts beneath the Philippine Sea plate. Along-arc variation in thickness of the arc crust has been observed by previous active seismic surveys beneath the Izu-Bonin arc. We have imaged three-dimensional (3-D) seismic velocity structures in the northern Izu-Bonin arc using arrival time data of the earthquakes during 80 days of observations by ocean bottom seismographs and three permanent island seismic stations. Our 3-D velocity model indicates heterogeneous structure in the mantle wedge along the arc. Low-velocity anomalies related to upwelling flow in the mantle wedge are not uniform beneath the volcanic front. Low-velocity anomalies extending down to the subducting slab beneath the volcanic front coincide with thicker parts of the arc crust north of Aoga-shima and south of Sumisu-jima. This coincidence suggests that heterogeneous mantle upwelling flow fundamentally controls the growth of the arc crust. Along topographic highs in the forearc, low-velocity anomalies in the crust and uppermost mantle coincide with positive magnetic anomalies suggesting the presence of a remnant arc, providing further evidence of this view of arc crust formation.

Seafloor seismometers monitor northern Cascadia earthquakes

The Mw = 9.0 earthquake of 11 March 2011 at the Japan Trench and its devastating tsunami underscore the importance of understanding seismogenic behavior of subduction faults and realistically estimating the potential size of future earthquakes and tsunamis. For the Cascadia subduction zone (Figure 1a), a critical knowledge gap is the level of microseismicity offshore, especially near the megathrust, needed to better understand the state of the locked zone. In 2010 the first detailed seafloor earthquake monitoring campaign along the northern Cascadia subduction zone recorded nearby earthquakes in the local magnitude (ML) range from possibly around zero to 3.8 (Figures 1b and 1c) and larger earthquakes from outside this region. Preliminary analyses indicate that the network appears to have yielded a fairly complete catalog for events with ML > 1.2. Only a few tens of these events occurred beneath the continental shelf and slope (Figure 1a). The majority of the earthquakes were located along the margin-perpendicular Nootka fault zone. The relatively low seismicity away from the Nootka fault is consistent with a fully locked megathrust. Land-based GPS measurements cannot resolve the question of whether the offshore part of the megathrust seismogenic zone is narrow and fully locked or wider and only partially locked (slowly creeping). If it were only partially locked, the seafloor seismometer data should show many more small earthquakes along the interface than were actually detected.