Yojiro Yamamoto

Seismicity near the hypocenter of the 2011 off the Pacific coast of Tohoku earthquake deduced by using ocean bottom seismographic data

We relocated hypocenters of the foreshock, mainshock, and aftershocks of the 2011 off the Pacific coast of Tohoku earthquake (M 9.0) in the middle part of the Japan Trench where the earthquake rupture initiated. Ocean Bottom Seismographs (OBSs), deployed in the area, recorded the earthquakes and these data provide improved images of the hypocenter distribution. The mainshock hypocenter was relocated slightly westward from that reported by Japan Meteorological Agency (JMA), placing it near the intersection between the plate boundary and the Moho of the overriding plate. The foreshock seismicity mainly occurred on the trenchward side of the mainshock hypocenter, where the Pacific slab contacts the island arc crust. The foreshocks were initially activated at the up-dip limit of the seismogenic zone ~30 km trenchward of the largest foreshock (M 7.3, two days before the mainshock). After the M-7.3 earthquake, intense interplate seismicity, accompanied by epicenters migrating toward the mainshock hypocenter, was observed. The focal depth distribution changed significantly in response to the M-9 mainshock. Earthquakes along the plate boundary were almost non-existent in the area of huge coseismic slip, whereas earthquakes off the boundary increased in numbers in both the upper and the lower plates.

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.

Hypocenter distribution of the main- and aftershocks of the 2005 Off Miyagi Prefecture earthquake located by ocean bottom seismographic data

The preliminary hypocenter distribution of the 2005 Off Miyagi Prefecture earthquake and its aftershocks is estimated using data from five ocean bottom and six onshore seismic stations located around the rupture area of the earthquake. The epicenter of the mainshock is relocated at 38.17°N, 142.18°E, and the focal depth is estimated to be 37.5 km. The aftershocks surrounding the mainshock hypocenter form several clusters that are concentrated along a distinct landward dipping plane corresponding to the plate boundary imaged by the previous seismic experiment. The strike and dip angles of the plane agree well with those of the focal mechanism solution of the mainshock. The size of the plane is about 20×25 km2 in the strike and dip directions, which is similar to that of the large coseismic slip area. The up-dip end of the planar distribution of the aftershocks corresponds to the bending point of the subducting oceanic plate, suggesting that the geometry of the plate boundary affects the spatial extent of the asperity of the 2005 earthquake

Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench

It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15–18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface.

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.

Aftershocks of the December 7, 2012 intraplate doublet near the Japan Trench axis

On December 7, 2012, a pair of large Mw 7.2 intraplate earthquakes occurred near the Japan Trench axis off Miyagi, northeast Japan. This doublet consisted of a deep reverse-faulting event followed by a shallow normal-faulting event. Aftershock observations using conventional and newly developed ultra-deep ocean bottom seismographs in the trench axis area showed that the shallow normal-faulting event occurred in the subducting Pacific plate just landward of the trench axis. The shallow normal-faulting aftershock activity indicated that in-plate tension in the incoming/subducting Pacific plate extends to a depth of at least 30 km, which is deeper than before the 2011 Tohoku-Oki earthquake, whereas in-plate compression occurs at depths of more than 50 km. Hence, we concluded that the neutral plane of the in-plate stress is located between depths of 30 and 50 km near the trench axis.

Geometry and segmentation of the North Anatolian Fault beneath the Marmara Sea, Turkey, deduced from long-term ocean bottom seismographic observations

Both the geometry and the depth of the seismogenic zone of the North Anatolian Fault under the Marmara Sea (the Main Marmara Fault (MMF)) are poorly understood, in part because of the faults undersea location. We recorded 10 months of microseismic data with a dense array of ocean bottom seismographs and then applied double-difference relocation and 3-D tomographic modeling to obtain precise hypocenters on the MMF beneath the central and western Marmara Sea. The hypocenters show distinct lateral changes along the MMF: (1) both the upper and lower crust beneath the Western High are seismically active and the maximum focal depth reaches 26 km; (2) seismic events are confined to the upper crust beneath the region extending from the eastern part of the Central Basin to the Kumburgaz Basin; and (3) the magnitude and direction of dip of the main fault change under the Central Basin, where there is also an abrupt change in the depth of the lower limit of the seismogenic zone. We attribute this change to a segment boundary of the MMF. Our data show that the upper limit of the seismogenic zone corresponds to sedimentary basement. We also identified several seismically inactive regions within the upper crust along the MMF; their spatial extent beneath the Kumburgaz Basin is greater than beneath the Western High. From the comparison with seafloor extensometer data, we consider that these regions might indicate zones of strong coupling that are accumulating stress for release during future large earthquakes.

Fracture Alignments in Marine Sediments Off Vancouver Island from Ps Splitting Analysis

Alignments of fractures and cracks in marine sediments may be controlled by various mechanisms such as horizontal compaction and extension and basement faulting. The orientation of these alignments can be estimated through analyses of S‐wave splitting. If sensors in ocean‐bottom observations are deployed through free fall, sensor orientation needs to be determined in order for the recorded data to be used for such analyses. Here, we estimate the sensor orientation from the linear particle motions of P‐to‐s (Ps) phases converted at the sediment–basement interface and also from T waves that are excited by earthquakes and propagate in the seawater. We examine waveforms of local earthquakes recorded by 32 ocean‐bottom seismometers (OBSs) that were deployed through free fall for three months in 2010 off Vancouver Island where the strike‐slip Nootka fault zone (NFZ) intersects the deformation front of the Cascadia subduction zone. Because the particle motion of the Ps wave was corrected by estimating splitting parameters, the fast polarization direction, which reflects S‐wave anisotropic structure within the sediment, can also be evaluated. Consequently, we could estimate the fast polarization direction at OBSs deployed near the NFZ and west of the deformation front. The obtained fast directions appeared to correspond to alignments of shear fractures in the marine sediments associated with the left‐lateral motion of the fault in the basement along the NFZ, margin‐normal cracks due to horizontal compression west of and slightly away from the deformation front, and frontal thrust faults within the accretionary prism near the deformation front.

S wave attenuation structure on the western side of the Nankai subduction zone: Implications for fluid distribution and dynamics

We estimated the S wave attenuation structure in southwestern Japan and the western Nankai Trough by analyzing maximum S wave amplitudes at 4–8, 8–16, and 16–32 Hz with a correction term for apparent amplitude attenuation due to multiple forward scattering. Because the estimated attenuation (Q−1) in our tomographic study was much larger than Q−1 due to wide-angle scattering, our estimated Q−1 was composed mainly of intrinsic attenuation. High-attenuation areas (Q−1 > 1/300 at 4–8 Hz) were imaged beneath Quaternary volcanoes and south off Shikoku. Low (<1/1500 at 4–8 Hz) or moderate Q−1 (1/500–1/1000 at 4–8 Hz) was imaged beneath Shikoku and nonvolcanic areas of Chugoku. High and moderate Q−1 in and around Shikoku are located near the top of subducting Philippine Sea Plate. This correspondence implies that these high and moderate Q−1 reflect fluid in the subducting slab. By applying a theoretical model of attenuation in water-saturated porous random media, we examined wave-induced fluid flow induced by lower frequency (<1 Hz) seismic waves that may be related with triggering of nonvolcanic tremor by surface waves. Even though Q−1 structure in this study cannot fully explain the tremor triggering by wave-induced fluid flow, large uncertainties of Q−1 in tremor zone suggest that high resolution imaging of Q−1 and random inhomogeneities would give some constraints for the spatial variation of permeability and other medium properties.

Giant rhyolite lava dome formation after 7.3 ka supereruption at Kikai caldera, SW Japan

Kikai submarine caldera to the south of the Kyushu Island, SW Japan, collapsed at 7.3 ka during the latest supereruption (>500 km3 of magma) in the Japanese Archipelago. Multi functional research surveys of the T/S Fukae Maru in this caldera, including multi-beam echosounder mapping, remotely operated vehicle observation, multi-channel seismic reflection survey, and rock sampling by dredging and diving, provided lines of evidence for creation of a giant rhyolite lava dome (~32 km3) after the caldera collapse. This dome is still active as water column anomalies accompanied by bubbling from its surface are observed. Chemical characteristics of dome-forming rhyolites akin to those of presently active small volcanic cones are different from those of supereruption. The voluminous post-caldera activity is thus not caused simply by squeezing the remnant of syn-caldera magma but may tap a magma system that has evolved both chemically and physically since the 7.3-ka supereruption.