Hiroki Miyamachi

Extensional structure in Northern Honshu Arc as inferred from seismic refraction/wide‐angle reflection profiling

A recent extensive seismic wide-angle experiment revealed a new image of crustal and uppermantle structure across Northern Honshu Arc, Japan. The western part of the arc recorded the crustal deformation by the Miocene back arc spreading of the Sea of Japan. The crust is composed of highly deformed Tertiary sedimentary layers, a relatively low velocity (5.75–5.9 km/s) crystalline basement and a 15-km thick lower crust with a velocity of 6.6–7.0 km/s. Clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading. The crust attains the maximum thickness (32–35km) east of the backbone range for which the magmatic intrusion/underplating since 10–15 Ma is a predominant factor. The eastern part of the arc has a less deformed upper crust and a reflective middle/lower crust, probably remaining a stable block since the time of the backarc spreading.

Finite frequency whole mantle P wave tomography: Improvement of subducted slab images

Received 23 July 2013; revised 14 October 2013; accepted 17 October 2013. [1] We present a new whole mantle P wave tomographic model GAP_P4. We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross correlation. Finite frequency kernels were calculated at the corresponding frequency bands for both long- and short-period data. With respect to an earlier model GAP_P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBSs) deployed in the western Pacific Ocean where station coverage is poor. This new model is different from a model in which the full data set is interpreted with classical ray theory. BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments. Citation: Obayashi, M., J. Yoshimitsu, G. Nolet, Y. Fukao, H. Shiobara, H. Sugioka, H. Miyamachi, and Y. Gao (2013), Finite frequency whole mantle P wave tomography: Improvement of subducted slab images, Geophys. Res. Lett., 40, doi:10.1002/ 2013GL057401.

Aftershock distribution of the October 4, 1994 Mw8.3 Kurile Islands Earthquake determined by a local seismic network in Hokkaido, Japan

On October 4, 1994, an earthquake with magnitude Mw8.3 occurred in the western part of Kurile Islands at 43.42°N, 146.81°E and 33 km in depth. The hypocenter parameters were determined by Hokkaido University in Japan. Aftershocks following this remarkable event were located using data from a local seismic network operated by Hokkaido University. We found that most of the aftershocks occurred (1) on the fault plane of the mainshock, (2) in the subducting plate around the fault plane of the mainshock, and (3) in the focal area of the largest aftershock, which occurred on October 9 with Mw7.3. Both (2) and (3) were not active immediately after the mainshock. Considering the time sequence of the aftershock activity, we identified one of the nodal planes of the Harvard quick CMT solutions as the fault plane of the mainshock; the strike is almost parallel to the trench axis and the dip angle is near vertical. It is obvious that this event is different from a low-angle thrust-type interplate earthquake. The distribution of aftershocks strongly suggests that it is an intraplate event.

Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface

Silent slip events get shallow Clues to help better predict the likelihood of devastating earthquakes and tsunamis may be embedded in a more gentle type of rumbling. Using oceanbottom seismometers, Yamashita et al. report rare observations of migrating tremors in the shallow part of a subduction zone off southern Kyushu, Japan. The tremors appear to be linked to a very low-frequency earthquake and seem to migrate to the region where big earthquakes are generated. The tremors may be tracing how and where stress gets concentrated onto the earthquake-producing portion of the fault. Science, this issue p. 676 Earthquake and tsunami hazard forecasts may benefit from shallow observations of seismic tremor migration in subduction zones. Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events.

Precise aftershock distribution of the 2005West Off Fukuoka Prefecture Earthquake (Mj=7.0) using a dense onshore and offshore seismic network

The 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) occurred on March 20, 2005 in the northern part of Kyushu, Japan. To study the aftershock activity, we deployed eleven pop-up type ocean bottom seismometers (OBSs), sixteen locally recorded temporary stations, and eight telemetered temporary stations in and around the epicenter region. We combined data from these stations and permanent stations located around the aftershock area, and determined the hypocenter of the mainshock and aftershocks. The mainshock was in the northwestern central part of the aftershock region, at a depth of 9.5 km. The mainshock was on a left-lateral strike-slip fault. Aftershocks were located in a depth range of 1–16 km and laterally extend for about 25 km in a NW-SE direction. We found that the aftershocks fell into four groups. This might be due to the heterogeneous structure in the source region. In the group that includes the mainshock, we estimated two fault planes bordering on the depth of the mainshock. There are 10-degree differences in both strike and dip angles between the lower and upper planes. From the aftershock distribution and the focal mechanisms, the rupture first propagated downward, and then propagated upward.

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.

Three-dimensional seismic velocity structure as determined by double-difference tomography in and around the focal area of the 2005 West off Fukuoka Prefecture earthquake

On March 20, 2005 the West off Fukuoka Prefecture earthquake (magnitude of 7.0 on the JMA scale) occurred in southeastern Japan. The earthquake fault was a left-lateral strike-slip having a nearly vertical fault plane and a strike in the WNW-ESE direction. The largest aftershock with a magnitude of 5.8 (JMA) followed 1 month later. To gain more detailed aftershock data, several teams from different Japanese universities jointly installed a number of temporary seismic stations and positioned Ocean Bottom Seismometers (OBSs) immediately above the focal area. Double-difference tomography was used to estimate the three-dimensional (3D) (Zhang and Thurber, 2003) velocity structures in and around the focal area based on the travel time data collected during seismic observations. The high-velocity regions estimated by the inversion are located on the edge of the aftershock area and on the shallow part of asperity, as inferred from the slip distribution. Conversely, the Vp/Vs ratio is not always as high as that found at the location of the asperity. This finding suggests that the construction of the medium is not uniformly elastic but complex, with different relations between elastic constants and strength.

Fine structure of aftershock distribution of the 1997 northwestern Kagoshima earthquakes with a three-dimensional velocity model

In 1997, two earthquakes (M6.5 and M6.3) occurred in the northwestern part of Kagoshima Prefecture, Japan. We carried out temporary seismic observation to obtain the detailed aftershock distribution. We constructed a 3-D P wave velocity model by inverting the travel times of aftershocks observed at 14 seismic stations in and around the focal area and relocated more than 14,000 aftershocks with the 3-D velocity model.The general features of the aftershock distribution are as follows: (1) Aftershocks of the first main shock (M6.5) are distributed with a strike of nearly E-W (N100°E) in a vertical plane with a horizontal length of 21 km and a depth range of 2 to 9 km; (2) The second main shock (M6.3) has an ‘L’-shaped aftershock distribution: one plane strikes nearly E-W, which is parallel to the aftershock zone of the first main shock, and the other is a conjugate plane; (3) An obvious seismicity gap of about 3 km wide is found between the aftershock zones striking E-W for the first and second main shocks; (4) The aftershock activities are generally low around the hypocenters of the two main shocks. Our results show that most of aftershocks occurred not in high or low velocity zones, but in intermediate velocity areas. Several vertical linear distributions of aftershocks are also confirmed in the two focal zones striking E-W. These peculiar distributions suggest that the aftershock activity is affected by the underground structural boundaries.

A subsurface structure change associated with the eruptive activity at Sakurajima Volcano, Japan, inferred from an accurately controlled source

Temporal variations of Green functions associated with the eruptive activity at Sakurajima Volcano, Japan, were estimated using an accurately controlled routinely operated signal system (ACROSS). We deconvolved 400 s waveforms of the ACROSS signal at nearby stations by a known source time function and stacked the results based on the time relative to individual eruptions and the eruption intervals; the quantities obtained by this procedure are Green functions corresponding to various stages of the eruptive activity. We found an energy decrease in the later phase of the Green functions in active eruptive periods. This energy decrease, localized in the 2–6 s window of the Green functions, is difficult to explain by contamination from volcanic earthquakes and tremors. The decrease could be more reasonably attributed to a subsurface structure change caused by the volcanic activity.

Deep Reflection Imaging beneath the Mizuho Plateau, East Antarctica, by SEAL-2002 Seismic Experiment

A seismic exploration was conducted on the Mizuho Plateau, East Antarctica, during the 2001/2002 austral summer season as the “Structure and Evolution of the East Antarctic Lithosphere (SEAL)” project by the 43rd Japanese Antarctic Research Expedition (JARE-43). The survey line of this exploration (SEAL-2002 profile) was almost perpendicular to the Mizuho inland traverse routes (JARE-41 refraction survey line; SEAL-2000) and was almost parallel to the coastal line along the Lutzow-Holm Bay. Several seismic shot records were obtained with clear arrivals of phases until a distance of 150 km in length. We have analyzed two shot data of both ends of the SEAL-2002 profile by using the conventional reflection method. Interval velocities were estimated by applying the normal-move-out (NMO) correction, then the obtained single-fold section obtained explicitly presents the horizontal reflectors originated from the middle crust, the lower crust and the Moho discontinuity. First, the reflector from the top of the middle crust was located at the depth of 23–24 km, which was corresponding to 8–9 s of two way travel time (TWT) in the single-fold section. Next, the reflector from the top of the lower crust was located at a depth of 31–34 km, corresponding to 11–12 s of TWT. The Moho reflector was observed in 13–14 s of TWT and the depth was estimated to be approximately 41–42 km.