Kimihiro Mochizuki

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.

New rates of western Pacific island arc magmatism from seismic and gravity data

Numerous studies have been conducted in order to look into the evolution of the continental crust. Some suggest that one of the mechanisms which contribute to the growth of continental crust is arc magmatism. It is in this context that Reymer and Schubert (Tectonics 3 (1984) 63) estimated arc magmatic addition rates to the continental crust. Their results suggest that island arc magmatism was producing material at an average rate of 20–40 km3/km/Myr (volume per unit width along the strike direction of arc). The present work utilizes the most recent worldwide marine gravity data, together with improved seismic data from some oceanic island arcs in the western Pacific region. The combined gravity and seismic data allow a more accurate image of the subsurface configuration beneath the oceanic island arcs and yield better estimates of crustal volumes created during arc magmatic processes. Oceanic island arcs investigated in this study show a crustal thickness ranging from 20 to 30 km. Utilizing this thickness, the relevant crustal volume for each island arc is then estimated. Dividing the crustal volume by the age of initiation of subduction of the arc gives arc magmatic addition rates of 30–95 km3/km/Myr. The estimates presented here are nearly twice as high as the previous estimates of arc magmatic addition rates.

Western Nankai Trough seismogenic zone: Results from a wide‐angle ocean bottom seismic survey

The Nankai Trough, southwestern Japan, is recognized as a vigorous seismogenic zone with well-studied historic earthquakes. This paper presents results of a wide-angle ocean bottom seismographs (OBS) study at the western Nankai Trough seismogenic zone. The OBS data used were acquired on a profile (250 km long) across the presumed coseismic slip zone of the 1946 Nankaido earthquake (Ms = 8.2). The main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake. The crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge. P wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities (Vp = 5.0–5.6 km/s and 6.6–6.8 km/s in oceanic layers 2 and 3, respectively). The maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with Vp = 3.4–4.6 km/s in the deeper part. The subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward: 3.2°and 7.2° at 0–50 km and 50–100 km from the trough axis, respectively. The oceanic crust is smooth to the hypocenter zone, down to 40 km depth beneath Shikoku Island. Our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust-island arc crust contact zone. Even though there is large uncertainty about the seaward limit of the coseismic slip zone, the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism.

Slow slip near the trench at the Hikurangi subduction zone, New Zealand

Applying pressure to plate tectonics The full range of deformation behavior of subduction zone faults that are responsible for great earthquakes and tsunamis is now clearer. Wallace et al. observed the heave of the ocean floor near the Hikurangi trench, off the east coast of New Zealand, with a network of absolute pressure gauges (see the Perspective by Tréhu). The gauges sit on the ocean floor and detect changes in pressure generated from slow-slip deformation events. Detailed geodetic observation of deformation events will finally clarify the role that such aseismic events play at major plate boundaries. Science, this issue p. 701; see also p. 654 Absolute pressure gauges detect a slow-slip event near the Hikurangi trench. The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world’s largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist.

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.

Deep crustal structure of the eastern Nankai Trough and Zenisu Ridge by dense airgun–OBS seismic profiling

Abstract An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough.

Heterogeneous crustal structure across a seismic block boundary along the Nankai trough

報告番号: 甲12448 ; 学位授与年月日: 1997-03-28 ; 学位の種別: 課程博士 ; 学位の種類: 博士(理学) ; 学位記番号: 博理第3228号 ; 研究科・専攻: 理学系研究科地球惑星物理学専攻

Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15–20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15–20 km from the aftershock distribution.

Along-trench structural variation and seismic coupling in the northern Japan subduction zone

Large destructive interplate earthquakes, such as the 2011 Mw 9.0 Tohoku-oki earthquake, have occurred repeatedly in the northern Japan subduction zone. The spatial distribution of large interplate earthquakes shows distinct along-trench variations, implying regional variations in interplate coupling. We conducted an extensive wide-angle seismic survey to elucidate the along-trench variation in the seismic structure of the forearc and to examine structural factors affecting the interplate coupling beneath the forearc mantle wedge. Seismic structure models derived from wide-angle traveltimes showed significant along-trench variation within the overlying plate. In a weakly coupled segment, (i) the sediment layer was thick and flat, (ii) the forearc upper crust was extremely thin, (iii) the forearc Moho was remarkably shallow (about 5 km), and (iv) the P-wave velocity within the forearc mantle wedge was low, whereas in the strongly coupled segments, opposite conditions were found. The good correlation between the seismic structure and the segmentation of the interplate coupling implies that variations in the forearc structure are closely related to those in the interplate coupling.