Eiichiro Araki

Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates

Detection of the presence of melt at variable depth beneath two oceanic plates reveals the vertical extent of old oceanic plates. Boundary Issues of the Lithosphere The depth of Earths tectonic plates is defined by the lithosphere-aesthenosphere boundary (LAB), but its seismic signature is more subtle compared with other deeper boundaries within Earth (see the Perspective by Romanowicz). Under oceanic plates, the LAB is often defined by where temperatures are hot enough to cause some melting. This boundary has been hard to detect in older oceanic plates, but it is important for understanding how these plates thicken with age or distance from ocean ridges, and for assessing heat flow through the oceanic crust. Kawakatsu et al. (p. 499) use a detailed seismic array to detect a seismic velocity reduction beneath the Philippine Sea and Pacific plates. The data imply that 5%, or less, melt forms horizontal layers, and that oceanic plate thicknesses do indeed deepen with age. Rychert and Shearer (p. 495) used 15 years of seismic data to explore the global distribution of an anomaly imaged by conversion of pressure waves to shear waves (waves associated with a sharp velocity drop). The data reveal a broad signal at depths of 70 kilometers (km) beneath ocean islands to 95 km beneath Precambrian shields. It is not clear whether this boundary is the lithosphere-aesthenosphere boundary or a layer with a distinct horizontal fabric. The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

Improvement of Seismic Observation in the Ocean by Use of Seafloor Boreholes

We developed a long-term, high-quality seismic ocean floor borehole observatory system, the `Neath Seafloor Equipment for Recording Earth9s Internal Deformation (NEREID). Four NEREID borehole observatories were installed in the Japan Trench off-Sanriku area (JT1, JT2), in the northwestern Pacific Basin (WP2), and in the Philippine Sea (WP1). The borehole sensors are cemented in the borehole to assure good coupling of sensors to the ground as well as to avoid effects of water flow around the sensors, which may have been a problem in previous borehole installations. The NEREID seismic records from two of the observatories (JT1, WP2) were free from long-period noise due to turbulence in the seafloor boundary current or to water flowing around the sensor that is significant on the seafloor. The infragravity wave noise clearly observed around 0.01 Hz on the horizontal components was significantly higher in the JT1 seismometer in the sediment because of the low shear modulus of the sediment. Ocean waves of long wavelength cause the infragravity wave noise. It is thus necessary to install seismometers in boreholes below the sediments to reduce the infragravity wave noise.

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.

Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust

Eight slow-slip events over 6 years accommodated up to 50% of the fault slip on the Nankai megathrust. Silently taking up the slack Megathrust earthquakes occur when locked subduction zone faults suddenly slip, unleashing shaking and causing tsunamis. However, seismically silent slow earthquakes also relieve slip on these dangerous faults. Araki et al. present data from ocean boreholes with which they analyze eight slow-slip events near the Nankai trench off the coast of Japan. These events accommodated up to half of the plate convergence over 6 years. The events appear to occur regularly, which has a long-term impact on hazard assessment for the region. Science, this issue p. 1157 The discovery of slow earthquakes has revolutionized the field of earthquake seismology. Defining the locations of these events and the conditions that favor their occurrence provides important insights into the slip behavior of tectonic faults. We report on a family of recurring slow-slip events (SSEs) on the plate interface immediately seaward of repeated historical moment magnitude (Mw) 8 earthquake rupture areas offshore of Japan. The SSEs continue for days to several weeks, include both spontaneous and triggered slip, recur every 8 to 15 months, and are accompanied by swarms of low-frequency tremors. We can explain the SSEs with 1 to 4 centimeters of slip along the megathrust, centered 25 to 35 kilometers (km) from the trench (4 to 10 km depth). The SSEs accommodate 30 to 55% of the plate motion, indicating frequent release of accumulated strain near the trench.

Mantle Discontinuity Depths Beneath the West Philippine Basin from Receiver Function Analysis of Deep-Sea Borehole and Seafloor Broadband Waveforms

We analyzed broadband waveform data recorded by a deep-sea borehole observatory (WP-1) and a long-term broadband ocean-bottom seismograph (NOT1) deployed in the west Philippine basin by the Ocean Hemisphere Project. We determined the depths of the 660-km discontinuity beneath the west Philippine basin using the receiver function method. The “660” depths determined from the WP-1 and NOT1 are consistent with each other, indicating that the estimated depths are reliable. The 660 depth determined using both WP-1 and NOT1 data was 669 ± 9 km, which is deeper by 9 km than the global averages, beneath the west Philippine basin. Interpreting the 660 depth in terms of temperature, the slightly deep 660 can be translated to mean lower temperatures by about 100 K at the 660, using the Clapeyron slope of the olivine to β -spinel and the post-spinel phase change. The cold temperature is qualitatively consistent with the tomographic image. When compared with previous regional studies of the 660 beneath the Philippine Sea, our results suggest the presence of significant topography on the mantle discontinuities beneath the Philippine Sea, which may be caused by a stagnant Pacific slab in the mantle transition zone. The present study demonstrates that data from deep-sea observations provide useful information for investigating deep Earth structure.

Development and application of an advanced ocean floor network system for megathrust earthquakes and tsunamis

Japan is prone to great earthquakes because of its position near two different subduction zones. The Philippine Sea plate subducts from the southeast, and the Pacific plate subducts from the east. The former was the source of a series of great earthquakes, of which the Tonankai earthquake of 1944 and the Nankaido earthquake of 1946 are the latest events. The latter was the source of the 2011 earthquake off the Pacific coast of Tohoku (Tohoku earthquake) of 11 March 2011 (M9).

Near‐field observations of an offshore Mw 6.0 earthquake from an integrated seafloor and subseafloor monitoring network at the Nankai Trough, southwest Japan

An Mw 6.0 earthquake struck ~50 km offshore the Kii Peninsula of southwest Honshu, Japan on 1 April 2016. This earthquake occurred directly beneath a cabled offshore monitoring network at the Nankai Trough subduction zone and within 25–35 km of two borehole observatories installed as part of the International Ocean Discovery Programs NanTroSEIZE project. The earthquakes location close to the seafloor and subseafloor network offers a unique opportunity to evaluate dense seafloor geodetic and seismological data in the near field of a moderate-sized offshore earthquake. We use the offshore seismic network to locate the main shock and aftershocks, seafloor pressure sensors, and borehole observatory data to determine the detailed distribution of seafloor and subseafloor deformation, and seafloor pressure observations to model the resulting tsunami. Contractional strain estimated from formation pore pressure records in the borehole observatories (equivalent to 0.37 to 0.15 μstrain) provides a key to narrowing the possible range of fault plane solutions. Together, these data show that the rupture occurred on a landward dipping thrust fault at 9–10 km below the seafloor, most likely on the plate interface. Pore pressure changes recorded in one of the observatories also provide evidence for significant afterslip for at least a few days following the main shock. The earthquake and its aftershocks are located within the coseismic slip region of the 1944 Tonankai earthquake (Mw ~8.0), and immediately downdip of swarms of very low frequency earthquakes in this region, illustrating the complex distribution of megathrust slip behavior at a dominantly locked seismogenic zone.

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

Deep sea borehole observatories ready and capturing seismic waves in the western Pacific

Digital broadband seismometer networks and GPS networks are the recent sources of rapid progress in solid Earth geophysics. But they do not exist beneath the oceans. The lack of observatories in deep oceans that cover 71% of the Earths surface has been considered a major deficiency in the seismic network by the scientific community. About 80% of the present plate boundary is located in the oceans and about 90% of seismic energy release is earthquakes beneath the sea floor, mostly at plate subduction zones. Ocean basins and continents that make up our environments are not mere superficial features. They are surface manifestations of plate tectonics, mantle mixing, and core-mantle interactions invigorated by the recycling of the oceanic plates.