Hitoshi Kawakatsu

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.

Seismic Evidence for Deep-Water Transportation in the Mantle

We report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern Japan. Our data indicate that water is released from the hydrated oceanic crust at shallow depths (<∼100 kilometers) and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle. Our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment.

Three‐dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi‐net tiltmeters

[1] We developed a three-step method for three-dimensional (3-D) S wave velocity tomography by fitting synthetic cross spectra to the observed ones of ambient seismic noise. We applied this method to the recording of Hi-net tiltmeters in Japan at 679 stations from June 2004 to December 2004. First, we calculated normalized cross spectra between radial components and those between transverse components for every pair of stations. The first step is local 1-D S wave velocity inversion for each station assuming small lateral heterogeneity under a 100-km circle of a station. We measured the dispersion curves of fundamental Rayleigh waves, fundamental Love waves, and first overtone of Love waves by fitting the synthetic cross spectra to the observed ones between pairs of stations within the circle. We inverted the measured dispersion curves for obtaining a 1-D S wave velocity model. The second step is the inversion of the observed cross spectra for obtaining path-averaged 1-D S wave velocity structure. The third step is the inversion of the resultant path-averaged structures for obtaining 3-D S wave velocity structure (0.1 � 0.1 � 1 km grid from the surface to a depth of 50 km) using ray approximation. The resultant S wave velocity structures show clear low-velocity anomalies along tectonic lines from the surface to a depth of 20 km. In particular, along the Hidaka mountain range, we observed S wave perturbation more extreme than � 20%. They also show low-velocity anomalies under volcanoes in Kyusyu and Tohoku. In the southwestern part of Shikoku, our results show a clear low-velocity anomaly corresponding to an accretional belt (Shimanto belt). Below 20 km, we observe a low-velocity anomaly in the center of Japan, which suggests a thick crust. Citation: Nishida, K., H. Kawakatsu, and K. Obara (2008), Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi-net tiltmeters, J. Geophys. Res., 113, B10302, doi:10.1029/2007JB005395.

Mechanism of Phreatic Eruptions at Aso Volcano Inferred from Near-Field Broadband Seismic Observations

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15 seconds. When phreatic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the conduit: gradual pressurization and long-period (approximately20 seconds) pulsations of the reservoir during the 100 to 200 seconds before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, where water and heat from the deeper magma chamber probably interact, appears to help control the surface activity at Aso volcano.

Global Surface Wave Tomography Using Seismic Hum

Long-period surface waves from oceanic or atmospheric disturbances can be used for seismic mapping of the upper mantle. The development of global surface wave tomography using earthquakes has been crucial to exploration of the dynamic status of Earth’s deep. It is naturally believed that only large earthquakes can generate long-period seismic waves that penetrate deep enough into Earth for such exploration. The discovery of seismic hum, Earth’s background free oscillations, which are randomly generated by oceanic and/or atmospheric disturbances, now provides an alternative approach. We present results of global upper-mantle seismic tomography using seismic hum and without referring to earthquakes. At periods of 100 to 400 seconds, the phase-velocity anomalies of Rayleigh waves are measured by modeling the observed cross-correlation functions between every pair of stations from among 54 globally distributed seismic stations. The anomalies are then inverted to obtain the three-dimensional S-wave velocity structure in the upper mantle. Our technique provides a new means for exploring the three-dimensional structure of the interior of terrestrial planets with an atmosphere and/or oceans, particularly Mars.

Depth variation of the mid‐mantle seismic discontinuity

Short-period array seismograms of deep events that occurred in the Indonesia, Japan and Izu-Bonin arcs are stacked and beam-formed to identify the near-source S-P converted waves that result from the mantle transition discontinuities. Most of the resulting images reveal the existence of a mid-mantle seismic discontinuity (“920 km discontinuity”) in these regions. Of the 15 events analyzed, three that occurred at the western end of the Indonesia arc show clear S-P arrivals observable even in individual seismograms. The mid-mantle discontinuity is characterized by large depth variation (900 ∼ 1080 km) and velocity contrast variation in different subduction zones. Especially, the depth variation of the mid-mantle discontinuity beneath the Indonesia arc, where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end, appears to be well correlated with the location of the high-velocity anomalies in recent tomographic models. However, the mid-mantle discontinuity cannot be simply coincided with the bottom of the high-velocity anomalies, because a velocity increase at the discontinuity is observed from the waveform analysis.

Direct evidence for the undulation of the 660-km discontinuity beneath Tonga: Comparison of Japan and California array data

Short-period seismograms of Tonga deep earthquakes recorded by Japanese and Californian seismic networks are stacked to identify the S-P converted wave associated with the 660-km discontinuity. The travel-time difference between this S-P converted wave and the direct P wave is used to constrain the depth of the 660-km discontinuity. Analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the Tonga subduction zone. Two events which exhibit clear S-P conversions in both Japan and California data are selected to show directly the depth variations of the 660-km discontinuity adjacent to the subducting slab. The S-P conversion points on the ray paths to Japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to California, which represents direct evidence for a slab-induced depression of the 660-km discontinuity.

Detection of a crack‐like conduit beneath the active crater at Aso Volcano Japan

To constrain the source of long period tremors (LPTs), we deployed a very dense broadband seismic network consisting of totally twenty-four stations around the active crater of Aso volcano in Kyushu, Japan. The spatial variation of the observed signal amplitudes reveals that the source of LPTs consists of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters. The detected crack has a dimension of 1 km and its center is located a few hundred meters southwest of the active crater, at a depth of about 1.8 km. The extension of the crack plane meets the crater chain including the active fumarole at the surface, suggesting that the crack has played an important role in transporting gasses and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.

Significance of non-double couple components of deep and intermediate-depth earthquakes: implications from moment tensor inversions of long-period seismic waves

Abstract Analysis of long-period seismic waves suggests that non-double couple components in source moment tensors of intermediate-depth and deep earthquakes are significant and appear to respond to the state of predominant strain release within slabs. The strain regime is partially or fully induced by the sources themselves or by slab structures near the sources. We observe consistency in the non-double couple components from three different seismic wave inversions. Among 21 earthquakes studied, 17 events have the same sign in the three non-double couple components, and the signs show a correlation with the state of strain release within the slabs. The effects of unmodeled propagation structure and instability of the inversion procedure are unlikely to be responsible for the consistent non-double couple components because the suites of seismic waves traverse different paths and the various inversion schemes have the different resolvabilities of the moment tensors.

Broadband converted phases from midmantle discontinuities

A technique for detecting intermediate-period (6–12 s) Sd P phases converted from S to P at a depth d in the source region is described. Previously, these phases were detected in short-period array recordings of deep events. The main idea of our technique is to deconvolve the vertical component of a single record by the S waveform, and to stack the deconvolved components of a number of records, with appropriate time-shift corrections accounting for the difference of epicentral distance. Using this technique, the phases converted from discontinuities at around 660 km, 860 km, 1070 km, and ‘1170’ km depths beneath Sunda arc are detected at seismograph stations in central and eastern Asia. Our data on ‘1070 km’ discontinuity are very consistent with those inferred from short-period recordings of the same events at the J-array in Japan (Niu and Kawakatsu, 1997), but favour a few different discontinuities in the midmantle, rather than one with a strongly variable depth. When compared with a tomographic model of the mantle for the same region, our data suggest that ‘1070 km’ discontinuity may act as a barrier for the downgoing lithospheric slabs.