Satoshi Kaneshima

Laboratory and seismological observations of lower mantle isotropy

We have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle. Based on observations of shear wave splitting, we conclude that the lower mantle is effectively isotropic. This result is surprising since the lower mantle is composed of elastically anisotropic minerals (silicate perovskite, MgO, SiO 2 ) that have been extensively strained by geologic processes. To reconcile the seismic observations, we have measured texture development during deformation and high temperature recrystallization of mantle silicates. We show that these experiments provide a direct explanation for the shear wave splitting measurements: the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

Detection of lower mantle scatterers northeast of the Marianna subduction zone using short‐period array data

Short period University of Washington, northern California, and southern California seismic array records of a sequence of deep Mariana subduction zone earthquakes show unexpected clear arrivals nearly 90 and 105 s following P. We measure slowness, arrival time, and arrival azimuth of these phases relative to P using array techniques to find where they emanate in the mantle. By combining slowness and back azimuth deviations and the move-out with depth of the arrivals from different earthquakes, we find their most probable origin is through S-to-P conversion at point scatterers in the mid- to lower mantle at 25.7°N 148.2°E 1590 km and 25.5°N 151.0°E 1850 km respectively, separated by ∼300 km. These features indicate elastically distinct material in the middle of the lower mantle, which, if fragments of subducted slabs, are not related to contemporary subduction. They may represent upwelling convective material from the deep mantle or D″, but they are not purely thermal in nature. As chemically distinct bodies, they may represent potential reservoirs of the isotopic and trace element components recognized geochemically.

Outer-core compositional stratification from observed core wave speed profiles

Light elements must be present in the nearly pure iron core of the Earth to match the remotely observed properties of the outer and inner cores. Crystallization of the inner core excludes light elements from the solid, concentrating them in liquid near the inner-core boundary that potentially rises and collects at the top of the core, and this may have a seismically observable signal. Here we present array-based observations of seismic waves sensitive to this part of the core whose wave speeds require there to be radial compositional variation in the topmost 300 km of the outer core. The velocity profile significantly departs from that of compression of a homogeneous liquid. Total light-element enrichment is up to five weight per cent at the top of the core if modelled in the Fe–O–S system. The stratification suggests the existence of a subadiabatic temperature gradient at the top of the outer core.

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.

Anisotropic loci in the mantle beneath central Peru

Abstract Seismic anisotropy of the upper mantle beneath the central part of Peru is examined by analyzing shear waves observed at broad-band stations and a temporary array of short-period stations. Shear-wave splitting is seen on various shear phases, such as direct S waves from local intermediate to deep earthquakes, ScS waves from regional deep earthquakes, and SKS waves from teleseismic earthquakes. It is inferred that the shear-wave anisotropy in the uppermost 100 km of the subcontinental mantle overlying the subducted Nazca plate is 0.5% at most, while the anisotropy in the subslab asthenosphere (depth range of about 150–350 km) is 2% or greater. The fast shear-wave polarization direction in this depth range, as observed at two broad-band stations, is 30°–40° different from the absolute motion of the Nazca plate. This does not fit simple two-dimensional (2-D) models of olivine alignment caused by slab-induced mantle flow, and implies either the existence of a complex flow pattern in the asthenosphere underneath the Nazca plate or the presence of an unknown mechanism for the anisotropy formation. The lower mantle beneath central Peru is found to be effectively isotropic for nearly vertically propagated shear waves.

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.

Small-scale heterogeneity at the top of the lower mantle around the Mariana slab

Abstract Short-period waveform data recorded at the western US seismic array for 14 Mariana deep earthquakes near 19°N show two anomalous wave packets within about 30 s after the arrival of the direct P-wave (simply called ‘P’ or ‘P-wave’): one at about 13 s (called ‘X1 phase’) and the other around 29 s (called ‘X2 phase’). We perform array analyses to locate the sources of these phases. In the first step, we measure arrival time, slowness, back-azimuth, and amplitude of these phases relative to P. The amplitudes of the X1 and X2 phases correlate with each other for the individual events, and vary more than an order of magnitude among the events which cluster in a region spanning less than 50 km. The amplitude correlation suggests that the two waves have a similar origin, and the large amplitude variation eliminates receiver side reverberation as the origin of the phases. The X1 phase has a slowness and an arrival azimuth which are not distinguished from those of P-waves across the entire array, and has nearly constant delay times regardless of the focal depths (from 567 to 605 km). The X2 phase arrives as a more emergent wave packet with slowness and azimuth different from those of P. In the second step, we compute composite semblance coefficients for the cases of P-to-P and S-to-P single scattering near the foci. The X1 and X2 phases are best interpreted as S-to-P scattered waves generated in the uppermost lower mantle north of the focal region based on three observations: (1) highest composite semblance values, (2) scatterer locations mutually consistent between the two event groups near the depths of 500 and 600 km, and (3) a reasonable amount of elastic property anomalies required. The X1 phase scatterer is determined at 19.8°±0.3°N, 145.7°±0.3°E, 710±30 km. On the other hand, the scatterer of the X2 phase appears to split into two objects: one at 20.4°±0.4°N, 145.4°±1.0°E, 900±80 km, and the other at 20.6°±0.4°N, 147.4°±0.5°E, 860±40 km. Although the geometries of the scatterers of X1 and X2 phases are not constrained, horizontal or subhorizontal discontinuities are unlikely. The changes in elastic properties associated with these heterogeneous objects probably occur within several kilometers, according to their high efficiency at converting short-period waves. They are thus likely to represent sharp chemical variations in major element composition. These objects are located within a thickened high-velocity anomaly at the top of the lower mantle, which has been determined by previous seismic studies. A plausible tectonic interpretation of these objects is that they are fragments of former oceanic crust which are entrained in the Pacific slab impinging on the more viscous lower mantle.

Seismic structure near the inner core-outer core boundary

A model of P-wave velocity of the Earths core 500 km above and below the inner core boundary (ICB) beneath North America is constructed from travel time analysis and broad-band waveform modeling of core phases at distances from 130° to 160°. Differential travel times between PKPBC (Cdiff) and PKPDF (TBC–TDF) are about 0.5 sec shorter than those computed from PREM at distances from 146° to 152° and increase with distance to nearly the same as those from PREM at 155°. Differential travel times between PKPCD and PKPDF (TCD–TDF) at distances from 130° to 143° are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKPBC (Cdiff) relative to PKPDF (ABC/ADF) at distances 150° to 156° are about 40% larger than those from PREM. These observations require a smaller P-wave velocity gradient (0.0005 sec−1) in the lowermost 300 km of the outer core than that from PREM (∼0.00065 sec−1), a slower P-wave velocity (10.96 km/sec) at the top of the inner core, and a larger velocity gradient (0.0005 sec−1) in the uppermost 300 km of the inner core.

Continuous high velocity aseismic zone beneath the Izu‐Bonin Arc

Since 1904, eleven unusually deep earthquakes have been reported near the southern end of the Izu-Bonin arc (about 24°N, 142°E). These isolated events are separated from ordinary deep earthquakes in the Wadati-Benioff (W-B) zone of the Izu-Bonin (I-B) arc by a distance of about 300 km parallel to the trench axis. They also are located 200 km closer to the trench than the natural extension of the shallower earthquakes above them. Using 3-D ray tracing, a detailed analysis is performed of the difference in travel time residuals between the anomalous events and the deep events within the W-B zone to the north. This analysis yields higher reliability and resolution than can be obtained from ordinary tomographic imaging. We find that these unusual events occurred in a high velocity zone which is connected to the northern W-B zone of the I-B arc. The aseismic high velocity zone has a P-velocity at least 3% faster than the surrounding mantle. Both our result and previous evidence for a horizontally lying slab west of the W-B zone at 27°N (e.g., Okino et al., 1989) are consistent with the existence of a prominent contortion of the subducted slab in the I-B region.

Evidence for the splitting of shear waves from waveform and focal mechanism analyses

Abstract A temporary seismograph station was sited above the aftershock area of an intermediate crustal earthquake (M = 4.9) in the Kinki district, Japan. The results from analysing digital three-component seismograms and numerically determining focal mechanisms of aftershocks have provided clear evidence for the splitting of shear waves attributed to crustal anisotropy. Faster shear waves from most of the aftershocks are polarized WNW-ESE, regardless of their azimuths and incident angles. Such shear wave motions are often significantly inconsistent with the focal mechanisms of the aftershocks determined using P-wave first motions. For many of the events, slower shear waves, polarized orthogonally to the faster shear wave direction (NNE-SSW), are observed to arrive about 0.1 s later. Two vectors showing particle motions of faster and slower shear waves in the horizontal plane are summed, to reconstruct polarizations of shear waves before splitting. For events with sufficiently high signal-to-noise ratios of shear waves and well determined focal mechanisms, the reconstructed polarizations are consistent with the focal mechanisms within the range of uncertainty. Such consistency is not clearly detected for the other events, possibly because of large noise effects and poorly constrained fault-plane solutions The direction of faster shear wave polarization (WNW-ESE) generally coincides with that observed at three other stations located within 15 km of the temporary station. In the study area, the fabric of crustal anisotropy is almost uniform over the scale of 15 km, but appears to exhibit small local fluctuations.