Miaki Ishii

Seismicity rate immediately before and after main shock rupture from high-frequency waveforms in Japan

[1] We analyze seismicity rate immediately before and after 82 main shocks with the magnitudes ranging from 3 to 5 using waveforms recorded by the Hi-net borehole array in Japan. By scrutinizing high-frequency signals, we detect � 5 times as many aftershocks in the first 200 s as in the Japan Meteorological Agency catalogue. After correcting for the changing completeness level immediately after the main shock, the aftershock rate shows a crossover from a slower decay with an Omori’s law exponent p = 0.58 ± 0.08 between 20 and 900 s after the main shock to a faster decay with p = 0.92 ± 0.04 after 900 s. The foreshock seismicity rate follows an inverse Omori’s law with p = 0.73 ± 0.08 from several tens of days up to several hundred seconds before the main shock. The seismicity rate in the 200 s immediately before the main shock appears steady with p = 0.35 ± 0.50. These observations can be explained by the epidemic-type aftershock sequence (ETAS) model, and the rate-and-state model for a heterogeneous stress field on the main shock rupture plane. Alternatively, nonseismic stress changes near the source region, such as episodic aseismic slip, or pore fluid pressure fluctuations, may be invoked to explain the observation of small p values immediately before and after the main shock.

The innermost inner core of the earth: Evidence for a change in anisotropic behavior at the radius of about 300 km

Since the discovery of the inner core in 1936, no additional spherical subshell of the Earth has been observed. Based on an extensive seismic data set, we propose the existence of an innermost inner core, with a radius of ∼300 km, that exhibits a distinct transverse isotropy relative to the bulk inner core. Specifically, within the innermost inner core, the slowest direction of wave propagation is ∼45° from the east-west direction. In contrast, the direction of the slowest wave propagation in the overlying inner core is east-west. The distinct anisotropy at the center of the Earth may represent fossil evidence of a unique early history of inner-core evolution.

Mantle avalanche as a driving force for tectonic reorganization in the southwest Pacific

Abstract The mechanism responsible for the recent, dramatic reorganization of the tectonic plate boundary in the New Hebrides region of the southwest Pacific has remained elusive. We propose that an ongoing avalanche of cold, dense slab material into the lower mantle, imaged by high-resolution seismic tomographic methods, provides the necessary driving force for this enigmatic evolution. Numerical experiments demonstrate that the avalanche model reconciles a broad suite of observational constraints, including the change in polarity of plate subduction, the rapid migration of the New Hebrides arc and opening of the North Fiji Basin, and the present-day geometry of slabs associated with both active and extinct subduction zones.

Mw 8.6 Sumatran earthquake of 11 April 2012: Rare seaward expression of oblique subduction

The magnitude 8.6 and 8.2 earthquakes off northwestern Sumatra on 11 April 2012 generated small tsunami waves that were recorded by stations around the Indian Ocean. Combining differential travel-time modeling of tsunami waves with results from back projection of seismic data reveals a complex source with a significant trench-parallel component. The oblique plate convergence indicates that ∼20–50 m of trench-parallel displacement could have accumulated since the last megathrust earthquake, only part of which has been taken up by the Great Sumatran fault. This suggests that the remaining trench-parallel motion was released during the magnitude 8.6 earthquake on 11 April 2012 within the subducting plate. The magnitude 8.6 earthquake is interpreted to be a result of oblique subduction as well as a reduction in normal stress due to the occurrence of the Sumatra-Andaman earthquake in 2004.

Multidisciplinary impact of the deep mantle phase transition in perovskite structure

A phase transition in (Mg, Fe) SiO3 (magnesium silicate-perovskite) for pressure-temperature conditions near the base of Earths mantle, first reported in May 2004, is stimulating strong multidisciplinary excitement and interactions. Experimentally and theoretically determined characteristics of this phase transition indicate that it may hold the key to understanding enigmatic seismological structures in the D” region of the lowermost mantle, with important implications for heat transport, thermal instabilities, and chemical properties of the lower mantle. All minerals undergo phase transitions with increasing depth into the Earth, reorganizing their crystal structures into denser-packed forms stable over a finite range of pressures and temperatures. The changes in material properties across such transitions often give rise to detectable contrasts in seismic velocities and density.

Continuous Wavelet Decomposition Algorithms for Automatic Detection of Compressional‐ and Shear‐Wave Arrival Times

Abstract The determination of P ‐ and S ‐wave arrival times is important for a variety of seismological applications including earthquake detection and seismic tomography. The method is based on the continuous wavelet transform of the waveforms. Unlike Fourier transform, the basis functions are localized in time and frequency, therefore, wavelet transform is suitable for analysis of nonstationary signals. For detecting the P ‐wave arrival, the wavelet coefficients are calculated using the vertical component of the seismogram. In the case of the S ‐wave arrival, we take advantage of the polarization of the shear waves, and cross examine the wavelet coefficients from two horizontal recordings. In addition, shear‐wave splitting, the time delay of polarized S waves, can be measured using real and imaginary wavelets. Because these steps can be automated, application of the technique can easily generate a large database of shear‐wave splitting measurements for studies of anisotropy.

DigitSeis: A New Digitization Software for Analog Seismograms

This article presents a new MATLAB software, DigitSeis, that converts digital, raster images of analog seismograms to readily usable, digital time series using image‐processing techniques. It classifies important features of analog seismograms, such as time marks, to simplify the generation of continuous time series and uses some of this information to assign accurate timing. Geometrical distortions associated with various stages of seismogram creation, storage, and digitization are detected automatically and corrected. Although the software is written to minimize human input, DigitSeis provides interactive tools for challenging situations such as when traces cross. Finally, the effectiveness of the software is demonstrated with digitization of a long‐period seismogram from the Harvard‐Adam Dziewonski observatory recorded in November 1938. At least five significant teleseismic events are identified, and its spectral analysis shows no spurious features.

Seismological Constraints on the Structure of the Earth's Core

Our knowledge of the Earth’s core has improved dramatically since its discovery, and this chapter attempts to provide a review of the current understanding as constrained by seismological observations. We also illustrate difficulties in studying this region, and uncertainties in the constraints. A short discussion of seismic data used to investigate the core is followed by sections focusing on properties of the outer core, the inner-core boundary, and the inner core. The range of observed values for compressional and shear wave speeds, density, and attenuation are presented.

Earth's core

The dense kernel of Earth comprised primarily of iron and nickel that contains nearly one-third of t…

Detection and classification of sub‐surface features based on their seismo‐acoustic signatures.

Complex subsurface structure makes seismo‐acoustic imaging difficult, raising the need for novel detection, classification, and imaging approaches. For the representation of range‐dependent seismo‐acoustic propagation, we employ a hybrid, coupled wavenumber integration approach to range‐dependent seismo‐acoustic modeling based on the OASES environmental modeling framework. In feature identification and classification, time series analysis frequently provides the sought‐after recognition clues. Nevertheless, the geometry and the structure of the subsurface features generate a variety of complexities to the wave field caused by different physical mechanisms, geometric constraints, and intrinsic properties. Based on the time series attributes such as the time of arrival, waveform amplitude, and the frequency content, a set of identification features is extracted and a set of corresponding classes is formed. Using the simulated data scenarios, general rules incorporating the identification features of subsurf...