Ground shaking depends strongly upon seismic wave speeds at the shallowest depths. This project examines polarization of seismic waves to constrain near-surface wave speeds.

We revisit the application of direct methods in solving tomographic inverse problems in view of significant developments in computer science and hardware in the last decade. These advances include efficient rank-revealing algorithms that take advantage of new architectures such as memory hierarchy and parallelism now exist, and new fill-reducing ordering algorithms that effectively propagate sparsity throughout common factorizations

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The directivity effect can provide important insights into characteristics of the earthquake mechanism by estimating the rupture properties. We consider the directivity effect in three-dimension, i.e., parameterizing in dip and azimuth. Our analysis shows that examining not only the azimuthal variation but also the dip dependency is crucial for robust estimatation of model parameters. Based upon the framework, we introduce an inversion scheme to obtain rupture properties; the duration, speed, dip and azimuth of the rupture propagation.

DigitSeis is a new open-source, interactive digitization software written in MATLAB that converts digital, raster images of analog seismograms to readily usable, discretized time series using image processing algorithms.

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Seismic phase arrivals are considered in both the time and frequency domains by decomposition using wavelets. This algorithm is designed to pick both P- and S-wave arrivals based upon their characteristics.

Back-projection technique takes advantage of the dense array of seismometers that are available around the world such as the Transportable Array in the United States and Hi-net array in Japan. The wavefront observed by the array is collapsed back in space and time (back-projected) to the target region to determine the timing and location of the energy source that generated the seismic waves. If an earthquake has large enough spatial and temporal extent, the rupture propagation can be imaged with great detail using this technique.

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Properties of the Earth's Core

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Accurate and complete cataloguing of aftershocks is essential for a variety of purposes, including the estimation of the mainshock rupture area, the identification of seismic gaps, and seismic hazard assessment. However, immediately following large earthquakes, the seismograms recorded by local networks are noisy, with energy arriving from hundreds of aftershocks, in addition to different seismic phases interfering with one another. We found that under certain conditions even large events can remain undetectable even from dense, sophisticated networks. We investigate this phenomenon in the case of Japan region.

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Constraining seismic properties of the 410- and 660-km discontinuities which delineate the mantle transition zone, is crucial in understanding the mantle composition and convection dynamics. One approach to study the transition zone is to use "triplicated" arrivals of seismic data. One of the challenging components in using triplication data, however, is to identify the three individual phases, since they arrive close in time and overlap with each other. Therefore, we analyze Radon transform of the data, which unwraps the bowtie shape in the original data and separates the three phases. Based on the transformed data, a new methodology to obtain the seismic structure around a discontinuity is introduced that utilizes the fact that spherical ray parameters at the two caustics of the triplication are sensitive to the depth and the velocity jump at the discontinuity.

Large earthquakes make Earth oscillate like a ringing bell for weeks, even months after the event. These oscillations are called normal modes or free-oscillations and they provide important insight about the properties of the causing earthquake.

Preliminary studies of seismic anisotropy using SKS splitting measurements made an assumption that the lower mantle is weakly anisotropic, and attributed the observed splitting to the upper mantle or crust. In the last decade, however, there has been growing evidence from S, Sdiff, ScS, SKS and SKKS waves indicating significant anisotropy at the base of the mantle.

An automated algorithm based on cluster analysis to obtain fast polarization azimuth for split shear waves and the delay time between the fast and slow polarized phases. This technique can be applied to any core-refracted shear wave, such as, SKS, PKS, SKKS, arrival.

The depth at which mantle constituents go through a phase change provides a very important constraint about the Pressure/Temperature structure of the mantle. Amongst many different metrics, amplitudes of surface reflected shear waves and their precursory arrivals offer the most complete global coverage due to their insensitivity to the first order source-station distribution.

This page provides summary of following topics:

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