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Inversion for Rupture Properties Based Upon Three-Dimensional Directivity Effect
research/images/slideshow/directivity.pngThe 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 estimation 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.
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Back-Projection Analysis of Earthquakes
research/images/slideshow/BP.jpgBack-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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Earthquake Detection and Hidden Earthquakes
research/images/slideshow/hidden1.pngAccurate 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, sofisticated networks. We investigate this phenomenon in the case of Japan region.
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Investigation of Earthquake Source Properties with Normal-Mode data
research/images/slideshow/sample2.pngLarge 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.
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Properties of the Earth's Core
research/images/slideshow/ic.jpgThe Earth's inner core has this remarkable property: Compressional waves travel faster along its spin axis than in the equatorial plane. Such a directional dependence of wave propagation, together with the anomalous splitting of core-sensitive normal modes, was explained by an anisotropic inner core model first proposed by Harvard Seismology Group in 1986. Since then, we have further investigated its anisotropy extensively using both travel-time anomalies and the normal modes splitting.
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Upper-Mantle Structure Based on Array Triplication Data
research/images/slideshow/triplication.pngAnalysis of the triplicated wave arrivals using Radon transform to separate different phases to study the transition-zone structure.
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Shear Wave Anisotropy at the Lower Mantle
research/images/slideshow/ray_paths_CMB_aniso_SLIDE.pngPreliminary 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.
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Fully Automated Splitting Analysis
research/images/slideshow/wav_4web_SLIDE.jpgAn 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.
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Mantle Reflectivity Structure around Convergent Plate Boundaries
research/images/slideshow/reflectivity_SLIDE.pngThe 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.
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Miscellaneous Topics
research/images/slideshow/misc_ALLsl.pngThis page provides summary of following topics:
Global Models of Surface Wave Group Velocity
Phase Velocity Maps
Normal-Mode Observations
Research Projects
Below are some research topics that are being or have been carried out by the Harvard seismology groupNear-Surface Seismic Wave Speed Based on Polarization Analysis | ||
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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. more |
Seismic Tomography using Sparse Direct Methods and High-Performance Computing | ||
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 more |
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Inversion for Rupture Properties Based Upon Three-Dimensional Directivity Effect | ||
![]() |
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. more |
DigitSeis: Software for Digitization of Old Analog Seismograms | ||
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. more |
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Wavelet Decomposition for Seismic Phase Picking | ||
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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. more |
Back-Projection Analysis of Earthquakes | |
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. more |
![]() |
Earthquake Detection and Hidden Earthquakes | |
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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Seismic Structure of Upper-Mantle Discontinuities Based on High-Frequency Triplication Data | ||
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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, the new methodology allows velocity jump, depth, and width of the discontinuities to be obtained. more |
Investigation of Earthquake Source Properties with Normal Mode Data | ||
![]() |
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. more |
Shear Wave Anisotropy at the Lower Mantle | |
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. more |
![]() |
Fully Automated Splitting Analysis | ||
![]() |
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. more |
Mantle Reflectivity Structure around Convergent Plate Boundaries | |
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. more |
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Miscellaneous Topics | |
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This page provides summary of following topics: more |