• Back-Projection Analysis of Earthquakes

  research/images/slideshow/BP.jpg

  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.

  

• Earthquake Detection and Hidden Earthquakes

  research/images/slideshow/hidden1.png

  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, sofisticated networks. We investigate this phenomenon in the case of Japan region.

  

• Investigation of Earthquake Source Properties with Normal-Mode data

  research/images/slideshow/sample2.png

  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.

  

• Properties of the Earth's Core

  research/images/slideshow/ic.jpg

  The 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.

  

• Shear Wave Anisotropy at the Lower Mantle

  research/images/slideshow/ray_paths_CMB_aniso_SLIDE.png

  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.

  

• Fully Automated Splitting Analysis

  research/images/slideshow/wav_4web_SLIDE.jpg

  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.

  

• Mantle Reflectivity Structure around Convergent Plate Boundaries

  research/images/slideshow/reflectivity_SLIDE.png

  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.

  

• Inversion for Rupture Properties Based Upon Three-Dimensional Directivity Effect

  research/images/slideshow/INV1.png

  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.

  

• Miscellaneous Topics

  research/images/slideshow/misc_ALLsl.png

  This page provides summary of following topics:
  Global Models of Surface Wave Group Velocity
  Phase Velocity Maps
  Normal-Mode Observations

  

Research Projects

Miscellaneous Topics

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

Properties of the Earth's Core
		The 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. 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. 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

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

Miscellaneous Topics
	This page provides summary of following topics: Global Models of Surface Wave Group Velocity Phase Velocity Maps Normal-Mode Observations more