Research Projects

Below are some research topics that are being or have been carried out by the Harvard seismology group
  • Inversion for Rupture Properties Based Upon Three-Dimensional Directivity Effect
  • Upper-Mantle Structure Based on Caustic Information from Array Triplication Data
  • Back-Projection Analysis of Earthquakes
  • Properties of the Earth's Core
  • Earthquake Detection and Hidden Earthquakes
  • Investigation of Earthquake Source Properties with Normal Mode Data
  • Shear Wave Anisotropy at the Lower Mantle
  • Fully Automated Splitting Analysis
  • Mantle Reflectivity Structure around Convergent Plate Boundaries
  • Miscellaneous Topics

    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.


    Upper-Mantle Structure Based on Caustic Information from Array Triplication Data

    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.


    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.


    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.


    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.


    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.


    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.


    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.


    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.


    Miscellaneous Topics

    This page provides summary of following topics:

  • Global Models of Surface Wave Group Velocity
  • Phase Velocity Maps
  • Normal-Mode Observations
  • more

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