Research Projects

Below are some research topics that are being or have been carried out by the Harvard seismology group
  • Near-Surface Seismic Wave Speed Based on Polarization Analysis
  • Instrument Calibration Based on Body-Wave Polarization Analysis
  • Relative Timing Between Analog Stations
  • DigitSeis: Software for Digitization of Old Analog Seismograms
  • Seismic Tomography using Sparse Direct Methods and High-Performance Computing
  • Inversion for Rupture Properties Based Upon Three-Dimensional Directivity Effect
  • Back-Projection Analysis of Earthquakes
  • Properties of the Earth's Core
  • Earthquake Detection and Hidden Earthquakes
  • Seismic Structure of Upper-Mantle Discontinuities Based on High-Frequency Triplication Data
  • Investigation of Earthquake Source Properties with Normal Mode Data
  • Miscellaneous Topics




    Near-Surface Seismic Wave Speed Based on Polarization Analysis

    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.

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    Instrument Calibration Based on Body-Wave Polarization Analysis

    Monitoring instrument performance and assessing the instrument response are vital for various seismological analyses that utilize the seismic signal recorded by the instrument. This project examines instrument gain of three-component seismometers using body-wave polarization measurements.

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    Relative Timing Between Stations with Analog Seismograms

    Absolute time prior to GPS was difficult to determine. With applications such as relative travel-time measurements and ambient noise tomography, a method for determining relative timing between two stations is developed.

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    DigitSeis: Software for Digitization of Old Analog Seismograms

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

    The use of DigitSeis by non-seismologists is piloted with high-school students in Japan.

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    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

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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.

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    Wavelet Decomposition for Seismic Phase Picking

    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.

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    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.

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    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.

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    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

    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.

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    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.

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    Miscellaneous Topics

    This page provides summary of following topics:

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



  • Department of Earth and Planetary Sciences / Harvard University / 20 Oxford Street / Cambridge / MA 02138 / U.S.A. / Telephone: +1 617 495 2350 / Fax: +1 617 496 1907 / Email: reilly@eps.harvard.edu