Harvard Seismology: Event Location
This project is currently funded by the Defense Threat Reduction Agency
(DTRA) with the goal of achieving high precision regional and teleseismic
event locations using 3D models of mantle structure. Primary motivation
for the research is successful monitoring of the proposed Comprehensive
Test Ban Treaty (CTBT). The CTBT has the stated goal of locating
events to a precision greater than
1000 km2. This goal is frequently not obtained using
conventional 1-D seismic velocity models and location techniques. Obtaining
better quality teleseismic event locations is also important for studies
which utilize the global distribution of seismicity, particularly in terms
Previously, we have demonstrated that absolute locations determined
using teleseismic arrival times can be improved through incorporation of three dimensional
(3-D) models of the P-wave velocity structure of the mantle, even when
these models are parameterized by a low-order
expansion of the velocity heterogeneity in terms of spherical harmonics.
In addition, a global study of Pn arrivals reported
ISC indicates that azimuthal anisotropy is an important feature in
upper mantle P-velocities. This has important implications for
the accuracy at which events can be located at regional distances.
Currently our research is focused in five main areas:
1. Development of new techniques for improving current Earth models,
such as multiple resolution algorithms and
incorporation of regional seismic phases.
2. Construction of higher resolution three-dimensional Earth models,
with emphasis on P-wave structure.
3. Evaulation of the new models using independent data.
4. Development of additional station and/or path corrections, in
conjunction with the 3D models, for use in location
- 5. Compilation of a high quality dataset of well located master events,
including both explosions and earthquakes,
for use in assessing improvements in location capability.
Current participants in this project are
Michael Antolik, Göran Ekström,
Yu J. Gu, Lapo Boschi, Jianfeng Pan,
Dr. Ivan Kitov, Institute for the Dynamics of the Geosphere, Moscow
Tests of location accuracy obtained using current models of P-velocity
These experiments are designed to test the location accuracy obtained using current mantle P-wave velocity models
of varying resolution. The data used are teleseismic phases from reference or "ground truth" events
(earthquakes and explosions) whose locations are known exactly or extremely well from local and regionazl
A new global P-velocity model parameterized in cubic splines
This model is based on a new dataset of summary P wave travel times and surface wave phase measurements.
The model is designed to recover long wavelength structure well, especially in the upper mantle to aid in
teleseismic event location.
- Improving locations of oceanic earthquakes
We are using a constrained inversion technique to relocate earthquakes on remote plate boundaries.
This should help compile reference events for testing 3-D models in areas with sparse station coverage.
G.P. Smith and G. Ekström, Improving teleseismic event locations using a three-dimensional Earth model
Bull. Seism. Soc. Am., 86, 788-796,
G.P. Smith and G. Ekström. Interpretation of earthquake epicenter and CMT centroid locations, in terms
of rupture length and direction, Phys.
Earth and Plan. Inter., 102, 123-132, 1997
Smith and G. Ekström, A global study of Pn anisotropy beneath continents, Journ. Geophys. Res., , in press, 1999)
Title Illustration. P-wave travel time residuals in 5o
x 5o caps predicted by the mantle model S&P12/WM13 for an
event in Kermadec. Red and blue triangles denote positive and negative
Department of Earth and Planetary Sciences,
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