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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Oral Presentation 2261
Local Velocity Ratio Calculation for California A Simple Approach for
Estimating Regional Vp/Vs
Mr. Gavin Hayes
Geosciences Penn State University, USA IASPEI
Kevin Furlong
For several decades, seismologists have used a variety of different methods to model the P-wave
velocity structure of the Earth, and in particular, the Earths crust. Modeling of the S-wave velocity field
has been (and remains) more difficult, and as a result S-wave velocity structure is often directly related
to P-wave models through empirical measurements of the velocity ratio in rocks. These velocity models
are subsequently used in many different types of analyses, such as the calculation of strong ground
motions in regional hazard maps, earthquake locations, and the inference of subsurface geology and
other crustal properties. In all cases, better estimates of S-wave velocity models lead to more accurate
models of the parameters they are used to describe, and can thus lead us to a more complete
understanding of crustal structure. Here, we develop a straightforward technique for computing
apparent velocity ratio of both the bulk seismogenic crust and shallow crust throughout California based
on P- and S-wave travel-times from earthquakes to the dense network of broadband and short-period
stations across the state. As the seismogenic zone is constrained to the shallow, brittlely deforming part
of the crust, we can use these earthquake travel-times to estimate the apparent velocity ratio of that
crust. Using this approach, we produce regional maps of Vp/Vs for all California, where station coverage
is dense enough and rates of seismicity high enough to allow this type of analysis. We can also isolate
the very shallow (generally aseismic) crustal section to construct a velocity ratio map of the near
surface.This method, which we call the Local Velocity Ratio Calculation (LVRC), yields a simple yet
powerful way to analyze the velocity ratio of the upper crust on a regional basis. Results may be related
to geologic structure, can be used in conjunction with bulk crustal studies (such as receiver function
analyses) to resolve upper and lower crustal properties and address issues related to crustal structure,
or used to infer parameters such as ground shaking susceptibility in California and other similar areas
worldwide, providing useful information to aid in the science of future hazard mitigation.