Seismic Attenuation in the Subduction Zone of Costa Rica

LeRoy Dorman, Allan Sauter, Heather DeShon, Susan Schwartz,
Andrew Newman, Sue Bilek, Marino Protti, Ernst Flüh, Tim Dixon

In 1999/2000, we undertook a study of the seismogenic zone beneath 
Costa Rica using permanent and temporary land stations as well as 
Ocean-Bottom Seismometers. The goal of this study was the 
determination of location and the properties of the the zone of 
nucleation of subduction zone thrust events. The seismic (and GPS) 
observations were made along two transects, the first NW of the Osa 
Peninsula and the second across the Nicoya Peninsula. Deshon and 
others (2000) have located thousands of events, many of them 
aftershocks of the 1999/08/20 Mw=6.9 underthrust earthquake, which 
was located near the Osa transect. We are beginning a study of 
seismic attenuation beneath the Osa transect to understand better the 
mechanical and thermal state of the source region.

Attenuation studies usually use the spectral ratio method, in which
spectra observed at two stations are divided, so that common elements
in the product-of-transfer-functions equation (source spectrum,
instrument corrections) are removed. There is, however, a way of
treating many observations from a single source in a single solution,
rather than pairwise. By taking the logarithm of the transfer function
equation, the product becomes a sum, and can be treated as a row in a
matrix equation containing data at multiple stations at multiple
frequencies (Dorman, JGR, 1968, 1969).  This way, a solution for mean
attenuation is extracted, as well as a source function.  It is
necessary, of course, to correct for the frequency response of the
several types of instruments and OBSs.  The land stations used STS-2
and L-22 sensors while the OBSs used L4C and PMD sensors.  The bulk of
the data processing is being done within the framework of the Antelope
relational database system (see http://www.brtt.com). In addition to
routines to associate arrivals with events and locate hypocenters, the
database offers a convenient framework for managing the housekeeping
tasks associated with matching individual components with their
respective frequency responses for conversion to ground motion.

We are obtaining values around 250 for compressional wave Q for arrivals
from a shallow event.