High-quality, digital seismograms from eight pairs of
co-located earthquakes in the northeastern United States
were analyzed to determine accurate source spectrum
corner frequencies. This was accomplished by applying
the empirical Green's function method to regional Pg
and Lg (or Sg) phases recorded by vertical
component seismographs of the U. S. National Seismographic
Network (USNSN) and the Lamont-Doherty Cooperative
Seismographic Network (LCSN) stations. The frequency
band used was 0.5 - 16 Hz for USNSN and 1 - 30 Hz for
LCSN records. The source spectrum corner frequencies
for the eight larger earthquakes of the event pairs
(magnitudes between m(Lg) =
2.5 - 4.1) range from about 4.3 to 16.3 Hz.
Based on the corner frequencies obtained independently
from the empirical Green's function analysis,
Sg- or Lg-wave displacement amplitude
spectra up to 30 Hz were used to determine the crustal
average Q factors along 87 event-station paths.
These paths crossed diverse tectonic features in the
northeastern U. S. and were in the epicentral distance
range of 41 to 1394 km. We found that within the
northeastern U. S., the crustal average
Q
we obtained was frequency dependent
and showed spatial variability which correlated fairly
well with the major tectonic features in the region.
Our attenuation measurements indicated low Lg
attenuation in the Adirondack Mountains with exposed
Precambrian Grenville basement with
Q
= 905f
, high Lg
attenuation in the central Appalachian Province with
Q
= 561-586f
, and
an intermediate Lg attenuation in northern New England
Appalachians with Q
=
705f
.
AGU Index Terms: 7245 Structure of the crust; 7230 Seismicity and seismotectonics; 8110 Continental tectonics-general; 9350 North America
Keywords/Free Terms: Attenuation,
JGR-Solid Earth 96JB02580
Vol. 101
, No. B11
, p. 25,231