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Group 2 – Tectonic Geodesy | Poster Abstracts
2-022
THE SLIP HISTORY OF SLOW SLIP EVENTS ALONG THE CASCADIA
SUBDUCTION ZONE FROM 1998 TO 2008 Gao H, and Schmidt DA
We estimate the slip distribution and propagation path of prominent slow slip events (SSEs) along
the Cascadia subduction zone from 1998 to 2008. We process continuous GPS data from the PBO,
PANGA and WCDA networks from January 1998 to June 2008 using GAMIT/GLOBK processing
package. We discretize the Juan de Fuca-North American plate interface into triangular subfaults
and interpret transient surface motions as slip on the interface. The slip distribution of 16 SSEs is
estimated using the Extended Network Inversion Filter (ENIF). Of these events, 13 events are
centered around the Puget Sound area, 2 events are resolved around the Columbia River and 1
event is located in southern Oregon. Other smaller events beneath Northern Vancouver Island,
Oregon and Northern California are not well resolved because of the limited station coverage. For
the major events analyzed in this work, the horizontal displacements observed at benchmarks
range from 2-6 mm and the maximum cumulative down-dip slip is estimated to be 2-6 cm for each
event. The propagation direction of SSEs is variable from one event to the next, indicating that the
propagation direction of SSEs is not systematic. Therefore, slab geometry or fault properties likely
do not dictate the propagation direction. The estimated moment magnitude of SSEs is between 6.1-
6.7 and the average stress drop of 0.06-0.1 MPa is nearly two orders of magnitude smaller than that
for the normal earthquakes (1-10 MPa). The stacked maximum cumulative slip for these 16 events
from 1998 to 2008 is ~ 25 cm, which is centered beneath the northern edge of the sedimentary
Olympic embayment between the 25-45 km depth contours on the slab interface. This is where the
subducting slab has a more gradual dip compared to the north and south, and the interface bends
along-strike. We hypothesize that the geometry of the slab plays an important role for focusing
transient strain release at this location along the subduction zone.
2-023
CHARACTERIZING NOISE LEVELS ON THE PLATE BOUNDARY OBSERVATORY
BOREHOLE SEISMOMETERS AND STRAINMETERS Barbour AJ, and Agnew DC
We are analyzing the spectral and temporal characteristics of data from the seismic band (0.001 to
10 Hz) recorded on the borehole strainmeters and seismometers installed by the Plate Boundary
Observatory (PBO). Our aim is to characterize these data as well as possible in order to determine
what can be detected using these sensors, and also to develop guidelines which can be used to
search for unusual behavior, such as episodic tremor. We use sine multitaper spectral estimation
(with prewhitening) to determine the power spectral density, since this provides estimates with
low variance and bias, without any loss of data. We summarize the spectrum by performing local
regressions in log-log space; this reduces the data volume by factors of 1000 or more, with little loss
of information. A preliminary analysis shows that some stations have had persistent operational
problems (e.g. B004 during early 2006), though others show consistently good behavior (e.g. B012
during early 2006). The noise levels for seismometers within tens of kilometers show similar
temporal changes but (apparently) different absolute levels; comparing records of teleseisms shows
that there are significant differences in relative gain (compared to the official values provided by
PBO). The seismic noise also shows a clear diurnal signal which may be correlated with coastal
proximity; at frequencies lower than the microseism band the spectrum appears to be dominated
by instrumental noise. The borehole strainmeters show noise levels that are also dominated by
instrumental noise (probably digitizer noise) at frequencies above the microseism band. At lower
frequencies (0.001 to 0.1 Hz) the noise level increases with decreasing frequency at levels higher
than is seen on long-base instruments at the surface. Future plans include: (1) analyzing as many
stations as possible, including stations in the Anza network, for the purposes of network
characterization; (2) removal of times with instrumental artifacts (and large earthquakes) to
2008 SCEC Annual Meeting | 149