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Poster Abstracts
COMPARISON OF CO-LOCATED PORE PRESSURE AND STRAIN OBSERVATIONS FROM EARTHQUAKES
RECORDED AT PBO BOREHOLE STRAINMETER SITES. (A-071)
F. Civilini
Water level fluctuations in response to earthquakes and tidal strains have been observed in wells for many decades. The
relationship between water level and dilatation can be used to characterize the well as a type of strainmeter. The correlation
between the water level of an aquifer and the induced strain due to the earth tidal force is a well documented relationship, and
can be used to calibrate various constitutive equations of a poroelastic medium to obtain Skempton’s coefficient, a variable
describing the effect of induced strain on pore pressure. The pore pressure response of the October 1999 Hector Mine
earthquake (Mw 7.1) at the Garner Valley Downhole Array (GVDA) is compared to pore pressure and strain observations
from two stations of the EarthScope Plate Boundary Observatory (PBO) for three magnitude 4.9 and above earthquakes in
2010: an El Mayor-Cucapah aftershock Mw 5.7 and two events of Mw 4.9 and Mw 5.4. These earthquakes were recorded at
distances of ~97 Km, ~20 Km, and ~14 Km, respectively. Sudden pore pressure steps, both positive and negative, ranging from
500 Pa to 10 KPa are observed in all but one of the records, with the later being a more gradual decrease. The strain records for
the same earthquakes reveal sharp and sustained changes of strain matching the pore pressure behavior. This suggests that in
at least some of the cases, the observed pore pressure responses correspond to tectonic deformation caused by the strain field
associated with the earthquakes. The similarity of the pore pressure step during the 1999 Hector Mine event to those of the
2010 earthquakes suggests that a this response was also related to the co-seismic tectonic strain field.
COMPARISON OF THE LOW-COST MEMS ACCELEROMETERS USED BY THE QUAKE-CATCHER
NETWORK AND TRADITIONAL STRONG MOTION SEISMIC SENSORS (B-025)
E.S. Cochran, J.F. Lawrence, A. Kaiser, B. Fry, and A. Chung
Accelerometers based on low-cost micro-electro-mechanical systems (MEMS) have improved swiftly, making the rapid
deployment of dense seismic arrays possible. For example, the Quake-Catcher Network (QCN) makes use of MEMS-based triaxial
sensors installed in homes and businesses to record earthquakes, with almost 2000 participants worldwide. QCN utilizes
an open-source distributed-computing system, called the Berkeley Open Infrastructure for Network Computing (BOINC), to
retrieve waveforms from continuous or triggered recordings back to the QCN server. Furthermore, the QCN approach can
also be used to augment existing seismic networks for rapid-earthquake detection purposes, as well as studies on seismic
source- and site-related phenomena.
Following the 3 September 2010 Mw7.1 Darfield earthquake, 192 QCN stations were installed in a dense array to record the
on-going aftershock sequence in and around the city of Christchurch. We examine the peak ground motions recorded during a
M5.1 aftershock and find that peak ground acceleration (PGA) is spatially variable, but with a clear decay in amplitude with
distance. In general, closely located GeoNet and QCN stations report similar PGA. Several QCN stations were located within 1
km of existing GeoNet stations, providing an opportunity to compare time series and amplitude spectra. For these closely
spaced pairs of stations, the amplitude spectra observed from the horizontal components are highly correlated with average
cross-correlation coefficients of 0.9 or higher. In addition, we find the correlation coefficient decreases with increasing distance
between station pairs.
COMPARISON OF SIMULATED SLOW SLIP EVENTS WITH OBSERVATIONS (A-101)
H.V. Colella, J.H. Dieterich, and K.B. Richards-Dinger
We model slow slip events (SSEs) after those observed along the Cascadia subduction zone using the 3D simulation code,
RSQSim, which employs rate- and state-dependent constitutive properties appropriate to the mode of fault slip. For
computational efficiency we impose the slip speed for SSEs, otherwise the simulations are fully deterministic in the nucleation,
propagation speed, extent of slip, and final distribution of slip. To develop robust statistical characterizations of slow slip
events, we generate long histories consisting of more than 105 events (spanning ~300-500 yrs). The simulated catalogs of SSEs
span a large range of moments, most of which would be too small to detected geodetically. The largest simulated SSEs
(Mw6.3-7.0) are in broad agreement with SSEs observed in Cascadia with a quasi-periodic recurrence interval ~10 months,
durations of 10-40 days, and a mean slip of ~3 cm. Results also show spontaneous, but transient, segmentation of SSEs. The
simulations also produce slip propagation characteristics similar to complex tremor patterns observed in Cascadia (i.e. rapid
tremor reversals and along-dip streaking). Stressing rates at the base of the seismogenic zone, directly adjacent to the region in
which SSEs occur, may be as much as 100x more than the stressing rate between events. This suggests a potential for
triggering of mega-thrust earthquakes by SSEs.
2011 SCEC Annual Meeting | 153