Annual Meeting - SCEC.org

Poster Abstracts
understanding the physics of the subsidence process, the role of faults, the geohydrology of the local aquifers, and the
mechanism of creep on the faults, as well as for decision-making for future investments in the Mexicali Valley.
Before the earthquake, faults were slipping vertically few centimeters per year, a rate of about half the subsidence rate
observed in the nearest subsidence bowl. Coseismic slip observed from the instruments and from manual measurements was
of the order of centimeters, suggesting a strong consolidation effect triggered by seismic waves. One year after the earthquake,
vertical slip observed on the Saltillo fault has been slower, while vertical slip on the Cerro Prieto fault ceased.
A precision leveling survey in the Mexicali Valley had been done 2 months before the earthquake and was repeated two
months after the earthquake. As expected for this earthquake, an uplift of about 30 cm towards the NE was observed along a
38 km line in the SW-NE direction. Three subsidence bowls differ from this general pattern. While two bowls lay within the
limits of subsidence area observed before the earthquake, the deepest one, with 36 cm relative depth, is situated to the
southwest outside of the subsidence zone. All the subsidence bowls are probably associated with liquefaction observed in the
area, with more liquefaction observed close to the epicenter.
We checked the subsidence pattern after the earthquake using InSAR images. The small number of available images does not
allow drawing definite conclusions, but suggests that the subsidence process continues at the previous rate.
IDENTIFYING FAULT HETEROGENEITY THROUGH MAPPING SPATIAL ANOMALIES IN ACOUSTIC
EMISSION STATISTICS (B-029)
T.H. Goebel, T. Becker, D. Schorlemmer, S. Stanchits, C. Sammis, E. Rybacki, G. Dresen
Frictional properties of fault surfaces fundamentally influence the local strength of the seismogenic crust. We investigate the
relationship between fault asperities and the creation of micro-cracks and damage by analyzing acoustic emission (AE) events
emitted during sliding of fractured Westerly granite surfaces in the laboratory. We developed a three stage procedure, for
which rock samples are initially fractured under triaxial loading, followed by fault locking due to pressure increase and a final
stage of fault reactivation. We observed three different types of behavior: (1) frictional sliding with decreasing differential
stress (creeping fault) (2) linear and non linear stress increase, slow as well as abrupt stress drops (3) approximately linear
stress increase followed by abrupt stress drop events between 10 to 300 MPa (stick slips with saw tooth pattern). We
performed a detailed spatial analysis of event clusters before and after stick slips, primarily focusing on their b-values, seismic
moment release and AE event densities. AE hypocenter distributions showed a high degree of spatial clustering close to low
b-value regions. Slip events and the connected acoustic emission “aftershocks” nucleated within or at the periphery of areas of
low b. To identify larger scale geometric asperities we combined fault structural information from post-experimental CT-scans
with AE statistics. Asperities were connected seismically to low b-value regions, high moment release and increased AE event
density and structurally to anomalous thin fault parts and point contacts of the host rock walls.
The rough fracture surfaces during laboratory experiments, strongly favor the creation of spatial and temporal distinct AE
clusters which have similar characteristics to seismicity observed on crustal scales. Specific crustal seismicity anomalies may
be an expression of fault heterogeneity and mark areas of increased seismic hazard.
CONNECTING THE SPATIAL DISTRIBUTION OF ACOUSTIC EMISSIONS TO FAULT ROUGHNESS
DURING STICK-SLIP EXPERIMENTS (B-036)
T.H. Goebel, C. Sammis, and T. Becker
Most continental earthquakes occur on or close to narrow fault zones within the seismogenic crust. Variations in size and
spatial distribution of seismic events are likely influenced by differences in roughness and degree of heterogeneity of faults.
The understanding of controlling factors of seismicity distributions around natural faults is limited due to largely unknown
crustal stress, fault zone structure and roughness. Unlike field studies, high pressure laboratory stick-slip experiments enable
the investigation of earthquake analog systems under controlled conditions. We connected spatial distributions of acoustic
emissions (AEs) to post-experimental fault structure and slip surface roughness of previously faulted and saw-cut Westerly
granite samples.
To create stick-slip events we locked the specimen by increasing the confining pressure and resumed axial loading. Our
experimental set-up enables the creation of a series of stick-slip events on a single fault plane. AEs were recorded and located
with high speed and accuracy using 14 piezo-ceramic sensors attached directly to the rock sample. We analyzed the degree of
localization of AEs with increasing stress level leading up to slip events. AE hypocenter distributions were quantified by
computing the fractal dimension of spatial AE distributions and the activity fall-off with fault normal distance. Fault structure
2011 SCEC Annual Meeting | 169