Mechanical controls on the distribution of earthquake afterslip from fault zone drilling and laboratory testing
Abstract
Earthquake afterslip can significantly damage surficial or shallowly buried infrastructure critical to emergency response and recovery. Few faults, however, are instrumented to record deformation with the spatiotemporal resolution required to distinguish between co- and post-seismic fault slip near Earth's surface (<1 km depth). Thus, predictions of shallow afterslip rely largely on rate-and-state friction theory, with few reliable data constraints. In this study, we use fault zone drilling and laboratory analyses to explore other mechanical factors - including composition, microstructures, and constitutive properties - that may have controlled the distribution of co- and post-seismic deformation at Earth's surface during the 2014 Mw 6.0 South Napa, California, earthquake.
During the South Napa earthquake, the distributions of co- and post-seismic surface deformation approximately corresponded to previously mapped bedrock units and alluvial basin deposits, respectively. We target two study sites, each from the dominantly co- or post-seismic sections to evaluate the mechanical factors contributing to their different slip behaviors. Previous work using mobile laser scans (MLS) and active-source seismic surveys determined that the two study sites share similar surface deformation fields, though the magnitudes and distributions of elastic moduli in the subsurface are distinct, with lithologic layering at the co-seismic site and a compliant fault zone at the post-seismic site. We use hollow-stem auger drilling to collect continuous core samples down to 15 m depth through both the fault zone and adjacent reference material at each site. Preliminary observations indicate that the fault zone at the co-seismic site occurs primarily within brown-grey silty- to sandy-clay overlying sandstone at 11 m depth, whereas the fault zone at the post-seismic site occurs primarily in green-grey silty clay with no shallow bedrock. We plan to split and sample the cores for textural and compositional analyses, along with mechanical testing to constrain variations in constitutive properties. Ultimately, the combined MLS, seismic tomography, and laboratory data will provide direct constraints for site-specific forward models of co- and post-seismic deformation associated with the South Napa earthquake.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T51J0339N
- Keywords:
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- 5112 Microstructure;
- PHYSICAL PROPERTIES OF ROCKSDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS