What is the contribution (if any) of off-fault damage to the earthquake energy budget?
Abstract
The structure of seismic faults comprises two main domains: a narrow slip zone, where most of the slip is accommodated, and a much wider surrounding damage zone, which is densely fractured. The slip zone weakens very efficiently during fast, coseismic slip, reducing frictional dissipation, promoting slip acceleration and rupture tip propagation. However, it is still questioned whether the energy dissipation due to off-fault damage production is a significant component in the earthquake energy budget.
Here we investigate the role of off-fault damage on slip mode and rupture propagation behaviour by conducting triaxial rupture tests on biomaterial samples (Westerly granite and Carrara marble), at confining pressure Pc = 5 and 50 MPa, under dry conditions. The experimental setup allows failure to initiate on a saw cut portion of the granite, and successively propagate through an initially intact portion of the sample (either marble or granite). Linear dynamic strain gauges and piezoelectric sensors were positioned along the sample to acquire strain and acoustic data in the low (Hz) and high frequency (MHz) domains, during fault propagation. Our results show that the brittle-ductile transition (5 MPa < Pc < 50 MPa) in the Carrara marble induces the switch from fast stick-slip to slow-slip behaviour in the composite samples. Stick-slip events (brittle field) generate larger slip (up to 45 microns) and stress drop (up to 6 MPa), and shorter rise time (microseconds). Conversely, silent slow slip events show lower rupture velocity (9 - 200 mm s-1), smaller stress drop (1.5 MPa), slip (< 10 microns) and slip velocity (< 35 microns s-1), and longer rise time (up to 5 s). Microstructural observations show that widespread off-fault damage, accommodated by distributed fracturing, developed in the surrounding of the main propagating fault in the ductile regime. Little (Carrara marble) to no (Westerly granite) damage developed in the samples that failed in the brittle regime. During failure in the ductile regime, the estimated critical nucleation length becomes larger than the size of the sample, if calculated using a slip weakening model. We interpret that energy dissipation associated to the generation of off-fault damage suppresses unstable,fast rupture propagation by significantly increasing seismic fracture energy.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S51E0437M
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7215 Earthquake source observations;
- SEISMOLOGY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 7290 Computational seismology;
- SEISMOLOGY