Migratory versus self-nucleation
Abstract
Where does earthquake nucleation begin? Simplified models of crack nucleation and frictional instability often embed the assumption that slip starts in the same location that ultimately becomes the hypocenter. However, in a heterogeneous fault zone, this assumption of self-nucleation may not hold. Creep can begin in weak areas of the fault with insufficient strain energy to initiate dynamic rupture, and then migrate into stronger areas of the fault to load and eventually trigger the hypocenter. Circumstantial observational evidence suggests that this second, migratory style of nucleation happened for at least some earthquakes. The 2014 M8.1 Iquique and 2011 M9.1 Tohoku earthquakes both had migrating foreshocks and geodetic signals that were interpreted as creep processes that moved towards the hypocenter. However, both cases of large, offshore rupture had insufficient long-term seafloor instrumentation to be certain of the observation.
A clearer example of the migratory nucleation style comes from the stick-slip cycle of the Whillans Ice Stream in Antarctica. A 100x150 km2 region near the grounding line has daily ~0.5 m slip events. Because the ice sheet is only ~800 m thick in this region, GPS and seismic instruments deployed at the surface can capture the spatial and temporal variability of slip across the rupture plane. We use data from 2014 and 2010 to find that a period of slow precursory slip can initiate either near the grounding line or at a particular location 40 km up-ice. After up to 10's of minute of slow slip at one location, the creep appears to trigger dynamic slip in the other nucleation location, in some cases after a clear expansion of the slowly slipping region. Importantly, creep and rapid slip can occur in both areas, and it is therefore not a material property difference that distinguishes creep or rapid slip. On Whillans, the oceanic tides oscillate the floating ice vertically creating an intrinsically heterogeneous stress and strain energy field conducive to migratory nucleation initiating in the low stress, creep-prone region. Migratory nucleation may also be common on faults where roughness and history create heterogeneities in the stress field.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S24B..03B
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7215 Earthquake source observations;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS