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Slip rate variability: Quantifying the past to understand the present

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In an ideal world, earthquakes would occur at regular intervals such that information on the rate and typical size of events could be used to make accurate forecasts of the future likelihood of earthquakes. However, examples from different fault systems suggest that the rate of earthquake occurrence varies more than would be expected using a classic model of stress accumulation. To explore this problem, I will show examples of fault behaviour from a range of scales in time and space. Central Italy is an excellent location to study active tectonics, where mid-magnitude earthquakes occur as a result of continental extension. This activity has produced numerous limestone bedrock fault scarps, preserved since the demise of the Last Glacial Maximum (about 10-20 kyr). Each earthquake exposes the fault scarp to cosmic radiation, which produces cosmogenic chlorine-36 in-situ at the Earth’s surface and down to a few meters depth. The concentration of 36Cl measured along the fault scarp can be used to infer past slip histories, which show that faults experience highly variable earthquake activity, with periods of greater intensity lasting probably thousands of years, separated by times of quiescence. I will place this slip rate variability in the context of the most recent sequence to affect the region, in 2016, which exhibited fault complexity at shorter timescales. Finally, in order to further understand the mechanics of faulting, we will look to the rock record of earthquakes recorded in faulting microstructures from faults that have been characterised by cosmogenic analyses. A key part of the earthquake cycle is the transition from very weak during the earthquake, to strong enough to accrue stress that ultimately builds to a subsequent earthquake. These observations give insight into the rheology of faults throughout the earthquake cycle, which is crucial for understanding how the state of stress evolves through time, and may help to explain non-uniform behaviour over multiple earthquake cycles. Suggested references: Cowie, P. A., Roberts, G. P., Bull, J. M., and Visini, F., 2012, Relationships between fault geometry, slip rate variability and earthquake recurrence in extensional settings: Geophysical Journal International, v 189, p 143-160. Cowie, P. A., et al., 2017, Orogen-scale uplift in the central Italian Apennines drives episodic behaviour of earthquake faults: Scientific Reports, v. 7:44858. Walters, R. J., et al., 2018, Dual control of fault intersections on stop-start rupture in the 2016 Central Italy seismic sequence: Earth and Planetary Science Letters, v. 500, p. 1-14.

This talk is part of the Bullard Laboratories Wednesday Seminars series.

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