Abstract
Large continental earthquakes necessarily involve failure of multiple faults or segments. But these same critically-stressed systems sometimes fail in drawn-out sequences of smaller earthquakes over days or years instead. These two modes of failure have vastly different implications for seismic hazard and it is not known why fault systems sometimes fail in one mode or the other, or what controls the termination and reinitiation of slip in protracted seismic sequences. A paucity of modern observations of seismic sequences has hampered our understanding to-date, but a series of three Mw>6 earthquakes from August to November 2016 in Central Italy represents a uniquely well-observed example. Here we exploit a wealth of geodetic, seismological and field data to understand the spatio-temporal evolution of the sequence. Our results suggest that intersections between major and subsidiary faults controlled the extent and termination of rupture in each event in the sequence, and that fluid diffusion, channelled along these same fault intersections, may have also determined the timing of rupture reinitiation. This dual control of subsurface structure on the stop-start rupture in seismic sequences may be common; future efforts should focus on investigating its prevalence.
Highlights
In regions of distributed continental faulting, networks of active faults are commonly segmented on length scales of 10–25 km, approximately equal to the seismogenic thickness of the Earth’s crust (Scholz, 1997; Stock and Smith, 2000; Klinger, 2010)
In particular we suggest this indirect structural control plays an important role for the southern-termination of major slip in the Norcia earthquake; the Pian Piccolo fault appears to have not stopped rupture during the Norcia earthquake and the slip maxima in this event instead terminates against the slip maxima of the Amatrice earthquake, which acts as a stress-shadow (Figs. 8b, 7)
Plotted spatially on the fault plane (Fig. 9b), the aftershocks appear to be propagating along the minor antithetic fault that ruptured in the Norcia earthquake, and the eastward dipping structure that we infer acted as a barrier to rupture in the Visso earthquake
Summary
In regions of distributed continental faulting, networks of active faults are commonly segmented on length scales of 10–25 km, approximately equal to the seismogenic thickness of the Earth’s crust (Scholz, 1997; Stock and Smith, 2000; Klinger, 2010). Hubert et al, 1996; King and Cocco, 2001; Wedmore et al, 2017) Both large earthquakes and seismic sequences require that all component faults are near-critically stressed, a condition that is thought likely to occur commonly in nature through stresssynchronisation of faults (Scholz, 2010). Ther dynamic and static stress transfer cause cascading failure of multiple critically-stressed faults or rupture is arrested before all these faults have failed In the latter case the start of rupture in subsequent subevents determines the temporal evolution of the seismic sequence. Our results suggest that structural complexity, namely the intersections between two sets of oblique faults, may have played an important dual role in the Central Italy seismic sequence: first by limiting the extent of individual ruptures and second by channelling fluid flow and controlling the timing of subsequent failure throughout the sequence
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