Abstract
Successive locations of individual large earthquakes (Mw > 5.5) over years to centuries can be difficult to explain with simple Coulomb stress transfer (CST) because it is common for seismicity to circumvent nearest-neighbour along-strike faults where coseismic CST is greatest. We demonstrate that Coulomb pre-stress (the cumulative CST from multiple earthquakes and interseismic loading on non-planar faults) may explain this, evidenced by study of a 667-year historical record of earthquakes in central Italy. Heterogeneity in Coulomb pre-stresses across the fault system is >±50 bars, whereas coseismic CST is <±2 bars, so the latter will rarely overwhelm the former, explaining why historical earthquakes rarely rupture nearest neighbor faults. However, earthquakes do tend to occur where the cumulative coseismic and interseismic CST is positive, although there are notable examples where earthquake propagate across negatively stressed portions of faults. Hence Coulomb pre-stress calculated for non-planar faults is an ignored yet vital factor for earthquake triggering.
Highlights
Successive locations of individual large earthquakes (Mw > 5.5) over years to centuries can be difficult to explain with simple Coulomb stress transfer (CST) because it is common for seismicity to circumvent nearest-neighbour along-strike faults where coseismic CST is greatest
If coseismic CST cannot or rarely overwhelms Coulomb pre-stress, the pre-stress must be taken into account when coseismic CST is calculated following large earthquakes and used to speculate on the location of future damaging earthquakes and associated seismic hazard[8]
If fault bends are not modeled, the coseismic CST calculated would not resolve important regions with raised or lowered stress at bends that may represent sites where subsequent ruptures associated with large earthquakes may nucleate or terminate
Summary
Successive locations of individual large earthquakes (Mw > 5.5) over years to centuries can be difficult to explain with simple Coulomb stress transfer (CST) because it is common for seismicity to circumvent nearest-neighbour along-strike faults where coseismic CST is greatest. The zero or uniform value assumption is likely to be erroneous and misleading because we know that, firstly, interseismic stresses will have accumulated over centuries to millennia due to tectonic loading[10], secondly, multiple earthquakes over many centuries will have contributed coseismic CST2, and thirdly, local bends in the fault geometry will have amplified or diminished the cumulative interseismic and coseismic CST11–15 These three factors suggest it is unlikely that Coulomb pre-stress is zero or spatially uniform as is commonly assumed when calculations of coseismic CST following large earthquakes are undertaken[5,15,16]. If coseismic CST cannot or rarely overwhelms Coulomb pre-stress, the pre-stress must be taken into account when coseismic CST is calculated following large earthquakes and used to speculate on the location of future damaging earthquakes and associated seismic hazard[8]
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