|Title||Local near instantaneously dynamically triggered aftershocks of large earthquakes|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Fan W.Y, Shearer PM|
|Type of Article||Article|
|Keywords||2004 parkfield earthquake; denali fault earthquake; distance; earthquake; foreshocks; japan; magnitude; rupture process; seismicity; stress transfer; tocopilla|
Aftershocks are often triggered by static- and/or dynamic-stress changes caused by mainshocks. The relative importance of the two triggering mechanisms is controversial at near-to-intermediate distances. We detected and located 48 previously unidentified large early aftershocks triggered by earthquakes with magnitudes between >= 7 and 8 within a few fault lengths (approximately 300 kilometers), during times that high-amplitude surface waves arrive from the mainshock (less than 200 seconds). The observations indicate that near-to-intermediate-field dynamic triggering commonly exists and fundamentally promotes aftershock occurrence. The mainshocks and their nearby early aftershocks are located at major subduction zones and continental boundaries, and mainshocks with all types of faulting-mechanisms (normal, reverse, and strike-slip) can trigger early aftershocks.
Twenty-seven of the 88 target earthquakes clearly triggered early aftershocks and include events at most of Earth’s subduction zones and continental boundaries. None of the 88 target earthquakes have cataloged aftershocks in the time/distance window that we examined for this study. Earthquakes with all types of faulting mechanisms are capable of triggering early aftershocks (16 reverse-faulting, 4 strike-slip, and 7 normal-faulting). Normal-faulting earthquakes have the highest triggering rate (50%), whereas triggering rates of reverse-faulting (28.1%) and strike-slip (23.5%) earthquakes are similar. For robustness, our back-projection approach focuses on the phase of the P wave arrivals, at the cost of losing absolute P wave amplitudes, which makes estimating the magnitudes of the very early aftershocks challenging. Nevertheless, by comparing with historical nearby earthquakes, the triggered early aftershocks are likely to be M 5 to 6.5 earthquakes.