Decay of aftershock density with distance does not indicate triggering by dynamic stress

• Published in: Nature (London) Vol. 467; no. 7315; pp. 583 - 586
• Main Authors: Richards-Dinger, Keith, Stein, Ross S., Toda, Shinji
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 30.09.2010; Nature Publishing Group
• Subjects: 704/2151/210; 704/2151/508; Decay; Density; Dynamics; Earth sciences; Earth, ocean, space; Earthquakes; Earthquakes, seismology; Exact sciences and technology; Faults; Humanities and Social Sciences; Internal geophysics; Japan; letter; multidisciplinary; Properties; Science; Science (multidisciplinary); Seismic activity; Seismic engineering; Seismic hazard; Seismic phenomena; Seismic waves; Strains and stresses; Stress relaxation (Materials); Stress relieving (Materials); Stresses; Tectonics. Structural geology. Plate tectonics; Temporal logic; United States; United States; Japan; Southern California; United States > US; stress; density; aftershocks; magnitude; mainshocks; earthquakes; seismic risk; seismicity
• DOI: 10.1038/nature09402
• ISSN: 0028-0836; 1476-4687

Static triggering of aftershocks The most predictable earthquakes are aftershocks, which invariably follow mainshocks. So what triggers aftershocks? It was recently argued — controversially — that the decay of aftershocks with distance from the main earthquake could only be explained by dynamic triggering. Keith Richards-Dinger and colleagues have tested this hypothesis and conclude that the observed decay can be better explained by static triggering. Resolving whether static or dynamic stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard. It has recently been argued that the decay of aftershocks with distance from the main earthquake could be explained only by dynamic triggering. This hypothesis has now been tested, the conclusion being that the observed decay can be better explained by static triggering. Resolving whether static 1 , 2 , 3 or dynamic 4 , 5 , 6 , 7 , 8 stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard 9 . Felzer and Brodsky 10 examined the distance distribution of earthquakes occurring in the first five minutes after 2 ≤  M  < 3 and 3 ≤  M  < 4 mainshocks and found that their magnitude M  ≥ 2 aftershocks showed a uniform power-law decay with slope −1.35 out to 50 km from the mainshocks. From this they argued that the distance decay could be explained only by dynamic triggering. Here we propose an alternative explanation for the decay, and subject their hypothesis to a series of tests, none of which it passes. At distances more than 300 m from the 2 ≤  M  < 3 mainshocks, the seismicity decay 5 min before the mainshocks is indistinguishable from the decay five minutes afterwards, indicating that the mainshocks have no effect at distances outside their static triggering range. Omori temporal decay, the fundamental signature of aftershocks, is absent at distances exceeding 10 km from the mainshocks. Finally, the distance decay is found among aftershocks that occur before the arrival of the seismic wave front from the mainshock, which violates causality. We argue that Felzer and Brodsky 10 implicitly assume that the first of two independent aftershocks along a fault rupture triggers the second, and that the first of two shocks in a creep- or intrusion-driven swarm triggers the second, when this need not be the case.