Megacity Megaquakes: Two Near-misses, and the Clues they Leave for Earthquake Interaction

Two recent earthquakes left their mark on cities lying well beyond the mainshock rupture zones, raising questions of their future vulnerability, and about earthquake interaction broadly. The 27 February 2010 M=8.8 Maule earthquake struck the Chilean coast, killing 550 people. Chile's capital of Santiago lies 400 km from the high-slip portion of the rupture, and 100 km beyond its edge. The 11 March 2011 M=9.0 Tohoku oki earthquake struck the coast of Japan, its massive tsunami claiming most of its 18,564 victims. Reminiscent of Santiago, Japan's capital of Tokyo lies 400 km from the high-slip portion of the rupture, and 100 km beyond its edge. Because of this distance, both cities largely escaped damage. But it may not have been a clean get-away: The rate of small shocks beneath each city jumped by a factor of about 10 immediately after its megaquake. At Santiago, the quake rate remains two times higher today than it was before the Maule shock; at Tokyo it is three times higher. What this higher rate of moderate (M<6) quakes portends for the likelihood of large ones is difficult--but imperative--to answer, as Tokyo and Santiago are probably just the most striking cases of a common phenomenon: Seismicity increases well beyond the rupture zone, as also seen in the 1999 Izmit-Düzce and 2010 Darfield-Christchurch sequences. Are the Tokyo and Santiago earthquakes, 100 km from the fault rupture, aftershocks? The seismicity beneath Santiago is occurring on the adjacent unruptured section of the Chile-Peru trench megathrust, whereas shocks beneath Tokyo illuminate a deeper, separate fault system. In both cases, the rate of shocks underwent an Omori decay, although the decay ceased beneath Tokyo about a year after the mainshock. Coulomb calculations suggest that the stress imparted by the nearby megaquakes brought the faults beneath Santiago and Tokyo closer to failure (Lorito et al, Nature Geoscience 2010; Toda and Stein, GRL 2013). So, they are aftershocks in the sense that they are contingent on the mainshock, and underwent at least an initial decay. But aftershocks do not necessarily signal a heightened likelihood of large shocks. They could instead accompany post-seismic creep, with the creep shedding the stress imposed by the megaquakes. These aftershocks are too deep for GPS observations to reveal unequivocally whether the faults are locked or creeping. But one clue is that the ratio of small to large shocks was not changed by the megaquakes. This distribution could be a reliable pointer for the probability of lager quakes, and so large shocks may now indeed be more probable than before the megaquakes--by a factor of at least two.