Reply to H. Acharya's "Comments on 'Seismic potential associated with subduction in the Northwestern United States'"

Abstract

Subduction in the northwestern United States presents us with a dilemma. Although there is good evidence of 3 to 4 cm/yr of convergence between the Juan de Fuca and North American plates, the occurrence of either historic or instrumentally recorded shallow thrust earthquakes is remarkably low. Why aren't there more earthquakes? Aseismic slip along the entire plate boundary provides a convenient explanation for this dilemma. Aseismic slip appears to be the predominant mode for plate interaction for many subduction zones. However, the Juan de Fuca subduction zone is clearly different from the most common class of aseismic subduction zone that is characterized by the subduction of very old oceanic lithosphere (Heaton and Kanamori, 1984). We noted that, in general, the subduction of young lithosphere is characterized by strong interplate seismic coupling. The Juan de Fuca subduction zone can be considered as an end member in that it involves some of the youngest subducted lithosphere observed anywhere. This, in itself, suggests that the Juan de Fuca subduction zone belongs in a class (perhaps aseismic) by itself. However, there are several other localities where comparably young crust appears to be subducting. These are southern Chile between 42° and 45° south latitude, Colombia near 2º north latitude, and the Rivera plate off western Mexico. All of these regions are seismically active and, in the case of Colombia and southern Chile, have involved earthquakes with energy magnitudes of 8.8 (1906) and 9.5 (southern half of the 1960 rupture zone), respectively. Furthermore, these regions of Colombia and southern Chile do not have bathymetric trenches, and there is no significant seismic activity deeper than 100 km observed on their Benioff-Wadati zones. There is also evidence that these regions have experienced significant periods of seismic quiescence. The NOAA catalog shows a remarkable absence of shallow activity between 41º and 45° south latitude along the Chile trench for at least 30 yr prior to the 1960 M_w 9.5 Chilean earthquake. Unfortunately, this catalog is not sufficiently complete to allow a comparison of seismicity at small magnitude earthquakes. However, at a magnitude cutoff of 6, the rate of seismicity in the 50 yr preceding the 1960 earthquakes in the region between 41º and 45º south latitude seems comparable to that reported for the Juan de Fuca subduction zone's 150-yr history (the Juan de Fuca convergence rate is about one-third that of southern Chile).

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