Earthquake swarms on the Mid‐Atlantic Ridge: Products of magmatism or extensional tectonics?

Abstract

By analogy with the microearthquake swarms which accompany terrestrial volcanic eruptions, teleseismically observed earthquake swarms along mid‐ocean ridges are commonly thought to be indicators of volcanism, particularly dike emplacement. We test this assumption by investigating the spatial and temporal patterns and other characteristics of earthquakes in 34 swarms on the Mid‐Atlantic Ridge. Resolution of the relative locations of swarm events is improved by the use of a multiple‐event relocation technique. Most swarms extend 5 to 15 km across the ridge axis and 5 to 20 km along it. None of the swarms displays the large length‐to‐width ratio and temporal migration of epicenters along axis which are characteristic of earthquake swarms in the well‐studied rift zones of Hawaii and Iceland. The relative locations, magnitudes, and source mechanisms of swarm events are most easily explained if teleseismically observable swarms are expressions of extension across a substantial portion of the median valley and perhaps into the rift mountains. The possibility that some of these swarms include events directly associated with magmatic intrusion cannot be excluded, however. Like their terrestrial counterparts, purely volcanic earthquakes on the mid‐ocean ridges probably occur dominantly at magnitudes below the teleseismic detection threshold of global seismic networks (mb < 4.5). The largest normal‐faulting earthquakes on mid‐ocean ridges and many of the teleseismically observed swarms likely occur on the faults marking the inner rift wall, where the crust first experiences significant faulting and is broken into numerous closely‐spaced normal faults. We speculate that the high degree of order in the dimensions, spacing, throws, and mechanical properties of these faults, combined with stress concentrations arising naturally from the mechanical behavior of the lithosphere at rifted, slow‐spreading ridges, may account for the common occurrence of earthquake swarms.