Contrasting rupture processes during the April 11, 2010 deep-focus earthquake beneath Granada, Spain

The Mw=6.3 deep-focus earthquake beneath Granada, Spain, in 2010 consisted of three resolvable sub-events occurring within a time span of 5 s. Estimated sub-event seismic moment partitioning is 12%, 7% and 81%, respectively. All sub-events had similar focal mechanisms with a vertical and a near-horizontal nodal plane, and all occurred within 5 km of each other at a similar depth, suggesting rupture on the near-horizontal nodal plane. However, directivity analyses indicate that the first sub-event ruptured unilaterally on the vertical plane. Its modeled rupture length of ∼9 km and stress drop of ∼2 MPa are typical of crustal earthquakes. In contrast, the following sub-events show no clear directivity. The third, best resolved, sub-event had a hypocenter ∼2 km from the first and a focal mechanism indistinguishable from the first, but it had a rupture dimension of <6.5 km and a stress drop of >40 MPa. This requires an ambient stress field significantly greater than the stress drop of the first sub-event, implying that the first sub-event ruptured as a slip pulse with a transient weakening mechanism. The large stress drops of the second and third sub-events suggest a crack-like rupture without fault healing and with nearly total stress drop. Fault-zone melting and metastable olivine are viable mechanisms for these ruptures. In contrast, the rupture characteristics of the first sub-event seem incompatible with most mechanisms currently under consideration for deep-focus earthquakes.