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.
Bibliographical noteFunding Information:
Data were made available through: the IRIS data management center (which provided data from USArray, PICASSO, Geofon, Geoscope, GSN and many local arrays); the TopoIberia Project (which provided data from IberArray); and the Spanish Instituto Geográfico Nacional (IGN) (which provided data from the Spanish National Array). Our regional seismic stations include Portuguese, Spanish and Moroccan permanent stations, 56 stations from IberArray and 36 stations from the PICASSO project deployed in southern Spain and Morocco. The authors wish to thank Alan Rempel, Cliff Frohlich and Allan Rubin for interesting discussions on the topic of this paper as well as three anonymous reviewers whose comments greatly helped this manuscript become more clear, thorough and persuasive. We also wish to thank Peter Shearer for his personal involvement in the review process. Our work was supported by the PICASSO project NSF Grant no. EAR-0808939 .
- Deep-focus earthquake
- Rupture mechanism