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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B01304, doi:10.1029/2007JB005028, 2008

Fault plane orientations of intermediate-depth earthquakes in the Middle America Trench

Linda M. Warren

Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D. C., USA


Meredith A. Langstaff

Department of Geophysical Sciences, University of Chicago, Chicago, Illinois, USA


Paul G. Silver

Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D. C., USA


Abstract

Intermediate-depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones. The orientation of presubduction faults is particularly well known offshore of Middle America, where seismic reflection profiles show outer rise faults dipping toward the trench and extending >20 km into the lithosphere. If water is transported along these faults and incorporated into hydrous minerals, the faults may be reactivated later when the minerals dehydrate. In this case, the fault plane orientations should be the same in the outer rise and at depth, after accounting for the angle of subduction. To test this hypothesis, we analyze the directivity of 54 large (M W ≥ 5.7) earthquakes between 35 and 220 km depth in the Middle America Trench. For 12 of these earthquakes, the directivity vector allows us to identify the fault plane of the focal mechanism. Between 35 and 85 km depth, we observe both subhorizontal and subvertical fault planes. The subvertical fault planes are consistent with the reactivation of outer rise faults, whereas the subhorizontal fault planes suggest the formation of new faults. Deeper than 85 km, we only observe subhorizontal faults, indicating that the outer rise faults are no longer being reactivated. The similarity with previous results from the colder Tonga-Kermadec subduction zone suggests that the mechanism generating these earthquakes, and controlling fault plane orientations, depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate-depth seismicity. Exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes.

Received 5 March 2007; accepted 24 September 2007; published 12 January 2008.

Keywords: intermediate-depth earthquakes; directivity; Middle America Trench.

Index Terms: 7209 Seismology: Earthquake dynamics (1242); 7240 Seismology: Subduction zones (1207, 1219, 1240); 8118 Tectonophysics: Dynamics and mechanics of faulting (8004); 8120 Tectonophysics: Dynamics of lithosphere and mantle: general (1213).

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Citation: Warren, L. M., M. A. Langstaff, and P. G. Silver (2008), Fault plane orientations of intermediate-depth earthquakes in the Middle America Trench, J. Geophys. Res., 113, B01304, doi:10.1029/2007JB005028.