Along Strike Heterogeneity of Seismic Slip Revealed by Oceanic Transform Fault Earthquakes

Abstract:

Oceanic transform faults (OTFs) are considered to have relatively simple structure [thermal, geometric, compositional], with the brittle-ductile transition defined by the 600-800ºC isotherm. Earthquakes on these faults account for less than half of the expected slip (Boettcher & Jordan, 2004), leaving the majority of motion to be accommodated aseismically.

The 2015 M_W7.1 Charlie-Gibbs transform earthquake is the latest of seven large [M≥6.25] earthquakes that form two quasi-repeating sequences dating back to 1920. These two sequences are separated by a region of persistent aseismicity in the center of the transform, interpreted to be a rupture barrier that prevents the full extent of the transform from rupturing in a single earthquake. However, aseismic rupture barriers alone cannot account for the inferred deficit in the seismic budget of OTFs.

A growing catalogue of slip distributions has revealed distinctive behavior for large OTF earthquakes. We present evidence from teleseismic body wave modeling for directivity and slip distribution of four MW ≥ 7.0 oceanic strike-slip earthquakes: the 2015 M_W7.1 Charlie-Gibbs transform earthquake in the North Atlantic, the 2015 M_W7.0 Fourier transform earthquake in the South Atlantic, and the 2013 M_W7.3 and 2006 M_W7.4 South Sandwich transform earthquakes in the Southern Ocean. Each earthquake initiates near the ridge with nominal slip then propagates unilaterally to rupture larger asperities nearer the middle of the transform, similar to behavior observed for the 1994 M_W7.0 Romanche transform earthquake. Significant continental strike-slip earthquakes, such as the 2002 M_W7.9 Denali earthquake and the 2001 M_W7.8 Kunlun earthquake, also exhibit unilateral ruptures with a small initial slip.

The slip distributions of large oceanic transform earthquakes suggest that seismic coupling of OTFs varies considerably along strike, with large slip asperities separated by areas of little or no slip. Substantial earthquakes are not observed in these areas of unresolvable co-seismic slip. This suggests that motion is accommodated both aseismically during the interseismic period and seismically during large earthquakes. Such multimode fault behavior has implications for quantifying seismic hazard along other faults with aseismic and creeping sections.