A Global Characterization of Physical Segmentation along Oceanic Transform Faults

Abstract

Wesnousky (2006) found that physical fault offsets of 3 - 4 km act as barriers to rupture propagation on continental strike-slip faults. Along oceanic transform faults (OTFs), step-overs, intra-transform spreading centers, and pull-apart basins can divide the fault system into a series of parallel or sub-parallel fault segments. We have characterized the segmentation of OTFs on a global scale and are investigating the effects of this segmentation on seismic behavior. The 1-arcmin Smith and Sandwell global seafloor topography dataset (v. 12.1), comprised of satellite altimetry data blended with depth estimates from ship-borne sonar, was used to measure physical parameters of 200 OTFs. For each OTF, fault length and distance from each endpoint to the nearest ridge discontinuity were measured. The 200 individual fault segments were classified into 101 single-segment faults and 34 multi-segment fault systems, each comprised of between 2 - 7 segments with offsets ≤ 50 km. Utilizing only ridge-transform-ridge segments, we constructed scaling relations for seismic parameters. To ensure a uniform minimum resolution for our global dataset, where some regions are constrained only by satellite altimetry data, any two adjacent fault segments with an offset ≤ 20 km, were combined into a single segment. The resulting dataset included 155 fault segments. An earthquake catalog was then generated from the global Centroid Moment Tensor database for each individual fault segment. A half-space cooling model was used to calculate the thermal area of contact above the 600-degree isotherm using slip rates acquired from the GSRM plate velocity model. Following the analysis of Boettcher and Jordan (2004), maximum-likelihood estimation was used to determine the largest expected earthquake for fault segments grouped by thermal area. Scaling relations between thermal area and the largest expected earthquake, as well as the seismic coupling coefficient, were calculated. Initial results show no significant differences in the scaling relations derived for the more segmented dataset. Future analyses utilizing OTF structure delineated from higher resolution sonar data will provide additional insight into the underlying mechanics of fault slip on OTFs