Seismic Behavior of Oceanic Transform Faults

We have conducted a comprehensive global investigation of oceanic transform fault (OTF) seismicity, which confirms that OTFs have fundamentally different seismic properties than continental transform faults (CTFs). We investigated frequency-magnitude relations, aftershock productivity rates, and the percentage of slip accommodated seismically for 75 OTFs using data from the International Seismological Centre, Harvard Centroid Moment Tensor Project, Oregon State University Moment-Tensor Solutions, and NOAA-PMEL/T-phase Project catalogs. OTFs have depleted aftershock sequences, high seismic deficits ( ~85%), few large earthquakes, low b-values ( ~0.85), and depleted high-frequency energy content compared to CTFs. Only ~15% of the slip on OTFs can be accounted for with typical high-frequency earthquakes, while CTFs (e.g. San Andreas and North Anatolian) have been shown to be fully seismic (i.e. no seismic deficit). No OTF earthquake greater than M_w 7.2 has occurred during the past 37 years and no evidence was found for characteristic earthquake behavior. We observed, on average, less than one mb >= 4.5 aftershock per large OTF earthquake. The limited ( ~4 years) equatorial Pacific T-phase data show that while many small aftershocks are detected following some events (e.g. Mw 5.9 on the Gofar TF), other OTF earthquakes have no recorded T-phase aftershocks (e.g. Mw 6.2 on the Gofar TF). Maximum event magnitude, seismic deficit, and b-value were observed to vary with slip rate on OTFs. These trends are likely to be related. Maximum event magnitude decreases with slip rate, thereby resulting in both high seismic deficits and sharp roll-offs in the frequency-magnitude distribution. High slip rates, seismic deficits, and the lack of large earthquakes require processes such as steady aseismic creep and slow aseismic transients to produce much of the long-term offset, especially on fast slipping transforms. The above properties suggest that OTFs can be characterized by a low degree of seismic heterogeneity, indicating a smooth rupture processes and relatively little structural complexity.