Moderate and large earthquake activity along oceanic transform faults
T. VANDEMARK, Penn State, firstname.lastname@example.org; C. AMMON, Penn State, email@example.com
Oceanic transform faults provide an excellent opportunity to explore strike-slip faulting processes in a relatively simple, but seismically productive faulting environment. Key parameters such as overall boundary geometry, plate age, and long-term slip rates are readily available. A number of transforms are quite active and provide plentiful data to investigate the effects of event clusters in time and space, and possibilities of earthquake interaction. The main drawback in such an investigation is the near total lack of local observations necessary to produce high-quality event locations, needed to complete detailed investigations. To circumvent this problem, we use a double-difference relocation method exploiting intermediate-period Rayleigh waveforms to estimate precise relative earthquake centroids for moderate and large magnitude events. Although a formal estimate of uncertainty is difficult to demonstrate, direction examination of the observed time shifts and geometric consistency suggest that many relative locations are accurate to better than five kilometers. The number of events available on each transform varies and the constraints are best during the last 15 years, a time corresponding to the installation of the current Global Seismic Network. We estimated improved relative locations for a total of 130 strike-slip events that occurred along the Romanche and Challenger transform fault systems. We observed spatial clustering of seismicity separated by possible aseismic gaps along each transform. To complement these observations made over a short observation time (about 15 years), we also investigated seismicity patterns using earthquake catalogs. We examined 862 strike-slip events from the Harvard CMT catalog distributed across 30 transforms accommodating movement over a range of relative plate velocity (V) to investigate the moment distribution along oceanic transforms. We compare the observed moment distribution with a simple uniform distribution estimated using NUVEL-1A. We estimate the fraction of asesimic motion for each transform using the ratio of observed to predicted cumulative seismic moment for each transform. All transforms with V ≥ 7.0 cm/yr have are approximately 90% aseismic; transforms with a V < 7.0 cm/yr have apparent aseismic fractions of about 80%. Slower moving transforms, (V < 7.0 cm/yr) have hosted all earthquakes of M ≥ 6.5. For Gutenberg-Richter analysis of the same events, we classified the transforms into three groups based on their associated relative plate offset speeds, V (0.0 to 3.9 cm/yr, 4.0 to 7.9 cm/yr, and ≥ 8 cm/yr). We find that as V increases, both the G-R slope and the corner magnitude decrease. An examination of the USGS earthquake catalog suggests that foreshock and aftershock sequences are rare for oceanic transform events M ≥ 6.0, with the exception of events along the Romanche Transform, which has aftershock sequences for 11 of 13 recent events. We are exploring the observed patterns in seismicity in concert with more modern precise locations to investigate rupture and plate motion accommodation along these tectonically important structures.