Temporal and spatial variations in seismicity of fast-slipping oceanic transform faults at the East Pacific Rise

Qiu Zhong¹, Jian Lin^{1, 2, 3}, Tingting Zheng³

¹ Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China.

² Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

³ State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China

 We analyzed hydro-acoustically recorded seismicity data from the East Pacific Rise (EPR) to investigate the temporal and spatial variations in seismicity of fast-slipping oceanic transform faults. The hydrophones in the Pacific Ocean have recorded earthquakes along the EPR from May 1996 to April 2002. Our investigation focused on four transform faults: Wilkes, Siqueiros, Gofar and Clipperton. Our analysis revealed that all of the four transform faults are associated with numerous foreshocks or/and aftershocks of mainshocks with magnitude M ~ 5 or greater. And the seismicity on the fault present segmented state on the fault or on the hook between transform fault and the ridge. First we fit the aftershocks time durations of more than 100 mainshocks with magnitude M > 4.5. Results of the analyses revealed the following: (1) The aftershock duration t_a of the investigated mainshocks ranges from about 5.5 days to about 3.1 years。The seismicity rate immediately following a mainshock event, R₀, varies from about 10 to 511, corresponding to average error of 10.7 % and the maximum error of 33.3 %, respectively. In addition, we calculate the rate between background seismicity, r, at each fault segment. Results revealed that the logarithm of R₀/r correlate with logarithm time duration of aftershocks but with almost no relation to the mainshock magnitude. We also calculated the energy accumulation of earthquakes on each segment of the transform faults. The cumulated energy differed from each other: Earthquake energy released in the last 6 years (from 1996 to 2002) are valuated at from 1.7×10¹⁹ N∙m/km on the Siqueiros Transform Fault, to 2.2×10¹⁷ N∙m/km on the Clipperton. Segments on each fault are calculated to have similar energy release. Together these investigations helped to advance our understanding of the earthquake mechanism and geodynamics of fast-slipping oceanic transform faults.