The Relationship Between Seismicity and Fault Structure on the Discovery Transform Fault, East Pacific Rise
On the Discovery transform fault, East Pacific Rise, absolute locations of one year of microseismicity recorded during an ocean bottom seismometer (OBS) deployment and 21 years of Mw ≥ 5.4 earthquakes, obtained from the Global Centroid Moment Tensor (CMT) catalog, are correlated with fault structure using high resolution multibeam bathymetry. Fifteen Mw 5.4 - 6.0 earthquakes occurred on Discovery between 1992 - 2013, with a mean repeat time of 7 years. Absolute locations for each of these events were obtained by employing a teleseismic surface wave relative relocation technique using events from the NOAA hydroacoustic catalog as empirical Green’s functions. Each event was found to occur within one of five distinct rupture patches on Discovery. Correlation of the rupture patch locations with the structural analysis of the fault zone indicates that the 8-km long intra-transform spreading center, located approximately in the center of Discovery, acts as a barrier to large rupture propagation. Secondary structural features within the fault zone (e.g. ridges that crosscut the transform valley) may also result in barriers to rupture propagation. The 2008 OBS deployment on Quebrada, Discovery, and Gofar transform faults recorded ~25,000 0.16 < mL < 4.58 earthquakes on Discovery, which were relocated using HypoDD. The relocated events cluster within 3 km of the fault trace and extend ~6 km outside of the western ridge-transform intersection. Approximately 4% of the microseismicity occurred beyond the active transform, and these events appear to be dominated by aftershock sequences. The rate of microseismicity varies significantly along strike and with time. The most striking region of the fault includes a 4.5-km wide zone on the western segment that is essentially devoid of any microseismic activity during the yearlong deployment period. The microseismic gap coincides with the largest, westernmost rupture patch, which was ruptured by 3 Mw 5.9 - 6.0 earthquakes in 1996, 2001, and 2012, suggesting that this fault segment may be fully locked between large events. By contrast, the centroid location for the second largest rupture patch (4 Mw 5.5 - 5.8 earthquakes in 1996, 2001, 2007, and 2012) on the western segment coincides with the area of highest microseismicity rate. This 5-km wide zone contains approximately 25% of the relocated microseismicity on Discovery. The smallest rupture patch on Discovery (3 Mw 5.4 - 5.5 earthquakes in 1998, 2003, and 2007) is located just west of the intra-transform spreading center and coincides with a zone of low seismic productivity, suggesting increased seismic coupling. The two rupture patches on the eastern segment of Discovery were outside the OBS array, and thus the microseismic activity in those regions is not well constrained. The results of this study suggest that physical fault structure influences the location and size of large repeating rupture patches, and provides a secondary control on the location of microseismicity. Fault structure, however, cannot account for all aspects of seismic activity on Discovery, and thus we suspect that varying mechanical properties along the fault, and specifically within the large rupture patches, are the primary controls on the location and rate of microseismicity.
Journal or Conference Title
Fall Meeting, American Geophysical Union (AGU)
9-13 December, 2013
San Francisco, CA, USA
American Geophysical Union Publications
Wolfson-Schwehr, M., Boettcher, M., McGuire, J., and Collins, J., The Relationship Between Seismicity and Fault Structure on the Discovery Transform Fault, East Pacific Rise, Abstract S51C-2387 presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9-13 Dec. 2013