The Relationship Between Structural Variations and Earthquake Rupture Properties Along the Gofar Transform Fault, East Pacific Rise

Abstract:

Gofar transform fault on the East Pacific Rise can be subdivided into multiple patches that slip during magnitude 5.5-6 earthquakes roughly every five years. It is noted that, however, these patches are separated by stationary rupture barriers that prevent ruptures propagating through. In 2008, Woods Hole Oceanographic Institution (WHOI) deployed an ocean bottom seismography (OBS) array for 1-year continuous measurements, which successfully captured the 18 September 2008 Mw 6.0 earthquake as well as its fore- and aftershocks. Previous studies showed that along-strike seismicity has apparent segmentation and quasi-period.

To better understand the relationship of structural variations and earthquake rupture properties along the Gofar transform fault, we use P and S first arrival times from 3874 earthquakes selected from this survey to determine three-dimensional Vp, Vs and Vp/Vs models and earthquake locations using a modified double-difference seismic tomography code tomoDDSP. Compared to tomoDD, tomoDDSP provides a more robust estimation of Vp/Vs model, which is a useful indicator of lithology and pore fluid pressure. In addition to the catalog picks, more accurate waveform cross-correlation data is also used in our inversion.

Overall, our result shows clear Vp/Vs segmentation along the fault strike. For each segment, the associated seismicity also shows different patterns. For the mainshock zone of the 2008 Mw 6.0 earthquake, it is associated with high Vp/Vs anomalies (1.7-2.1) and the seismicity is concentrated within a narrow zone around the depth 7 km. The adjacent eastern and western zones are associated with low Vp/Vs anomalies (<1.6), where in addition to the concentration of seismicity around 7 km depth there is also some seismicity distributed all over the upper crust. Further to the east, the foreshock zone identified in previous studies is associated with higher Vp/Vs anomalies with most of seismicity diffusively located above 7 km. To the west of the foreshock zone, a clear low velocity zone extending deep to 10 km is imaged, which could act as a barrier for the rupture propagating from the mainshock zone to the foreshock zone. The cause of the Vp/Vs anomalies will be further investigated, which could help explain the distribution pattern of seismicity and rupture patches along the strike.