Spatial and Temporal Variations in Earthquake Stress Drop on Gofar Transform Fault, East Pacific Rise: Implications for Fault Strength

Abstract

On Gofar Transform Fault on the East Pacific Rise, the largest earthquakes (6.0 ≤ MW ≤ 6.2) have repeatedly ruptured the same portion of the fault, while intervening fault segments host swarms of microearthquakes. These long‐term patterns in earthquake occurrence suggest that heterogeneous fault zone properties control earthquake behavior. Using waveforms from ocean bottom seismometers that recorded seismicity before and after an anticipated 2008 MW 6.0 mainshock, we investigate the role that differences in material properties have on earthquake rupture at Gofar. We determine stress drop for 138 earthquakes (2.3 ≤ MW ≤ 4.0) that occurred within and between the rupture areas of large earthquakes. Stress drops are calculated from corner frequencies derived using an empirical Green's function spectral ratio method, and seismic moments are obtained by fitting the omega‐square source model to the low frequency amplitude of the displacement spectrum. Our analysis yields stress drops from 0.04 to 3.2 MPa with statistically significant spatial variation, including ~2 times higher average stress drop in fault segments where large earthquakes also occur compared to fault segments that host earthquake swarms. We find an inverse correlation between stress drop and P wave velocity reduction, which we interpret as the effect of fault zone damage on the ability of the fault to store strain energy that leads to our spatial variations in stress drop. Additionally, we observe lower stress drops following the MW 6.0 mainshock, consistent with increased damage and decreased fault strength after a large earthquake.

Stress Drops on the Blanco Oceanic Transform Fault from Interstation Phase Coherence

“…This median is similar to or higher than other estimates on oceanic transform faults (Boettcher and Jordan, 2004;Allmann and Shearer, 2009;Moyer et al, 2018). The median stress drop is a factor of 1.7 (0.8 to 3.5) higher on the more seismically active southeast segment of the Blanco Fault.…”

“…This factor of difference should be carefully considered due to the scatter of individual stress drop, and the large uncertainty in the factor itself. Nevertheless, we note that one possible explanation for the lower stress drops on the more aseismic segment, which were also observed on the East Pacific Rise (Moyer et al, 2018), is that the more aseismic segment has higher temperatures, which lead to a shallower seismogenic zone, a shortened seismic cycle, less time for healing and thus less potential for large strength and stress drop in the earthquakes.…”

“…Despite the uncertainty on our stress drop estimates, it is interesting to note that Moyer et al (2018) identified a similar spatial variation in stress drops for East Pacific Rise transform faults, where stress drops were a factor of 2 larger in higher seismic moment release areas. They explained their results using the model of Hardebeck and Loveless (2018), where creeping faults had reduced strength and therefore lower stress The transition to velocity-strengthening frictional sliding and thus to aseismic creep is thought to be temperature dependent, occurring at 500 -600 • C on transform faults (Abercrombie and Ekström, 2001;Boettcher et al, 2007;He et al, 2007;Braunmiller and Nábělek, 2008).…”

“…Earthquakes on oceanic transform faults also display a wide variation in stress drops, with both unusually high stress drops (e.g., Allmann and Shearer, 2009;Chen and McGuire, 2016) and some of the lowest recorded stress drops (e.g., Pérez-Campos et al, 2003). Stress drops may also vary between areas with more or less aseismic slip (Moyer et al, 2018). But these behaviors remain poorly understood, as only a modest number of studies have examined earthquake properties on these faults because they are often far from observing stations.…”

Spatio-temporal Characteristics of Frictional Properties on the Subducting Pacific Plate Off the East of Tohoku District, Japan Estimated from Stress Drops of Small Earthquakes

“…Yamada et al (2017) analyzed stress drops of small earthquakes off the Pacific coast of Hokkaido, Japan and found that the spatial pattern in stress drops has a good correlation with spatial characteristics of coseismic displacements during individual historical large earthquakes. Moyer et al (2018)…”