Using the CoCo metric of relative structural complexity of major plate-boundary fault networks to explore potentially time-variable fault loading rates on major strike-slip faults

Abstract

The Coefficient of Complexity (CoCo) is a metric that quantifies the relative structural complexity of the fault system surrounding a specific study site on a primary fault extending through the system. Specifically, the CoCo metric has been used successfully to correlate the relative constancy or non-constancy of incremental slip rates on major strike-slip faults with the proximity, number, and slip rates of other neighboring active faults within a given radius of observation. Interestingly, our analysis shows that faults that extend through more structurally complex plate-boundary fault systems are characterized by more temporally variable slip behavior than faults that are embedded within simpler settings. Complex stress interactions within structurally complicated tectonic networks, as well as possible temporal changes in fault strength and kinematic interactions amongst mechanically complementary faults, likely explain these different behaviors. The CoCo metric thus not only provides a potential means for better evaluating the future behavior of large plate-boundary faults in the absence of well-documented incremental slip-rate behavior, but also can be used to differentiate faults that typically slip at a constant rate from the ones which do not. Using these results, we explore the relationship between incremental fault slip rates averaged over both short and large displacements on major strike-slip faults and geodetic estimates of strain accumulation rate on faults with different CoCo values. As might be anticipated, our analysis shows that the relatively constant slip rates on faults embedded within structurally simple strike-slip tectonic networks (i.e., low-CoCo faults) generally match rates of elastic strain accumulation of the faults’ shear zones, as measured by geodetic slip-deficit rates. In marked contrast, geodetic slip-deficit rates for faults embedded within structurally complex fault systems (high-CoCo faults) are less consistent with geological rates, whether averaged over short or large displacement scales, indicating significant variations in strain-accumulation rate on high-CoCo faults. We use these data to suggest possible patterns of geodetic-to-geologic rate ratios that may be indicative of the likely near-future behavior of the fault in question.