Building Earth Structures from Earthquakes: Understanding Deformation within the Santa Cruz Mountains Across Space and Time - Livestream

How do earthquakes build mountains and geologic structures? This is an important fundamental question in earth sciences that has implications for hazards posed by motion along active faults. In this talk, I’ll present a decades’ worth of data collection from my group, which image the deformation, uplift, and erosion of the Santa Cruz Mountains over time-scales ranging from decades to millions of years. These myriad data are then used to address two questions - one applied and one fundamental. First, we ask whether topographic and thermochronologic data can be used to estimate the slip rate distribution along reverse faults that bound the western flank of Silicon Valley using models of crustal deformation and erosion. These structures are difficult to characterize directly, because they are either blind or do not preserve sediments that can be used to deduce their slip rates. We find that our methodology for inverting topography for fault slip rates yields estimates for plate-boundary motions and rock erodibilities that agree with published estimates. We infer that these faults are capable of producing a Loma-Prieta type earthquake every 300 years, and so constitute a substantial hazard to Silicon Valley given the proximity and geometry of these faults. Second, we ask whether a small modification to the traditional elastic earthquake-cycle model that allows crust to yield over geologic time-scales can reconcile conflicting information about deformation, uplift, and erosion of the Santa Cruz Mountains when measured over decades versus millions of years. We find that a crustal rheology that includes strain-hardening plasticity successfully bridges observations over these vastly different time-scales. Interestingly, the model implies that interseismic intervals build the mountains in the area, while elastic strain release during earthquakes lowers them, such that interseismic yielding allows the steady accrual of long-term uplift of the range. Our studies show that decadal time-scale measures of surface deformation cannot discriminate the appropriate rheology of the crust - it is only by including geologic observations that we can deduce this information. As such, geologic observations constitute a cornerstone of crustal deformation studies that seek to understand the rheology of Earth’s crust.

Speaker: George Hilley, Stanford University