Earthquake Science Center Seminars

Yihe Huang, University of Michigan

The Cascadia subduction zone is considered to pose great earthquake and tsunami hazards in the Pacific Northwest, but we have a limited understanding about where the earthquake will initiate and how large the earthquake magnitude and tsunami amplitudes can be. I will present physics-based simulations of Cascadia earthquake rupture and tsunamis to discuss the important parameters used to constrain the simulations and their implications. We show in 2-D dynamic rupture simulations that given our current understanding of the shear stress rate in the gap region it is unlikely for Northern Cascadia rupture to stop right above the gap region. By developing for the first time 3-D dynamic Cascadia rupture simulations, we find the level of interseismic shear stress accumulation in the central Cascadia and in the shallow region near trench controls the final rupture size of Cascadia earthquakes. Our rupture simulations can provide a reasonable fit to the 1700 A.D. coastal subsidence data using different combinations of locking depths and initial shear stress distribution. We also use geodetic coupling models and earthquake rupture properties to constrain tsunami simulations of M7.5–9.2 Cascadia earthquakes. While it is often believed that the worst-case tsunami scenario results from the largest M9 earthquake, our results highlight the importance of considering M>8.5 earthquakes for tsunami hazard mitigation, as they can create comparable coastal tsunami amplitudes due to the focusing effect of the concave coastline geometry near Oregon.

Alexis Ault, Utah State University

Above the seismogenic zone, earthquake ruptures may propagate to the surface along discrete fault planes or earthquake energy is attenuated in fault damage zones. Forecasting seismic hazards requires knowledge of the slip distribution and mechanical behavior of the shallow crust. Iron is the 4th most abundant element in Earth’s crust and hematite grows in a range of shallow fault rocks due to the redox potential of diverse Fe-bearing minerals. Hematite in fault rocks exhibits grain morphologies and nano- to microscale textures that preserve evidence for different slip rates and deformation conditions. Hematite is also amenable to (U-Th)/He thermochronometry, a powerful tool that, when combined with textural observations, informs the timing and temperatures (and thus rates) of fault slip. Hematite deformation experiments at ambient conditions and seismic and subseismic slip rates show low coefficients of quasi-static and dynamic friction, as well as generally velocity neutral behavior. In this talk, I share examples of how we fuse textures and thermochronometry data patterns from natural and experimental surfaces to inform fault mechanics. Investigation of hematite in shallowly exhumed, seismogenic faults, such as parts of the southern San Andreas fault (CA), eastern Denali fault zone (Yukon), and Intermountain Seismic Zone (UT) zone imply a broad range of hematite rheology, variable slip rates, and evolving mechanical properties that may promote or inhibit rupture propagation in the shallow crust.

Yolanda Lin, University of New Mexico

Current policies, risk management practices, emergency preparedness, and scientific research interests are often shaped by the effects of past events. However, our past experiences and current methodologies may not prepare us for low probability events that have the potential to affect our natural, built, and social environments. This is especially true in regions of low- to moderate- seismicity, where uncertainties regarding the long-term seismicity of these regions, unknown faults, and limited historical records complicate the selection of an earthquake scenario, which are often needed for purposes of emergency management and planning. What are some ways that we can identify effective earthquake scenarios in these uncertain contexts? In this talk, I will highlight how we can leverage downward counterfactual thinking in these data-sparse contexts. Additionally, I will present a new consequence-driven approach to earthquake scenario selection that can be used to integrate stakeholder interests directly within the scenario selection process. Finally, I will discuss how we can use scenarios in order to quantify and celebrate mitigation and resilience efforts that can uncover "invisible benefits" and help move us beyond a focus on short-term loss avoidance.

Alex Hatem, USGS

Geologic slip rates represent time-averaged fault displacement and are a handy metric to define a fault’s activity. Useful as they are, slip rates aggregate numerous earthquakes that have occurred between field-based observation points, which are typically dated offset geomorphic features (e.g. stream channels or terrace risers). This means that time-displacement paths - that is, the potentially erratic stick-slip journeys of displacement-time pairs between geomorphic features - are usually unknown.

Here, I present a field study of incremental geologic slip rates along the Hope fault (Marlborough fault system, New Zealand), discuss some pitfalls of geologic slip rates as a metric, and present a potential path forward toward quantifying variability when we calculate geologic slip rates. The numerical approach presented (STEPS) is a complement to geologic field observations and is a data-driven method to provide reasonable estimates of fault time-displacement histories between field measurements.

Curtis Baden, Stanford University

Relative crustal motions along active faults generate earthquakes, and repeated earthquake cycles build mountain ranges over millions of years. However, the long-term summation of elastic, earthquake-related deformation cannot produce the deformation recorded within the rock record. Here, we provide an explanation for this discrepancy by showing that increases in strain facilitated by plastic deformation of Earth’s crust during the earthquake cycle, in conjunction with isostatic deflection and erosion, transform relative fault motions that produce individual earthquakes to geologic deformations. We focus our study on the data-rich Santa Cruz Mountains, CA, USA, and compare predicted and observed quantities for rock uplift, apatite (U-Th)/He thermochronology, topographic relief, 10Be-based erosion rates, and interseismic surface velocities. This approach reconciles these disparate records of mountain-building processes, allowing us to explicitly bridge decadal measures of deformation with that produced by millions of years of plate motion.