Earthquake Science Center Seminars

Haiyang Kehoe, USGS

Seismograms contain information of an earthquake source, its path through the earth, and the local geologic conditions near a recording site. Ground shaking felt on Earth’s surface is modified by each of these contributions–the spatiotemporal evolution of rupture, three-dimensional subsurface structure, and site conditions all have a substantial impact on hazards experienced by exposed populations. In this talk, I highlight three studies that have improved our understanding of ground motion variability arising from source, path, and site effects. First, I describe the rupture process of the 2017 Mw 7.7 Komandorsky Islands earthquake, which reached supershear speeds following a rupture jump across a fault stepover, and demonstrate the enhanced hazard associated with supershear ruptures across Earth’s complex transform fault boundaries. Second, I compare high-frequency wavefield simulations of Cascadia earthquakes using various tomography models of the Puget Sound region, Washington State to highlight the role of basin structure on ground motion amplification. Third, I show horizontal-to-vertical spectral ratio maps of the continental United States and emphasize the continued importance of region-specific constraints on site characterization. While each study demonstrates progress towards understanding the individual roles of source, path, and site effects on damaging earthquake ground motions, together they underscore distinct challenges for improving seismic hazard models and their uncertainties.

Tara Nye, USGS

Models of earthquake ground motion (both simulations and ground-motion models) can be likened to a puzzle with three primary pieces representing the earthquake source, site conditions, and source-to-site path. Early versions of these models were developed using average behavior of earthquakes across a variety of regions and tectonic environments. Although informative, such models do not capture the unique source, path, and site effects that are expected to have a significant influence on resulting ground motion. This talk highlights several approaches for improving modeling of ground motion by focusing efforts on the different pieces of the ground-motion puzzle. Segments of the talk include (1) constraining rupture parameters of rare tsunami earthquakes, (2) estimating site-specific high-frequency attenuation in the San Francisco Bay Area, and (3) investigating relationships between path effects and crustal properties in the San Francisco Bay Area. With continued refinement to models of ground motion, we can improve confidence and reduce uncertainty in seismic hazard and risk assessments.

Rashid Shams, University of Southern California

Site response in sedimentary basins is partially governed by mechanisms associated with three-dimensional features. This includes the generation of propagating surface waves due to trapped and refracted seismic waves, focusing of seismic energy due to basin shape and size, and resonance of the entire basin sediment structure. These mechanisms are referred to as basin effects and they lead to a significant increase in the amplitude and duration of the observed ground motions from earthquake events. Currently, ground motion models (GMMs) incorporate basin effects using the time-averaged shear-wave velocity in the upper 30 m (V_S30), and the isosurface depths (depth to a particular shear wave velocity horizon, z_x). This approach captures site response features associated with the basin but uses parameters that are one-dimensional in nature and therefore are limited in their description of the lateral and other three-dimensional (3D) contributing effects. This work explores geometric features as predictive parameters in the development of region-specific models to improve the characterization of site response in sedimentary basins. In this work we constrained basin shape using depth to sedimentary basement (depth to a particular shear wave velocity horizon i.e., z_1.5 and z_2.3) and depth to crystalline basement (z_c,b) which are derived and validated using systematic exploration of geological cross sections and Community Velocity Model (CVM) profiles over Los Angeles Basin (LAB). Finally geometric parameters such as includes Standard deviation of zcb, Standard deviation of Absolute difference between z_1.5 and z_cb, distance from basin margin, and Spatial Area of Influence based on V_S30 are computed based on finalized shape. Residual analysis is employed to access derived geometric parameters for their ability to reduce bias and uncertainty in basin site response analysis.

Amy Williamson, University of California Berkeley

Alerts sent through earthquake early warning (EEW) programs provide precious seconds for those alerted to take simple protective actions to mitigate their seismic risk. Programs like ShakeAlert have been providing alerts for felt earthquakes across the west coast of the US for almost 5 years. Earthquakes are also one part of a multihazard system and can trigger secondary natural hazards such as tsunamis and landslides. However in order to be effective and timely, EEW and tsunami forecast algorithms must rely on the smallest amount of data available, often with variable quality and without analyst input. This talk focuses on potential advances to EEW algorithms to better constrain earthquake location and magnitude in real time, providing improved alerts, particularly in network sparse regions. Additionally, this talk highlights work using real time data to generate rapid tsunami early warning forecasts, its feasibility, and the benefit of unifying earthquake and tsunami alerts into one cohesive public-facing alerting structure.

James Biemiller, USGS

An unresolved aspect of tsunami generation in great subduction earthquakes is the offshore competition between coseismic deformation mechanisms, such as shallow megathrust slip, slip on one or more splay faults, and off-fault plastic deformation. In this presentation, we first review results from data-constrained 3D dynamic rupture modeling of an active plate-boundary-scale low-angle normal fault, the Mai’iu fault, that show how stress, fault structure, and the strength and thickness of overlying sediments influence shallow coseismic deformation partitioning in an extensional setting. Similar modeling approaches can shed light on shallow coseismic deformation in contractional settings, such as the Cascadia subduction zone (CSZ). Along the northwestern margin of the U.S., robust paleoseismic proxies record multiple M>8 paleoearthquakes over the Holocene, despite limited modern interface seismicity. Additionally, growth strata in the outer wedge record Late Quaternary slip on active landward- and seaward-vergent splay faults inboard of prominent variably-vergent frontal thrusts at the deformation front. The relative importance of megathrust vs. splay fault slip in generating tsunami hazards along the Pacific Northwest coastline is relatively unconstrained. Here, we develop data-driven 3D dynamic rupture models of the CSZ to analyze structural controls on shallow rupture processes including slip partitioning across the frontal thrusts, splays, and underlying decollement. Initial simulations show that trench-approaching ruptures typically involve meter-scale slip on variably oriented preexisting planar splay faults. Splay slip reduces slip on the subduction interface in a shadowed zone updip of their intersection, with greater splay slip leading to stronger shadowing. We discuss two structural controls on splay faults’ coseismic slip tendency: their dip angle and vergence. Gently dipping splays host more slip than steeply dipping ones and seaward-vergent splays host more slip than landward-vergent ones. We attribute these effects to distinct static and dynamic mechanisms, respectively. Finally, we show initial results from simulations with newly mapped frontal thrust geometries from CASIE21 seismic reflection data and discuss future directions for our CSZ dynamic rupture modeling project.

Jaeseok Lee, Brown University

Field observations indicate that fault systems are structurally complex, yet fault slip behavior has predominantly been attributed to local fault plane properties, such as friction parameters and roughness. Although relatively unexplored, emerging observations highlight the importance of fault system geometry in the mechanics governing earthquake rupture processes. In this talk, I will discuss how the geometrical complexities of fault networks impact various aspects of fault slip behavior, based on the analysis of surface fault trace misalignment. We discover that surface fault traces in creeping regions tend to be simpler, whereas those in locked regions are more complex. Additionally, we find correlations between complex fault geometry and enhanced high-frequency seismic radiation. Our findings suggest the potential for a new framework in which earthquake rupture behavior is influenced by a combination of geometric factors and rheological yielding properties.

Thomas Lee, Harvard University

Since the first seismograms were recorded in the late 19th century, the seismological community has accumulated millions of ground motion records on both paper and film. While almost all analog seismic recording ended by the late 20th century, replaced by digital media, the still-extant archives of paper and film seismograms are invaluable for many ongoing scientific applications. This long-running record of ground motion is crucial for developing understanding of how both natural and anthropogenic events have changed the Earth and its processes throughout the last century.

Today, most of these records are housed in institutions with limited resources, which must prioritize certain objects or types of objects for preservation and access. For example, when seismologists today are forced to triage collections, the bulky paper-records are oftentimes more at-risk for deaccessioning than more compact film copies. However, alterations introduced in reformatting (i.e., paper to film) as well as preservation requirements of the various records are not often fully understood or appreciated. To make these decisions in an informed way, it is vital to know the stability of the recording media and the level of accuracy that can be obtained from these different records. For example, image distortion and available color depth in paper and microfilm copies can result in discrepancies in derived time series which could lead to significant errors in products such as earthquake magnitude and location.

We present lessons learned from recent experiences with modern archiving and processing of legacy seismic data. These include techniques for data rescue (including both scanning and conversion to time series), the importance of characterizing the full processing chain, and the importance of involving archivists and citizen science in preservation efforts.

Ross Maguire, University of Illinois Urbana-Champaign

Seismic source parameters – including hypocentral locations and focal mechanism solutions – provide the most direct constraints for understanding tectonic stresses and deformation processes within planetary interiors. The SEIS (Seismic Experiment for Interior Structure) seismometer deployed by the InSight mission to Mars detected and located approximately 40 high-quality marsquakes. However, inferences about the present-day deformation and seismotectonics of Mars have been hindered by the non-uniqueness and technical challenges that arise when using seismograms recorded by just a single seismometer. In this talk, I will review what we have learned about seismic activity on Mars from InSight and discuss how waveform-based inversions of data from a single station have helped us gain a clearer understanding of martian tectonics. Several high-quality marsquakes from the Cerberus Fossae region appear to be consistent with an active extensional tectonic setting, while the largest marsquake observed by InSight – S1222a – was likely due to compressional stresses near the hemispheric dichotomy boundary. Ongoing work is aimed at gaining a better understanding of the uncertainties involved in single station moment tensor inversions and developing best practices for obtaining robust solutions.

Roland Burgmann, University of California Berkeley

Decadal changes in aseismic fault slip rate on partially coupled faults reflect long-term changes in fault loading and/or fault-frictional properties that can be related to earthquake cycle processes. We consider constraints on aseismic fault slip rates from historical alignment array measurements, InSAR measurements since 1992, and repeating micro-earthquakes since 1984 along the Hayward fault, California. During recent decades, creep rates consistently increased along the whole Hayward fault. Accelerated fault creep associated with M > 4 earthquakes on the northern Hayward fault in 2007, 2010 and 2018 may explain some of the creep-rate accelerations, but the acceleration on the remaining Hayward fault does not seem to be directly tied to small-scale afterslip transients. Dynamic models of partially coupled faults through earthquake cycles suggest non-stationary asperities that continue to decrease in size late in the earthquake cycle. We explore such asperity erosion models to explain the apparent decadal acceleration of aseismic Hayward fault slip.

Savvas Marcou, University of California Berkeley

MyShake is a free smartphone application, developed at UC Berkeley, that serves as one of the main delivery mechanisms for earthquake early warning (EEW) alerts issued to the US West Coast by the USGS ShakeAlert system. While it is most well-known for delivering alerts to the public, MyShake was originally conceived as a platform for crowdsourcing earthquake data. MyShake currently collects crowdsourced shaking experience reports, EEW message delivery receipts, as well as triggered acceleration waveforms using the onboard smartphone accelerometer. In this talk, I will present the progress made in taking advantage of the growing MyShake crowdsourced database, in two key areas: 1) Studies of ground motion variability in California, 2) Earthquake early warning performance assessment and predictive modeling.

In the first part of the talk, I will introduce the MyShake Ground Motion Database, a database of over 1500 acceleration observations collected from 2019 to 2023 using triggered waveforms from MyShake phones globally, with a focus on California. Past research shows that the accelerations recorded by MyShake phones are systematically higher than accelerations recorded by ground-based (i.e. “free-field”) stations, likely due to the modifying effect of buildings. My work thus treats these accelerations as a distinct intensity metric, rather than equivalent to free-field acceleration. I show the development of a bespoke MyShake Ground Motion Model, and how it can be used to map the spatial variability of ground motion with remarkable correspondence to results from the free-field. I will then discuss the potential applications in ground motion products such as ShakeMap, as well as the validation of next-generation non-ergodic (i.e., location specific) ground motion models.

In the second part of the talk, I will focus on MyShake’s role as an EEW delivery platform. I will present a new methodology that uses alert delivery data to rapidly assess the end-to-end performance of the US West Coast alerting pipeline every time MyShake sends out an alert. I then introduce a thought experiment where we combine our understanding of delivery latencies with the network-based, point source EEW algorithm EPIC to demonstrate the potential effectiveness of EEW in the February 2023 Turkiye earthquake doublet. Finally, I will show what the results could mean for EEW performance in a large California earthquake.