Earthquake Science Center Seminars

Brendan Meade, Harvard University

Computational earthquake sequence models provide generative estimates of the time, location, and size of synthetic seismic events that can be compared with observed earthquake histories and assessed as rupture forecasts. Here we describe a three-dimensional probabilistic earthquake sequence model that produces slip event time series constrained across geometrically complex non-planar fault systems. This model is kinematic in nature, independent of rheology, and without explicit treatment of stress. Instead, the model's foundation is based on integrating geometric moment accumulation and release with empirical earthquake scaling laws. We use this model to generate synthetic earthquake sequences and show an extension to afterslip where post- and co-seismic slip may be spatially both separate and collocated.

Daniel Trugman, University of Nevada, Reno

The level of ground motion felt during an earthquake is controlled primarily by the size of the event and the distance from the source. However, it has long been understood that variations in rupture properties like stress drop and rupture directivity can cause earthquakes of equal size to generate significant differences in ground motion. This seminar will focus in detail on the relation between the rupture process and ground motions through case studies of M5-M6 earthquakes in California and Nevada. In California, we use a Bayesian spectral inference technique to reveal regional variations in stress drop and directivity among recent M5 earthquakes. We then focus in detail on a large normal faulting earthquake near the California-Nevada border, where precariously balanced rocks provide a unique perspective on near-fault ground motions. Taken together, these studies provide new insights into rupture and ground motion that inform both fundamental earthquake science and more applied hazard calculations.

Kenneth Hudson, UCLA

Soil liquefaction and resulting ground failure due to earthquakes presents a significant hazard to distributed infrastructure systems and structures around the world. Currently there is no consensus in liquefaction susceptibility or triggering models. The disagreements between models is a result of incomplete datasets and parameter spaces for model development. The Next Generation Liquefaction (NGL) Project was created to provide a database for advancing liquefaction research and to develop models for the prediction of liquefaction and its effects, derived in part from that database in a transparent and peer-reviewed manner, that provide end users with a consensus approach to assess liquefaction potential within a probabilistic framework. An online relational database was created for organizing and storing case histories and laboratory data which is available at http://nextgenerationliquefaction.org. Additionally, a supported modeling team was formed and has been working to define new liquefaction triggering and manifestation models. The models are approaching triggering and manifestation prediction using new methodologies to improve upon existing models.

Gary Fuis, USGS

The 1989 M 6.9 Loma Prieta earthquake is the first major event to occur along the San Andreas fault (SAF) zone in central California since the 1906 M 7.9 San Francisco earthquake. Given the complexity of this event, uncertainty has persisted as to whether this earthquake ruptured the SAF itself or a secondary fault. Recent work on the SAF in the Coachella Valley, in southern California, has revealed similar complexity, arising from a non-planar, non-vertical fault geometry, and has led us to re-examine the Loma Prieta event. We have compiled data sets and data analyses in the vicinity of the Loma Prieta earthquake including the 3-D seismic velocity model and aftershock relocations of Lin and Thurber (2012), potential-field data collected by the U.S. Geological Survey (USGS) following the earthquake, and seismic refraction and reflection data from the 1991 profile of Catchings et al. (2004). The velocity model and aftershock relocations of Lin and Thurber (2012) reveal a geometry for the SAF that appears similar to that in the Coachella Valley (although rotated 180 °): at Loma Prieta the fault dips steeply near the surface and curves with depth to join the moderately southwest-dipping main rupture below 6-km depth, itself also non-planar. The SAF is a clear velocity boundary, with higher velocities on the northeast, attributable to Mesozoic accretionary and other rocks, and lower velocities on the southwest, attributable to Cenozoic sedimentary and volcanic rocks of the La Honda block. Rocks of the La Honda block have been offset right-laterally hundreds of kilometers from similar rocks in the southern San Joaquin Valley and vicinity, providing evidence that the curved northeast fault boundary of this block is the plate boundary. Thus, we interpret that the Loma Prieta earthquake occurred on the SAF and not on a secondary fault.

Mustafa Erdik, Bogazici University

On Feb. 6, 2023, a magnitude Mw7.8 earthquake struck near Pazarcık City in south-central Türkiye, followed by a magnitude Mw 7.5 event about 9 hours later. The presentation will cover the explanation of what happened, including the fault rupture, strong ground motion, building codes, and damages. The reasons behind extensive damages, associated casualties, and the ways of reducing them will also be discussed.

Ayşegül Askan, Middle East Technical University

On Feburary 6, 2023, two earthquakes with M7.8 and M7.5 occurred in Southeastern Türkiye only hours apart and caused significant damage and fatalities. The effects of these events were very widespread with complex source and site effects as well as major geotechnical and structural damages. The mainshocks and their aftershock sequences were recorded in a large region by the national strong motion network of Türkiye, operated by AFAD. This presentation will provide a general initial assessment of the earthquakes, focusing mainly on the strong ground motions and examples of observed damages. Initially, the regional tectonics, past seismic activity in the area and characteristics of the recorded strong motions of the will be highlighted. Next, comparisons of observed data against the most recent design spectra and selected ground motion models will be presented. Field observations of common geotechnical and structural damages will be briefly demonstrated. Finally, implications for future hazard assessments, possible building code updates and risk mitigation practices will be discussed.

Susan Hough, USGS

At estimated magnitude 6.4, the Long Beach earthquake was only moderately large. Whereas the 1906 San Francisco earthquake had a profound impact on earthquake science, leading to the development of elastic rebound theory, the Long Beach earthquake arguably had a larger impact on earthquake risk reduction. The earthquake occurred at a pivotal time, after the start of earthquake monitoring in southern California, but at a time when the severity of earthquake hazard in southern California was still debated. Moreover, at the time, earthquake professionals did not appreciate the potential severity of hazard from moderately large earthquakes. The earthquake that rocked southern California 90 years ago this month took a grim toll on lives and property, including 120 deaths. But the earthquake also knocked the region out of its complacency. By virtue of its timing, early in the emergence of the Los Angeles region as a preeminent population center, the 1933 earthquake had an outsized impact on earthquake risk reduction. The earthquake set California on a path towards improved resilience that continues to this day. In this talk I present an overview of the historical context of the event, its impact on science and risk reduction, and modern scientific results that shed new light on the earthquake and distribution of ground motions.

Paula Burgi

Remote sensing observations of the 2019 Ridgecrest, California, earthquake sequence revealed a significant amount of surface ejecta in the nearby Searles Lake, including one area where the surface ejecta was arranged in a repeating hexagonal “honeycomb” pattern. This pattern is collocated with injection wells from a solution mining operation, suggesting anthropogenic activities influenced the spatial distribution of surface ejecta. Lithology, geotechnical soil behavior, and the spatial distribution of long-term InSAR-derived subsidence indicate that surface ejecta in Searles Lake is not likely related to liquefaction. We propose a process, similar to liquefaction, that results in surface ejecta: (1) dissolution of evaporites increases the void/cavity space that is filled with fluid, (2) ground shaking causes void/cavity collapse (i.e., a volume reduction), (3) the collapse increases the fluid pressure, and (4) the increased pressure results in fluid flow to the surface.

Haeyong Noh, Stanford

With increasing urbanization, civil infrastructure plays key roles in connecting, protecting, and supporting this growing population in cities. However, the large stock of aging infrastructure poses significant challenges to the economy and public safety. Smart structures are designed to sense, understand, and respond to the structural state and the human needs, but traditional monitoring approaches using dedicated sensors often result in dense sensing systems that are difficult to install and maintain in large-scale structures. This talk introduces non-dedicated sensing approaches that utilizes vehicles and pre-existing telecommunication cables as sensors to indirectly infer infrastructure health, such as bridges and railways. Challenges lie, however, in creating robust inference models for analyzing noisy vibration response data. To this end, we created physics-guided data analytics approaches that combine statistical signal processing and machine learning with physical principles. We evaluate our system with field experiments through the collaboration with the City of San Jose.

David Schwartz, Scientist Emeritus, USGS

The Mw 7.9 Denali fault, Alaska earthquake of Nov. 3, 2002 was one of only nine earthquakes in shallow continental crust, worldwide, larger than magnitude 7.8 since 1900. The complex faulting, which involved the Susitna Glacier thrust fault, the principal Denali strike-slip fault, and the Totschunda branch of the Denali fault occurred along 351 km of the 1200 km-long Denali fault system. Right-lateral surface offsets of 4.5–5.1 m, with a maximum offset of 8.8 m, were measured. The rupture traversed the Trans-Alaska pipeline, which had been successfully designed to accommodate Denali slip. The three-fault rupture complexity has given impetus to the inclusion of multi-fault ruptures in seismic hazard analysis. The seminar will provide an overview of the coseismic faulting and the post-seismic geologic investigations, including implications of the 2002 event for the behavior of long strike-slip faults.