Earthquake Science Center Seminars

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.

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.

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.

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.

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.