SCEC Award Number 18218 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Surface displacement and ground motion from dynamic rupture models of thrust faults with variable dip angles and burial depths
Name Organization
Julian Lozos California State University, Northridge
Other Participants
SCEC Priorities 4a, 4b, 4c SCEC Groups GM, FARM, Seismology
Report Due Date 04/30/2020 Date Report Submitted 11/10/2020
Project Abstract
Thrust fault earthquakes are particularly hazardous in that they produce stronger ground motion than normal or strike-slip events of the same magnitude due to a combination of hanging wall effects, vertical asymmetry, and higher stress drop due to compression. Additionally, vertical surface displacement occurs in both blind and emergent thrust ruptures, and can potentially damage lifelines and infrastructure.
Our 3D dynamic rupture modeling parameter study focuses on planar thrust faults of varying dip angles and burial depth, in order to establish a physics-based understanding of how ground motion and permanent ground surface displacement depend on these geometrical parameters. We vary dip angles from 20º to 70º, and burial depths from 0 km to 5 km. We conduct rupture models on these geometries embedded in a homogeneous half space using different stress drops but fixed frictional parameters, and with homogeneous initial stresses versus stresses tapered toward the ground surface.
Ground motions decrease as we bury the fault under homogeneous initial stresses. In contrast, under tapered initial stresses, ground motions increase in blind thrust faults as we bury the fault, but are still highest in emergent faults. As we steepen dip angle, peak particle velocities in the homogeneous stress case generally increase in emergent faults, but decrease in blind thrust faults. Meanwhile, ground motion consistently increases with steepening dip angle under the stress gradient.
Due to the simple geometry of a planar fault, our results can be applied to understanding basic behavior of specific real-world thrust faults.
Intellectual Merit This project addresses questions of how fault geometry influences not only rupture behavior, but also resulting hazards. While there have been many geometrical parameter studies for strike-slip faults, fewer exist for dip-slip faults, and this helps to fill in that gap. This is particularly relevant to SCEC given the many thrust faults of different geometries in the Transverse Ranges and directly under Los Angeles. While this study focuses on very simplified faults, it may still be useful for improving understanding of specific individual faults in the absence of a site-specific study, and it can certainly provide a basis or motivation for developing future site-specific modeling studies or ground motion/displacement assessments on blind and emergent thrust faults alike.

In addition to helping fill a gap in the modeling literature, this work can also hopefully help fill in the gaps in observations of ground motion and permanent surface displacement from thrust fault ruptures. By showing systematic relationships between fault burial depth, dip angle, and initial stress conditions, this can allow for a first-order physics-based assessment of possible shaking and displacement from real-world faults, even in the absence of a site-specific study for that fault. These results could be useful in improving the range of fault geometries and distances-from-source in ground motion prediction equations. They may also be helpful in the development of surface displacement prediction equations.
Broader Impacts From an education/teaching standpoint:
This work constitutes CSUN student Sirena Ulloa's Masters research. We designed the project and wrote the grant together, then she conducted all of the modeling and post-processing. She also wrote the entire first draft of the manuscript. This project was central in Sirena's learning how to conduct research and analyze results, as well as for her grant and paper writing skills. All of these skills will serve her well in searching for and working in a job in hazard assessment (her current desired career path).
It is also worth noting that Sirena is a Black woman and the first person in her family to attend graduate school. This project directly represents the excellent hard work of someone from an underrepresented group, and she will continue to bring her skillset and expertise from this project into whatever job she gets.

From a societal standpoint:
By providing some physics-based tools for estimating ground motions and permanent surface displacement from a wide range of possible thrust fault ruptures, this work can potentially influence ground motion prediction equations and ground displacement prediction equations, which can, in turn influence seismic hazard maps. This can allow for a better sense of ground motion and displacement hazards around known thrust faults without much recorded activity, which can help planners and people living near such faults to make better plans and strategies for protecting lives and infrastructure near those faults.
Exemplary Figure Figure 1.
Peak vertical particle velocity (top), peak horizontal particle velocity (second from top), maximum permanent uplift (second from bottom), and maximum permanent subsidence (bottom) for the homogeneous stress case (left column) and the tapered stress case (right column). Note that the relationship between which dip angle and depth has the highest value for each parameter switches between the two stress cases. Also note that emergent ruptures have significantly higher values for each parameter than emergent ruptures, regardless of initial stress case.

Credits: Sirena Ulloa and Julian Lozos