SCEC Award Number 22013 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Numerical Modeling of Ground Surface Deformation Related to Large Thrust and Reverse fault earthquakes in California
Name Organization
John Shaw Harvard University Robb Moss California Polytechnic State University, San Luis Obispo
Other Participants Kristen Chiama
SCEC Priorities 3e, 3g SCEC Groups Seismology, Geology, EEII
Report Due Date 03/15/2023 Date Report Submitted 03/14/2023
Project Abstract
This project seeks to develop a mechanical basis for our understanding of permanent ground surface deformation that occurs during large thrust and reverse fault earthquakes, with a particular emphasis on past and future events in southern California. Our effort builds on SCEC’s extensive experience in identifying and characterizing active thrust and reverse faults and their ground surface deformation signatures (e.g., Shaw and Suppe, 1994; Ruben et al., 1988; Shaw and Shearer, 1999; Mueller et al., 1999; Oskin et al., 2000; Shaw et al., 2002; Dolan et al., 2003; Hubbard et al., 2014; Sorlien et al., 2015; Rockwell et al., 2016; Marshall et al., 2017; Levy et al., 2019). Based on this foundation, we developed a new application of geomechanical modeling tools (Distinct Element Method; DEM) to simulate ground surface rupture. DEM is well suited to reproduce the granular mechanics of near surface sediment and soil deformation. Our work contributes towards a physics-based understanding of ground deformation phenomena that can help to assess and mitigate these hazards. In addition, we focused on developing numerical simulations that describe patterns of deformation expected for future ruptures on large thrust and reverse faults in California. In this report, we present results focused on the Cucamonga fault, based on trench excavations presented by Dolan et al., (1996 and unpublished).
Intellectual Merit Our study improves understanding of the physical processes that control the style, distribution, and intensity of ground surface ruptures on thrust and reverse faults during large earthquakes. We explore how parameters related to fault geometry and sediment properties control ground deformation characteristics such as scarp dip, width, and patterns of secondary folding and fracturing. Our results show that localized fault scarps are most prominent in cases with stiff sediment strengths and steeply dipping faults, whereas broader deformation is more prominent with softer sediment strengths and shallowly dipping faults. Based on insights from dozens of experiments, we propose a fault scarp classification system that describes the general patterns of surface deformation observed in natural settings and reproduced in our models, including: monoclinal, pressure ridge, and simple scarps. We note that each fault scarp type is modified by hanging wall collapse. These results can help to guide both deterministic and probabilistic assessment in Fault Displacement Hazard Analysis (FDHA), an area of focus for SCEC.
Broader Impacts The displacement magnitude, width, and degree of tilting or warping of the ground surface associated with fault ruptures have an important impact on the ability of built structures to withstand large earthquakes. Our numerical models aim to provide a quantitative basis to relate observed ground deformational features with fault displacements at depth, which can be linked to earthquake magnitudes. These relationships can help to forecast patterns of ground surface deformation associated with future large magnitude thrust and reverse fault earthquakes. Specifically, they can help inform PFDHA analyses, which are based on limited data for this class of fault. Ultimately, the goal of our research is to help better forecast ground rupture hazards in ways that can help protect life and property in future earthquakes in southern California.
Exemplary Figure Figure 1: Classification system for fault scarp types (1: Monoclinal Scarp, 2: Pressure Ridge; and 3: Simple Scarp) with a representative DEM model of the scarp morphology reproduced from Chiama et al., (in review). Plots show fault scarp profiles from the 45 DEM experiments organized by scarp type and colored by sediment strength mechanics (0.1 MPa dark blue; 0.5 MPa light blue; 1.0 MPa grey; 1.5 MPa orange; 2.0 MPa dark red). The representative example DEM model profiles are identified as black lines in the plots. (1a) DEM model with sediment strength of 0.1 MPa on a fault dipping 40º. Field photo from the 2008 Wenchuan, China earthquake (reproduced from Fu et al., 2011). (2a) DEM model with sediment strength of 0.5 MPa on a fault dipping 20º. Field photo from the 1999 Chi-Chi, Taiwan earthquake (reproduced from Chen et al., 2001). (3a) DEM model with sediment strength of 2.0 MPa on a fault dipping 60º. Field photo from the 2008 Wenchuan, China earthquake depicting the fault plane and striations (reproduced from Li et al., 2010). (1b) DEM model with sediment strength of 2.0 MPa on a fault dipping 40º. Field photo from the 1988 Armenian earthquake courtesy of Dr. Arkady Karakhanyan (reproduced from the Institute of Geological Sciences, Republic of Armenia ca. 1988). (2b) DEM model with sediment strength of 2.0 MPa on a fault dipping 20º. Field photo from the 1999 Chi-Chi, Taiwan earthquake (reproduced from Lee et al., 2001). (3b) DEM model with sediment strength of 1.0 MPa on a fault dipping 60º, and field photo from the 2016 Kaikoura earthquake in New Zealand with along-strike variation in the fault rupture (photography by Kate Pedley, reproduced from Nicol et al., 2018).