SCEC Award Number 18074 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Improved 3-D Vp models near the southern San Andreas Fault from SSIP explosive shots and local earthquakes
Investigator(s)
Name Organization
Patricia Persaud Louisiana State University
Other Participants Rasheed Ajala (PhD student)
SCEC Priorities 3a, 4a, 2b SCEC Groups SAFS, Seismology, CXM
Report Due Date 03/15/2019 Date Report Submitted 10/25/2018
Project Abstract
 The Coachella Valley is a high seismic hazard area. Knowledge of the seismic velocity structure, basin geometry and fault zones is required to improve earthquake hazard estimates in this region. We carried out a joint inversion of first P-wave travel times from the SCSN (39,998 local earthquakes) and 251 explosive shots from the 2011 Salton Seismic Imaging Project to produce a detailed 3-D VP model. Velocity contrasts in the top ~3 km correlate with the surface geology, including the low-velocity (<5 km/s) sedimentary basin. Peninsular Ranges rocks have higher VP than Eastern Transverse Ranges rocks. Sediment thickness is ~4 km near the Salton Sea and decreases to <2 km at the northwestern end of the valley. Eastward thickening of sediments towards the San Andreas fault within the valley reflects the basin asymmetry. In the Peninsular Ranges, zones of relatively high VP (~6.4 km/s) between 2 to 4 km depth probably represent Late Cretaceous mylonitic rocks. We confirm a NE-dipping San Andreas fault and identify the new Lost Horse Valley fault zone.
Our 3-D model and basement surface have been contributed to the SCEC-CVM and provide much needed data constraints and quantification on crustal heterogeneities at different spatial scales that will help to determine the relative roles of fault geometry and crustal properties in controlling ground motions. Our model will allow numerical simulations of ground motions to be adapted for local geologic conditions including meaningful basin shapes and structural domains, and also provides improved shallow-crustal properties important for constraining nonlinear effects.
Intellectual Merit The basement beneath the Coachella Valley is complex and is overlain by ~4 km thick sediments near Mecca Hills at the northern end of the Salton Sea (Figure 3; Ajala et al., 2018). Sediment thickness decreases to a maximum of ~2 km at the northern end of the valley. Basin geometry is asymmetric with more sediment accumulation on the eastern side against the San Andreas fault, consistent with previous gravity and 2-D seismic refraction studies. Crystalline basement structure in the Coachella Valley region is highly heterogeneous and is characterized by high velocity regions that may represent Peninsular Ranges mylonitic rocks and the Eastern Transverse Ranges Orocopia Schists (Ajala et al., 2018). Seismic velocity contrasts show a northeast dipping San Andreas fault down to depths of at least 9 km, consistent with gravity and fault imaging studies at shallower depths (Dorsey & Langenheim, 2015; Fuis et al., 2017). Our velocity model also shows an apparent offset across the sinistral Blue Cut fault, and evidence for a complex San Andreas fault zone structure beneath the northeast side of the Coachella Valley basin. Interpretation of seismicity lineaments, lateral velocity contrasts, and apparent geologic offsets, reveal a previously unnamed fault zone in the Little San Bernardino Mountains striking northeast from the Eureka Peak fault, which we refer to as the Lost Horse Valley fault zone (Ajala et al., 2018).
Our model has practical significance for improving earthquake hazard studies by providing a more accurate seismic velocity model for the northern Salton Trough. The accuracy of ground motion estimates strongly depends on the seismic velocity structure, especially the basin structure which is key in determining shaking intensity (Lee et al., 2014). The Coachella Valley basin asymmetry and the irregular basement structure defined in our velocity model will result in different ground shaking estimates than for a symmetric basin and regular basement structure in current regional community velocity models used in seismic hazard analysis for Southern California. Crustal heterogeneities characterized in our model can be used to improve modeling of the rheological properties of the crust and their interaction with the main faults in the area. Finally, our velocity model presents a good starting point for further research on the anisotropic properties of the mylonites within the Peninsular Ranges batholith.
Broader Impacts This award supported a PhD student and the research program of an early-career tenure-track faculty. A manuscript was submitted to JGR - Solid Earth with revisions submitted in October, 2018. Project results were presented at the 2018 SCEC Annual Meeting, the 2018 IRIS Workshop and the 2018 Student Seismology Workshop held at Columbia University.
We have made the 3-D velocity model as well as the derived basement and estimated Z2.5 surfaces important for seismic hazard assessment available for download at our LSU research webpage (https://www.geol.lsu.edu/persaud/Data.html) and have provided these files to the organizers of the SCEC CVM for incorporation into the Community Modeling Environment.
Our model reveals the detailed 3-D basin geometry and strong crustal heterogeneities that can be used to improve earthquake ground shaking estimates of seismic hazard. Sedimentary basins focus and amplify seismic waves, and thus generate larger amplitude motions that persist for longer times. Therefore, our estimated basement surface or Z2.5 surface (available at our web page) has practical applications for structural engineers interested in incorporating basin amplification terms in earthquake-resistant constructions. The Z1.0 surface can be estimated from the Z2.5 surface. Crustal heterogeneities in the model can also help improve our understanding of fault rupture processes and can provide better accuracy in ground motion predictions.
Exemplary Figure Figure 3. Perspective view of the our estimated basement depths using the 4.50 km/s isovelocity contour from our 3-D velocity model. The view is looking at the surface from N42˚W towards the southeast at 35˚ elevation. Black lines are surface traces of mapped faults in the area from Jennings and Bryant (2010) that are plotted at their surface elevation. Contour interval is 1 km. The Coachella Valley basin asymmetry is evident, and the mountain ranges and basement geometry align with major faults. BCFZ - Blue Cut fault zone; BF - Banning fault; GHF - Garnet Hill fault; MCF - Mission Creek fault; PMFZ - Pinto Mountain fault zone; SAFZ - San Andreas fault zone; SJFZ - San Jacinto fault zone. Source: Ajala et al., (2018).