Improved 3-D VP models near the southern San Andreas Fault … · 2018-10-26 · !1- 2017 SCEC Proposal FINAL REPORT Improved 3-D V P models near the southern San Andreas Fault from

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2017 SCEC Proposal FINAL REPORT

Improved 3-D VP models near the southern San Andreas Fault from SSIP explosive shots and local earthquakes

SCEC Award No. 18074

PI: Patricia Persaud Department of Geology and Geophysics

Louisiana State University E235 Howe-Russell

Baton Rouge, LA 70803 [email protected]

Proposal Categories: A: Data Gathering and Products

SCEC Research Priorities: P3.a, P4.a, P2.b, 2.c

Interdisciplinary Working Groups: San Andreas Fault System (SAFS) SCEC Community Models (CXM)

Proposal period: February 1, 2018 – January 31, 2019

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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.

Figure 1. Location map of the Salton Trough and the Salton Seismic Imaging Project (SSIP) shots (red stars), scaled by their explosives weight in kilograms, and receivers (blue circles). Black lines are surface traces of mapped faults in the area from Fenby and Gastil (1991), Jennings and Bryant (2010) and Rockwell et al. (2015). Pink box shows our study area in the Coachella Valley in Figure 2. Black arrows in the inset map show the relative motion of the Pacific and North American plates and the red line represents the plate boundary. Source: Ajala et al., (2018).

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TOMORAPHIC INVERSION Our preferred starting

velocity model for the tomographic inversion is the 1-D average of the 3-D velocity model from Lin (2013). A finite difference scheme for the eikonal equation (Hole & Zelt, 1995; Vidale, 1990) is first used to compute the travel times in the starting velocity model. We then iteratively invert the travel time data for velocity model perturbations using the nonlinear travel time tomography algorithm from Hole (1992) and Hole et al. (2000), which allows for hypocenter inversion.

The inversion strategy is based on the minimum structure approach (Hole et al., 2006; Hole et al., 2000). Our velocity model was gridded at 0.5 km fixed spacing within a 214 by 198 km area to ensure that the model velocities are accurately located and that boundaries in the model are correctly delineated. Depths are given with negative values above sea level and range from -5 km to 16 km. Topography is included in the velocity model so that station time corrections are not used. The smoothing dimensions of the model were changed after every three tomographic iterations. Large smoothing operators of 300 by 300 by 30 grid cells were used in the early iteration to model the large-scale features. The smoothing size was reduced to 10 by 10 by 2 grid cells in the later iterations to reveal the small-scale structures, thus ensuring a slow, stable and global minimum convergence. In local earthquake tomography, it is generally

considered incorrect to invert for model parameters without hypocenter relocations (Hole et al., 2000; Thurber, 1992), because the original catalog hypocenter locations are deemed inaccurate.

Figure 2. Locations of the data sets used in our 3-D velocity models for the Coachella Valley. Purple dots represent earthquakes from 1981 to 2011 in the Hauksson et al. (2012) catalog, selected to a maximum depth of 10 km. Light blue triangles are the Southern California Seismic Network (SCSN) stations. Black lines are surface traces of mapped faults in the area from Jennings and Bryant (2010). Yellow lines are gravity-derived basement contours from Dorsey and Langenheim (2015) and Langenheim et al. (2005). BCFZ - Blue Cut fault zone; BF - Banning fault; BRF – Buck Ridge fault; CCF - Coyote Creek fault; CF - Clark fault; DH - Durmid Hill; EPF - Eureka Peak fault; EWCF - East Wide Canyon fault; GHF - Garnet Hill fault; IH - Indio Hills; IHF - Indio Hills fault; LCF - Long Canyon fault; LSBM - Little San Bernardino Mountains; MCF - Mission Creek fault; MH - Mecca Hills; OM - Orocopia Mountains; PMFZ - Pinto Mountain fault zone; SAFZ - San Andreas fault zone; SCF - Salton Creek fault; SJFZ - San Jacinto fault zone; SJM - San Jacinto Mountains; SRM - Santa Rosa Mountains; SWF - Smoketree Wash fault. Source: Ajala et al., (2018).

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In our study, we simultaneously invert explosion and earthquake travel times and experimented with relocating the Hauksson et al. (2012) hypocenters, but our final velocity model fixes the hypocenters (for more details see Ajala et al., 2018). We employ a combination of ray coverage maps and synthetic model reconstruction tests to assess the resolution of our final velocity model.

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).

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).

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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. PEER-REVIEWED PUBLICATIONS RESULTING FROM THIS AWARD Ajala, R., P. Persaud, J. Stock, G. Fuis, J. Hole, M. Goldman and D. Scheirer, Three-Dimensional

Basin and Fault Structure From a Detailed Seismic Velocity Model of Coachella Valley, Southern California (Submitted to Journal of Geophysical Research - Solid Earth. Revisions submitted - Oct 20, 2018.)

The 3-D Velocity model, estimated basement depths, and Z2.5 surface are available at:

https://www.geol.lsu.edu/persaud/Data.html PRESENTATIONS RELATED TO THIS PROJECT Ajala, R., Persaud, P., Stock, J. M., Fuis, G. S., Hole, J. A., Goldman, M. R., & Scheirer, D. S.

(2018), 3-D upper crustal velocity structure of the Coachella Valley, Southern California: results from the Salton Seismic Imaging Project, Poster Presentation at 2018 SCEC Annual Meeting.

Ajala R., P. Persaud, J. M. Stock, G. S. Fuis, J. A. Hole, M. R. Goldman, and D. S. Scheirer (2018), On Improving Seismic Risk Assessment for a Large Magnitude Rupture on the San Andreas Fault in the Northern Salton Trough, Southern California. Presentation at the 2018 IRIS Workshop, June 12-14, 2018.

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Ajala R., P. Persaud, J. M. Stock, G. S. Fuis, J. A. Hole, M. R. Goldman, and D. S. Scheirer (2018), Basin, Fault Zones, and Adjacent Mountain Ranges: Seismic Velocity Structure of the Coachella Valley, Southern California. Presentation at the Student Seismology Workshop, Columbia University, March 13-14, 2018.

ACKNOWLEDGMENTS

The Salton Seismic Imaging Project (SSIP) was funded by the U.S. Geological Survey Multihazards Demonstration Project, and the National Science Foundation Earthscope and Margins Programs through grants OCE-0742253 (to California Institute of Technology) and OCE-0742263 (to Virginia Tech). This research was supported by Southern California Earthquake Center (SCEC) award 18074 and the U.S. Geological Survey Grant G15AP00062. SCEC is funded by NSF Cooperative Agreement EAR-1033462 and USGS Cooperative Agreement G12AC20038. The SCEC contribution number for Ajala et al., (2018) is 8269.

The 3-D velocity model as well as the derived basement and estimated Z2.5 surfaces important for seismic hazard assessment are available for download at our LSU research webpage (https://www.geol.lsu.edu/persaud/Data.html). REFERENCES Ajala, R., P. Persaud, J. Stock, G. Fuis, J. Hole, M. Goldman and D. Scheirer, Three-Dimensional

Basin and Fault Structure From a Detailed Seismic Velocity Model of Coachella Valley, Southern California (Submitted to Journal of Geophysical Research - Solid Earth. Revisions submitted - Oct 20, 2018.)

Dorsey, R. J., & Langenheim, V. E. (2015). Crustal-scale tilting of the central Salton block, southern California. Geosphere, 11(5), 1365-1383. doi:10.1130/ges01167.1

Fenby, S.S., and Gastil, R.G., (1991), Geologic-tectonic map of the Gulf of California and surrounding areas, in Dauphin, J.P., and Simoneit, B.R.T., eds., The Gulf and Peninsular Provinces of the Californias: American Association of Petroleum Geologists Memoir 47, p. 79–83, doi: 10 .1306 /M47542C6.

Fuis, G. S., Bauer, K., Goldman, M. R., Ryberg, T., Langenheim, V. E., Scheirer, D. S., et al. (2017). Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations. Bulletin of the Seismological Society of America. doi:10.1785/0120160309

Hauksson, E., Yang, W., & Shearer, P. M. (2012). Waveform Relocated Earthquake Catalog for Southern California (1981 to June 2011). Bulletin of the Seismological Society of America, 102(5), 2239-2244. doi:10.1785/0120120010

Hole, J. A. (1992). Nonlinear high-resolution three-dimensional seismic travel time tomography. Journal of Geophysical Research: Solid Earth, 97(B5), 6553-6562. doi:10.1029/92JB00235

Hole, J. A., Brocher, T. M., Klemperer, S. L., Parsons, T., Benz, H. M., & Furlong, K. P. (2000). Three-dimensional seismic velocity structure of the San Francisco Bay area. Journal of Geophysical Research: Solid Earth, 105(B6), 13859-13874. doi:10.1029/2000JB900083

Hole, J. A., Ryberg, T., Fuis, G. S., Bleibinhaus, F., & Sharma, A. K. (2006). Structure of the San Andreas fault zone at SAFOD from a seismic refraction survey. Geophysical Research Letters, 33(7), n/a-n/a. doi:10.1029/2005GL025194

Hole, J. A., & Zelt, B. C. (1995). 3-D finite-difference reflection traveltimes. Geophysical Journal International, 121, 427-434.

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Jennings, C. W., & Bryant, W. A. (2010). Fault activity map of California: California Geological Survey Geologic Data Map 2, scale 1:750,000.

Langenheim, V. E., Jachens, R., Matti, J., Hauksson, E., Morton, D., & Christensen, A. (2005). Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California. Geological Society of America Bulletin, 117(11), 1554-1572. doi:10.1130/b25760.1

Lee, E., Chen, P., & Jordan, T. H. (2014). Testing Waveform Predictions of 3D Velocity Models against Two Recent Los Angeles Earthquakes. Seismological Research Letters, 85(6), 1275-1284. doi:10.1785/0220140093

Lin, G. (2013). Three-Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California. Bulletin of the Seismological Society of America, 103(5), 2694-2708. doi:10.1785/0120120286

Rockwell, T.K., Fletcher, J.M., Teran, O.J., Hernandez, A.P., Mueller, K.J., Salisbury, J.B., Akciz, S.O., and Štěpančíková, P., (2015), Reassessment of the 1892 Laguna Salada earthquake: Fault kinematics and rupture patterns: Seismological Society of America Bulletin, v. 105, p. 2885–2893, doi: 10 .1785 /0120140274.

Thurber, C. H. (1992). Hypocenter-velocity structure coupling in local earthquake tomography. Physics of the Earth and Planetary Interiors, 75, 55-62.

Vidale, J. E. (1990). Finite difference calculation of traveltimes in three dimensions. Geophysics, 55(5), 521-526. doi:10.1190/1.1442863