CHIRP Paleoseismic Investigation of Lake Elsinore PIs: Neal Driscoll (Scripps/UCSD) and Graham Kent (UNR) Final Report

Motivation for Proposed Work

Even though the Elsinore Fault is subordinate to the San Andreas and San Jacinto faults systems, it accommodates approximately 5 mm/yr of the total strain and as such poses a significant geohazard for Southern California. We conducted a CHIRP seismic survey of Lake Elsinore to image this fault system, which was designed to complement the onshore paleoseismic investigations and allow for a more complete event characterization for this segment of the fault. The survey was designed to test whether events are multi-segment ruptures or if Lake Elsinore acts as a baffle that isolates the Glen Ivy Segment to the north from the Temecula Strand to the south (segment offset is ~2.5 km). Extensive radiocarbon dating of several long cores (>10 m) recovered from Lake Elsinore would have allowed us to establish a chronostratigraphic framework for the sedimentary sequences, the critical first step toward developing recurrence intervals. Even though our group has successfully employed this acoustic paleoseismic approach in several other tectonically active regions (Lake Tahoe; Kent et al., 2005, Dingler et al., 2009; Fallen Leaf Lake, Brothers et al., 2009; Offshore La Jolla, Hogarth et al., 2007; Le Dantec et al., 2011; Salton Sea, Brothers et al., 2009; 2011; Pyramid Lake and Great Salt Lake, in prep.), the surfical sediment in Lake Elsinore was gas charged, which precluded imaging the subsurface reflectors. After a day of surveying a regional grid in Lake Elsinore, we discovered that the gas-charge sediment was pervasive around the lake.

Figure 1. USGS survey boat that was chartered for the Lake Elsinore survey. (Left) The CHIRP is deployed off a J-frame on the starboard side out of the propeller wash. Pictures are from Walker Lake. Upon discovery of the gas-charged sediment and lack of acoustic penetration we contacted the SCEC office and proposed to move up to the “finger lakes” near Palmdale, CA to examine fault history along the San Andreas. We secured the CHIRP and mobilized the vessel and crew and drove north to Elizabeth Lake (Figure 2).

Figure 2. CHIRP seismic system secured on the vessel on Lake Elsinore. After discovering that gas-prone sediments were pervasive around the lake, we migrated north to explore other tectonic lake systems.

We explored Elizabeth Lake first. We attended the local town meeting to explain why we were surveying the lake and we secured permission from the Fish and Game Ranger on duty. We deployed the vessel along an extremely steep and narrow boat launch. We spent the day surveying Elizabeth Lake and were met with little success again. The lake floor had high acoustic reflectivity probably reflecting gas and hardpan developed during exposure, which limited subbottom imaging.

Figure 3. Location map showing Elizabeth Lake along the San Andreas Fault system. Inset shows enlargement of Elizabeth lake and a picture from the loading dock looking southwest.

2 Based on the lack of acoustic penetration in Elizabeth Lake, we decided against surveying the other finger lakes in the region along the San Andreas Fault system. We decided to move north once again and survey Walker Lake. Previous seismic reflection surveys in the lake had successfully imaged the subsurface geology and there is long standing question regarding the occurrence of strike-slip faults verses dip-slip faults in the region (Wesnousky et al., in press). Walker Lake is located within the Walker Lane deformation zone, which is a northwest trending zone of discontinuous active faults between the Sierra Nevada on the west and the north-northeast trending faults of the Great Basin to the east. This region accommodates about 20% of the right-lateral transform motion across the Pacific-North American plate boundary.

Transtensional deformation in the region is evidenced by the occurrence of both normal and strike-slip faulting within the Walker Lane. Wesnousky et al. (in press) provide an intriguing explanation of how tens of kilometers of localized shear may be accommodated in the absence of strike-slip faults and their research highlight the difficulty in melding geologic and geodetic observations and the analysis of seismic hazard.

Figure 4. Bathymetry of Walker Lake showing the location of Profile D01L02.

To examine the deformation within Walker Lake, we acquired two days of CHIRP seismic reflection data across the lake. CHIRP seismic reflection data acquired from Walker Lake are high quality and penetration was on the order of 50 m or greater.

Figure 5. CHIRP reflection profile acquired in Walker Lake. Location of profile is shown in Figure 4.

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The CHIRP data reveal several interesting deformational features in the lake that are indicative of strike-slip faults. Note the intense folding and deformation that increases down section along the eastern portion of the profile. This could be some of the missing strike-slip deformation associated with 5 mm/yr of dextral slip in the region. The reflectors diverge to west documenting the increase in accommodation that appears to be tectonically controlled. We also acquired dual-frequency side-scan data in Walker Lake. We will interpret the geophysical further and will publish the results later in the summer. Gulsen Ukarkus, a Turkish post-doctoral researcher at Scripps will be working on the acquired data set. We will also use this data to submit a NEHRP proposal to acquire additional CHIRP data in Walker Lake to determine where the strike-slip faults go onshore. We will then use ground-base LiDAR to quantitatively map shoreline offsets; these potential piercing points will place important constraints on slip rates in the region.

References Brothers, D., D. Kilb, K. Luttrell, N. Driscoll, and G. Kent (2011) Loading of the San Andreas Fault by flood-induced rupture of faults beneath the Salton Sea. Nature Geoscience, v. 4: 486-492. Brothers, D.S., Driscoll, N.W., Kent, G.M., Harding, A.J., Babcock, J.M., and Baskin, R.L., 2009. Tectonic evolution of the Salton Sea inferred from seismic reflection data. Nature Geoscience 2, 581-584; doi:10.1038/ngeo590. Brothers, D.S. G. M. Kent, N.l W. Driscoll, S. B. Smith, R. Karlin, J. A. Dingler, A. J. Harding, G. G. Seitz, and J. M. Babcock, 2008. New Constraints on Deformation, Slip-Rate, and Timing of the Most Recent Earthquake on the West Tahoe–Dollar Point Fault, Lake Tahoe Basin, California. Bulletin of the Seismological Society of America, Vol. 99, No. 2A, pp. –, March 2009, doi: 10.1785/0120080135 Dingler, J., Kent, G., Driscoll, N., Babcock, J., Harding, A., Seitz, G., Karlin, B., and Goldman, C., 2009, DOI: 10.1130/B26244.1 A high-resolution seismic CHIRP investigation of active normal faulting across the Lake Tahoe Basin, California-Nevada: Geological Society of America Bulletin, v. 121. Hogarth, L.J., Babcock, J., Driscoll, N.W., Le Dantec, N., Haas, J.K., Inman, D.L., and Masters, P.M., (2007). Long-term tectonic control on Holocene shelf sedimentation offshore La Jolla, California. Geology, V. 35, 3: 275–278 doi: 10.1130/G23234A.1. Kent, G.M., J.M. Babcock, N.W. Driscoll, A.J. Harding, J.A. Dingler, G.G. Seitz, J.V. Gardner, L.A. Mayer, C.R. Goldman, A.C. Heyvaert, R.C. Richards, R. Karlin, C.W. Morgan, P.T. Gayes, and L.A. Owen, 2005. 60 k.y. record of extension across the western boundary of the Basin and Range province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe Geology V.33, 5: 365–368. Le Dantec, N., Hogarth, L., Driscoll, N., Babcock, J., Barnhardt, W., and Schwab, W., In Press. Tectonic Controls on Nearshore Sediment Accumulation and Submarine Canyon Morphology Offshore La Jolla, Southern California. Marine Geology. Wesnousky, S.G., 2006, Predicting the endpoints of earthquake ruptures: Nature, v. 444, doi:10.1038/nature05275. Wesnousky, S.G, J. M. Bormann,C. Kreemer,, W. C. Hammond,and J. N. Brune, (in press) Neotectonics, geodesy, and seismic hazard in the Northern Walker Lane of Western North America: Thirty kilometers of crustal shear and no strike-slip? Earth and Planetary Science Letters xxx (2012) xxx–xxxx.

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