PURPOSE
The objective of this project is to evaluate the earthquake potential of the San Joaquin Hills (SJH) and environs in densely populated Orange County, California. The SJH are located at the southern margin of the Los Angeles basin. Within the last decade several active or potentially active blind thrust faults have been discovered in the Los Angeles region around the western, northern and northeastern margins of the Los Angeles structural basin (1, 2). Despite their relatively low slip rates, these faults pose significant danger to the inhabitants of southern California because of their potential to generate large amplitude ground motions in densely populated and economically important areas (1). Because blind faults are not accessible for direct study unless they generate earthquakes such as the 1994 Northridge and 1987 Whittier Narrows earthquakes, their relative contribution to seismic hazard in southern California has been difficult to assess (3). Better constraints are needed on the recency and rate of deformation of blind thrusts in the LA basin. We address this problem for the southernmost LA basin by combining methods of stratigraphic and geomorphic analysis, U-series dating and structural modeling to constrain the age and rate of deformation of the San Joaquin Hills by blind thrust faulting.
Preliminary findings from the first year of investigation confirmed that the San Joaquin Hills have been uplifted and deformed during the late Quaternary, implying the existence of an underlying blind thrust fault. The goal of the second year of effort is to determine rates and styles of late Quaternary deformation of the SJH by mapping and age dating late Quaternary marine deposits and erosion platforms. The Quaternary data will provide constraints for modeling the causative underlying structure and slip rate.
RESULTS
This report covers the first half of the second year of this project. We have made significant progress mapping and dating late Quaternary deposits and developing preliminary models of a blind thrust beneath the SJH. Preliminary mapping and dating results are summarized in Figure 1. This report contains a summary of my findings and a synthesis of results for the overall interdisciplinary collaborative investigation. Refer to reports by Mueller and Gath for additional information.
With assistance from Munro and Gath, I compiled data on the location and elevation of marine terrace erosional platforms and the thickness and characteristics of the overlying terrace sediments around the northwestern and northern margins of the SJH and Newport Mesa. Much of this data was obtained from geotechnical investigations because many of the original exposures have been obliterated by construction. Natural and temporary construction exposures were used whenever possible. Most of the platforms are covered with marine and non-marine sediments. The abrasion platforms and sediments were first classified by relative age based on geomorphology, soil characteristics, preliminary fossil assemblage analysis, elevation, vertical sequence and tentative correlation with sea level highstands. Tentative ages of the 1st, 2nd, and 4th coastal terraces were assigned based on amino acid racemization and zoogeographic correlations from Barrie et al. (4)
In parallel with the mapping, I coordinated efforts to date the marine terrace deposits using U-series methods supplemented by Kennedy's zoogeographic analysis of fossil assemblages. I did not submit fossils for amino acid racemization dating, as originally proposed, because we were fortunate to obtain corals for more accurate U-series dating. The final results will have regional as well as local significance because we are dating well known fossil localities. Dating efforts were focused on 1) the coastal terraces of Barrie et al.(4) and 2) the Eastbluff area above Newport Back Bay because it is the best documented locality for late Quaternary marine invertebrate fossils in Orange County (5). The Los Angeles County Museum of Natural History (LACMNH) houses an extensive collection from Eastbluff marine terrace deposits and several investigators have run amino acid racemization analyses of fossils from Eastbluff and the coastal terraces (6). Therefore, U-series dates from our project will provide age calibration for both the Orange County fossil assemblages and amino acid analyses.
Three solitary corals were submitted to Edwards for dating. An altered specimen ofBalanophilia elegans, collected from a coastal terrace by Kennedy (GK on Fig. 1), yielded a nominal age of 170 ka. Uncertainty in the age and the field relationships allow plausible correlations of the specimen with either stage 5e or stage 7 sea level highstand. Although a correlation with stage 7 highstand cannot be ruled out, we prefer a stage 5e interpretation because it is consistent with other published and unpublished age data (4,6) for this terrace, and because the sample was clearly altered and possibly reworked from an older terrace above.
Two solitary corals were dated from Eastbluff. A large unaltered specimen (FP on Fig. 1) of Paracyathus stearnsii from a published private collection (7) has an age of 122 + 1 ka. Several other species at the collection locality were in life positions, and there is no reason to suspect that this specimen was significantly out of place or appreciably altered. Therefore, this date is readily accepted as representative of the age of the terrace, and the lower Eastbluff terrace is correlated with the stage 5e sea level highstand, with high confidence. A surprising age of 17 - 19 ka was reported for a small specimen of Astrangia from the LACMNH. The sample reportedly came from Eastbluff locality 66-9 (Fig. 1) of Kanakoff and Emerson (5) and the isotopic composition indicates minimal alteration. The age of this specimen correlates with the last glacial maximum, when sea level was at least 150m below present (8a) so it is very difficult to explain how it was deposited more than 25 m above sea level in Eastbluff. Several hypotheses, including tsunami, unusual U uptake, and cataloging errors are under consideration to explain the reported age. We do not accept the age of this specimen as representative of the terrace deposits because the age conflicts with all other reasonably well- constrained data. Additional work is needed to resolve this discrepancy.
Late Quaternary uplift rates can be derived for the SJH and Newport Mesa from measured shoreline elevations, age of the shorelines, and the elevation of sea level when the shoreline was cut. The stage 5a, 5e and 7 sea level highstands occurred at approximately 81 ka, 124 ka and 212 ka (8). From construction exposures at Newport Mesa and Corona Del Mar, the elevation of the stage 5a shoreline is 21 - 22 m above present sea level. A third natural shoreline exposure within lower Newport Back Bay near Galaxie Park is consistent with these measurements, but the elevation is poorly constrained and could be lower. Estimates of paleo sea level at 81 ka range from -12 m to approx.0 m, leading to uplift rates of 0.26 to 0.42 m/ka. Along the coast of the SJH, the 36 m elevation of the 5e shoreline is well constrained by grading exposures. If sea level was +5 to + 6m (8) then the uplift rate over the last 124 ka was approx. 0.24 - 0.25 m/ka. Stage 7 shorelines at elevations of 50 - 57 m have risen from - 5 m over 212 ka at an average uplift rate of 0.26 - 0.29 m/ka.
There is evidence that uplift continued during the Holocene, implying that the SJH blind thrust is active. The most compelling evidence is documented in a 1954 study of marshlands in Newport Bay (9) which concluded that a "ledge or bench of ancient marsh deposits" around the margins of Newport Bay must have formed by tectonic uplift (Figure 2a). A leveling survey showed that the marsh bench "averages 38 inches above the present marsh on the western shore and 62 inches on the eastern bank. It is approximately 6 inches higher in the central part of the Bay than at the north and south ends." Analysis of the sediments, flora and elevation of the marsh bench suggest that it formed when freshwater flowed through Back Bay, and then raised above sea level to its present elevation. The extant salt marsh was established at sea level after diversion of the freshwater drainage (9). From the frequency of historical floods of the Santa Ana River and several "Indian kitchen middens... uncovered beneath and at the base of the marsh bench" Stevenson proposed that uplift of the marsh bench occurred within the last few centuries (9). There is a similar feature cut into rocky cliffs at low elevation, but above high tide, along the modern shoreline in Laguna and Newport Beach (Fig. 2b, 2c). Active downcutting by coastal streams provides additional evidence of late Holocene uplift of the SJH. Sea level rose approx. 150 m. during the early to middle Holocene, thereby inducing sedimentation in coastal drainages. Erosional exposures of bedrock in coastal canyons of the SJH suggest that uplift has occurred since sea level stabilized approximately 6 ka, during the late Holocene. Because of the potential significance of seismogenic uplift of the densely populated SJH, evidence of Holocene activity deserves further investigation.
References
(1) Dolan, J. et al, (1995), Prospects for larger of more frequent earthquakes in the Los Angeles metropolitan region, Science, v. 267, p. 199 - 205.
(2) Working Group on California Earthquake Probabilities (1995) Seismic hazards in southern California: Probable earthquakes, 1994 to 2024, Bull. Seism. Soc. Amer., v. 85, no. 2, pp. 379-439.
(3) Abrahamson, N., W. Foxall and A. Cornell, in progress
(4) Barrie D., T. S. Tatnall and E. Gath (1992). "Neotectonic Uplift and Ages of Pleistocene Marine Terraces, San Joaquin Hills, Orange County, California", in The Regressive Pleistocene Shoreline Southern California, Heath, E. and L. Lewis eds., South Coast Geological Society Annual Field Trip Guidebook No. 20, South Coast Geological Society, Santa Ana, CA, pp.115-121.
(5)Kanakoff G. P. and W. K. Emerson (1959) Late Pleistocene Invertebrates of the Newport Bay Area, Calif., L. A. Co. Mus. Contrib. in Science, no. 31; Bruff, S. C. (1946) "The Paleontology of the Pleistocene Molluscan Fauna of the Newport Bay Area, California", Calif. Univ., Pubs. Geol. Sci, v. 27, p. 213 - 240; Wallis, M. C. (1984) "The Eastbluff Fossil Story", inThe Natural Science of Orange County, Memoirs of the Natural History Foundation of Orange County, vol. 1, Butler, B., J. Gant and C. Stadum eds., pp 124-129.
(6) Wehmiller, J. F. et al. (1977) USGS OFR 77-680; Ponti and Lajoie, unpublished data
(7)Peska, F. (1984). "Late Pleistocene Fossils from Upper Newport Bay, California" in The Natural Science of Orange County, Mem. of the Natural History Foundn. of Orange Co., v. 1, Butler, B., J. Gant and C. Stadum eds., pp 55 - 60; and written communication, 1997.
(8) (a)Lajoie et al., (1991), in Quaternary Nonglacial Geology: Coterminous U.S., R. B. Morrison, ed., p. 190-214 (b) Muhs, D. R., G. Kennedy and T. Rockwell (1994), Quaternary Res., v. 42, 72-87.
(9) Stevenson, R. E. (1954). The Marshlands at Newport Bay, California, unpublished Ph.D. thesis, Univ. of So. Calif, Los Angeles.
Publications:
Grant, L. B., E. Gath, R. Munro, and G. Roquemore, Neotectonics and Earthquake Potential of the San Joaquin
Hills, Orange County, California, Seism. Res. Lttrs, v. 68, no. 2, p. 315, 1997
Grant, L., E. Gath, R. Munro, K. Mueller, G. Kennedy and L Edwards,Uplift and earthquake potential of the San Joaquin Hills, Orange County, California, Abstr, 1997 Ann. Meeting, So. Calif. Earthquake Center.
Annual Report, 1997
Historic and paleoseismic behavior of the southcentral San Andreas Fault between Cholame and the Carrizo Plain
Submitted by Lisa B. Grant
Natural Sciences, Chapman University, Orange, CA 92866; lgrant@chapman.edu
Collaborative project with
Ramon Arrowsmith, Arizona State Univ.
Dallas Rhodes, Whittier College
PURPOSE
To evaluate seismic hazard in California, it is essential to understand the behavior and earthquake potential of the San Andreas fault. Despite the tectonic complexity of southern California and the large number of active faults, the San Andreas fault dominates seismic hazard assessments because of its high moment release rate and documented historic and prehistoric frequency of generating large magnitude earthquakes (1). In addition, data on the earthquake history of the San Andreas fault forms the basis of numerous models of fault behavior and calculations of seismic hazard. Despite the large amount of data available, there are many unanswered questions about the validity of segmentation models for the San Andreas, the rupture patterns of prehistoric earthquakes, and the patterns, or lack thereof, of earthquake recurrence (2). The objective of this research is to provide data to help determine the size and extent of past ruptures along the San Andreas fault between the northwestern Carrizo Plain and Cholame; to test the hypothesis that the San Andreas consists of separate Carrizo and Cholame segments; and to provide information on the distribution of slip near Cholame from the 1857 earthquake.
METHODS AND RESULTS
During the last year we have worked collaboratively to address these questions by attempting to directly measure the 1857 displacement using historic cadastral survey data, using geomorphology to analyze offset streams; and searching for a good site to excavate for a paleoseismic record. Although we have worked together on all aspects of the project, Arrowsmith has directed the project and concentrated on geomorphic analysis and field reconnaissance of sites (see annual report by Arrowsmith and Rhodes). In this report I summarize my primary contribution of reviewing cadastral surveys and attempting to measure the displacement associated with the 1857 earthquake.
Data from pre-1857 cadastral surveys was successfully used by Grant and Donnellan (3) in combination with paleoseismic data (4) to measure the amount of displacement associated with the 1857 earthquake. In this project, we are attempting to repeat a similar analysis to resolve the discrepancy between reported amounts of slip on the Cholame "segment" of the San Andreas fault. (5,6).
In 1855 and 1856 James E. Freeman surveyed and established township and range lines across the San Andreas fault between Rancho Cholame (approximately at Highway 46 on Figure 1) and the northern Carrizo Plain, approximately 6 km southeast of Wallace Creek (south of Highway 58 on Figure 1). He subdivided the township and range lines into sections approximately 1 mile long. He also subdivided portions of the interior of some of the townships into one mile long sections. At the endpoints of each section line, he established monuments to mark the section corners. In all, Freeman surveyed and established at least 26 section lines across the San Andreas fault between Highways 46 and 58. Each of the 26 section lines was defined by two monuments spanning all or a portion of the fault zone.
Records of the original surveys were filed with the General Land Office and are maintained on microfiche by the U.S. Bureau of Land Management, Cadastral Survey Office in Sacramento, CA. I obtained and reviewed all the microfiche records of Freeman's original pre-1857 surveys in the vicinity of the San Andreas fault in California. After identifying pairs of monuments established prior to 1857 that might span all or a portion of the 1857 rupture, I attempted to research the history of each monument to determine if it could be used to reliably measure the 1857 displacement and post-1857 strain accumulation. After reviewing all related post-1857 surveys or resurveys on file with the BLM, I visited the San Luis Obispo County Surveyor's office and checked the county resurveys, other officially recorded surveys, and county monumentation files. Some of the monuments along the San Luis Obispo and Kern County boundary had been reset by Kern County. According to general practice, records of remonumentation along the county line are supposed to be filed with both counties. Records of the Kern County remonumentation of the county line are incomplete in the San Luis Obispo County Surveyor's office. Presumably, these records are on file in Kern Co.
Records of monumentation and resurveys were used to establish or reject the authenticity of corner monuments. For our purposes, an authentic monument is one that is preserved in its original location. From the records, it appears that many of Freeman's original fault-spanning monuments were "lost", meaning that they were never recovered or found after the 1857 earthquake. Many of these monuments were later proportionally reset so they are not in their original locations and cannot be used to measure fault related displacement. Some of the original monuments are in remote or inaccessible areas that have not been surveyed since 1855 or visited by us, so the condition of those monuments is not known. Fortunately, a few monuments survived the earthquake, and were reset in their original locations. Several of these monuments survive to the present day. Arrowsmith and Rhodes have visited some of these monuments to compare them with their officially recorded location and description to test their authenticity (see report by Arrowsmith and Rhodes). Some have been modified, without record, so their authenticity is doubtful. The records and field observations of a few monuments are consistent with preservation in their original 1855 locations. In theory, the distance between these monuments can resurveyed to measure the total amount of displacement from the 1857 earthquake and post-1857 strain accumulation.
Figure 1 summarizes the available information about monuments which reportedly survived the 1857 earthquake and might be in their original locations. Circled monuments were found during field checks by Arrowsmith and Rhodes. Solid dots denote monuments that were preserved in their original locations up to the date of the most recent record. Question marks indicate monuments that have been reset without record but are probably in their original locations. The record of reset might be on file with Kern Co. rather than San Luis Obispo Co.
There are five pairs of monuments which span the fault zone and are candidates for resurveying. Three of these pairs have been field checked and one pair (T28S R18E sec6 N boundary) was resurveyed by Arrowsmith and Rhodes. The line length measured with a total station was only 0.67 m longer than the line length reported by an 1893 survey using poles and chains. This suggests that errors in the old chained surveys were less than 1 part in 1000, similar to the errors reported by Grant and Donnellan (3). However, other measurements appear to contradict this and suggest that the typical measurement errors in the more rugged Cholame region were substantially larger. All recorded resurveys of lines between candidate authentic monuments indicate an increase in the distance between monuments after the 1857 earthquake. The resurveys occurred in 1893, 1942, 1979 and 1997 and used various types of equipment and methods. Some of the resurveys are poorly documentd and may contain large errors. The apparent post-1857 line length increases range from a minimum of 2.5 m to a maximum of 31 m. The lines are at an angle to the fault, so changes in line lengths are not the same as measurements of fault displacement.
It is difficult to interpret the available data. Additional surveys with a total station would help to constrain the magnitude of errors in the original and subsequent surveys. The line resurveyed by Arrowsmith and Rhodes is approximately 23.2 m longer than it was when it was established in 1855. This line appears to have measurement error of less than 1 m (although further work is necessary to confirm this) and it is near the proposed Bitterwater Valley paleoseismic excavation site. If the proposed paleoseismic excavation is fruitful, then it would be worthwhile to carefully evaluate errors in the original surveys to to obtain an accurate measurement of displacement in Bitterwater Valley associated with the 1857 earthquake.
References
(1) Working Group on California Earthquake Probabilities (1995) Seismic hazards in southern California: Probable earthquakes, 1994 to 2024, Bull. Seism. Soc. Amer., v. 85, no. 2, pp. 379-439.
(2) Grant, L. B. (1996). Uncharacteristic earthquakes on the San Andreas Fault, Science, v. 272, 826 -827.
(3) Grant, L. B. and A. Donnellan (1994). 1855 and 1991 surveys of the San Andreas fault: implications for fault mechanics, Bull. Seism. Soc. Amer, v. 84, no. 2, 241-246.
(4) Grant, L. B. and K. E. Sieh (1993) Stratigraphic evidence for seven meters of dextral slip on the San Andreas fault during the 1857 earthquake in the Carrizo Plain, Bull. Seism. Soc. Amer, v.83, no. 3, 619 - 635.
(5) Lienkaemper, J. J. and T. A. Sturm (1989). Reconstruction of a channel offset in 1857(?) by the San Andreas fault near Cholame, California, Bull. Seism. Soc. Amer, v.79 no. 3, 901-909.
(6) Sieh, K. E. (1978). Slip along the San Andreas fault associated with the great 1857 earthquake, Bull. Seism. Soc. Amer., v. 68, 1421 - 1448.
Publications and products:
Arrowsmith, R., L. Grant and D. Rhodes, Investigation of historic and paleoseismic behavior of the San Andreas Fault between Cholame and the Carrizo Plain, 1997 Annual Meeting Southern California Earthquake Center.
Arrowsmith, R., L. Grant and D. Rhodes, Cholame and Carrizo SAF field trip stops, Handout for participants, SCEC Group C San Andreas Field Trip Meeting, Sept. 1997.
During the last year we have completed this project and continued to disseminate the results of our investigation to scientists, interested professionals, and the public. Residual funds were spent in support of publication and dissemination of results, including preparation of final figures, page charges for publication, and partial expenses for presenting results at meetings.
Final results were published in the Bulletin of the Seismological Society of America, and presented at the Annual Meeting of the Seismological Society of America in 1997. John Waggoner is scheduled to present the results to Orange County geologists at a monthly meeting of the South Coast Geological Society, and Lisa Grant gave a public lecture in Newport Beach and seminars at the U.C. Irvine School of Engineering and at UC Berkeley. Lisa presented a summary of the results to Congressman Christopher Cox of Newport Beach (Orange County) in November. In addition, Lisa Grant and John Waggoner gave several interviews with print and broadcast journalists to increase public understanding and awareness of seismic hazards associated with the Newport-Inglewood fault in Orange County. Within the last 2 years scientific findings were also presented at the AGU Fall Meeting (1995), at the U.S.G.S. in Menlo Park (1996), at Calif. State Univ. in Fullerton (1996), and at SCEC meetings. Lisa also co-led a SCEC field trip on the Newport-Inglewood fault zone and co-authored the companion field trip guidebook. The general public has been educated about the Newport-Inglewood fault by Lisa's 3 part LA Underground KFWB radio series on Orange County seismic hazards, produced by Jack Popejoy. This project also provided research experience for two SCEC undergraduate summer interns, Chris Sykes and Carmen von Stein.
The PI expresses her sincere appreciation to SCEC for supporting this project.
Publications:
Grant, L. B., J. T. Waggoner, T. K. Rockwell and C. von Stein, Paleoseismicity of the North Branch of the Newport-Inglewood Fault Zone in Huntington Beach, California from Cone Penetrometer Test Data, Bull. Seism. Soc. of Amer, v. 87, no. 2, p. 277-293, April 1997.
Grant, L. B., J. T. Waggoner, T. K. Rockwell and C. von Stein, Paleoseismicity of the North Branch of the Newport-Inglewood Fault Zone in Huntington Beach, California from Cone Penetrometer Test data, Seism. Res. Lttrs, v. 68, no. 2, p. 300, 1997.
Grant, L. B., J. T. Waggoner and C. von Stein, Paleoseismicity of the North Branch of the Newport-Inglewood Fault in Huntington Beach, California, EOS Trans. Am. Geophys. Union, Vol 76, No 46, p.F362, 1995.
Forrest, M., T. Rockwell, L. Grant and E. Gath, The Newport-Inglewood and Whittier-Elsinore Fault Zones, Southern California Earthquake Center Shattered Crust Series, No. 1, July 1996.
