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The San Andreas Fault Zone in the Carrizo Plain, California: Review of Quaternary Geologic Investigations, Landforms, and Fault Activity.

Arrowsmith, J R., 1995, Appendix C, Coupled tectonic deformation and geomorphic degradation along the San Andreas Fault System, PhD. Dissertation, Stanford University, pp. 312-346.

C.1. Introduction

C.1.1. The Carrizo Plain as the Cadillac of the San Andreas fault system

Just as a Cadillac is a lavish automobile with many luxurious options, the Carrizo Plain area, situated in the southeastern California Coast Ranges ( Figure C.1), is the premier example of the San Andreas Fault (SAF) system in California. The area has been long regarded as a site of world-class examples of strike-slip faulting. The SAF follows the northeastern side of the plain and all along it are offset, beheaded, and abandoned drainages that reflect that recent strike-slip motion. That motion apparently has been accommodated recently by great earthquakes; the most recent was the 1857 Fort Tejón earthquake [Sieh, 1978c]. The SAF in the Carrizo Plain has the largest accumulated post-early Miocene offset and is the oldest reach of the entire active fault system (315 km and ~15 Ma; [Page, 1990]). The area has provided evidence for both sides of the argument regarding the sate of stress along the SAF ([Wilcox et al., 1973]­a strong SAF with early folding oblique to its strike; and [Zoback et al., 1987]­a weak SAF unable to sustain much shear traction and thus with folds, thrust faults, and the minimum horizontal stress striking nearly parallel to it).
This appendix provides descriptions of the location and setting of the Carrizo Plain; a summary of early research along the SAF zone there, and in particular, the Elkhorn Hills of the southeastern portion of the Carrizo Plain (Figure C.1); a review of the historic and paleoseismologic investigations along the SAF; and a discussion of the slip deficit along the Cholame segment and potential for rupture of M 7 earthquakes (prepared for the California Earthquake Prediction Evaluation Council­CEPEC­as part of its review of the Parkfield Earthquake Prediction Experiment). This appendix is meant to complement the research presented in Chapters 3 and 4.

Figure C.1. Shaded relief map of central California showing the situation of the Carrizo Plain in the Coast Ranges. The orange line shows the surface trace of the SAF, and is thicker along the portion that is inferred to have ruptured in the 1857 earthquake [Sieh, 1978c]. TR is Temblor Range; CR is Caliente Range; CV is Cuyama Valley; LP is La Panza Range; SV is Salinas Valley; CP is Cajón Pass; TP is Tejón Pass; W is Wrightwood. Original relief map is from Relief map of the world: cylindrical projection; elevation data from ETOPO5 dataset; by Ray Sterner, Johns Hopkins University Applied Physics Laboratory.

C.1.2. Location and setting

The Carrizo Plain is located about 90 km west of Bakersfield and about 85 km north of Santa Barbara (Figure C.1). Most of it is in San Luis Obispo County, but the extreme northeastern portion is in Kern County. It is a narrow, undulating, and mostly undrained plain, about 15 km wide (NE-SW) and 75 km long (NW-SE) where elevations range from 1900' (580 m) in the alkali wetland of the Soda Lake basin to 2500' (760 m) in the Southern Elkhorn Hills of the extreme southeastern Carrizo Plain (Figures C.1 and C.2). The northwesternmost Carrizo Plain is dissected by the San Juan drainage which flows to the Salinas River. The ~100 km long Temblor Range separates the Carrizo Plain from the southern San Joaquin Valley on the northeast, with elevations of 3000' to 4,300' (914 to 1310 m). The Temblor Range tapers in elevation to the northwest where it merges into Table Mountain and Orchard Peak near Cholame. On its southwest side, the Carrizo Plain is separated from the Cuyama Valley by the ~80 km long Caliente Range. The maximum elevation of the Caliente Range is 5,100' (1550 m). The southern portion of the range is higher and more rugged, while the northwestern range is lower and merges with the northwestern Carrizo Plain. The La Panza Range lies west of the Cuyama Valley [Dibblee, 1962]. The Carrizo Plain is a perched basin, about 1000' higher than the adjacent Cuyama Valley to the southwest, and about 1500' above the Southern San Joaquin Valley (Dibblee, 1962 suggested that the region was uplifted about 1000' since the late Pliocene).
The area was extensively farmed and grazed in the earlier part of this century, but now 90% of it is within the Carrizo Plain Natural Area: cooperatively managed by the Nature Conservancy, the United States Bureau of Land Management, and the California Department of Fish and Game. The objectives of this management include preservation of a representative sample of the historic southern San Joaquin Valley flora and fauna; conservation of threatened and endangered species and their habitat (including San Joaquin Kit Fox, San Joaquin Antelope Squirrel, Giant Kangaroo rat, short-nosed Kangaroo rat, Blunt-nosed leopard lizard, Greater and lesser Sandhill Cranes, and the California Condor); resolution of conflicts between intensive oil and gas development and endangered species habitat; provision for multiple use (including recreation and grazing); and protection and restoration of resource values (including cultural, soils, vegetation, paleontological, and geological). Indeed, the preservation of the distinctive landforms and geologic record of deformation along the San Andreas fault in the Carrizo Plain may be as important to society as the protection of endangered species.
The earliest known inhabitants of the Carrizo Plain area were the aboriginal population. Sometime 500 to 1000 years ago, a sophisticated native population flourished,

Figure C.2. Panchromatic SPOT image (courtesy of SPOT Image Corp.) of the southeastern Carrizo Plain illustrating the geography, physiography, and geomorphology of the Caliente Range foothills (at the bottom); the southeastern Carrizo Plain (center); and the Elkhorn Scarp­defined in the northwest by the pressure ridges, including the Dragon's Back, and in the southeast by the southwest side of the Elkhorn Hills (northeast of the SAF, NEH is northern Elkhorn Hills, and SEH is Southern Elkhorn Hills). The Elkhorn Plain separates the Elkhorn Hills from the Temblor Range (at the top right).

produced distinctive rock art (e. g., Painted Rock), and probably hunted the many animals attracted to the Carrizo Plain in wet times for its Soda Lake and gathered nuts and other vegetation in the surrounding mountains [Cawley, 1962]. The early aborigine culture that flourished in the Carrizo Plain was surely affected by great earthquakes, and may have even departed as the result of one (further research in the native descriptions of great earthquakes in this area could be interesting, similar research has been done for the coastal Indians of northwest California and their oral records of Cascadia subduction zone earthquakes). Spaniards came through the area in the 1700s and named the area "Carrizo" after the lush reeds growing there [Cawley, 1962].

C.2. Early geologic Investigations of the "San Andreas Rift" and the Elkhorn Hills

The Carrizo Plain area has long provided evidence for the recent right lateral motion along the fault, the amount of that slip, its diversion of drainage, and vertical motions resulting in interesting and unique topography. The recognition of the area's geomorphic and structural similarity and continuity with the SAF zone along much of its length was clear in 1906 [Lawson and others, 1908]. In January 1906, A. C. Lawson and H. W. Fairbanks led a field trip to the Carrizo Plain, and pictures from that trip are reproduced in the so-called Lawson Report [Lawson and others, 1908, volume I, plate 21]. It contains an interesting chapter entitled ³The San Andreas Rift as a Geomorphic Feature.² The rift cuts across California from Humboldt County in the north to the Colorado Desert in the south and is

The San Andreas Rift was recognized as the result of the interaction between geomorphic and tectonic processes. This text, written by Harold W. Fairbanks, records clear thinking about the geomorphology of the Carrizo (or Carissa) Plain:

This clear and interesting description of the pressure ridges along the Elkhorn Scarp (³...deeply dissected slope toward the southwest, and long gentle slopes toward the northeast. This ridge is clearly a fault block, and now separates the southeastern arm of Carissa Plain from Elkhorn Plain²), and its coalescence into the Northern and Southern Elkhorn Hills (not yet so named; ³Except for one slight bend, the ridge which we have been describing follows a straight course toward the southeast for a distance of nearly 20 miles, finally blending in a much larger mountain-like elevation.² These features have been investigated in detail by Arrowsmith, 1991b; Arrowsmith, 1991a; Rhodes and Arrowsmith, 1991; Arrowsmith, 1992a; Arrowsmith, 1992b; Arrowsmith and Rhodes, 1992 and Chapter 4 of this Dissertation (Figure C.2). The conclusions of Fairbanks regarding the origin of these features are impressive in that, even in 1906 (and perhaps a realization from the 1906 earthquake), he recognized the scale of and suggested the nature of the physical deformation associated with the San Andreas Fault.

The next records of an early scientific visit to the Carrizo Plain come from Arnold and Johnson, 1910 who describe the area in passing, and propose names for some of the geographic elements there. Their investigation of the Carrizo Plain was peripheral to a study of the oil fields on the eastern side of the Temblor Range: the developed and well- known McKittrick, Midway, and Sunset fields. The authors spent the summer and fall of 1908 performing a detailed reconnaissance of parts of the Carrizo Plain district. Examination of their Plate I (Preliminary Geologic and Structural Map of the McKittrick- Sunset Oil Region, California), indicates that their mapping of the Carrizo Plain area was incomplete. They only covered the northern Carrizo Plain to the Caliente Range, and the western Temblor Range foothills in the central and southern Carrizo Plain. They did recognize definite evidence of horizontal offset along the SAF (including the 400'­120 m offset of Wallace Creek [Arnold and Johnson, 1910]). They also proposed the names of some places, including the Panorama Hills, Panorama Point (in the central Carrizo Plain along the SAF), the Elkhorn Hills, the Elkhorn Scarp (³The name proposed for the terrace- like escarpment.... This feature is purely structural, following the great San Andreas fault or rift, and hence the application of the term 'scarp¹ (p. 20).² According to their definition, the pressure ridges are only the northern portion of the escarpment separating the Carrizo Plain from the Elkhorn Hills (Figure C.2).
Arnold and Johnson's description of the topography along the San Andreas Fault in the Carrizo Plain and Elkhorn Hills region emphasizes their inference that its character resulted from the interaction of the tectonic and geomorphic processes:

The discussion of the Elkhorn Scarp¹s striking aspect followed by the description of its continuation to the south (into what is now called the Big Bend), reflects the possible structural association of the Elkhorn Hills and the beginning of the Big Bend. The Elkhorn Scarp is composed of several pressure ridges in the northwest, and a steep and incised southwest facing scarp to the southeast (see Chapter 4 and Figure C.2).

H. O Wood and J. P. Buwalda presented an abstract in the proceedings of the Cordilleran section of the Geological Society of America in 1931 in which they discussed the drainage patterns and other physiographic features along the SAF in the Carrizo Plain. They concluded that the drainage patterns were the result of ³at least several thousand feet² of right lateral slip along the SAF, and that ³the interpretation of the drainage patterns as merely subsequent streams is entirely inadequate as the offsets are practically without exception in the same direction² [Wood and Buwalda, 1931].
Several wells were spudded into the area of the southern Carrizo Plain during the heyday of oil exploration in the 1930s and 1940s, but none was successful [Graff, 1962].
In the 1950s and 1960s, Tom Dibblee mapped the region, and noted both the SAF and a southwest dipping fault parallel to the SAF in the southern Elkhorn Hills, as well as some of the grabens in the northern Elkhorn Hills. In their classic paper of 1953, Mason Hill and Dibblee noted both the offset drainages of the Carrizo Plain and ³Recently developed trenches which trend southward into the fault have been observed on the southwest side of the Temblor Range. These are oriented correctly to be tensional in origin and due to right lateral movement along the San Andreas² [Hill and Dibblee, 1953, p. 446]. They refer to the grabens in the Elkhorn Hills that are investigated in Chapter 4 (and are evident in Figure C.2). As a part of the presentation of Dibblee¹s work, the Carrizo Plain was the area of the 1962 field trip of the San Joaquin Geological Society and Pacific Section of AAPG and SEPM [Dibblee, 1962]. He comments about the geomorphology of the Carrizo Plain and vicinity:

Robert E. Wallace was the next geologist to turn his attention to the SAF in the Carrizo Plain area. Wallace, 1968 investigated the offset channels and is discussed in more detail below. The 1973 contribution was a short report on the en échelon geometry of adjacent segments of the SAF (an example of which came from the Carrizo Plain) in which he observed that the length of the longest continuous fractures was 10 to 18 km­similar to the depth of the deepest earthquake foci­suggesting that the fractures were roughly equidimensional [Wallace, 1973].
In 1975, Bob Wallace presented a less detailed description of the right laterally offset landforms than his 1968 paper, and included a discussion of the patterns of fault related stream channels in the Carrizo Plain area. He notes drainages offset right-laterally, drainages deflected by sliver ridge and pressure ridge uplift and offset, and false offsets that could result from differential uplift deflecting drainages, or en échelon fractures over the fault zone followed by subsequent streams. He included a discussion of vertical displacements that referred to sag ponds between strands of the SAF, the observation that the ratio of vertical to horizontal slip was commonly 1 to 10 or 1 to 20 (probably as a consequence of juxtaposed, horizontally offset topography), and an interesting discussion of the Elkhorn Hills:

Along with Wallace's textual contributions, three maps of the area were published in the early 1970s. For the first Vedder and Wallace, 1970, they combined field and aerial photographic studies to produce a map of the recently active breaks along the SAF and related faults in the Carrizo Plain area. They also commented on the locations of diverted and offset drainages, notches, cracks, depressions, benches, scarps, elongate ridges, moletracks and landslides. Associated with that work, and published in the same year, was the geologic map of the Wells Ranch and Elkhorn Hills quadrangles­areas in the southeastern Carrizo Plain ranging from about the crest of the Temblor range to the crest of the Caliente Range [Vedder, 1970]. The map shows the structure within the uplifted and deformed Paso Robles formation of the Elkhorn Hills, as well as the locations of wells that were used to generate the geologic cross-sections presented with the map. Because the map covers two quadrangles, the Elkhorn Hills are not covered in great detail; however, it is useful in its illustration of the neighboring structures. Three things of interest are: 1) Vedder shows an area parallel to the SAF and on the northeast margin of the Elkhorn Hills as being as "Zone of Large Scale Landslides"­an area whose morphology and structure is interpreted in Chapter 4 to result from repeated slip along southwest dipping reverse faults. 2) In Vedder's cross-sections, the area underneath the SAF is described as "Zone of probable intense deformation. Lack of subsurface data precludes structural and stratigraphic interpretation." 3) The Paso Robles formation in the Elkhorn Hills is shown to overlie the Bitterwater Creek shale, an upper Miocene marine unit; the contrast in material properties between it and the Paso Robles formation may contribute to the localization of the deformation.
Another geologic map of importance is Dibblee, 1973. This map of the regional geology (Carrizo Plain, Temblor, Caliente, and La Panza ranges) shows the juxtaposition of different rock types across the SAF, and that the SAF truncates some of the earlier structures (e.g., the Wells Ranch Syncline in the southeastern Caliente Range).
With the suggestion and inspiration of Richard H. Jahns, Kerry Sieh carried out a detailed investigation or the chronology of large earthquakes along the SAF in southern California. Much of this work, initiated as his PhD. research, is described below in the historic and paleoseismology and Cholame segment sections. As part of the research about the 1857 earthquake, Kerry Sieh noted that along the SAF in the area of the Elkhorn Hills, right-lateral strike-slip offsets apparently formed in the 1857 earthquake abruptly decreased 10-20% (9 to 7 m; although later work has indicated that it may have been 7 to 6 m -- see Chapter 4; nevertheless, slip apparently decreased in this area) compared to those in the northwest. That observation led him to look to the side of the SAF for distributed deformation that might account for the decreased slip. He found good evidence for secondary faulting in the Elkhorn Hills:

Thomas and Sieh, 1981 investigated the Quaternary formation of the Elkhorn Hills as a consequence of sequential development of folds and thrusts:

See Chapter 4 for a discussion of detailed mapping and inferences regarding the structure and temporal development of the Elkhorn Hills.

C.3. Historic and Paleo-seismicity

The mode of strain release along the principal traces of the San Andreas fault system varies from continuous creep to infrequent large earthquakes. Allen, 1968 pointed out the distribution of the portions of the SAF with different modes of strain release, and discussed several effects and hypothesized causes. The two sections of the fault with the least modern seismicity were those that have recently experienced a great earthquake (the 1857 and 1906 breaks). These segments have fairly well-defined geomorphic traces (in contrast to the broader zones of creeping sections, e.g., the SAF in the Carrizo Plain versus the central creeping section that lies between San Juan Bautista and Parkfield), and both have bends in their strike. The 1906 segment has the Santa Cruz Mountains bend, and the Carrizo/Fort Tejón segment has the Big Bend.
The boundaries of the 1857 stretch were defined by investigations of felt reports and observations of the surface rupture ([Wood, 1955], and later by Sieh, 1977; Agnew and Sieh, 1978; Sieh, 1978b; Sieh, 1978c; Figure C.3 ). The southern end was correlated geologically with the increasing complexity of the SAF system as it interacts with the northernmost spreading of the Gulf of California in the Salton Trough. The transition is in the area of Cajon Pass, northwest of San Bernardino. The northern end of the rupture may be defined geologically by the 1-2 km right step in the SAF trace in the Cholame Valley. In this area, the contrast in basement types changes, with southeasternmost significant Franciscan rocks on the northeast juxtaposed against the Salinian basement [Hanna et al., 1975]. However, anecdotal reports of the event suggested that it ruptured northward into the creeping section, and thus the significance of the Cholame stepover as a rupture barrier or segment boundary is still debated. The other gross structural feature of the 1857 break that may affect its style of strain release is the Big Bend in which the SAF changes strike from ~N40W (Southern Santa Cruz Mountains to the Northern Carrizo Plain) to ~N70W near Tejon Pass [Allen, 1968; Jennings and Strand, 1969] ( Figure C.1). Allen, 1968 suggested that the bend in strike and basement contrast were important controls in the style of strain release in both the 1906 and 1857 earthquakes.

C.3.1. 1857 earthquake

Investigations of the 1857 event conclude that it was part of a major earthquake sequence. These studies have included documentation of the felt effects of the earthquake sequence [Wood, 1955; Agnew and Sieh, 1978; Sieh, 1978c], (Figure C.3). The felt reports were limited by the sparse population of central California at the time (the majority of the state's population was in the area between San Francisco, Sacramento, and the North-central Sierra Nevada­responding to the Gold Rush; e. g., [Agnew and Sieh, 1978]). At least 5 foreshocks occurred in the 9 hours before the event, including a late evening shock on January 8 of ~M¾3.5 near San Francisco; possibly 2 moderate events during the early morning and pre-dawn hours; a dawn shock; and a sunrise shock on January 9 (Figure C.3). The latter two events may have occurred along the Parkfield- Cholame section of the SAF, immediately northwest of the Carrizo segment, based upon comparisons of felt reports of the foreshocks with isoseismals produced by twentieth century Parkfield events (Figure C.3). Thus, the Parkfield-Cholame section of the SAF is

Figure C.3. Felt intensity distributions for the 1857 earthquake sequence showing the two most prominent foreshocks [Sieh, 1978a], and the mainshock: the Fort Tejón Earthquake [Agnew and Sieh, 1978].

a transitional reach with both creep and earthquake events and 1857-type events may be triggered by such a moderate earthquake [Sieh, 1978c; Sieh and Jahns, 1984].

Based upon foreshock locations, main shock isoseismals, and peak slip; the epicenter of the main event (~8:24 am PST, January 9, 1857) is inferred to be in the northwestern Carrizo Plain or along the Parkfield-Cholame section [Agnew and Sieh, 1978; Sieh, 1978c; Goter, 1988], and the rupture probably propagated unilaterally southeastward ([Ellsworth, 1990]; Figure C.3). Modified Mercalli Intensities of V (felt by nearly everyone, some dishes and plaster broken, and tall objects disturbed) were noted over much of California (an area approximately bounded by the Northern Sierra Nevadas; Las Vegas, Nevada; San Diego; and San Francisco). Intensities of VI (felt by all, many frightened, a few instances of damaged chimneys and fallen plaster) were felt in Los Angeles, VI-VII (slight to considerable structural damage, everybody runs outdoors) were felt in the southern San Joaquin Valley, and IX and greater (considerable to complete damage and panic) were felt along the fault. The duration of shaking was between 1 and 3 minutes which is consistent with unilateral southeastward rupture [Ellsworth, 1990]. Many aftershocks were felt in the weeks and months following the event, although as time from the event became greater, it is harder to differentiate the felt events from "normal" moderate California earthquakes. However, the two best identified aftershocks were: 1) A large event northwest of Tejon Pass that was felt in Cajon Pass, Los Angeles, Ventura, Visalia, and Sacramento; and caused damage at Fort Tejon (10:30 to 11 pm PST, January 9, 1857); and 2) A M~6 event located between Tejon and Cajon Passes and felt in San Diego, San Bernardino, Los Angeles, Santa Barbara, and Castaic Junction (~5 pm PST, January 16, 1857) [Agnew and Sieh, 1978].
Along with the investigations of the felt effects of the 1857, earthquake, abundant geomorphic evidence for the event is preserved in the Carrizo Plain (e. g., [Arnold and Johnson, 1910; Dibblee, 1962; Wallace, 1968; Sieh, 1977; Sieh, 1978c; Wallace and Schulz, 1983; Wallace, 1990; Grant and Sieh, 1993]). H. W. Fairbanks, in [Lawson and others, 1908, p. 41, V. 1], described the geomorphology of the SAF in the Carrizo Plain and recognized that it had "probably originated during some of the earlier movements along the Rift;" and that "Plainly visible along the steep front of the line of hills described are the lesser ridges and hollows produced during the last violent earthquake in this region, probably in 1857." [Arnold and Johnson, 1910], p. 20] also identified the evidence for recent offsets: "The topography produced by the latest movements in this region is unique and interesting. Low ridges on both or one side of elongated depressions, sunken areas in flat land, and great furrows along hill slopes are some of the evidences of the fracturing which occurred, as near as can be determined, in 1857." Wallace, 1968 presented a study on the offset streams of the Carrizo Plain: 130 streams along the SAF in the Carrizo Plain were noted to have been affected by right lateral slip. 40 of those channels were offset 20 to 50 feet, which Wallace suggested corresponded to slip from the 1857 Fort Tejón earthquake. The observation that many streams had more than 50 feet of offset suggested that recurrent earthquakes along this reach of the SAF had slipped within a narrow zone and consistently offset the drainages.
While many other geologists surely recognized the geomorphic effects of the 1857 earthquake, Kerry Sieh [Sieh, 1977; Sieh, 1978c] was the first to thoroughly investigate the distribution and extent of the smallest offsets along this reach of the SAF (although Wood, 1955 compiled some of the contemporary accounts of surface offsets). Sieh hypothesized that the smallest offsets of the many reference features such as stream channels, landslides, and alluvial fans along this reach were attributable to coseismic and postseismic slip in the 1857 earthquake, assuming that they were formed before that earthquake and after the penultimate one [Sieh, 1978c]. Features were first identified in aerial photographic studies, and then the entire reach from Parkfield to Cajón Pass was traversed and the 150 measurements of offsets were based upon at least one quarter hour field investigation including detailed sketching and careful measurement using a tape. The resulting estimated distribution of offset was illustrated in Sieh and Jahns, 1984 which included data added after the publication of Sieh, 1978c.
With regard to the inferred 1857 slip distribution, a few points are worth mentioning.
1) The northern extent of the rupture is inferred to be at least Highway 46 in the Cholame area based upon an observation of a ~3.5±0.2 m gully 1.3 km SE of Highway 46, and contemporary accounts of the "earthquake crack" extending 80 km northwest of Cholame (into the creeping reach-[Wood, 1955; Sieh, 1978c]; Figures C.3 and C.4). This offset includes ~12 cm cumulative offset in the 1901, 1921, 1934, and 1966 Parkfield earthquake sequences. However, offsets along the SAF for ~20 km SE of Highway 46 are ambiguous, and other interpretations of this site suggest that geomorphic processes have accentuated the current offset [Sims, 1989]. Lienkaemper and Sturm, 1989 also argue that the Sieh, 1978c estimates for offset 21.5 km SE of Highway 46 are 2 m too low. See below for more discussion of these offsets.
2) Maximum right-lateral offsets in the 1857 earthquake are at least 7 m, and two examples of 9 m offsets are located a few hundred meters SE of Wallace Creek [Sieh, 1978c]. Again, the interpretation of offset gullies is difficult, and subsequent studies have argued that offsets determined geomorphically may be a maximum [Grant and Sieh, 1993; Sims et al., 1993]; see below.

Figure C.4. Important sites around the northern portion of the Carrizo Plain and the Cholame segment of the San Andreas Fault. The definition of the segmentation of the SAF by WGCEP, 1988. That for the Cholame segment is illustrated along with segment boundaries suggested by geologic evidence: the 1857 slip increased by a few meters and the surface trace bends in the area of Bitterwater Canyon. The Watertank site is a paleoseismic site [Sims, 1987]. Still Lake is a site that has been reconnoitered and is recommended for paleoseismic investigation. Note that it is 73 km between the existing paleoseismic sites in the Carrizo Plain and the Watertank site. KS 15 and the associated number refers to offset channel #15 from Sieh, 1978c and L&S '89 refers to the higher offset determined for the same channel by Lienkaemper and Sturm, 1989. James E. Freeman surveyed township boundaries from township 24 south in 1855 and 1856, and Grant and Donnellan, 1994 recovered original monuments from that survey spanning the SAF in the Carrizo Plain near Wallace Creek. The background is from the 1:250,000 scale San Luis Obispo and Bakersfield sheets of the state geologic map.

3) Evidence throughout the 1857 break (and particularly in the southeastern Carrizo Plain) is consistent with a single, large offset, rather than more, smaller offsets or creep [Sieh, 1978c].
4) Many subsidiary faults along the SAF must have also slipped in the 1857 break, but only two areas show geomorphic evidence significant secondary deformation: the Littlerock, Nadeau, and North Branch San Andreas faults in the western Mojave desert region between Littlerock Creek and Leona Valley; and in the Elkhorn Hills (bounded on their southwest margin by the SAF) of the southeasternmost Carrizo Plain where secondary normal and reverse faults display fresh scarps [Sieh, 1978c], and [Arrowsmith, 1991b; Arrowsmith, 1991a; Arrowsmith, 1992a; Arrowsmith, 1992b; Arrowsmith and Rhodes, 1992; Arrowsmith, 1995]; Figures C.2 and C.5. Slip along the SAF adjacent to the Elkhorn Hills apparently decreased abruptly by 10-20%, and that deformation may be accommodated by distributed deformation across the Elkhorn Hills [Sieh, 1978c] and [Arrowsmith, 1991b]. Recall the discussion above.
5) The southeastern extent of the 1857 break was probably as far southeast as Wrightwood (evidence from disturbed trees [Sieh, 1978c; Meisling and Sieh, 1980]) where the smallest offsets attributable to the 1857 event are about 3 m.
6) The inferred rupture length was at least 300 km.
7) Separating the 1857 slip distribution into 4 segments of similar slip magnitude, Sieh, 1978c determined a seismic moment for the event (including any afterslip) of 5.3 to 8.8 x 1027 dyne-cm­somewhat larger than the 1906 event (the variation results from different possibilities for the locations of the ends of the rupture, as well as its depth­10 or 15 km). A moment magnitude of M = 7.9 was determined by Hanks and Kanamori, 1979, while M = 7.8 was determined by Ellsworth, 1990.

C.3.2. Slip rates along the SAF

Starting with Wallace, 1968, analyses of the average Holocene slip rate along the SAF have found good sites in the Carrizo Plain. Sieh and Jahns, 1984 used radiocarbon dating and stratigraphy to obtain time constraints for incision of downstream channel segments at Wallace Creek (the name of which was proposed by them, because it was identified and discussed by Wallace, 1968; Figure C.6). The present and previous (now beheaded and abandoned) channels at Wallace Creek were occupied 3,700 and 10,000 yr ago and are now offset ~130 m and ~350 m respectively. These data and the 475 m offset of an alluvial fan give a late Holocene slip rate for this segment of the San Andreas fault of 33.9 ± 2.9 mm/yr for the past 3,700 yr and 35.8 + 5.4/-4.1 mm/yr for the past 13,250 yr.
Wallace, 1968 also suggested an investigation of the offset and abandoned channels of Phelan Creeks­approximately 2 km southeast of Wallace Creek (Figure C.6). In an

Figure C.5. Side Looking Airborne Radar image of the Carrizo Plain area. Look Direction is northeast. The rugged topography of the Temblor Range along the north side of the image is a contrast to the flatter Carrizo Plain. The patchwork texture of the Carrizo Plain results from variable grazing and farming practices on different properties. Soda Lake is the lowest portion of the Carrizo Plain (~1900' elevation). The SAF follows the southwestern foothills of the Temblor Range. Local relief along the SAF and a more westerly SAF strike increase toward the southeast. In the area of Wallace Creek in the northwest, little local relief is evident and Recent fault activity has been confined to dominantly strike-slip along a narrow fault zone. Toward the southeast in the Elkhorn Hills, on the other hand, local relief and secondary fault activity increase, possibly the result of increased contraction across the SAF zone caused by a more westerly SAF strike as it enters the Big Bend.

Figure C.6. Aerial photograph of the SAF zone in the central Carrizo Plain. The locations of the important landforms and paleoseismic sites of Wallace Creek [Wallace, 1968; Sieh and Jahns, 1984]; Phelan Creeks [Sims et al., 1989; Sims et al., 1993; Sims et al., 1994]; and Phelan Fan [Grant and Sieh, 1993] are shown. The trace of the SAF is indicated by the triangles on the lower photo. Note the en échelon, right stepping pattern of the SAF in this area. Local extension associated with a right step in the southeastern portion of the lower photograph has produced a small graben [Sims et al., 1990]. The upper two photos are blow-ups to show the detail of Wallace Creek and Phelan Creeks. Photograph courtesy of the Fairchild Aerial Photography Collection at Whittier College. Original photograph scale 1:24,000; photograph date 26 February, 1936.

extensive paleoseismic investigation of 23 trenches, Sims et al., 1989; Sims, 1994; and Sims et al., 1994 developed a chronology for the offset of the three sets of downstream channels at Phelan Creeks and determined a slip rate of 35.1 ± 2.6 mm/yr from the 238 ± 15 m offset of a channel system incised before 6778 ± 62 CAL B.P. For a channel incised a few decades after 3,764 ± 45 yr CAL BP or 3335 ± 129 yr CAL B.P. and offset 122.2 ± 1.5 m, slip rates of 32.5±2.1 mm/yr and 36.4±5.0 mm/yr are determined. Grant and Donnellan, 1994 recalculated the 14C dates from Sieh and Jahns, 1984 to determine a revised late Holocene slip rate of 33 ± 3 mm/yr. Clearly, these data are similar to that of Sieh and Jahns, 1984, and an average late Holocene slip rate of ~33 mm/yr may be assumed.

C.3.3. Previous events­slip and timing

The tectonic landforms along the 1857 reach and evidence for significant geologic offset along the SAF suggest that slip events must recur. Along much of the 1857 reach, and in particular in the Carrizo Plain, offsets of gullies are apparently clustered about integer multiples of the inferred 1857 offset and suggested that each part of the fault experienced a characteristic amount of slippage in the past few earthquakes. Around Wallace Creek, the offset has been 7 to 12.3 m during the last 3 large earthquakes [Sieh and Jahns, 1984]. However, with the typical dating and measurement errors and possibly variable slip distribution, it becomes difficult to differentiate between 3 7m events and 2 10.5 m events that might have caused a 21 m offset.
A re-interpretation of the offset in the last few events was presented in Grant and Sieh, 1993. They suggest that the 9 m offsets in the Carrizo Plain may result from either highly variable slip in 1857, or more regular 7 m 1857 slip, and 2 - 3 m slip in a previous event. Observations from the 1992 Landers, California earthquake favor a highly variable slip distribution (e.g., [Antonellini et al., 1992; Sieh and others, 1993; Arrowsmith and Rhodes, 1994]); while interpretations of a complex rupture history favor different events with different magnitudes of slip ([Grant and Sieh, 1994], see below and Figures C.7 and C.8). Geodetic reoccupation of historic survey markers by Grant and Donnellan, 1994 favor the highly variable slip distribution because they independently determined that at least 9.5 ± 0.5 m of slip occurred along the SAF near Wallace Creek in 1857.
The conclusion of recurrence of great earthquakes and the implications for the model of characteristic earthquakes [Schwartz and Coppersmith, 1984], as well as the threat to the growing population of southern California, drove efforts to determine the recurrence interval for these events. Given the slip rate of ~35 mm/year, a slip predictable recurrence interval for earthquakes might be determined by assuming that the slip in 1857 would be repeated and the event would not occur until that slip had accumulated (for

Figure C.7. Inferred slip deficit and geomorphic offset observations along the SAF in the area of the Cholame segment. The bold dashed line shows the inferred historic slip deficit assuming that all slip was released in the 1857 earthquake. The northern portion is constrained by observations of historic offset by Lienkaemper and Prescott, 1989, and the southern by assuming that the deficit accumulates at the long term slip rate of 33 mm/yr. Geomorphic observations are shown with error bars: circles are from Sieh, 1978c; square is from Lienkaemper and Sturm, 1989; the thin dashed line is the summary value from J. Lienkamper, unpublished field notes, 1987-1995; and the triangles show the results of Grant and Sieh, 1993; Grant and Sieh, 1994. The result from Grant and Donnellan, 1994 was 11 ± 2.5 m over a one mile aperture spanning th SAF.

Figure C.8. Inferred temporal and spatial distribution of surface rupturing earthquakes along the SAF in central and southern California. (modified from Grant and Sieh, 1994 by adding inferences from Sims et al., 1989; Sims, 1994; Sims et al., 1994). Dots indicate ages of events from historic records. Vertical bars indicate possible ages within 1 s (Phelan Creeks), and 95% confidence (all others) for pre-historic events. Letters refer to events defined in individual studies. Horizontal lines indicate possible correlations of events between sites; dashes and question marks indicate greater doubt. Phelan Creeks data is from Sims et al., 1989; Sims, 1994; Sims et al., 1994; Bidart Fan data is from Grant and Sieh, 1994; and Wrightwood data is from Fumal et al., 1993; Weldon, 1991. All other data is from published summaries compiled in WGCEP, 1988.

example, 9 m of slip would imply 257 year recurrence interval; [Sieh and Jahns, 1984]). Paleoseismologic studies of offset stratigraphy and landforms have provided data on the activity and applicability of simplistic recurrence models, as well as the spatial distribution of large events on the Carrizo Plain. The quality and distribution of the paleoseismologic data are dependent on the fortuity of preservation, as well as the skill and technology of interpretation. In this regard, the data are limited to several sites and have been reinterpreted. As more data becomes available, the picture of slip events along the SAF will no doubt be reinterpreted.

The current interpretation of paleoseismology along the SAF in Southern California is summarized in Sieh et al., 1989; Fumal et al., 1993; Grant and Sieh, 1994; Sims, 1994; Sims et al., 1994 and in Figure C.8. Apparently, at least 5 large, surface-rupturing earthquakes have occurred in the Carrizo Plain since ~1218 A.D. [Grant and Sieh, 1994; Sims, 1994; Sims et al., 1994]. And, at least 2 more occurred after ~200 B.C. (possibly dated by Sims, 1994; Sims et al., 1994 at 1003±20 and 838±35). Both of these recent studies in the Carrizo Plain have difficulties of interpretation. The events dated by Sims, 1994; Sims et al., 1994 are based upon the assumption that a large offset rupture event perturbs the stream channels enough to trigger deposition of a coherent sediment package, or abandonment of the channel. While this may appear a difficult hypothesis, it is defensible on several accounts. 1) horizontal offsets of ~7 m can change the local slope of the channel and deposition should occur as the stream power (proportional to slope and drainage basin area­[Horton, 1945]) decreases during the offset event. 2) the deposition may not occur immediately, but the moderate frequency storms that must do much of the geomorphic work have recurrence intervals on the order of tens of years­an order of magnitude less than that of the earthquakes and thus the event dated may be only within a few tens of years of the earthquake (similar as the 14C dating precision). 3) The offset to cause abandonment occurs when the stream is nearly ready to abandon itself because of the flow processes around the bends in the channel, so it is plausible that a large slip event could easily push the channel across that threshold, as well as possibly juxtaposing a new low across the fault from the upstream channel [Sims et al., 1993].
The investigation of Grant and Sieh, 1994 relies upon more typical paleoseismic analysis of fracture terminations and stratigraphic disruptions. However, it suffered from relatively poor radiocarbon control, as well as a depositional hiatus between 200 B.C. and 1218 A.D.­possibly concealing earthquake events. Despite these caveats, these studies have provided control on the late Holocene record of faulting in the Carrizo Plain. Their data may be correlated with that from further southeast and two conclusions drawn. 1) the Carrizo segment may rupture independently, in conjunction with the Parkfield-Cholame segments to the northwest (see below), or in conjunction with the Mojave segment to the southwest (e. g., 1857) and 2) the occurrence of these events may not be regular [Grant and Sieh, 1994]. Different ruptures from the northwest and southeast may explain the discrepancy in the interpretation of the offset in the last two events [Grant and Sieh, 1993].

C.4. Slip deficit along the Cholame segment and potential for rupture of M 7 earthquakes

The Parkfield segment of the SAF stretches from the southern end of the central creeping section (at about Middle Mountain) 30 km southward to the apparently locked Cholame segment that begins near State Highway 46 [WGCEP, 1988] (Figure C.3). The Cholame segment extends about 55 km SE to near State Highway 58 in the north central Carrizo Plain. Assuming that all of the accumulated elastic strain along the SAF in the Cholame segment was released in the 1857 earthquake, and assuming a long term slip (and thus loading) rate of ~35 mm/yr, Sieh and Jahns, 1984 suggested that a surface slip deficit of ~4 m (accumulated since the 1857 earthquake) exists along the Cholame segment (this was also suggested by Harris and Archuleta, 1987; WGCEP, 1988; Lienkaemper and Prescott, 1989). Harris and Archuleta, 1987 investigated data from geodetic networks spanning this segment and determined that it is probably locked to a depth of 15 km and thus accumulating elastic energy. From Parkfield northwestward, the surface slip deficit (since 1857) is small (accommodated by creep and the previous Parkfield earthquakes), while along the Cholame segment, very little (near Highway 46) to no historic slip since 1857 has been observed [Lienkaemper and Prescott, 1989] (Figure C.7). If we assume that a fault segment will be ready to slip again once the slip deficit accumulates to the same magnitude as the slip released in the last earthquake. Therefore, depending on the amount of slip in the 1857 earthquake along the Cholame segment, the area may or may not be ready to slip again.
The essential piece of ambiguous data is the slip magnitude along the Cholame segment in 1857. Several interpretations vary from 1 to 6 m. For the northern portion of the segment, John Sims suggests that only about 1 m of slip occurred in 1857 (personal communications, 1993-1995). The Sieh, 1978c survey of offset landforms along the SAF concluded that there was about 4 m of slip along this segment in 1857 and in the penultimate event. The Sieh, 1978c observations of ~9 m in the Carrizo Plain near Wallace Creek have been corroborated by Grant and Donnellan, 1994, although the slip in that area varied from ~9 m at Wallace Creek to ~7 m at Phelan Fan 2.6 km SE [Grant and Sieh, 1993]. Lienkaemper and Sturm, 1989 made a careful investigation of an offset stream channel along the Cholame segment and inferred that it was offset ~6 m in the 1857 earthquake. In spring 1987, a blue ribbon committee of USGS geologists visited the area and "discussed the evidence with Sieh. They concluded that the geomorphic features recorded major fault slip, but differed among themselves in their interpretations, emphasizing the ambiguous nature of the geomorphic data" [WGCEP, 1988, p. 33].

C.4.1. Recommendations for future research: paleoseismology, continued geomorphic investigations, and investigation of historic land surveys

While there are many possible avenues of research concerning the segmentation and seismic rupture potential of the central SAF, I recommend the following as potentially fruitful avenues of research that may help determine the timing of slip events and the slip in the 1857 earthquake along the Cholame segment.

C.4.1.1. Paleoseismology

The Cholame segment has not received much attention in paleoseismic investigations. The Watertank site, Sims, 1987 is about 3 km NW of Highway 46 (Figure C.4). The next paleoseismic sites are on the Carrizo Plain at Phelan Creeks [Sims et al., 1994], Phelan Fan [Grant and Sieh, 1993], and the Bidart site [Grant and Sieh, 1994], about 70 km SE of Highway 46. Within those ~73 km, there are candidate sites for paleoseismic investigations. For example, the area around Still Lake (~11 km SE of Highway 46) has been surveyed by John Sims and colleagues (Figure C.4).

C.4.1.2. Continued geomorphic investigations

Between Highways 46 and 58, Sieh, 1978 observed 19 offset landforms. Lienkaemper and Sturm , 1989 investigated one of them in great detail by generating detailed topographic maps from repeated aerial photography. That type of thorough geomorphic and sedimentologic investigation should be continued, and further work accompanied by paleoseismic investigations may begin to eliminate the ambiguity associated with the geomorphic offset indicators.

C.4.1.3. Investigation of historic land surveys

This is an intriguing possibility. In 1855 and 1856, James E. Freeman surveyed Township and Range lines in the region of the Carrizo Plain and possibly northward. Grant and Donnellan, 1994 recovered original monuments from that survey spanning the SAF in the Carrizo Plain near Wallace Creek and determined that the 1857 slip along the SAF between those monuments was 9.5 ± 0.5 m. While most of the monuments have been moved, destroyed, or otherwise lost, the possibility of recovering a few more along the Cholame segment is interesting. Based upon the work of Grant and Donnellan, 1994 (whose errors were ~1-2 m), we could differentiate between offsets of 1 and 6 m along the Cholame segment in the 1857 earthquake.

C.5. Acknowledgments

Kerry Sieh reviewed an early version of this manuscript, and his comments (especially with regard to the history of the scientific research in the Carrizo Plain) are appreciated. Ron Lyon provided the radar image shown in Figure C.5. I am grateful to Mark Zoback, Bill Ellsworth, and Steve Hickman for their invitation to participate in the San Andreas Fault Zone Deep Drilling Project Workshop, and to the California Earthquake Prediction Evaluation Council (CEPEC) and the California State Geologist, Jim Davis, for the invitation to comment on the geologic and geomorphic observations along the Cholame segment. These invitations encouraged me to summarize the research presented in this appendix.

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