CHAPTER 4
CALCULATION OF SEISMIC SLIP RATES, RECURRENCE INTERVALS, AND UPLIFT GRADIENT RATES FOR THE CRISTIANITOS AND MISSION VIEJO FAULTS USING AGE-DATED FLUVIAL TERRACE LEVELS AT SAN JUAN CREEK, CALIFORNIA
Abstract
Ages and elevations of 13 near-coastal fluvial terrace levels occurring in the San Juan Creek drainage basin in southern California are used to evaluate regional uplift trends, seismic slip, and recurrence intervals along the Mission Viejo and Cristianitos faults. The displacement of terrace levels along the Mission Viejo fault indicates that the western block has been downthrown relative to the eastern block. One offset terrace couple on the Cristianitos fault indicates that the east block of this fault has been downthrown relative to the western block creating a graben between the 2 faults. The slip rate for the Mission Viejo fault is increasing, and its Holocene rate is estimated to be 0.057 m/ka. Projection of the slip rate trends indicates activity commenced on the Mission Viejo fault ~345 ka. Average recurrence interval for this fault is calculated to be ~19.5 ka. These data suggests that although the recurrence interval on the Mission Viejo fault is long, the fault is still experiencing active slip at an ever increasing rate. One pair of offset terraces on the Cristianitos fault yields a slip rate of 0.03 m/ka, and an average recurrence interval of ~57 ka. The uplift rate has remained uniform over the last 500 ka at ~0.29 m/ka, but small localized differences in the tectonic uplift rate have produced uplift rate gradients (tilting) expressed on most terrace levels between the 2 faults. Two trends in the uplift rate gradients occur, the first ranging from 0 to 195 ka and appearing essentially uniform at 0.038 m/ka-km, and the second expressing an inclined gradient ranging from 195 to 310 ka indicating the initiation of tectonic tilting to be ~313 ka. The directions of uplift rate gradients frequently reverse through time possibly indicating seismically controlled tilting of the middle graben block.
4.1. Introduction
The San Juan Creek drainage basin lies within the coastal zone of southern California near San Juan Capistrano (Fig. 1.1) and has a significant number of fluvial terraces contained within its tributaries occurring in association with marine terraces at the mouth of the drainage system. These terraces have been mapped and correlated into former-longitudinal profiles (Chapter 1), and dated (Chapter 2) using a variety of dating techniques including; 1) cosmogenic radionuclide dating; 2) correlation with previously dated marine terraces; and 3) correlation with the glacio-eustatic sea level fluctuation curve.
4.1.1. Climate, Vegetation, Hydrology, and Topography
The San Juan Creek watershed is situated primarily in the southern portion of Orange County, California. The eastern extremities extend into Riverside County and the southern into San Diego County. The total relief of the drainage basin is 1,715 m asl. This number reflects the height of Santiago Peak, the highest point in the basin. In its entirety, the basin comprises an ~456 km2 drainage area. The watershed tilts generally southwest with characteristics that can be described in sections. The upper third of the drainage basin has steep slopes with deep cutting canyons and is extremely rugged. The central portion of the watershed features rolling hills, and the remaining lower third exhibits a highly developed floodplain. San Juan Creek originates in the Santa Ana Mountains with its main stream channel beginning in the Cleveland National Forest. From there it flows ~43.5 km from the headwater at ~1,280 m asl, to the Pacific Ocean where it empties at Doheny State Beach near Dana Point Harbor. Trabuco Arroyo, Oso Creek, Bell Canyon, and Canada Gobernadora are the major tributaries to San Juan Creek. All of these represent intermittent streams that now often bear water almost year-round because of current urban and agricultural development (U.S. Army Corps of Engineers, 2002).
The climate of southern California is a mild Mediterranean one. It is typified by dry, warm summers and wet, cool winters. Largely, stream flow occurs in conjunction with rains with rapid runoff increases corresponding to heavy rainfalls (Kochel et al., 1997). Occasionally, the uppermost part of the watershed may receive small amounts of snow, but this should be considered a negligible contributor to runoff. Near the coast precipitation is ~33 cm and this increases to ~45 cm in the mountains. The months between November and April represent the main flood season and are accompanied by frequent rainless periods in summer that can last as long as several months (U.S. Army Corps of Engineers, 2002). Overlaying annual variations are episodic but irregular cycles of wet and dry periods related to the climate phenomenon known as the Southern Oscillation (El Niño). These periods last for 3-6 years and 10-30 years respectively. (Kochel et al., 1997).
The calculated storage capability of the San Juan Creek groundwater basin is expected to be ~0.111 km3. This unconfined groundwater generally exists in the shallow, narrow, alluvial valley fill in the area of San Juan Canyon and its tributaries. This fill, when measured from the coast up throughout the tributaries, varies from ~61 to 0 m. The groundwater alluvium can be separated into an upper and a lower area where San Juan Canyon is traversed by the Cristianitos fault (U.S. Army Corps of Engineers, 2002).
Characteristic of the area are chaparral and coastal sage/grassland communities. Chaparral dominates in the upper portion of San Juan Creek watershed, and a combination of sage scrub (chaparral, riparian vegetation and sage/grassland) is exhibited in the lower portion. Coastal sage shrub (often called southern coastal shrub or soft chaparral) and chaparral are the major shrubland types of this lower section. Remnants of oak woodland appear only in small valleys and potreros on moister hillsides (Lewis, 1942). These oak woodlands are thickly enclosed within the chaparral and coastal sage. There is considerable development of the Trabuco Arroyo watershed in areas downstream of the Cleveland National Forest (U.S. Army Corps of Engineers, 2002).
4.1.2. Geologic and Geomorphic Setting
Southern California can be divided into the Transverse Ranges, the Peninsular Ranges, and the Los Angeles Basin (Fig. 1.1). The Peninsular Ranges south of the Los Angeles Basin are comprised mainly of several Cretaceous granitic blocks that have been uplifted along major faults. These blocks extend southward from the Los Angeles Basin to the end of Baja California and are bounded on the east by the San Andreas fault. The location of this study area lies on the western border of the Santa Ana Mountains, which occur in the northeast portion of the Peninsular Ranges near the coast, 32 km south of the Los Angeles Basin. Progressive uplift and westward tilting of the Santa Ana Mountains have resulted in entrenchment and headward erosion of the river and tributary system, resulting in the establishment of numerous fluvial terraces throughout the drainage basin.
San Juan Creek drains into the Pacific Ocean at the city of Dana Point. This study is based on the fluvial terraces found along San Juan Creek which begin to occur ~9.6 km upstream from the Pacific Ocean. All terraces mapped are fill terraces having thicknesses of 6 m or more. In this regard, they represent terraces created by glacio-eustatic sea level fluctuation rather than strath and cut terraces which are principally the result of the natural river system growth, incision, and extension (Merritts et al., 1994; Chapter 2).
The coastal zone of southern California near San Juan Capistrano comprises Late Cretaceous to Recent rocks and sediment (Fig. 1.2). In this area a sedimentary veneer of Miocene rocks has been laid across the Cretaceous Williams Formation from east to west, with some exposures of igneous Cretaceous plutons in the highlands of the Santa Ana Mountains. Two fault zones strike primarily north-south across the drainage basin, including the Mission Viejo fault zone, consisting of several branches that crosscut San Juan Creek and Bell Canyon, and the Cristianitos fault zone, consisting of several branches that crosscut San Juan Creek and Trabuco Arroyo (Morton and Greensfelder, 1976; Morton and Miller, 1981).
The San Joaquin Hills form the southern margin of the Los Angeles Basin fault system (Dolan et al., 1995) ~16 km northwest of San Juan Creek. Barrie, et al. (1992) established ages for the first and second terraces (80-85 ka, and 120-130 ka, respectively) in the marine terrace suite on the western flank of the San Joaquin Hills between Newport Beach and Laguna Beach in Orange County. The ages for these terraces are established using amino acid racimization techniques on gastropod shells found within the marine sediments on the terraces. From this data, they determine the uplift rate of 0.25 m/ka.
Grant et al. (1997) uses Barrie’s data to examine the Quaternary deformation and potential blind thrust fault in the San Joaquin Hills on the north border of this study area (Fig. 1.1). The hills are formed by a northeast-vergent anticline that has uplifted and deformed marine terraces. Grant et al. (1999) proposed that this anticlinal fold developed above an active, southwest-dipping blind thrust that slips at a rate of 0.42-0.79 m/ka based on uplift rates of 0.21-0.27 m/ka. Rivero et al. (2000) confirms the existence of the blind thrust, and suggests that the San Joaquin Hills anticline is the northern onshore extension of the Oceanside thrust detachment, having an average southwest dip of 23o. Grant et al. (1997) interprets the San Joaquin Hills anticline and blind thrust to be the product of partitioned strike-slip and compressive shortening across the southern Newport-Inglewood fault zone (Grant et al., 1997; 1999). These recent studies indicate the possibility of a blind thrust fault beneath the San Joaquin Hills on the northern boundary of this study area which may extend within the boundaries of the San Juan Creek basin (Grant et al., 1997).
4.1.3. Previous Work
At San Onofre State Beach, the Cristianitos fault is exposed in the beach cliff beneath an overlying, unbroken, regressive marine terrace. The terrace has been dated by U/Th, amino acid, faunal association, and soil-stratigraphic techniques (Shlemon, 1992) to be ~125 ka occurring during marine oxygen isotope stage (MIS) 5e. This marine terrace and wave-cut platform is exposed between Oceanside and Laguna Beach and is uplifted northward. The platform is 6 to 10 m above sea level at San Onofre having an uplift rate of ~0.09 m/ka, and is 17 to 29 m above sea level at Dana Point (to the north) having an uplift rate of 0.26 m/ka. Barrie et al. (1992) delineates 8 elevated marine terraces south of Newport Beach and reports a shoreline angle elevation of 37 m for their 125 ka Terrace No. 2. They calculate an uplift rate of 0.25 m/ka, based presumably on an assumed paleo-terrace height of +6 m for the 125 ka interglacial.
The development of near-coastal fluvial fill terraces as a response to glacio-eustatic sea level fluctuations have been noted and discussed on various river systems in both tectonically active and non-active regimes (Ramsay, 1931; Zeuner, 1945; Clayton, 1964; Merritts et al., 1994; Taylor et al., 2006, to name a few). Merritts et al. (1994) astutely observes that the deposition of fluvial terraces along river-reaches near a marine coastal system will be controlled by aggradation resulting from glacio-eustatic sea level fluctuation (fill terraces), while terraces along upstream reaches of the river will be controlled by regional uplift and headward erosion (strath terraces). The downstream fill terraces will be parallel to each other and to the modern stream gradient, while the upstream strath terraces will be inclined to the modern stream gradient and will converge in the downstream direction.
As indicated in Chapter 2 of this study, all of the Quaternary terrace deposits observed in the Bell Canyon area consisted of alluvial material ranging from 6 to 13 m in thickness and, in this regard, all are fill terraces as opposed to cut terraces or strath terraces. Fluvial terraces in the lower San Juan Creek (SSJ) have been examined by Taylor et al. (2006) who report thicknesses ranging from 5 to 15 m again indicating fill terraces. The presence of thick, near-coastal fill terraces indicates that this reach of the San Juan Creek drainage system has been strongly influenced by glacio-eustatic sea level fluctuations superimposed on a slowly uplifting coastline.
Within the San Juan Creek study area, fluvial terraces have been surveyed, correlated, and age dated by 2 authors including Taylor et al. (2006), and this present study (Section 2.3.3). Taylor et al. (2006) surveyed terraces from just above the floodplain to the highest elevations in the hills of the lower San Juan Creek. Surveyed terraces within this present study (Chapter 2) range from ~20 m above the floodplain in both lower and upper San Juan Creek, and in Bell Canyon. These 2 surveys overlap by ~38 terraces within the lower San Juan Creek that occur ~20 m above the modern floodplain (Fig. 2.11). Taylor et al. (2006) established 6 fluvial terrace levels, 5 of which occur 20 m above the floodplain. These authors provide a broad variety of possible dates for each terrace level based on a variety of dating techniques including luminescence dating, correlation with marine terraces, and use of the glacio-eustatic sea level fluctuation curve. They conclude that the regional uplift throughout the area is uniform and is 0.40 m/ka (a value much higher than other studies throughout the region; Chapter 3), and that no slip has occurred on the Cristianitos and Mission Viejo faults over the last 500 ka.
This present study (Chapter 2) establishes 13 terrace levels throughout the San Juan Creek and Bell Canyon area (as opposed to 5), and evaluates the timing of the fluvial terraces (Chapter 3) based primarily on correlation with marine terraces and the glacio-eustatic sea level fluctuation curve. This current study indicates that the regional uplift rate is 0.29 m/ka. The conclusions established in this study regarding the number of terrace levels, regional uplift rate, terrace chronology, and displacement of terraces by the Cristianitos and Mission Viejo faults are quite different than those postulated by Taylor et al. (2006).
4.2. Methodology
Terrace levels present in San Juan Creek and Bell Canyon can be equilibrated to sea level by subtracting the stream gradient elevation from the terrace elevation at the point where each fluvial terrace is projected onto the stream gradient (Fig. 3.4; Section 3.2.2). As previously discussed, near-coastal terraces dominated by glacio-eustatic sea level changes will be parallel to one another. Variations from this geometry are indicative of changes resulting from either seismic displacement (opposite sides of a fault) or local variations in regional uplift. Terrace levels along SSJ between the Cristianitos and Mission Viejo faults are tilted higher to the west (downstream) and lower to the east (upstream) expressing a variable uplift rate dependent on location (Fig. 4.1).
The tilt of the terraces in these 2 study areas is in the opposite direction that would be expected if their inclination were the result of natural headward expansion of the stream system. That is, location in the stream system and formative depositional environment (dominated by glacio-eustatic or headward erosion) cannot explain the tilt of the terrace levels.
Very little data is available for evaluating the tilting of terrace levels east of the Mission Viejo fault (NSJ). However, these terraces do show indications of displacement across the fault. The potential for fault displacement on the Mission Viejo and Cristianitos faults will be discussed in section 4.3.2.
4.2.1. Uplift Rate Gradient
The rate of uplift between individually uplifted terraces is sometimes unequal (Fig. 4.1), and when trends exist on various levels, it is helpful to evaluate them by looking at the trends between different levels. The uplift rate of any terrace tread can be established using the following formula:
ur = el-psl
T
where uris the uplift rate, el is the terrace elevation above the stream gradient (equilibrated to sea level, Chapter 3), T is the age of the terrace level, and psl is the ancient sea level highstand at the time of formation. Once the uplift rate for individual terraces is known, the relationship between any 2 uplift rates on the same level can be expressed in terms of the tectonically tilted slope of that level (uplift rate gradient); that is, the slope of the line expressed in terms of its uplift rate. The uplift rate gradient can be calculated by subtracting the uplift rate of 1 individual terrace from that of another on the same terrace level, and then dividing the difference in uplift rates by the distance between them. The equation for this slope can be written as:
urg = (ur1 – ur2)/d
where urg is the uplift rate gradient (change in the uplift rate), uris the uplift rate of 2 terraces (ur1 is downstream, ur2 is upstream) found on the same terrace level, and d is the distance in kilometers between the 2 locations where the terraces are projected onto the modern stream gradient (mid-valley axis). This equation can also be rewritten by combining it with the equation above into the formula:
ugr = ((el1-psl)/T - (el2 – psl)/T)/d
where el is the elevation of 2 terraces (el1 is downstream, el2 is upstream) above the stream gradient found on the same terrace level. Note that since the terraces are on the same level, psl (ancient sea level) and T (age of the terrace level) are the same for both terraces. This equation can be reduced to:
urg = (el1 – el2)/T • d
This formula simply expresses the gradient or slope of each line as uplift per kilometer so that slopes of various terrace levels can be evaluated in terms of their uplift rate. As the formula above indicates, a positive urg represents a change in which the downstream side is uplifted higher than the upstream side causing the terrace level to be tilted downward in the upstream direction (the normal case in this study area). A negative urg represents tilt downward in the downstream direction. Additionally, the larger the urg, the more tilt in the terrace level slope, and the smaller the urg, the less tilt in the slope. Numbers that are at or near zero express no tilt. The urg of all terrace levels having sufficient data have been calculated for both the San Juan Creek and Bell Canyon study areas, and can be found annotated on Table 4.1.
Table 4.1 also shows that the average urg can be calculated by dividing the sum of all uplift rate gradients calculated in a study area by the number of terrace levels used in the calculation. In SSJ, the average urg (slope) is 0.006 m/ka-km, while the average urg in Bell Canyon is 0.008 m/ka-km.
4.2.2. Partitioning of Uplift Rate Gradients
All of the terraces examined in the urg calculations (Fig. 4.1a) are oriented along the 2 primary directions, each in a separate study area (Fig. 4.1b). The first set of terraces is located in the SSJ and trends east to west. The second set of terraces is located in Bell Canyon and trends north to south. These 2 partitioned orientations are very nearly perpendicular, and for the purposes of this analysis, a right angle orientation will be assumed. The total urg between the 2 sides of the right triangle can be calculated using vector addition and the Pythagorean theorem; the hypotenuse of the triangle representing the total urg. This analysis can provide an estimate of total urg for the entire study area and local region. For example, when applied to the overall average urg for SSJ and Bell Canyon (calculated above), this calculation yields:
(overall urg)2 = (0.006 m/ka-km)2 + (0.008 m/ka-km)2
overall urg = 0.01 m/ka-km
Calculations indicating the total urg for those individual terrace levels (where data is available) are shown in Table 4.1.
4.2.3. Interval Uplift Rate Gradient
The total urg establishes a pattern of cumulative change throughout this study area, but that total cumulative change disguises the true geomorphic history of the area because the magnitude of each total urg consists of both the magnitude of that level and the magnitude of all other levels below it. This total urg can be segmented into discrete time-segments representing the intervals between terrace levels, and it is this calculation of the segmented urg for each level that will reveal the urg geomorphic history of the area (Fig. 4.2).
As a flat alluvial surface (incipient terrace level No. 1) begins to be uplifted, it may experience tilting from uneven tectonic uplift (flat as expressed on the sea level equilibrated graph, Fig. 3.4). When the next alluvial surface (incipient terrace level No. 2) forms below the newly uplifted terrace, the new terrace may be tilted while the lower formative surface is a flat floodplain with no tilt. Once the lower surface begins to uplift, if it experiences tilting, the new tilt will be added into the inclination of the terrace above it. Therefore, by subtracting the urg of the lower terrace from the terrace above it, the lower terrace is recalibrated to a horizontal plain, and the original interval of inclination for the higher terrace can be determined (Fig. 4.2). From these various intervals of inclination a urg geomorphic history can be determined. This simple formula can be expressed as the following:
urg terrace1 (higher) – urg terrace2 (lower) = interval urg
where urg terrace1 is the upper terrace, and urg terrace2 is the lower terrace. The interval urg has been calculated for each terrace level for which there is data (Table 4.2).
4.2.4. Direction of Uplift Rate Gradient
The above analyses provide the overall and interval urg and, by examining the vectors (Fig. 4.1b), a general idea of the direction of increasing uplift can be obtained. To calculate the actual direction (in degrees from north; 360o scale) of the urg, a trigonometric formula (arctan) can be used. This calculation is possible because of the right angle formed by the 2 study areas, the use of vectors to evaluate the urg, and the observation that Bell Canyon trends due north. The formula is:
Do = (arctan SSJ/BC) + Qo where Do is the direction of increasing uplift in degrees from north, SSJ is the urg occurring along southern San Juan Creek (opposite side of the triangle), BC is the urg occurring in Bell Canyon (adjacent side of the triangle), and Qo is a constant added to the final calculation to place it in the proper quadrant depending on the direction of increasing uplift expressed by the partitioned vectors SSJ and BC (Fig. 4.3).
Applying this formula to the average urg calculated above, for example, yields:
Do = (arctan 0.006/0.008) + 180o = 189.6o
Combining the results obtained from the overall urg calculation with this directional calculation, reveals that throughout the study area, while the total uplift occurred at a rate of ~0.29 m/ka, uplift rate gradient (or tilting) also occurred at a total rate of 0.01 m/ka-km increasing overall in a nearly due south direction (190o). The urg interval directions have been calculated for all terrace levels with sufficient data (Table 4.2).
4.2.5. Displaced Terrace Levels Terrace levels have been equilibrated to sea level and graphically projected onto the stream gradients of lower San Juan Creek and Bell Canyon in Figure 3.4 (Chapter 3). The graphical representation of the possible displacement along the Cristianitos and Mission Viejo faults is shown in Figure 4.4a, and the zones indicated in Figure 4.4b.
Possible displacements of both the upstream and downstream terrace levels within the study area are in Table 4.3.
4.2.6. Slip Rates on the Mission Viejo Fault
The western side of the Mission Viejo fault has been downdropped while the eastern side has been uplifted (Fig. 4.4). In a zone of active seismicity where terraces are displaced by faults, slip rates can be calculated from the equation:
R=D/T
where R is the slip rate, D is the amount of displacement across terrace levels, and T is the age of the terrace level. Terraces on various former longitudinal stream profiles that may have been displaced by the Mission Viejo fault have been analyzed using this formula and are noted in Table 4.3. This table displays 2 sets of calculations and assesses both areas of the Mission Viejo fault as it trends back and forth across the terraces within the study area. The slip rates displayed in Table 4.3 represent the cumulative slip rate for each terrace level analyzed.
When the slip rate for more than one terrace level is known, it is possible to evaluate the slip rate for discrete intervals through time by establishing the amount of displacement that occurred between time intervals. The interval slip rate can be calculated by modifying the equation above:
IR = (d1-d2)/( t1-t2)
where IR is the interval slip rate, d1-d2 is the amount of displacement between 2 terrace levels (d1 higher terrace, d2 lower terrace), and t1-t2 is the interval of time between the formation of the 2 terrace levels (t1 older terrace level age, t2 younger terrace level age). Table 4.4 shows the assessment of interval displacement and the calculation of the interval slip rate for the Mission Viejo fault.
4.2.7. Recurrence Interval for the Mission Viejo Fault
The recurrence interval for seismic activity on a particular fault can be calculated from the equation:
t=d/R
where t is average recurrence time, R is slip rate, and d is average displacement-per-earthquake. The average displacement per earthquake can be calculated using the empirical regression relationship between fault length and maximum displacement-per-earthquake (Fig. 4.5) established by Wells and Coppersmith (1994).
4.3. Results and Discussion
4.3.1. Uplift Rate Gradients
A history of the uplift activity is recorded by the uniform uplift rate in the study area, the uplift rate gradient interval per level, and the direction of the uplift rate gradient interval per level. The uplift rate has remained constant over the last 500 ka at ~29 m/ka, but small, localized differences in the tectonic uplift rate have produced uplift rate gradients that are expressed throughout the study area on each terrace level.
All of the T11 terrace levels express this uplift rate gradient, as do levels T10, T9, T8, and T6. The only terrace level to not express downstream uplift rate gradient is T7 where 3 terraces are essentially horizontal. The multiple terrace levels occurring in terrace level T11 (including T11a, T11abc, T11b, T11bc, T11c, and T12) are spaced very close together and a detailed numeric analysis with values so close in elevation would not be productive. Therefore, the only terrace level of this terrace group that has been analyzed is T11a (Tables 4.1 and 4.2). Terrace levels T5, T4, T3, T2, and T1 do not have enough terraces in this location to evaluate uplift gradiant trends. Terrace levels within Bell Canyon between the Cristianitos and Mission Viejo faults are tilted (higher downstream, lower upstream) expressing an uplift rate gradient which increases southward (inclined dark lines Fig 4.1a; dashed arrow pointing south Fig. 4.1b). The uplift rate gradients expressed in Table 4.1 are cumulative rates, and in order to assess the tectonic history of any particular level, the cumulative influence of previous levels must be subtracted from it to establish its uplift rate gradient interval (Table 4.2). The magnitude and direction of these intervals have been plotted in Figure 4.6, which shows 2 renderings of each uplift rate gradient interval including a line covering the entire time span of each gradient, and also a point representing the average time and interval gradient for each dated terrace level.
Two trends in the uplift rate gradients can be seen in this figure, the first ranging from 0 to 195 ka and appearing essentially uniform, and the second inclined gradient ranging from 195 to 310 ka. Both of these trends have been highlighted with trendlines showing the average of the gradient points in each group. The intersection of the first uplift rate gradient interval trendline and the y-axis represents the current magnitude of the uplift rate gradient today (0.038 m/ka-km), while the intersection of the second gradient and the x-axis indicates the initiation of tectonic tilting throughout the study area beginning ~313 ka.
The directions of the uplift rate gradients are also shown in Figure 4.6. This figure indicates that frequently reversals in direction occur in the gradients from one terrace level to the next. This oscillation in almost opposite directions indicates that the controlling mechanism for the uplift rate gradient moves from one end of the study area to the other. Since this block lies between the Mission Viejo and Cristianitos faults, seismic activity, or a combination of seismic and tectonic activity, may be responsible for the oscillation of uplift occurring at opposite ends of the faults. This topic will be discussed further below when the relationship between the Mission Viejo and Cristianitos faults is established.
In addition to the frequent oscillation of the uplifted rate gradients, older terrace levels (125-310 ka) have an uplift rate gradient magnitude with directions that oscillate in the NE/SW quadrants while younger terrace levels (125 ka to recent) have gradient magnitudes that oscillate in the NW/SE quadrants. This change of gradient orientation may be signaling an accompanying change in seismic or tectonic uplift orientation.
4.3.2. Fault Displacement
The Cristianitos fault intersects the San Juan Creek/Bell Canyon stream gradient in the downstream portion of Figure 4.4a (~10.5 km from the ocean), while the Mission Viejo fault intersects the stream gradient farther upstream in 2 locations, crossing San Juan Creek trending north/south and then crossing Bell Canyon again trending north/south. These faults are projected on the graph in Figure 4.4a tracing vertically up from the valley floor (stream gradient equilibrated to 0 m elevation) and passing up the canyon walls separating individual terraces onto fault-bound blocks east and west of the Mission Viejo fault. This graph is not a representation of a vertical wall, but rather represents an incline plane (sloping canyon wall) viewed only in 2 dimensions. The Mission Viejo fault, trending across this slope at an oblique angle to the stream gradient, appears to be a wandering low-angle line trending diagonally across the graph. The odd appearance of this fault results from the fact that the wall is an incline plane rather than vertical, and the fault does not intersect the plane at a perpendicular angle like the vertical line represented by the Cristianitos fault.
Displacement across the Mission Viejo faults can be seen on several of the T11 terrace levels (south and north strands), but for the purposes of this discussion only the T11a level has been highlighted to be used in displacement calculations (Fig. 4.4a; Table 4.3). Additional terrace levels showing displacement across the Mission Viejo fault include terrace level T9 (south strand) and T6 (south and north strands). Terrace levels T8, T7, T3 and T2 have offset terraces that have distances >3 km from each other which may obscure accurate displacement determination, so these terrace levels have not been included in the offset assessment. Terrace levels T10, T5, T4, and T1 do not have terraces on opposite sides of the Mission Viejo fault and, so, provide no data for displacement analysis.
4.3.3.Slip Rates on the Mission Viejo fault. Figure 4.4a indicates that the Mission Viejo fault has offset terraces in this study downdropping terrace levels to the west. Table 4.3 shows an increasing trend in fault slip rate for the Mission Viejo fault over the last 345 ka. This increasing fault slip rate is illustrated in Figure 4.7 which is a graph of interval slip rates.
A mean-average trendline has been placed on the graph and extrapolated to the X and Y axis intercepts. The intersection of this trendline with the y-axis indicates that the current slip rate on the Mission Viejo fault is ~0.057 m/ka. The intersection of this trendline with the x-axis indicates that slip on the Mission Viejo fault initiated ~345 ka.
4.3.4. Recurrence Interval for the Mission Viejo Fault
The Mission Viejo fault is ~25 km long (Shlemon, 1992). Using a surface rupture length (SRL) equal to the full length of the fault, the average displacement per earthquake can be calculated with the empirical regression relationship between fault length and maximum displacement-per-earthquake (MD) established by Wells and Coppersmith (1994). Applying this regression equation:
log(MD) = -1.38 + 1.02 • log(SRL)
yields a maximum displacement of 1.11 m for the Mission Viejo fault. If this maximum displacement is used as the average displacement-per-earthquake for the Mission Viejo fault, and the estimated current slip rate of 0.057 m/ka is used, then the calculated average recurrence interval is 19.5 ka. This analysis suggests that, although the recurrence interval on the Mission Viejo fault is long, this fault is still experiencing active slip at an ever increasing rate. Additionally, it must be noted that this average displacement-per-earthquake is the maximum that the fault can yield, and therefore, this average recurrence interval should be viewed as the maximum possible. If smaller displacement-per-earthquake ruptures are commonplace on this fault, then a smaller recurrence interval (more frequent) will be required in order to achieve the total displacement indicated by the offset terraces.
4.3.5. Recurrence Interval for the Cristianitos Fault
Information is quite sparse within the study area indicating displacement on the Cristianitos fault owing to the fact that very few terraces occur immediately downstream of this fault. Although several terrace levels mapped in this study cross this fault and appear to be offset, only the T8 terrace level has terraces within 1 km of each other occurring on opposite sides of the fault. These 2 terraces occur at elevations of 59.5 m downstream, and 53.2 m upstream of the fault. The terraces are offset from each other by 6.3 m with the western side of the Cristianitos fault displaced upward relative to the eastern side. The T8 terrace level has been dated at 210 ka (Section 3.3.2) and, based on the formulas provided above, this date provides a slip rate calculation of 0.03 m/ka. The Cristianitos fault is ~35 km in length (Shlemon, 1992) and applying the Wells and Coppersmith (1994) regression model to this fault yields a maximum displacement of 1.57 m. Using this maximum displacement as the average displacement-per-earthquake for the Cristianitos fault, an estimated recurrence interval for this fault is calculated to be 52.3 ka.
4.4. Conclusions
Displacement analysis of terrace levels along the San Juan Creek and in Bell Canyon indicate that the western side of the Mission Viejo fault has been downthrown relative to the eastern side. One offset terrace couple on the Cristianitos fault indicates that the east side of this fault has been downthrown relative to the western side creating a graben between the 2 faults. These 2 faults systems appear to cut across the Bell Canyon and Gobernadora Canyon Lineaments (Chapter 2), indicating that the faults and the graben they form postdate the lineaments.
An overall uplift gradient of ~0.29 m/ka has been acting to raise this area over the last 500 ka years. Beginning ~345 ka, activity on the Mission Viejo fault (and presumably the Cristianitos fault) has had the effect of tilting the downdropped graben block into various orientations throughout its seismic history and preserving a record of these variable tilt orientations in the uplift rate gradients present on the terrace levels today. Uplift rate gradient tilting begins ~315 ka, just after the initial activity on the Mission Viejo fault commenced. Before 125 ka, tilting oscillations were oriented within the NE/SW compass quadrants, but after this time, oscillation became oriented in the NW/SE compass quadrants (levels T10 and T11) signaling a possible change in the stresses controlling seismic and tectonic activity within the downdropped graben between these 2 faults. Specifically, this orientation may indicate that before 125 ka the northern portion of the Mission Viejo fault and the southern portion of the Cristianitos fault were more active orienting gradient tilting in these 2 directions. After 125 ka activity may have shifted to the northern Cristianitos fault and the southern Mission Viejo fault. If this assessment is valid, then it must be recognized that these faults do not commonly rupture through their entire length with each earthquake.
Analysis suggests that activity on the Mission Viejo fault is still present, and the slip rate is continuing to increase. Assessment of the slip rate intervals present between various terrace levels indicates that the slip on the Mission Viejo fault began ~345 ka and has attained a current level of 0.057 m/ka. The recurrence interval is calculated to be 19.5 ka. Assessment of 1 terrace couple offset by the Cristianitos fault yields a slip rate of 0.03 m/ka and a recurrence interval of 52.3 ka. Given the dearth of data, these numbers for the Cristianitos fault can be considered only as estimates. However, if the slip rate on the Cristianitos fault is increasing as it seems to be on the Mission Viejo fault, then the current slip rate may be larger than this calculation estimates. As noted previously, these average recurrence intervals represent the maximum possible displacement-per-earthquake, but given the above assessment, the displacement-per-earthquake may be smaller with rupture alternating on the 2 faults between the north end of one and the south and of the other, and vice versa. The average recurrence intervals therefore, will be shorter (more frequent) than those expressed in the calculations above.
Shlemon (1992) states that on the coast adjacent to San Onofre, an intensive study using a variety of dating techniques has indicated that no movement has occurred at this location for at least 125 ka. Since the 52.3 ka recurrence interval is only a statistical estimate, it could represent the correct interval even though a 72 ka hiatus has occurred between fault ruptures. However, this assessment may indicate that the northern portion of the Cristianitos fault is currently active while the southern portion is locked. Ultimately, much more data and analysis would be needed to establish the validity of any recurrence interval estimated for the Cristianitos fault.
Considering the slip rate and recurrence interval established for the Mission Viejo fault, however, this study may indicate that a more accurate determination of fault rupture, fault displacement per seismic episode, and seismic slip rate is warranted for this fault. Such a study could well take advantage of the lower, more recent flight of terraces occurring near the level of the San Juan Creek floodplain.