|San Andreas Fault|
|Central California Last 90 Million Years of Tectonic Events|
|Recent Horizontal Displacement|
The San Andreas Fault delineates the boundary between the Pacific Plate and the North American Plate just west of the central California Valley. The fault is parallel by other smaller faults mostly on the west side running in a north-south direction and by Garlock Fault which runs perpendicular to San Andreas Fault along the Traverse Ranges (see map below). The geology on the central coast side of California is complex; in a generalized geologic map (see Fig 2), the geology of the eastern side of California is preserved intact dating back to Late Cretaceous while on the coast side, most geologic features now locked together appear to be out of place. The Monterey Terrane, which dominates the central coast California could date back to Late Cretaceous since it shows similarities to the Mojave-eastern arc located at lower latitudes. The tectonic blocks of the central coast could have been transferred northward several degrees of latitude via the tectonic activities in the last 20 million years. The tectonic activities continue today as the Pacific Plate and the North American Plate slip past each other reshaping the landscape and generating earthquakes along the fault.
The current relative movement of the plates creates several small earthquakes in the upper crust and every so often the fault builds enough stress allowing the plates to move past the average rate generating moderate to large size earthquakes (see map below), some of which have been shown to be catastrophic in populated areas in the last 160 years. The current horizontal displacement between the Pacific Plate and North American Plate has reshaped surface features in recent years. Man-made structures that have been built in the last century along the fault and streams that flow on the landscape across the fault are continuingly being twisted and shifted in directions relative to the horizontal displacement.
San Andreas Fault Map. The San Andreas Fault, other active faults and related earthquakes are shown for California (taken from USGS).
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The San Andreas Fault extends more than 1,100 km and is located along the western side of central California; the fault begins at the Sea of Cortez branching out from the East Pacific Rise and cuts across central California exiting at Point Reyes and continuing northwest until it meets with the Mendocino Fracture Zone. The fault separates the North American Plate (east) from the Pacific Plate (west) and depths extend to at least 18 km (USGS, 1997). The San Andreas Fault is relatively young in geologic terms, it dates back to the Early Miocene about 15-20 million years ago (Ma). The strike-slip horizontal displacement of the fault in which the North American Plate moves in a southeast direction and Pacific Plate moves in a northwest direction (see Fig 1) has a displacement rate of approximately 5cm/year (USGS, 1997). As the plates on opposite sides of the fault move, hundreds of small earthquakes related to plate movement occur in the upper crust near the fault every year with some moderate-size catastrophic earthquakes with magnitudes > 6 occurring around every 20-22 years and magnitudes > 8 occurring around every 150 years based on historic events. USGS (1997) estimates the accumulated displacement caused by the fault to be 550 km over the past 15-20 million years. By comparing the geology on opposite sides of the San Andreas Fault, the mismatch of geologic features indicate the terranes on the west side of the fault have been transported more than 200 km via the San Andreas Fault (see Fig 2).
Figure 1. Central California coast false-color composite. Landsat 5 TM dataset from Sep. 09, 2009 with relatively low cloud cover were used to produce the composite using IDRISI Taiga using Bands 2,3,4. The active vegetation features mostly on the windward slopes and valleys are shown with red-bright red tones, water features are dark blue, bare soils, rocks, sediment, paved surfaces, and dry vegetation are shown in gray tones. The San Andreas Fault Zone is shown with a solid yellow line and arrows show direction relative to plate movement and the plates labeled in bold yellow. Major mountain ranges of the Pacific Plate are labeled in white along with the Salinas Valley labeled in green. The city of Parkfield is located just east of Cholame Hills on the edge of the North American Plate.
Figure 2. California Geology (taken from Ducea, 2007). The generalized geologic map of California shows the major tectonic features. The geology of Mojave-eastern arc east of the Peninsular Ranges Batholith (PRB) correlates with the Salinia arc west of the San Andreas Fault. Figure 1 (above) is located mostly west of the Great Valley forearc basin on the Franciscan Complex, which is overlapped geologically by the Salinia basement (see Fig 5).
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The complex geology of the modern California central coast can be summarized only by generalizing the tectonic events that have taken place in the last 90 million years. Assuming that the Farallon Plate was being subducted under the North American Plate in the Late Cretaceous period (see Fig 4) before the San Andreas Fault became active some 20 Ma. The subducting motion and relative direction of the Farallon Plate created the Eastern California arc followed by the Western arc. As the Farallon Plate subducted under the North American Plate, the existing Franciscan Complex wedged between the Western arc and the Farallon Plate. The Great Valley forearc basin accumulated sediments from the ancient seas over the Franciscan Complex (see Fig 1), which now form most of the California Valley that extends from the Traverse Ranges to the south up to the northern California Valley.
During the Late Oligocene period some 30 Ma, the older dense ocean crust of the east Pacific Plate split the Farallon Plate as it was moving east towards the North American Plate. The two new emerged plates formed the Juan de Fuca Plate to the North split from the Pacific Plate by the Mendocino Fracture and the Cocos Plate on the south Pacific split from the Pacific Plate by the East Pacific Rise Fracture. During the Early Miocene some 20 Ma the realignment of the plates on the west side of North America forced the relative movement of the Pacific Plate and the North American Plate into a strike-slip horizontal displacement forming the San Andreas Fault.
Based on the relative movement of the Pacific Plate and North American Plate and geologic evidence present on terranes on opposite sides of the San Andreas Fault we can assume that the coast ranges maybe related to the south. According to Ducea (2007), the Coast Ranges west of the San Andreas Fault could date back to the Late Cretaceous and may have been transferred to the Pacific Plate via the San Andreas Fault. The complex geology of the Monterey Terrane (see Fig 3) is composed Late Cretaceous basement blocks that have accreted over the Pacific Plate. Cross-sections of geology maps of central California show the Monterey Terrane and terranes of Southern California have similar tectonic underplating. The Underplating of the Monterey Terrane west of the San Andreas Fault (see Figs 3-5) is believed to be composed of sediments wedged by the early subducting Farallon Plate some which are exposed as the Schist of the Sierra de Salinas (see Figs 3 & 5) in the Salinas area between the Santa Lucia Range and the Gabilan Range (see Figs 1 & 5D). The faults parallel to the San Andreas Fault and the Salinas Shear Zone (see Fig 5C) mark the boundaries of the structurally collapsed accreted wedges that formed prior to the San Andreas Fault. The Coast Ranges that now dominate the landscape along the coast are believed to have formed after the San Andreas Fault in the last 20 million years.
Figure 3. Central California Coast Terrane (taken from Ducea, 2007). The map shows the three main basement rocks that compose the Monterey Terrane and the Late Cretaceous and Cenozoic faults west of the San Andreas Faul.
Figure 4. Pre-San Andreas tectonic profile (taken from Ducea, 2007). A (95-80 Ma) shows the normal subduction of the Farallon Plate under the North American Plate, which generated the great California arc. B (76-73 Ma) shows a shallower subduction and accretion of wedge sediments.
Figure 5. Tectonic reconstruction of the modern Monterey Terrane (taken from Ducea, 2007). C (74-68 Ma) shows the collapse of the margin that resulted in deposition of marine sediments. D (modern) shows modern Monterey Terrane after the collapsed margin and other tectonic events that took place during the Late Cenozoic along the San Andreas Fault and after the current mountain ranges formed.
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The horizontal displacement continues in the central California coast. The Pacific Plate and North American Plate continue to slide past each other in opposite directions. Along San Andreas Fault the recent horizontal displacement is evident on surface features. The slow horizontal movement of the plates deforms the linear shapes of the streams that carry runoff from the adjacent mountain slopes. Driving along highway 46 towards Paso Robles oil fields signal the location or areas rich in sediments deposited prior to San Andreas (see Fig 6) towards the end of the North American Plate. A few kilometers before reaching the end of the North American Plate westbound highway 46 changes direction and runs parallel to the San Andreas Fault in a northwest direction. Driving along the fault the apparent displacement along the fault appears to be vertical along the Temblor Range, which runs parallel to the fault on the Pacific Plate side (see Figs 7-8). The displacement could be related to the fault since it is consistent throughout the range appearing as if someone sliced the range with a knife. In addition to the vertical displacement shown on the mountain ranges, the depression formed along the fault shows the banks along North American Plate side to be slightly lower than those on the Pacific Plate side (see Figs 11-12). The vertical displacement does not have to be related to the tectonic activities along the fault, however, we could assume that the vertical displacement if not caused by subsidence as the plates move apart, could be influenced by the differences in weight of the buoyant plates and/or the accreted geology west of the San Andreas Fault.
Of the several hundred earthquakes that are generated in the upper crust along the San Andreas Fault every year, only a few reach magnitudes greater than 6 around every 20-22 years in the Parkfield area (USGS, 1997). The most evident recent horizontal displacement can be measured on the man-made structures as they are twisted or modified by moderate-size earthquakes. About 25 km northwest of the intersection of highway 46 and highway 41 a small bridge crosses the San Andreas Fault to access the small city of Parkfield (see Figs 9-14). Recent tectonic activity along the fault triggered strong 6 M + earthquake twisting the metal barriers and moving the columns that hold the Parkfield bridge in place (see Fig 9). The bridge shows evidence of horizontal displacement relative to the direction of the moving plates; the Pacific Plate sliding northwest and North American Plate sliding southeast twisted the bridge in the same direction.
Tectonic activities along the San Andreas Fault will continued as the plates buildup stress moving in opposite directions. Based on historical events, the next moderate-size > 6 M earthquake is expected to hit the Parkfield area in the 15 years, while a big earthquake is overdue in the southern California area just east of the Mojave-eastern arc, south of where the Garlock Fault and San Andreas Fault intersect (see Fig 2), an area which has an expected > 8 M earthquake interval of 150 years based on historical events.
Figure 6. Oil Field between Temblor Range and Lost Hills. The oil fields are located between the Temblor Range and the small city of Lost Hills along Highway 46. Several oil fields are located in the fossil rich areas located between the Great Valley forearc basin and the Franciscan Complex (photo by: J. Alvarez).
Figure 7. North end of Temblor Range. The Temblor Range is located at the edge of the Pacific Plate and runs parallel to the San Andreas Fault. Vertical and horizontal displacement is common along the San Andreas side of the Range. In this photograph the vertical displacement on the slopes is about 2-4 meters (photo by: J. Alvarez).
Figure 8. North end of Temblor Range & beginning of Cholame Hills. At the end of the Temblor Range another the Cholame Hills emerge also running parallel to the San Andreas Fault on the edge of the Pacific Plate (photo by: J. Alvarez).
Figure 9. Parkfield bridge crossing the San Andreas Fault. The sign marks the entrance to the North American Plate. The bridge crosses the San Andreas Fault just south of Parkfield. The bridge has been retrofitted after strong earthquakes >6 magnitude twisted the bridge and offset the columns, which were once aligned horizontally (photo by: J. Alvarez).
Figure 10. Parkfield bridge showing the twisted metal barriers and offset columns caused by the horizontal displacement of the Pacific and North American Plates (photo by: J. Alvarez).
Figure 11. South view of the San Andreas Fault. The photograph was taken from Parkfield bridge which runs perpendicular to the fault. Cholame Creek runs along the fault. The banks on the Pacific Plate (right) seem to be about 3 meter higher than the banks on the North American Plate (left) (photo by: J. Alvarez).
Figure 12. North view of the San Andreas Fault. The photograph was taken from Parkfield bride facing north and it shows the depression along the fault filled with sediment from runoff and mountain ranges that emerge on both sides of the fault (photo by: J. Alvarez).
Figure 13. Entrance sign to the city of Parkfield. The small city of Parkfield is assumed to be located on the edge of the North American Plate (photo by: J. Alvarez).
Figure 14. Southwest view from Reason Mountain. The photograph was taken from the peak of Reason Mountain located on the North American Plate about 10 km north of Parkfield. Parkfield is to the left, three mountain ranges are visible in the background; Middle Mountain (front) runs along the San Andreas Fault, the Cholame Hills are on the background and the Santa Lucia Range are on the far background (photo by: J. Alvarez).
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