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The San Andreas Fault is the world's most studied and famous fault. Here just south of the San Gorgornio Pass, the fault splits into three main strands. The northern most is the Mission Creek Strand which many geologists consider the most active of the three. Each strand is separated by quite a distance, with all the strands merging near the base of Mt. San Gorgornio and then spreading outward as the fault moves towards the southeast. I visited this super unique site while on a graduate level geology field class and I wanted to share to others the uniqueness of this site.
The San Andreas Fault is a transform plate boundary entirely within the state of California. Its southern end is about a mile southeast of Bombay Beach in the Salton Sea. Its northern end is about 50 miles south of the city of Eureka in northern California, along the coast. The fault connects the Salton Trough and the spreading center (more on this later) in the Gulf of California to the Mendocino Fracture Zone and the Cascadia Subduction Zone. The town of Bombay Beach is built directly on the fault.
This part of the fault, from Bombay Beach to about Wrightwood has not had a major earthquake in over 400 years. This includes the Mission Creek strand of the San Andreas Fault. The recurrence interval of large earthquakes, capable of slipping more than 10 meters(33 feet) generating a moment magnitude (Mw) up to an 8.3, is about 150 years. That means the last major earthquake (or two) should have already occurred. This is why geologists are very worried about the near future of the San Andreas Fault. Let's first discuss how plates move before discussing the evolution of the modern San Andreas Fault.
There are two types of plates on Earth: Continental and Oceanic. Continental plates are much thicker (30-70km) and are much lighter. They are mostly made of felsic materials such as granites, which contain feldspars and quartz minerals. Unlike continental crust, oceanic crust is much denser and thinner (5-25km thick). This difference in density allows oceanic plates to slide beneath and under continental plates at subduction zones where oceanic plates can be recycled back into the mantel. A common misconception is that the mantel is liquid when in fact it is solid. It has different physical properties than the crust in that the crust is rigid and tends to fracture (brittle behavior) while the mantle can "flow" and deform (strain) under stress (ductile behavior). The mantel is similar to silly putty, which is solid but has some properties of a liquid. It can flow away from an input of stress, while the crust cannot.While the temperature of the mantel is high enough to melt most minerals (and rocks), the high pressure from the kilometers of rocks above allow the material to stay a solid rather than melt into a liquid. Changing a phase (solid, liquid, gas, plasma) is dependent only on the temperature and pressure of the material. The ductile properties of the solid mantle allow the plates to effectively float on top of it.
The mantle, being the largest structure of earth's interior, making up over 50% of Earth's volume, has currents (called convection currents) similar to ocean currents. Hotter material from the lower part of the mantel is less dense and will begin to rise towards the upper mantle. As is rises, its temperature drops and begins to cool and shrink, becoming less dense. Eventually it will fall back down to the lower where it is heated up again and the process starts over. These cycles, known as convection currents, are what drives plate tectonics. Earth is the only place in the universe where this occurs, although some have argued Jupiter's icy moon Europa may also have plate tectonics to an extent.
There are three main types of plate boundaries. The plates can collide with one another in a head-on collision (convergent), they can pull apart from one another (divergent) or they can slide past one another (transform). A divergent plate boundary occurs when these mantle convection currents push up the land allowing it to crack and split apart. The weight of the plate down slope of these "push up" areas will break up the crust even more and lava can start to fill in these cracks. Eventually, if the land starts to become below sea level, it will fill up with water as a new ocean basin is formed. This is how the Atlantic Ocean was created. The Gulf of California, up to the Salton Sea area, is an example of a divergent plate boundary which is about 6 million years old. Iceland is a prime example of this occurring.
A convergent plate boundary has three possible outcomes. First is when a continental plate collides head on with another continental plate. Beacuse both plates are thick and are not very dense they cannot be subducted. In stead, large mountains begin to form as the land between the two plates begin to uplift. The Himalia Mountains, and Mt. Everest are perfect examples of this occur. The Alps are another place of two continental plates colliding. The second outcome is when an oceanic plate collides head on with a continental plate. Because the oceanic plate is much thinner and more dense, it always subducts under the continental plate. As the oceanic plate subducts, it carries trapped sea water which lowers the melting temperature of the surround rocks. Eventually this sea water can melt the rocks, which are less dense than the solid rock around them. The melted rocks begin to rise and create volcanic arcs where enormous volcanoes can erupt. The Cascadia subduction zone is a prime example, creating volcanoes such as Mt. Shasta and Mt. St. Hellens. The third case is when an oceanic plate collides head on with another oceanic plate. One of them will subduct, and this will depend on the dencity of both (which can vary), but also the momentum (which is a function of volume, thickness, and velocity). A prime example of this is Japan. Again, a volcanic arc will form and create tall volcanoes such as Mt. Fuji.
Most major plate boundaries are convergent or divergent. In rare cases, do transform boundaries occur. These really only occur in linking divergent and convergent boundaries. They link the other types of boundaries to accommodate for the shape of the Earth (sphere). The San Andreas Fault does exactly this: connecting the rift zone in the Gulf of California (of which extends to the Salton Sea) to the subduction zone in Cascadia (off the coast of Northern California, Oregon, Washington, and British Columbia). It is one of, if not, the largest transform plate boundary on earth, stretching nearly 800 miles.
About 30 million years ago, the final stages of what would become the west coast of North America were forming. The entire west coast of the continent had a large subduction zone as the Farrallon Plate (an oceanic plate) subducted under the current North American Plate (a continental plate). This plate, was irregularly shaped in the center and was wider on the edges. As it subducted, the area of the plate in the center became less and less until about 26 million years ago, the center of the Farralon Plate fully subducted under North America, leaving the larger of the two parts of the plate to the north (Juan de Fuca Plate) and the south (Cocos Plate). These two plates are still subducting under North America and were once part of the same plate, the larger Juan de Fuca.
During this time, the modern west coast of North America began to take shape. The plate boundary reorganized itself and created the modern day San Andreas Fault where the crust was the weakest. It has been in motion since about 25 million years ago. It is possible to visit and see other faults that may have been a temporary plate boundary. In Los Angeles County, Devils Punchbowl County Park was considered to be a former plate boundary (an early San Andreas Fault) before the majority of the motion was confined and moved slightly to the modern San Andreas Fault a few miles away.
Part of that reorganization is still evident here with the multiple stands of the San Andreas Fault between San Gorgonio and Indio. Multiple strands accommodate the plate motion, some more than others. Like previously mentioned, it would appear that the Mission Creek Strand is the most active of the three in this area (Garnet Hill, Mission Creek, and Banning).
Now that we've discussed the fundamentals of how plate boundaries form, the different types of plate boundaries, and how the San Andreas came to be, we can go into details on why this specific site along the San Andreas is super special. As I previously mentioned, this part of the San Andreas Fault is composed of three strands which together make up the larger San Andreas Fault zone from Mt. San Gorgonio to roughly the Indio area, just northeast of Palm Springs and I-10.
It is remarkably easy to see the fault from the air in this region due to the topography and geology of the land. To the north is the high Mojave desert, which receives snow and higher precipitation than the lower Colorado Desert of the Salton Trough. Rain and snowmelt drain into these foothills and they can infiltrate the dry, porous soil to become part of the groundwater. As the water flows underground, downslope, it will have to cross these strands of the San Andreas. This first of which is the Mission Creek Strand. A fault acts as a physical boundary in the soil. On one side of the fault (east) the soil is permeable and groundwater can flow easily. On the other side of the fault (west) the soil is less permeable. This difference in soil permeability allows water to become trapped by the fault. Groundwater has a difficult time flowing across the fault, as evident by the change in plants and greenery across the fault, which can be seen from the air.
The water collects along the fault and overtime larger plants can grow, such as palm trees. To many locals, these are known as palm tree oasis and they dot the landscape in this area. Why? Because of the many strands of the San Andreas Fault and the natural runoff of snow and rainwater from the higher elevations. The fault can be mapped by "connecting the dots". Instead, however, these dots are changes in vegetation and the linear palm tree oasis'. Note in the image above the amount of vegitation north of the fault along the path that runoff flows (dotted blue line) verses the other side of the fault which has noticeably less vegetation. The side that receives both runoff and groundwater is much healthy, and greener, than the side that only receives seasonal runoff.
Water can only flow directly across the fault during storm events, when the groundwater becomes saturated with surface runoff. Most of the time, however, groundwater is not saturated and the groundwater's path is altered due to the San Andreas Fault blocking its natural flow. Also note that groundwater can only cross the fault once the ground becomes saturated and the water makes it to the surface and becomes runoff, where it can then flow over the fault. This occurs where water seeps to the surface then drains over the fault before going back down into the groundwater again. Notice where the path of groundwater steps over the fault, the vegetation becomes noticeably less and less as the water is returned to the ground.
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