Aromas, Ca Earthquake: A Transform Boundary Tremor

Aromas, California, is an area of very high seismic activity, with an average of about 1,700 earthquakes per year. On December 5, 2023, a 1.7-magnitude earthquake was recorded in the town, with its epicenter just 2.2 km (1.2 mi) from Aromas. This earthquake was too small to be felt by people. The San Andreas fault, a transform plate boundary, is responsible for earthquakes in the Bay Area, where two tectonic plates, the Pacific Plate and the North American Plate, meet and cause relative horizontal motion.

The San Andreas fault line

The San Andreas Fault is a continental right-lateral strike-slip transform fault that extends roughly 1,200 kilometers (750 mi) through the U.S. state of California. It forms the tectonic boundary between the Pacific Plate and the North American Plate. The Pacific Plate, to the west of the fault, moves in a northwest direction, while the North American Plate, to the east, moves towards the southwest. This relative motion between the plates results in earthquakes along the fault.

The San Andreas Fault slices California into two parts, with San Diego, Los Angeles, and Big Sur on the Pacific Plate, and San Francisco, Sacramento, and the Sierra Nevada on the North American Plate. Interestingly, despite San Francisco's association with the 1906 earthquake, the San Andreas Fault does not actually pass through the city. However, several communities, including Desert Hot Springs, San Bernardino, Wrightwood, and Palmdale, are located directly on the fault line.

The fault has traditionally been divided into three main segments: northern, central, and southern. Each segment exhibits different characteristics and varying degrees of earthquake risk. The southern segment, stretching from Parkfield in Monterey County to the Salton Sea, is capable of producing a magnitude 8.1 earthquake. An earthquake of this magnitude would result in devastating consequences for Los Angeles, San Bernardino, Riverside, and surrounding areas.

The San Andreas Fault is well-known for its geological complexity and the variety of rock types found along its length. The fault is easily visible in certain areas, such as the Carrizo Plain and the Olema Trough, where it appears as a series of scarps and pressure ridges. In other regions, the fault's presence is more subtle, concealed by alluvium or overgrown with brush.

The San Andreas Fault is also unique in that it is one of the few places in the world where faults "creep," meaning the fault slip occurs slowly and continuously without resulting in earthquakes. This phenomenon has provided valuable insights into the behaviour of faults and the mechanisms of earthquakes.

The Earth's crust and upper limits

The Earth's crust is the outermost solid shell of the planet, accounting for less than 1% of the Earth's volume. It is the top component of the lithosphere, which includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates whose motion allows heat to escape from the interior of the Earth. The crust is what we live on and contains all known life in the universe.

The Earth's crust is between 20 and 120 km thick, with an average thickness of about 15 to 20 km. It is made up of two distinct types of crust: continental and oceanic. Continental crust is thicker, ranging from 30 to 50 km thick (and even thicker in some regions), and is composed mostly of less dense, more felsic rocks such as granite. Oceanic crust is thinner, ranging from 5 to 10 km thick, and is composed primarily of denser, more mafic rocks such as basalt. Both types of crust "float" on the mantle because they are less dense than the mantle below.

The temperature of the crust increases with depth, ranging from about 100 °C to 600 °C at the boundary with the underlying mantle. The lower crust should be the hottest part, and if there is little to no compositional stratification, it should have the lowest effective viscosity. The crust's temperature can be estimated using measurements of surface-heat flow, models of heat production, and thermal conductivity distributions with depth.

The crust is usually distinguished from the underlying mantle by its chemical makeup, specifically the contrast in seismic velocity. The boundary between the crust and the mantle is called the Mohorovičić discontinuity. In the case of icy satellites, the crust may be distinguished based on its phase (solid crust vs. liquid mantle).

The crust is formed through igneous processes and is constantly modified by erosion, impact cratering, volcanism, and sedimentation. The Earth's first crust, called the primary or primordial crust, was likely formed from the solidification of a magma ocean and was repeatedly destroyed by large impacts. The current crust is called the secondary and tertiary crust, which correspond to oceanic and continental crust, respectively.

The upper crust to lower crust transition represents a key lithological change and includes the boundary between the magmatic heat source and the upper crustal hydrothermal system. This transition zone is associated with evolved lithologies and partial melting experiments, providing insights into the composition and structure of the lower crust.

The lower crust is denser than the upper crust, and its density increases with depth. The lower crust is also more mafic, with a higher proportion of ferromagnesian minerals such as pyroxenes and olivine. The P-wave velocity of the lower crust is variable but generally high, suggesting a dominance of mafic lithologies. However, the presence of evolved compositions and partial melting processes is also evident.

The crust is in constant motion due to plate tectonics, with the plates moving relative to one another. This motion results in earthquakes, which occur mostly in the upper 10-12 miles (15-20 km) of the Earth's crust. Earthquakes are caused by the failure on faults induced by the strains of plate motions. California is unique in that it is one of the few places in the world where faults "creep," meaning the fault slip is slow and continuous and occurs without earthquakes.

Plate tectonics and the Pacific Plate

Plate tectonics is a scientific theory that describes the motions of the 15 to 20 large, rigid, and brittle tectonic plates that make up the Earth's outermost layer, or lithosphere. The Pacific Plate is the largest of the seven major tectonic plates, covering 103 million square kilometres (40 million square miles). It is an oceanic tectonic plate that lies beneath the Pacific Ocean, stretching from the west coast of North America up to Alaska and reaching the east coast of Japan and Indonesia in the west.

The Pacific Plate was formed around 190 million years ago at the triple junction of the Farallon, Phoenix, and Izanagi Plates. Over time, the growth of the Pacific Plate reduced the Farallon and Phoenix Plates to small remnants, and the Izanagi Plate was destroyed through subduction under Asia. The Pacific Plate is composed of almost entirely oceanic crust, with some continental crust in New Zealand, Baja California, and coastal California.

The Pacific Plate contains all types of plate tectonic boundaries along its edges, including convergent, divergent, and transform borders with other plates. Its northeastern side forms a divergent boundary with the Explorer, Juan de Fuca, and Gorda Plates, creating the Explorer, Juan de Fuca, and Gorda Ridges, respectively. In the middle of its eastern side, the Pacific Plate shares a transform boundary with the North American Plate along the San Andreas Fault. This transform boundary accommodates the horizontal relative motions of the plates, with the North American Plate moving southwest and the Pacific Plate sliding horizontally northwest.

The southeastern side of the Pacific Plate has a divergent boundary with the Nazca Plate, forming the East Pacific Rise as the plates pull apart and create new oceanic crust. The southern edge of the plate is a divergent boundary with the Antarctic Plate, forming the Pacific-Antarctic Ridge. In the southwest, the Pacific Plate has a complex but predominantly convergent boundary with the Indo-Australian Plate, subducting under it north of New Zealand and forming the Tonga and Kermadec Trenches.

The Pacific Plate plays a significant role in the Pacific Ring of Fire, a pattern of plate tectonics known for its powerful earthquakes and volcanoes. The movement and interaction of the Pacific Plate with other tectonic plates contribute to the seismic activity and volcanic eruptions associated with the Ring of Fire.

Earthquakes and volcanic activity

The San Andreas Fault is a principal fault system in western California, separating the Pacific Plate to the west from the North American Plate to the east. The Pacific Plate slides horizontally northwestward relative to the North American Plate, resulting in seismic activity along the San Andreas Fault and its associated faults. This motion has led to California experiencing a high level of seismic activity, with the San Andreas Fault being responsible for significant earthquakes in the state's history.

The Aromas region in California is no stranger to earthquakes. While the specific boundary type of the Aromas earthquake is not explicitly mentioned, it is safe to assume that it occurred due to the nearby San Andreas Fault system. The area surrounding Aromas has a very high level of seismic activity, with numerous earthquakes occurring annually.

For instance, in 2023, the area experienced 722 earthquakes within a 365-day period. While most of these quakes had lower magnitudes, larger earthquakes do occur but less frequently, with an average recurrence interval of approximately every 5 to 10 years. The largest earthquake in the Aromas region in recent memory was a 5.5 magnitude quake in Westwood, California, in 2022.

California's unique geological setting, where plates creep slowly and continuously, also contributes to its high seismic activity. This aseismic slip means that earthquakes in the state can occur without warning, making it essential for residents and authorities to be vigilant and prepared for potential seismic events.

Seismic activity and magnitude

Aromas, California, is an area of very high seismic activity. On average, there are about 1,700 earthquakes per year in or near the city. Most of these are minor, with magnitudes of less than 2.0, and are not felt by people. However, larger earthquakes do occur, albeit infrequently.

On March 31, 2017, a magnitude 4.0 earthquake struck near Aromas. The quake occurred shortly before 4 a.m. at a depth of about 3 miles. There were no reports of damage or injuries as a result. This earthquake was felt by people in the area, with some taking to social media to share their experience.

In the past 24 hours as of March 24, 2024, Aromas experienced three quakes with magnitudes up to 2.8. In the past seven days, there were 39 quakes with magnitudes up to 3.3 within up to 100 km (63 mi) distance. Of these, two quakes were above magnitude 3, four were between magnitude 2 and 3, and 33 were below magnitude 2, which are usually not felt.

The strongest earthquake in the past seven days as of March 24, 2024, measured 3.3 on the Richter scale. It occurred on March 18, 2024, 38 km (24 mi) away from Salinas, California. This quake had a very shallow depth of 5.5 km (3.4 mi) and was not felt by anyone.

The San Andreas fault, a transform plate boundary, is the principal fault in western California, where the Pacific Plate and the North American Plate meet. The horizontal relative motion of these plates causes earthquakes along the San Andreas fault and associated faults. Earthquakes in the Bay Area reflect the relative motion of these two plates.

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