Magnitude 5.2 Earthquake in Mono County, California.

The United States Geological Survey recorded a Magnitude 5.2 Earthquake at a depth of about 8.8 km roughly 30 km to the southeast of the town of Bodie in Mono County, California, slightly after 7.35 am local time (slightly after 2.35 pm GMT) on Saturday 11 April 2020. There are no reports of any damage or injuries relating to this quake, but people have reported feeling it across much of northern California, southern Oregon. and western Nevada.

The approximate location of the 11 April 2020 Mono County Earthquake. USGS.

California is extremely prone to Earthquakes due to the presence of the San Andreas Fault, a tectonic plate margin that effectively bisects the state. The west of California, including Santa Barbara and Los Angeles, is located on the Pacific Plate, and is moving to the northwest. The east of California, including Fresno and Bakersfield is on the North American Plate, and is moving to the southeast. The plates do not move smoothly past one-another, but constantly stick together then break apart as the pressure builds up. This has led to a network of smaller faults that criss-cross the state, so that Earthquakes can effectively occur anywhere.

Tectonic boundaries and faults in California and the surrounding area. USGS.

Witness accounts of Earthquakes can help geologists to understand these events and the underlying structures that cause them. If you felt this quake (or if you were in the area but did not, which is also useful information) then you can report it to the United States Geological Survey here.

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Rockfall kills two climbers in the Sierra Nevada mountains of California.

The bodies of two climbers missing since Sunday 27 October 2019 were recovered from Red Slate Mountain, in the Sierra Nevada range of California, on Wednesday 30 October. Friends notified Mono County Search and Rescue when Jennifer Shedden, 34, of Mammoth Lakes, and Michelle Xue, 22, of Los Angeles, failed to return from a climbing trip on the mountain. The bodies of the two women were sighted in the in a couloir (narrow, steep-sided gully) on Monday by a California Highway Patrol helicopter. They were still attached to the rockface by ropes, and had apparently been killed by a rockfall, although rescuers were unable to recover them due to continuing instability on the rockface, with a second attempt by the National Guard failing the next day. The bodies were eventually retrieved by a Yosemite Search and Rescue helicopter. Both women are described as having been highly experienced climbers, and well equipped for the expedition.

A rescue team on Red Slate Mountain following the deaths of two climbers this week. Mono County Sheriff's Office.

Red Slate Mountain forms part of the Sierra Crest, a ridgeline that runs 800 km north-to-south through the Sierra Nevada Mountains, slightly to the south of the Long Valley Caldera. The mountain is not actually made of slate, but rather hornfels and marble, forms of mudstone and limestone that have been metamorphosed by heating and pressure. The area is seismically active; mountains appear to be permanent structures to humans, but are actually dynamic active systems, being constantly uplifted (in the case of the Sierra Nevada by the subduction of the last chunks of the ancient Farallon Plate along the Pacific Coast of North America), and constantly worn down by erosion, with the area around the Long Valley Caldera being particularly prone to movements. This combination of regular rock movements and a strong seasonal freeze-thaw cycle makes the area particularly prone to rockfalls (heating and cooling of the rock on a seasonal, or even daily, cycle, causes the minerals to expand at different rates, leading to the formation of cracks).

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British tourist killed by rockfall on El Capitan, Yosemite National Park.

A British tourist has been killed and his female companion, also British, has been seriously injured, following a rockfall on El Capitan, a mountain in Yosemite National Park, California, on Wednesday 27 September 2017. The couple were initially thought to have been climbing the peak when the incident occurred, but are now known to have been walking beneath a cliff popular with climbers when a chunk of rock about 40 m high, 20 m wide and 3 m thick, and estimated to weigh about 1170 tonnes, fell from the rockface. The couple will not be named until relatives in the UK can be contacted. About 30 climbers were on the cliff-face when the incident occurred, though none of them are reported to have suffered any injuries.

Dust from a rockfall on El Capitan, Yosemite National Park, on 27 September 2017. Tom Evans/US National Park Service.

El Capitan is a granite outcrop reaching 2308 m above sea-level, and about 900 m above the valley bellow, in the Sierra Nevada Mountains. Mountains appear to be permanent structures to humans, but are actually dynamic active systems, being constantly uplifted (in the case of the Sierra Nevada by the subduction of the last chunks of the ancient Farallon Plate along the Pacific Coast of North America), and constantly worn down by erosion. The granite outcrops of the Sierra Nevada ate the remains of an ancient batholith, formed in the Cretaceous Period by a mantle plume that rose up beneath the area, but did not reach the surface to any great extent. This allowed the liquid magma of the plume to cool slowly, allowing the large mineral crystals that make up granite to form. This granite has been exposed at the surface by uplift and erosion over the last four million years. Because granite is made up of large crystals of different minerals, with different physical properties, it is particularly prone to erosion, as heating and cooling of the rock on a seasonal, or even daily, cycle, causes the minerals to expand at different rates, leading to the formation of cracks. These cracks can be further widened by water entering them and freezing, pushing them further open, and allowing more water to enter on the next cycle.

Granite from the Sierra Nevada, showing the large crystals that give it both its distinctive appearance and make it vulnerable to erosion. Roc Doc Travel.

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