Magnitude 6.2 Earthquake off the west coast of Vancouver Island.

Natural Resources Canada reported a Magnitude 6.2 Earthquake at a depth of 5 km roughly 182 km off the west coast of Vancouver Island, British Columbia, slightly after 7.35 pm local time on Tuesday 24 December 2019 (slightly after 3.35 am on Wednesday 25 December, GMT). This is a large quake, and was felt across much of southern British Columbia and northwest Washington State, although there are no reports of any damage or casualties, and no tsunami warning was issued.

The approximate location of the 24 December 2019 Vancouver Island Earthquake. USGS.

Vancouver Island is located on the western margin of the North American Plate, close to the Challenger Trench, along which the Explorer and Juan de Fuca Plate are being subducted. These are two remnant parts of the ancient Farallon Plate, which formerly underlay part of the eastern Pacific, and which has now been largely subducted beneath North America. The Explorer an Juan de Fuca Plates pass under the North American Plate as they sink into the Earth, but this is not a smooth process, with the plates constantly sticking together then breaking apart as the tectonic stresses build up.

 The subduction of the Juan de Fuca Plate beneath North America. United States Geological Survey.

On this occasion the Earthquake occurred on the northern part of the Juan de Fuca Plate, close to the Juan de Fuca Ridge. The Juan de Fuca ridge marks the extensional margin between the Juan de Fuca Plate and the Pacific Plate. The Juan de Fuca Plate is thought to be a remnant of an ancient plate known as the Farallon Plate, a once vast plate now largely subducted beneath the North American Plate. There are three remnants of this plate remaining off the west coast of the US; the Juan de Fuca Plate, the Gorda Plate to the south and the Explorer Plate to the north. The Juan de Fuca Ridge was once part of the longer Farallon Ridge, which ran along the eastern margin of the Farallon Ridge before it was fractured into three parts.

Spreading on the Juan de Fuca Ridge, and its relationship to the volcanoes of the North American west coast. Wikimedia Commons.

Witness reports can help geologists to understand the processes going on in Earthquakes and the structures in the rocks that cause them. If you felt this quake you can report it to Natural Resources Canada 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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Worker killed at Arizona copper mine.

A worker has died at the Ascaro-operated Mission Mine near Sahuarita in Pima County, Arizona, following a vehicle collision at about 9.30 am on Thursday 27 July 2017.  Gabriel Benitez, 41 was reportedly sitting in a Ford 550 truck when it was crushed by a much larger mine vehicle. The driver of the other vehicle has tested clean for drugs and alcohol, and the incident is being investigated by the Mine Safety and Health Administration and the Arizona State Mine Inspector’s Office.

Mine vehicles at the Mission Mine in Pima County, Arizona. Ascaro.

The Mission Mine consists of a complex of open pits covering an area roughly 4 km by 2.4 km, and about 365 m deep, located about 30 km to the south of Tuscon. The copper is extracted from a porphyry deposit (large grained igneous rock formed by the slow cooling of magma, usually in magma chambers beneath volcanoes on subductive plate margins) that also produces small amounts of zinc, lead and molybdenum minerals. This is intruded into a folded complex of Triassic sedimentary rocks and Palaeozoic limestones, and is associated with the Laramide Orogeny, a period of mountain building along the western margin of North America from Mexico to Canada, associated with the subduction of the Kulla and Fallaron Plates beneath the North American Plate, between about 80 and 35 million years ago.

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Magnitude 5.1 Earthquake off the coast of northern California.

The United States Geological Survey recorded a Magnitude 5.1 Earthquake at a depth of about 26.5 km roughly 74 km offshore of the city of Ferndale in Humboldt County, California, slightly after midnight on 5.00 pm on Friday 28 July 2017, local time (slightly after midnight on Saturday 29 July local time). There are no reports of any damage or injuries relating to this quake, but people have reported feeling it across much of northwest California and parts of southwest Oregon.

The approximate location of the 28 July 2017  Californian offshore 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.

However the  28 July 2017 Earthquake happened on the southern part of the Cascadia Subduction Zone, a northward extension of the San Andreas Fault, where the Gorda Plate is being subducted beneath the North American Plate. Along this boundary the rocks of the two plates continuously stick together, then become stressed as the motion of the two plates draws them apart. This stress builds up until the rocks are forcibly snapped apart, which we experience as Earthquakes.

Map showing the progress of the San Andreas Fault to the north, where it becomes the Gorda Plate subduction zone. Humbolt State University.

 The Gorda Plate, along with the Explorer and Juan de Fuca Plates are remnants of an ancient oceanic plate, the Fallaron Plate which has almost completely disappeared beneath North America. The Fallaron Plate formerly diverged from the Pacific Plate along the Fallaron Ridge, but as the plate has been subducted both it and the ridge have broken up. The remnants of the plate are now the Explorer Plate in the north, then the Juan de Fuca Plate, then the Gorda Plate in the south. This borders onto the Pacific Plate along the Mendocino Fracture Zone, which extends on land as the San Andreas Fault. 

 North of California the San Andreas Fault becomes the Mendocino Fracture Zone. USGS.

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

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