Daily BrewEarthquakes, swarms shake both sides of the Pacific
In the early afternoon of August 26th, the southern California town of Brawley experienced a magnitude 5.5 earthquake, which was preceded and followed by hundreds of other tremors — from magnitude 1.4 up to 5.3. The residents in the region are still experiencing tremors from this 'earthquake swarm' today!
At 8:47am Eastern Daylight Time, there was a magnitude 7.9 earthquake 93 kilometres east of the Philippines, which has been followed by at least 7 aftershocks so far — ranging from magnitude 5.5 down to 4.9 — in the time since. The earthquake triggered both a tsunami watch and a tsunami warning from the Pacific Tsunami Warning Center, but both were canceled over the next two hours, as thankfully sea level readings did not indicate any danger.
Earthquakes are caused by the rubbing together of the different tectonic plates that make up the Earth's crust.
[ Related: Earthquake record shakes up Pacific Northwest predictions ]
My first year college geology professor used to describe tectonic plates as being like the skin that forms on the top of a pot of cooking beans. The skin will move and crack as the beans and sauce heat underneath it. The same happens with the Earth's crust, which is cooled rock from the planet's mantle — the thick molten layer between the crust and the planet's core — and as the mantle boils and churns underneath it, the crust moves and cracks. The large "pieces" that are still intact are the tectonic plate, and the cracks are the various faults where most earthquakes and volcanoes happen.
The most active region for quakes is around the massive Pacific Plate, and it is frequently called the 'Ring of Fire' because of the number of active volcanoes around it. The earthquake swarms in California and this massive earthquake east of the Philippines were both caused by the movement of this plate. In California, the Pacific Plate rubs against the western edge of the North American Plate, along a strike-slip fault, causing frequent tremors and earthquakes. Over in the Philippines, today's earthquake and aftershocks were specifically caused by the subduction of the Philippine Sea Plate under the larger Eurasian Plate to the west.
Earthquakes are very common — there have been 40 of magnitude 2.5 or greater just in the past day — and while earthquake swarms like the one currently going on in Brawley are uncommon, they aren't very rare. They happened frequently in the 1960s and 1970s, when the Brawley Seismic Zone was quite active, and there have been several smaller swarms in the area over the past decade. They sometimes precede larger earthquakes, when they are called 'foreshocks', and they sometimes come before volcanic eruptions, but they aren't specifically associated with any event and can happen completely on their own.
There have been reports of injuries and a few deaths due to today's quake in the Philippines, as well as evacuations both there and in Brawley. Having never experienced an earthquake beyond feeling a slight sway from thousands of kilometres from the epicentre, I can only imagine what it's like to feel the ground violently shake beneath your feet. My thoughts are with all those affected by these disasters.
[ Related: Japanese government says giant offshore quake could kill hundreds of thousands ]
