The Earth Story

Trembling

Every dot on this map of the US and Canadian Pacific Northwest is an earthquake that took place during the months of April and May. Just to make sure – no, as of right now that area has not been totally destroyed by these events, they are all small earthquakes with moment magnitudes from 0-3. Most, if not all, are too small to be felt by people at the surface. While these small earthquakes have not damaged the region, they are indicating something else – slow movement on the Cascadia Subduction Zone, of a sort that seems to occur every 14 months or so. Oregon, Washington, California, and British Columbia all sit above a subduction zone, where the oceanic plates to the West (The Gorda plate and the Juan de Fuca plate) are being pushed down into the planet. This subduction zone, like others around the world, is capable of producing great earthquakes with magnitudes >9; the last major one occurred in the year 1700 and was powerful enough to trigger a recorded tsunami in Japan, giving an exact date for the Earthquake.

Large earthquakes like those occur when a patch on the fault becomes locked. The plates are always slowly moving, trying to slide past each other at this boundary but the force of friction between two parts of rock sliding past each other is strong. Even though the plates move, the rocks don’t instantly break; instead, the whole plate bends as the stress builds up, locking energy in the plates much like stretching a rubber band. Only when the stress on the fault becomes too great do the rocks fail, releasing that energy as an earthquake.

These small earthquakes are occurring in a different part of the fault from the zone that locks up and produces major earthquakes. They are occurring inland, where the fault is deeper and the stress conditions on the rocks are different. Here, the rocks build up a little bit of stress and then break in small earthquakes, in a pattern that occurs every 14 months. These small earthquakes allow some motion – right now these quakes are allowing the surface parts of those states to move to the west by a few millimeters.

This motion is taken up on one part of the fault, but it does not propagate throughout the entire subduction zone. Only part of the fault is moving in these earthquakes, so this is likely transferring additional stress to the locked zone in the fault. It is thought that the risk of a major earthquake is higher during these events, but the record of monitoring them is too short to be able to really say how much these events change the chances of a great earthquake. They can be a good “Every 14 month” reminder to double-check your emergency supplies and evacuation plan if you live in this area or could be exposed to tsunami risk.

These sets of small earthquakes were discovered on the Cascadia subduction zone a little over a decade ago and have since been recognized on other subduction zones, including Japan, Mexico, and New Zealand. The timing at each fault and the pattern of earthquakes is different at each fault, but in all cases they are likely an important process; they allow the deeper parts of the fault to move and transfer stress up to the shallow, locked zones. The technical term for this repeating pattern of minor quakes is “Episodic Tremor and Slip”.

-JBB

Image credit: https://pnsn.org/earthquakes/recent

Read more: https://www.seattletimes.com/seattle-news/series-of-small-earthquakes-detected-in-washington-oregon-and-canada/ https://www.king5.com/article/weather/earthquakes/quakes-you-cant-feel-being-recorded-south-of-seattle/281-0c9be3cd-6b81-406d-96bd-e6328ad3684d https://science.sciencemag.org/content/300/5627/1942

Want to check out an active mid-ocean ridge? The University of Washington has a series of geophysical monitoring stations on the Juan de Fuca ridge off the Pacific Northwest US Coastline. In 2015 they were able to track a series of eruptions due to the presence of earthquakes and other seismic signals. Here you can travel along the ocean floor as they pick up the equipment on that ridge - lots of pillow basalts erupted underwater and a bunch of organisms living off the energy supplied by these eruptions.

Oregon Coast Range Pillow Basalts

The Coast Range in Oregon lies to the west of the more well-known Cascade Range, creating the other side of the Willamette Valley and providing spectacular views of the Cascades and the seashore on a clear day. However, unlike the Cascades, which are composed of andesites and rhyolites (silica-rich, viscous rocks characteristic of continental volcanic arcs), the Coast Range is composed of basalts and sandstones. How did these oceanic rocks end up forming mountains up to 1249 m (4097 ft) high?

The northwest coast of North America is defined by the subduction of smaller plates, including the Juan de Fuca, Explorer, and Gorda plates, beneath the behemoth North American continent. 400 km west of the Coast Range lies the divergent plate boundary between the Juan de Fuca plate and the Pacific plate, and 150 km offshore lies the Cascadia megathrust subduction zone, in which the Juan de Fuca plate sinks back down into the asthenosphere (the uppermost part of the mantle, typically found at 100 km deep).

However, not all rocks are doomed to melt. The oldest rocks in the Coast Range, the Siletz River volcanics, formed during the Paleocene and middle Eocene (60-45 million years ago). These Siletz River volcanics provide clear examples of pillow basalts, indicating that these rocks were formed underwater. Note the radial jointing in the second picture -- the inside of these round bubbles looks like a bomb blast because of even, outside-in cooling.

Mary’s Peak, the highest summit of the Coast Range, is actually an old hot spot volcano, formed from a weak point in the ocean floor similar to modern Hawaii. The island chains towered above the sea floor, moving east in a conveyor-belt like system. Over time, these rocks were slammed into the continent at a rate of 4 cm/year, and instead of subducting, accreted onto the side of the North American plate. With more accretion, these sea mounts were uplifted and are now the Coast Mountain Range. These oceanic basalts define the west coast of Oregon, as the Columbia River basalts define the east and the Cascade range defines the center.

AGB

Photo Credit: 1 - Loren Kerns - https://flic.kr/p/ebwoJD 2 - Amanda Barker - https://flic.kr/p/EH9vMf References: Bishop, Ellen Morris, "In Search of Ancient Oregon", 2003 http://www.oregongeology.org/sub/publications/ims/ims-028/unit07.htm