The moving pieces under Humboldt

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by Carlos Pedraza

Cal Poly Humboldt sits on top of a very seismically active part of the world known as a the Cascadia subduction zone. This area is composed of three tectonic plates under the ocean off of the Pacific coast.

The Juan de Fuca, Explorer, and Gorda plates are subducting beneath the continental North American Plate, where the Cal Poly Humboldt campus is located.

As the oceanic plates push against the continental plate, the friction created leads to deformation and faulting.

“We live on that boundary where this is taking place, which leads to frequent earthquake activity,” said Cal Poly Humboldt geology professor Amanda Admire.

In addition to the deformation from the Cascadia subduction zone, the Humboldt region is also influenced by the movement along the San Andreas Fault to the south. Humboldt stands on top of an intersection of three different plates pushing against each other.

The plates themselves move very slowly, only a few centimeters every year. However, they still generate friction as they move against each other. This is the energy released during an earthquake and tsunami.

Graphic by Carlos Pedraza and August Linton

In the Pacific Northwest, both earthquakes and tsunamis are important to prepare for. The Redwood Coast Tsunami Work Group, an organization made up of local government officials, tribes, and relief groups, gives information and warnings in their “Living on Shaking Ground”magazine.

The magazine states that “more than two-thirds of our large historic earthquakes have been located offshore within the Gorda plate.”

A tsunami is created when an earthquake along a fault ruptures the seafloor, moving the entire water column and releasing that built-up energy, which moves out in all directions.

The primary local tsunami hazard, the Cascadia subduction zone, is very close to Humboldt’s coastline compared to other regions in the Pacific Northwest.

According to Admire, a tsunami produced along the fault between the Gorda and North American plates would only take approximately ten minutes to reach the Humboldt shoreline. In Oregon and Washington the fault is further from shore, allowing for more warning time should there be a tsunami.

This much seismic activity can be exciting to study for geologists and scientists, but for people living in Humboldt it may be nerve racking. Admire said there is no need for panic, but that residents should prepare.

The last mega earthquake along the Cascadia subduction zone was in 1700. However, there are still smaller instances of seismic activity as the plates move and push against each other.

So when an earthquake happens: drop, cover and hold on. If you’re near the coastline, head for higher ground in case of a tsunami.

To find more preparedness tools and tsunami evacuation maps for the region, check out the Redwood Coast Tsunami Work Group.

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One Comment

  1. David David Thursday, February 10, 2022

    25 years ago our neighborhood in Simi Valley had an emergency evacuation due to a chemical spill. My wife grabbed our one preschooler and our dog and was driving out of our tract within 2 minutes. There were 3 streets to get out for about 200 houses. It was lucky the chemical cloud did not get there as due to the traffic and narrow streets she got less than 1/2 mile in 2 hours. A good experiment for people on the peninsula would be to see if you can actually run to your designated dune top in 10 minutes with the last 100 feet uphill in fine sand, because there is no chance that you could drive away. There are far more people and long narrow roads.

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