by Donna on December 11, 2021 12:42 am
I've felt some pretty strong jolts over the 37 years I've lived in SoCal. Arizona rarely if ever experiences earthquakes.
I just checked the USGS website and counted 33 quakes of 4.5 magnitude or greater over the past week along that fault off the coast of northern Oregon. Tracking quakes is a sort of hobby of mine. I've seen and read about quake swarms comprised mostly of quakes with magnitudes less than 3.0, but not any like this swarm.
Seismologists aren't worried about it, though.
"The Blanco Fault Zone is about 200 miles (322 kilometers) from the Cascadia Subduction Zone. At Cascadia, the Juan de Fuca plate dives under the North American plate. This diving motion is known as subduction, and it can produce large, devastating earthquakes. The up-down motion of the crust at subduction zones can also create dangerous tsunamis.
But Blanco is a kinder, gentler fault zone. It sits at the spot where the Juan de Fuca plate and the Pacific plate rub against each other. It's a transform fault zone, also known as a strike-slip boundary. As the name suggests, this means the plates slip past each other with very little up-and-down motion. While strike-slip faults (such as the San Andreas Fault) can lead to dangerous quakes, the lack of vertical motion on the fault means the risk of a tsunami from a Blanco quake is low, according to a 2019 blog post from Scientific American."
Here's why they're almost certain about the date and time of the Cascadia quake of 1700:
"...We have found several tsunami records in Japan from AD 1700 with no indication of a local cause. Historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in South America, Alaska or Kamchatka, leaving Cascadia as the most likely source of this tsunami. The estimated time of the earthquake is the evening (about 21:00 local time) of 26 January 1700. The magnitude is estimated as 9 from the tsunami heights, in which case the earthquake ruptured the entire length of the Cascadia subduction zone. These estimates are consistent with Native American legends that an earthquake occurred on a winter night."