There are three categories of earthquakes, all of which occur here.
o Shallow or crustal quakes happen along faults near the surface, up to about 10 miles deep. Such faults include the Mount St. Helens Seismic Zone, the Lacamas Creek Fault and the Portland Hills Fault. Shallow faults can trigger by themselves, or could be triggered by deeper earthquakes created through plate tectonics.
o Interplate quakes happen when one geologic plate affects another, such as parts of Juan de Fuca melting and scraping beneath the North America plate. Those quakes tend to be deeper, perhaps 30 miles below Earth’s surface.
o Subduction zone earthquakes happen when plates stick and then suddenly slip against one another at plate boundaries, such as the Cascadia fault.
Every year, the Pacific Northwest Seismograph Network records about 2,000 earthquakes in Washington and Oregon. Most are shallow quakes with magnitudes of less than 3.0.
Larger shallow quakes are far less frequent but can be dangerous. The strongest shallow quake recorded since white settlers came to the region was an estimated magnitude 7.4 back in 1872. It was felt in Oregon, Idaho and Washington. More recently, the “Spring Break Quake” on March 25, 1993 — a shallow magnitude 5.6 centered southeast of Portland — caused $28 million in damage.
Interplate quakes can cause even more damage. On Feb. 28, 2001, the magnitude 6.8 Nisqually earthquake centered 32 miles beneath the Puget Sound region killed one and injured 700, creating between $1 billion and $4 billion in damage.
Scientists think both of those types of earthquakes happen at a rate of about one per 50 years in the Pacific Northwest.
A 3.7 magnitude earthquake about two miles northeast of Amboy was widely felt in the Vancouver-Portland metro area according to the U.S. Geological Survey.
The quake struck at 5:03 p.m. at an estimated depth of 4.7 miles. The magnitude was originally estimated at 3.5, but the intensity was upgraded twice, to 3.7, within the first few minutes of the temblor.
Shaking was reported in a wide area of Clark County, including Yacolt, northern areas of Vancouver, Battle Ground, Salmon Creek, Northwest, Hazel Dell, Orchards and Washougal.
No damage has been reported, and no emergency calls resulted from the quake, according to the Clark Regional Emergency Services Agency.
A community-generated intensity map compiled by reports to the USGS website reported light shaking. Did you feel it? Report to the USGS here.
As of about 5:45 p.m., almost 800 Clark County residents had responded to the U.S. Geological Survey’s “Did you feel it?” website, which sorts the responses according to ZIP codes; about 470 of those who reported feeling the quake live in Vancouver. The most distance response was from Lummi Island, about 190 miles away.
Seth Moran, a seismologist with the USGS’ Cascades Volcano Observatory in Vancouver, said Wednesday’s quake was a tectonic earthquake. That means it was caused by the squeezing and grinding of plates, not by the movement of magma. The earthquake’s epicenter is outside the Mount St. Helens seismic zone, Moran said.
There have been similar events in Southwest Washington before, Moran said. Smaller quakes often spring from the many “microfaults” that criss-cross the entire region, he said. Those tend to produce relatively minor events, Moran said, but not always.