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Recent Earthquakes off the Oregon Coast

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Posted: June 2nd, 2015
Author: Dulcy Berri

First, a little background on the geological setting and plate tectonics. In the Pacific Northwest, the Juan de Fuca plate includes a submarine mountain range that defines the spreading center (divergent boundary), where the plates divide and new crust is formed. The plate moving east then plunges beneath the North American Plate. As the denser plate of oceanic crust is forced deep into the Earth’s interior beneath the continental plate (convergent boundary), “subduction” occurs as it encounters high temperatures and pressures that partially melt the solid rock.

Credit: USGS

Figure 1. | Credit: USGS

Some of this newly formed magma rises toward the Earth’s surface to erupt, forming a chain of volcanoes above the deep subduction zone (Cascade Range: Mt. Baker, Mt. Rainier, Mt. St. Helens, Mt. Hood, etc.).

The third plate boundary is that of “strike-slip” fractures of the Blanco fracture zone in the Pacific Northwest and the Mendocino and San Andreas fracture zones in California (Figure 1 ). At these boundaries, plates move oceanic and continental crust in a lateral direction. Movement is often sticky and strain builds up until the fault breaks. With enough slip over a large enough area, a significant earthquake will occur.

Five shallow earthquakes occurred off of the Oregon coast from late Sunday night through Monday afternoon, ranging from 5.8 magnitude followed by three aftershocks than a 5.9 magnitude quake a 1:11 PM. No tsunamis were generated; it is generally believed that a 7.0 is required to cause a tsunami. The cluster of quakes occurred near the boundary of the Blanco fracture zone and the Juan de Fuca ridge at depths of six miles deep1, which is relatively shallow compared to subduction zone quakes that are the type most feared to eventually rock the Northwest.

Author: Dulcy Berri, LG, LHG, RG

Dulcy Berri is a Senior Hydrogeologist at PBS with over 25 years of technical experience performing geologic and environmental studies throughout the Pacific Northwest.

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