Magnitude-4.6 Earthquake Rattles Seattle Area

Magnitude-4.6 Earthquake Rattles Seattle Area

July 12, 2019– Mary Greeley News – A magnitude-4.6 earthquake awakened people across the Seattle area just before 3 a.m. local time on Friday. Several smaller aftershocks also hit the region.

The epicenter was near Three Lakes, Wa., about 35 miles north of Seattle, and tremors were felt across the Puget Sound region. There were no early reports of injuries and the U.S. Geological Survey (USGS) said there was no tsunami risk.

The largest aftershock so far today of 12 earthquakes was a M3.5 at a depth of 30.6 km or 19.01 miles.

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A 4.6-magnitude quake is relatively benign, but it comes just six days after a magnitude-7.1 quake struck Southern California, the region’s worst in 20 years.

Seattle is close to Mount St. Helens, Mount Baker, Glacier Peak and Mount Rainier, four of the U.S.’s most active volcanoes. Earthquakes can increase the risk of volcanic eruption, according to the USGS.

Over 14,720 people reported feeling this earthquake on the USGS “Did You Feel It” web page.

This was a thrust fault movement where the one block moved up and over another. East moved up over the west. Compression was in the center of the Moment Tensor ball. This cats eye focal mechanism is the signature on a thrust fault produced by compressional tectonic forces.

Body Waves: P-waves

Magnitude-4.6 Earthquake Rattles Seattle Area

Primary Wave: P wave is a compressional (or longitudinal) wave in which rock (particles) vibrates back and forth parallel to the direction of wave propagation. P-waves are the first arriving wave and have high frequencies, but their amplitude tends not to be very large

P-wave first motions focal mechanisms

P-waves will radiate in all directions away from a fault. In some directions the first motion of the P-waves will initially be compressional (C) (the earthquake pushes the ground in the direction of motion). In other directions the P-waves will be dilitational (D) (the earthquake pulls the ground away from the direction of wave motion. The dilitational and compressional first motions are divided into quadrants. Seismologist can use this pattern of first motions to infer the orientation of the fault

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