M4.6 - 2.23 mi NW from Monroe, WA
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Widely felt in greater Seattle area and with some felt reports east of the Cascades and north to the Canadian border. No reports of damage have been recieved.
About 17 located aftershocks in the first 24 hours. See below for plot.
Earthquake deep (~29 km or ~18 miles) and not clearly associated with any particular surface-mapped fault (although close to the Southern Whidbey Island Fault Zone (SWIFZ), Rattlesnake Mt. FZ and an unnamed NE-trending fault zone).
In Washington there have been only 2 other earthquakes of larger than this one in the past 10 years: A deep intraslab M4.9 near Poulsbo in the Puget Lowlands in 2009 and an M4.7 near Okanagan in Northeast Washington State in 2011.
Strongest recorded Peak Ground Acceleration (PGA) was about 52 gals (cm/s/s) at station UW.LEOT, 15 km from epicenter. This is characterized as light-to-moderate shaking with the potential to, although unlikely to, cause very slight damage.
Felt reports from >13,000 (as of 12:30 pm PDT on the 12th). Maximum felt intensities exceeded MMI IV at a few sites close to the epicenter, indicating light shaking.
Although not yet a public system, ShakeAlert EEW system released its 1st alert to pilot users 7.8 seconds from Origin Time (M4.4, location 3km from true epicenter). If the ShakeAlert system had been charged with issuing public alerts, Seattle would have received 5-10 s of advanced warning before shaking was felt.
- Careful review of first motions results in a first motion focal mechanisms for both larger events in agreement with the moment tensor generated from regional broad-band data by Bob Hermann; ie: thrust motion on NE-SW oriented fault planes.
Cumulative number of aftershocks starting with main shock, July 12, 09:51 Z up to July 26 2019 Z.
Time versus event depth plot for the same period.
Tectonic Summary of Area
“Crustal” earthquakes originate from slip on faults within the crust of the North American Plate. Some of these earthquakes reflect stresses that are generated by the convergence of the Juan de Fuca and North America plates but most are related to stresses originating from the interaction of the North American plate and the Pacific plate in California and Nevada. This interaction results in north–south oriented compressive stresses in the crust throughout the western and northern region. These crustal earthquakes occur in the upper 25 km of the earth's crust on faults oriented roughly east–west and northwest–southeast. In southern Oregon, extensional (pull apart) stresses also cause faulting and crustal earthquakes. Many crustal fault zones have been mapped, including the Seattle Fault Zone, the South Whidbey Island Fault, the Devil's Mountain Fault, the Tacoma Fault in the Puget Sound lowlands, and the Spencer Canyon Fault in central Washington. However, not all of the active faults are mapped, and many crustal earthquakes occur on faults that don't reach the Earth's surface. The largest historically documented crustal earthquake was the 1872 M˜7 earthquake near Lake Chelan. Other crustal earthquakes have included the 1993 M5.6 Scott's Mills, Oregon, the 1993 M5.9 and M6.0 Klamath Falls, Oregon, and the 1996 M5.4 Duvall, Washington earthquakes.
Depth within the Earth where an earthquake rupture initiated. PNSN reports depths relative to sea level, so the elevation of the ground above sea level at the location of the epicenter must be added to estimate the depth beneath the Earth's surface.
A measure of how well network seismic stations surround the earthquake. Measured from the epicenter (in degrees), the largest azimuthal gap between azimuthally adjacent stations. The smaller this number, the more reliable the calculated horizontal position of the earthquake.
How well the given earthquake location predicts the observed phase arrivals (in seconds). Smaller misfits mean more precise locations. The best locations have RMS Misfits smaller than 0.1 seconds.
A P first motion is the direction in which the ground moves at the seismometer when the first P wave arrives. We distinguish between upward and downward first motions. This is the number of observations that were used to obtain the fault plane solution.
Orientation of first possible fault plane
The strike is the angle between the north direction and the direction of the fault trace on the surface, while keeping the dipping fault plane to your right.
The dip is the steepness of the fault plane measured as an angle between the fault plane and the surface. For example, 0 degrees is a horizontal fault and 90 degrees is a vertical fault.
Rake is the angle, measure in the fault plane, between the strike and the direction in which the material above the fault moved relative to the material on the bottom of the fault (slip direction).
Orientation of second possible fault plane
The orientation of the two possible fault planes is the best solution we can find to match the observed first motions at the seismometers using a grid search method. The uncertainty of the strike, dip, and rake indicate the number of degrees by which those values can vary and still match the observations satisfactorily.
Code, or name, to designate a particular seismic station
Network Code indicates the organization responsible for a particular station, the PNSN consists of UW=University of Washington, UO=University of Oregon, and CC=Cascade Volcano Observatory
The quality of an observed P arrival polarity indicates how well you can tell whether it is up or down and can range from 0 (poor) to 1 (good).
The channel name allows one to distinguish between data from different kinds of sensors. The first character indicates the sample rate of the data, examples are E=100Hz, B=40 or 50Hz, H=80 or 100 Hz. The second character indicates whether the channel is a high (H) gain or low (L) gain velocity channel or a strong-motion acceleration channel (N). The third character indicates the direction of motion measured, Z=up/down, E=east/west, N=north/south.
Polarity means the direction of motion, in this context it means whether it is up (U) or down (D).