Earthquake Report

Magnitude: 4.6 Ml
Date: Fri July 12, 2019 02:51:38 AM (PDT)
Event Id: 61535372
Version #6: This report supersedes any earlier report of this event
This event has been reviewed by a seismologist
Magnitude 4.6 Ml
Time Fri July 12, 2019 02:51:38 AM (PDT)
Fri July 12, 2019 09:51:38 (UTC)
Distance From 3.6 km (2.23 miles) NW from Monroe, WA
18.4 km (11.41 miles) SE from Everett, WA
Coordinates 47.872, -122.014
Depth 28.97 km (17.67 miles)
Location Quality Excellent
Event ID 61535372
Horizontal Uncertainty 0.51 km
Depth Uncertainty 0.81 km
Azimuthal Gap 38.0 deg
Number of Phases 108
RMS Misfit 0.43

Historic Activity

Shake Map

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Summary

  • 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.

Aftershocks

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.

 
Magnitude
Measure of the energy released in an earthquake, obtained from interpretation of seismograms.  For technical reasons several different magnitude scales are in common use.  At PNSN we use the following:  Md (Duration Magnitude) - based on the duration of shaking.  Ml (Local Magnitude) - based on the peak amplitudes of high frequency seismograms, and Mw (Moment Magnitude) - based on matching waveforms of the lowest frequency ground motions in broad-band seismograms.  More information at: http://earthquake.usgs.gov/earthquakes/glossary.php#magnitude and http://neic.usgs.gov/neis/phase_data/mag_formulas.html
(Origin) Time
Date and Time when the earthquake rupture initiated.  Large earthquake ruptures can take many seconds to finish.  Seismologists usually use Grennwich Mean Time (GMT) to avoid confusion ariasing from mixing observations from different time zones.  However, the local time is also given as a reference for what local residents experience.
Distance From
Distances and directions from hearby geographical reference points to the earthquake.  The reference points are towns, cities, and major geographic features.  The accuracy of these distances are limited both because of earthquake location uncertainties (typically for PNSN earthquakes, less than 1 km.) and because of the geographic spread of reference points such as cities.
Coordinates
Location in geographic coordinates (as Latitude, Longitude in decimal degrees) of the position on Earth's surface directly above where an earthquake rupture initiated.  PNSN coordinates are referenced to the WGS84 ellipsoid.
Depth

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.

Location Quality
To assist non-seismologists in evaluating the reliability of an earthquake location, we assign a "quality" to each location.  The quality types are (in decreasing order of reliability) "excellent", "good", "fair", "poor" and "unknown".  This description is determined from the formal uncertainties produced by the earthquake location program.
Horizontal Uncertainty
The horizontal uncertainty of the earthquake location, given in km, is an estimate of how well the observed data constrain the location.  The estimate includes information about data quality and the arrangement and proximity of stations to the earthquake.
Depth Uncertainty
The uncertainty in depth of the earthquake location, given in km.  The depth is often the least well constrained of the location parameters, and trades off with uncertainties in the Origin Time.
Azimuthal Gap

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.

Number of Phases
Number of P and S arrival-time observations used to compute the hypocenter location.  In general, more arrival-time observations result in improved earthquake locations.
RMS Misfit

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.

Number of P First Motions

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.

Plane A

Orientation of first possible fault plane

Strike

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.

Dip

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

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).

Plane B

Orientation of second possible fault plane

Uncertainty

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.

Station

Code, or name, to designate a particular seismic station

Network

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

Quality

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).

Channel

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

Polarity means the direction of motion, in this context it means whether it is up (U) or down (D).