Earthquake!

Arrow points approximately to our location!

In the early hours of this morning (2:51 a.m.), a M4.6 earthquake struck under the town of Monroe, east of Everett (red star).  We and many friends (black arrow), were awakened by shaking and rattling! I haven't been woken up by an earthquake in more than 50 years! We reported it to the USGS site where they gather data on how far the earthquake was felt; the map to the left compiled from such reports shows that the shaking intensity was V or less around Puget Sound.
    This was a deep earthquake--24.3 km.  A M3.5 aftershock occurred less than 2 minutes later at a depth of 30.1 km, within the crust of the North American plate. There have been no reports of injuries, and a few reports of cracks in foundations near the epicenter. The quake was not on the Cascadia subduction zone. The Nisqually earthquake of 2001 with M6.8 near Olympia was in the subduction zone and was within the Juan de Fuca plate.
     There is an excellent summary of the types of earthquakes that occur in the Pacific Northwest here. As of 10:30 PDT this morning, there have been a number of small aftershocks.
      The nearest big fault is the South Whidbey Fault, which runs southeast from the Strait of Juan de Fuca toward Monroe (to the south of Monroe). The motion along this fault is strike-slip/reverse thrusting.  According to UW seismologist Bill Steele, it looks like the quake originated "from a cluster of faults running north-south from Duvall", that is, it was not along the South Whidbey Fault.  In 1996 a M5.4 earthquake near Duvall caused millions in damage, but that quake was shallower as well as stronger than this one. In a video on this King5 site, he explains that the motion was tensile, that is, the ground pulled apart. The Early Warning System gave 3-4 seconds warning in Seattle.
    Here is a short article on the South Whidbey Island fault zone from the Department of Natural Resources, WA. And, below is a reproduction from the University of Washington website on the South Whidbey Fault:
    

"Much of the Southern Whidbey Island fault zone (SWIF), which runs in a north-westward direction from Woodinville to near Port Townsend, Washington, remains mostly hidden. Geologists conclude that the SWIF is capable of producing a M6.5 to M7.4 earthquake (Kelsey et al., 2004).  The ground shaking expected for a M7.4 earthquake is shown in the ShakeMap below. As with other crustal faults, any moderate or large earthquake on the SWIF will likely be followed by numerous felt aftershocks, some that could be damaging, and hundreds to thousands of smaller ones detectable only by sensitive instruments.

‘ShakeMap’ showing the intensity of ground shaking (colors) expected for a M7.4 earthquake on a segment of the South Whidbey Island fault (white line indicates intersection of the causative fault with the surface), overlain on topography.

     "The SWIF was first discovered because movements along it juxtaposed older crystalline bedrock next to younger volcanic basalts (Johnson et al., 1996). These rocks have contrasting densities and magnetic properties that were measured and mapped by Gower et al. (1985), and attributed to motions along a single fault. Subsequent studies showed that numerous fault strands comprise the SWIF, located within a 6-11 km (3.7-6.8 mile) wide band.

      "These faults dip steeply to the northeast and have had north-side-up and lateral displacements, and are visible at the Earth’s surface only about every 35 km (22 miles).These studies used seismic reflection data, sea cliff exposures, and sparse borehole data to map the SWIF to the eastern Strait of Juan de Fuca (Johnson et al., 1996), while others used seismic imaging methods to steer the fault along the northwestern margin of the Port Townsend basin, where it may merge with the Darrington-Devils Mountain fault zone near Victoria, British Columbia (Broker at Al, 2005; Ramachandran et al., 2005). If these interpretations are correct, the SWIF extends a minimum of 150 km (92 miles) from Victoria, British Colombia, to near Woodinville, Washington.

     "Evidence that the SWIF has been recently active comes from high-resolution seismic images and measurements documenting uplift of the shorelines that straddle the faults, along two coastal marshes on Whidbey Island, at Hancock Lake on the south side of the SWIF and Crockett Lake on the north side (Kelsey et al., 2004). If no movement on the fault strand occurred in the latter part of the last 10,000 years (Holocene epoch) both sites should have comparable sea-level histories. However, stratigraphic observations and radiocarbon dates used to construct relative sea level curves for each site diverge between 2800 and 3200 years ago, suggesting uplift of about 1 to 2 m (3.3 to 6.6 feet) along the north side of the fault strand. This amount of uplift was likely generated by a M6.5 to M7.0 earthquake, according to empirical relationships between vertical displacement versus magnitude for historical earthquakes (Kelsey et al., 2004).

"Earthquakes on the SWIF probably caused at least three episodes of strong ground shaking and one tsunami in the last 1200 years. Geologists studied the stratigraphy of channel bank exposures along the Snohomish River near Everett, Washington reveal and infer that a widespread pairing of sand overlain by clay that correlates over 20 km² was left behind by a tsunami surge across the delta between 1200 - 1020 years ago (Bourgeois and Johnson, 2001). Multiple episodes of strong ground shaking also have been inferred from liquefaction features, sand dikes and sand-filled cracks up to 1 m (3.3 feet) wide, some of which terminate below and others that cut across the tsunami deposit and thus, pre- and post-date it (Bourgeois and Johnson, 2001).

     "More recently studies extend the record farther back in time and southward.  These suggest that the SWIF produced at least four earthquakes since deglaciation about 16,000 years ago, the most recent being less than 2700 years ago. High-resolution topography (LiDAR) and measurements of the magnetic properties of the rocks reveal lineaments indicative of fault movements.  These show that the SWIF forms a 20 km (12 miles) wide swath of parallel fault strands, that project onto the mainland near Everett and continues to the southeast towards Woodinville (Blakely et al., 2004; Sherrod et al. 2008).  The most prominent feature, the Cottage Lake lineament, extends at least 18 km (11 miles) and lies on strike with the SWIF on Whidbey Island. Excavations across visible scarps that exhibit north-side-up vertical relief of 1-5 m (3.3 to 16.4 feet) show these were created in multiple earthquakes that post-date deglaciation. 

     "Although highly speculative, geologists have suggested that the SWIF is part of a larger system of faults that extends from Victoria, reddish Columbia to Hanford, Washington a distance of about 385 km (236 miles). However, while such a system may reflect very large-scale geologic processes, no evidence exists indicating multiple zones have failed together in a single earthquake. A series of faults and folds in the Snoqualmie area of the Cascades likely correlate with the SWIF (Dragovich et al., 2007, 2008), merge with mapped faults on Rattlesnake Mountain (mapped by Tabor et al., 2000) near North Bend and continue southeast into the Cascade Mountains. Others suggest that faults in the Yakima fold and thrust belt correlate with faults west of the Cascades, based on lineaments in magnetic measurements and other observations (Blakely et al., 2009).

     "The HAZUS program provides quantitative estimates of some of the impacts of a M7.4 earthquake on the SWIF.  Examples include ~97800 buildings (~5% of the inventory) at least moderately damaged, with 6% of these damaged beyond repair.  A handful of bridges will be destroyed completely, significant fractions of the utility system will be only partially functional in the first day after the earthquake but mostly fixed within a week.  However, in excess of 100,000 households will be without potable water or power in the first day and tens of thousands still without both after a week.  Almost 14,000 households will be displaced and 58% of these will require public sheltering.  Fatality estimate range from 90 to 432 depending on the time of day the earthquake strikes.  Economic losses will be in the range of many billions of dollars."