ShakeMapGeologicSummaries

2012 EVERGREEN EARTHQUAKE EXERCISE SCENARIOS; BACKGROUND SUMMARIES

SEATTLE FAULT ZONE M6.7 EARTHQUAKE SCENARIO

The Seattle Fault zone includes several fault strands within a 4 to 7 km-wide (2.5 to 4 miles) east-trending band extending from the Cascade Range foothills on the east across the Puget Lowland to Hood Canal, crossing Lake Sammamish, Lake Washington, Puget Sound, Bainbridge Island, and the Kitsap Peninsula. Most of the evidence of these faults lies hidden beneath the major population centers of Seattle, Bellevue, and Bremerton. The most recent and largest known earthquake within the Seattle Fault zone occurred about 1050-1020 ago, with a magnitude greater than M7 (Bucknam and others, 1992; Atwater, 1999; Nelson and others, 2000). While perhaps the most well studied crustal fault zone in the Puget Sound region, new information and ideas about the locations, orientations, and history of motions of the Seattle Fault zone continue to emerge. All the available information indicates that there is about a 5% chance of having a M>6.5 earthquake on one of the strands of the Seattle Fault zone in a 50 year timeframe. Any moderate or large earthquake on the Seattle Fault zone 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.

Seattle ShakeMap

‘ShakeMap’ showing the intensity of ground shaking (colors) expected for a M6.7 earthquake on a fault within the Seattle Fault zone (causative fault outlined in white), overlain on topograp

A M6.7 earthquake, like that assumed in the Evergreen Exercise Series (see ShakeMap on left), thousands of years ago on the northernmost strand of the Seattle Fault Zone might be what caused surface displacement of about 6.5 feet mapped just west of Lake Sammamish in southeast Bellevue, near Southeast 38th Street. The causative fault rupture extends for about 23 km (14 mile), from Harbor Island to just east of Lake Sammamish. These same characteristics were used to as the basis for this earthquake scenario. The starting point of the scenario is a model of the ground shaking, displayed as ShakeMap (left).

The Seattle fault was originally identified by abrupt changes in measurements of the Earth’s gravity field, caused by the juxtaposition of rocks of contrasting densities (Danes et al., 1965). Fault slippage during an earthquake uplifted older volcanic rocks of Tertiary age (65-2.6 million year old) on the south side of the fault and down-dropped younger sediments to the north. More recently additional, parallel fault strands were identified within an east-trending zone, evident in a variety of geophysical measurements (Blakely et al., 2000).

The Seattle Fault Zone has hosted multiple larger earthquakes. The clearest evidence of one of these can been seen in a conspicuous flat surface, or platform, bordering the shoreline of southern Bainbridge Island, parts of Kitsap County, and Alki Point in West Seattle. This platform initially formed as wave action cut the surface while at a lower, intertidal elevation. About 1100 years ago a single earthquake uplifted this surface as much as 8 meters (Bucknum et al., 1992), and caused a tsunami, liquefaction, and landslides. Examination of the geologic record extending as far back as 8000 years ago reveals evidence for another comparable sized earthquake on the Seattle Fault Zone about 6900 years ago (Sherrod et al., 2000), suggesting a recurrence interval on the order of thousands of years. More recent studies of newly identified fault scarps (scars on the surface left when faulting cuts the Earth’s surface) within the Seattle Fault Zone confirmed these inferences. Analyses of high-resolution topographic imagery (LiDAR), followed-up on with trenching studies, led to the discovery of a fault scarp on southern Bainbridge Island that probably formed in as many as three surface-rupturing earthquakes in the past 2500 years (Nelson at all, 2003, 2003). Additional LiDAR and trenching studies identified fault scarps at Islandwood on Bainbridge Island, Waterman Point and Point Glover in Kitsap County, which probably formed as a result of two surface-rupturing earthquakes about 1100 years ago (Nelson et al, 2003).

Some of the impacts of a M6.7 Seattle Fault zone earthquake have been quantified using the HAZUS program. Because this is a moderate earthquake, the serious damage will result in places and the financial cost of recovery very significant. Example impacts estimated by the HAZUS program include about 7% (96,412) of inventoried buildings at least moderately damaged (i.e., have a 50% chance of being functional) and ~3% of these damaged beyond repair. One week after the earthquake, 90% of the 9,008 hospital beds in use on the day of the earthquake will be back in service and after 30 days 99% will be operational. No critical facilities, transportation systems or lifelines will be completely destroyed. Schools will experience the greatest damage among critical facilities, with ~13% experiencing moderate or greater damage. Among the transportation networks and utilities that have moderate or greater damage are ~2% of the bridges and 15% of the port facilities, and 35% of communications and 31% of potable water systems. 5% and 11% of households will be without potable water and electricity, respectively, in the first day after the earthquake, but after a week these percentages will both have dropped to 3%. 166,920 truckloads of debris will need to be removed. 12,771 households will be displaced with 57% of these requiring temporary sheltering. Between 68 to 345 fatalities are forecast, depending on the time of day the earthquake strikes. The total building-related losses are estimated to cost 15 billion dollars.

Darrington-Devils Mountain fault zone M5.7 Earthquake Scenario

Darrington – Devils Mountain Fault Zone (DDMFZ) extends westward for more than 125 km (77 miles) from the Cascade Range foothills to offshore Vancouver Island. It was initially identified as two separate fault zones; at its east end the Devils Mountain fault merges with the Darrington fault zone (Tabor, 1994) and at its west end, the Devils Mountain fault may merge with the Leech River and (or) San Juan faults on Vancouver Island (Johnson and others, 2001). Any moderate or large earthquake on the DDMFZ will likely be followed by numerous felt aftershocks and hundreds to thousands of smaller ones detectable only by sensitive instruments.

Darrington-Devils Mountain ShakeMap

‘ShakeMap’ showing the intensity of ground shaking (colors) expected for a M5.7 earthquake on a segment of the Darrington –Devils Mtn fault (white line indicates intersection of the causative fault with the surface), overlain on topography.

The M5.7 earthquake scenario for the DDMFZ assumes a 5-10 km (3-6 mile) long section of fault ruptures between Mt. Vernon and Darrington (see ShakeMap on right). The DDMFZ forms the northern boundary of the sediment-filled, Everett basin. This boundary separates rocks with differing magnetic properties and thus can be ‘seen’ as changes in the magnetic field measured over the area that occur in a linear band extending from the Cascade Mountains to Vancouver Island, British Colombia (Johnson et al. 2001). This juxtaposition of rocks with different magnetic signatures likely resulted from slippage along a fault surface. The Darrington – Devils Mountain fault zone was initially identified and named independently by two different geologists, who both noted contrasting rock types and ages juxtaposed along northeast trending faults. Separate exposures on Devils Mountain near Mt. Vernon, Washington (Loveseth, 1975) and near the town of Darrington (Vance et al., 1980) were named accordingly, but later recognized as the same fault zone and the names merged (Tabor, 1994).

High-resolution LiDAR topographic maps have since revealed several potential faults scarps, and subsequent studies provide more detailed information about the fault zone’s past. Trenches across scarps on Whidbey Island exposed faulted and folded glacial sediments, and mapping of these indicate the causative fault strands slipped in a variety of directions, with total offsets of ~1.0-4.5 m (3.1 to 14.8 feet) vertically and ~2 m (6 feet) horizontally (Johnson et al., 2004). Radiocarbon ages from these trenches show that these offsets likely resulted from two earthquakes, one ~1100-2200 years ago and the second earthquake 100-500 years ago (Johnson et al., 2004). Other strands underlie a low scarp (< 1 m tall) in Skagit County east of Mount Vernon, and three trenches across this scarp exposed faulted glacial deposits and sheared bedrock, with similar complexity and vertical offsets of ~0.5 m (1.6 feet) (Personious et al. 2009) and horizontal displacements between 1-3.5 m (3.3-9.8 feet) (Personious et al., 2009).

The impacts of this earthquake are forecasted to be quite mild, in part because of its location remote from large population centers. Example impacts quantified using the HAZUS program indicate that about 1,468 buildings will be at least moderately damaged (50% chance of being functional) and only 2 buildings that will be damaged beyond repair. However building-related losses may be as much as $165 million dollars. 99% of the impacted regions 11,301 hospital beds will be available for use within a day after the earthquake and 100% within a week. No critical facilities or transportation systems will experience even moderate damage. Some pipelines (<1%) may break and 2,362 households will be without potable water the first day, although all will have it restored within a week. None should experience loss of electricity. 27 households could be displaced. No fatalities and only a few serious injuries are expected. 1,160 truckloads of debris will require removal.

SOUTHERN WHIDBEY ISLAND FAULT M7.4 EARTHQUAKE SCENARIO

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.

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.

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.

Local and regional experts have identified several strands of the Tacoma Fault Zone in the southern Puget Sound region, although the eastern extent of the zone remains uncertain. Changes in elevations of coastal marshes surrounding the Tacoma Fault Zone document a large, ~M7 earthquake that occurred on the fault about 1,100 years ago. Studies are underway to identify evidence of earlier quakes and to more accurately describe the fault system.

The M7.1 earthquake scenario for the Tacoma fault zone is based on a 56 km (34 miles) long rupture running between Kent and Union, Washington (ShakeMap on right). A M7.1 earthquake within the Tacoma Fault zone 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. Evidence for the Tacoma fault zone consists of several linear features (lineaments) observed in a variety of measurements, that appear to bound a geologic structure called the ‘Seattle uplift’ along its southern and western flanks. The Seattle uplift is a region where volcanic rocks (basalts) laid down flat about 56 to 34 million years ago and buried, have been uplifted by as much as 6-7 km (3.7 to 4.3 miles). Geologic evidence for past activity of the Tacoma fault includes raised tidal flat deposits and shorelines along Hood Canal, Case Inlet, and Carr Inlet. Geologists infer that these tidal flats uplifted between 900-1300 years ago, based on radiocarbon ages of peat and delicate plant fossils that formed over former tidal flat muds (Bucknam et al.,1992).

Scarps along the Tacoma fault zone are visible in high-resolution topographic maps (LiDAR surveys) near Belfair and Allyn, Washington. These scarps, as high as 4 m (12 feet) in places, suggest that the Tacoma fault ruptured the ground surface in the recent past. Trenches across one scarp — the Catfish Lake scarp — show evidence for a geologically recent (in the last ~10,000 years, or the Holocene epoch) earthquake that folded glacial deposits and young soils. This earthquake probably also locally uplifted shorelines along Case Inlet and Hood Canal, raising them as much as a 4 m (12 feet) between 1240 and 850 years ago. Additional evidence of a large earthquake on the Tacoma fault zone at this time comes from trenches across two other scarps, which between 600-1300 years ago (Nelson et al., 2008).

Some geologists speculate that the Tacoma fault zone may merge with the White River fault zone at Enumclaw and continue eastward through the Cascade Mountains, eventually merging with structures in the Yakima fold and thrust belt. However, this does not imply that an earthquake traversing the Cascades has happened in the past or is likely to happen in the future.

The ground shaking and fault offset of several yards (meters) during a future major earthquake on the Tacoma Fault Zone would cause landsliding, liquefaction, and possibly a tsunami in Puget Sound. The HAZUS program provides quantitative estimates of some of the impacts of a M7.4 earthquake on the SWIF. Examples include ~84300 buildings (~4% of the inventory) at least moderately damaged, with 5% of these damaged beyond repair. A dozen 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 50,000 households will be without potable water or power in the first day and tens of thousands still without both after a week. Almost 11,600 households will be displaced and 60% of these will require public sheltering. Fatality estimate range from 64 to 328 depending on the time of day the earthquake strikes. Economic losses will be in the range of many billions of dollars.

This scenario models what is considered to be a deep earthquake, like the 2001 M6.8 Nisqually earthquake (see ShakeMap below). Unlike the other earthquakes considered in the Evergreen Exercise, which occur on shallow crustal faults within the over-riding North American plate, this type of earthquake occurs on a hidden fault deep within the oceanic Juan de Fuca plate.

Earthquakes often are named for the nearest impacted, well-known city or town rather than the causative fault, particularly when the fault has not been mapped (in this case because it has no clear expression at the surface). These ‘intraplate’ earthquakes occur because the plate deforms as it descends, or subducts, beneath the North America plate. Beneath the Puget Sound region, deep earthquakes usually occur at depths of about 30 to 50 miles (45 to 80 km) and likely are smaller than M7.5. These are the most frequent type of damaging earthquake in the Puget Sound region, having an 84% of occurrence in a 50-year time period.