Mercer Island Seismic Hazard Assessment geomapnw.ess.washington.edu 0 0.5 1 by Kathy G. Troost & Aaron P. Wisher 1:12,000 April 2009 Kilometers Miles 0 0.5 1 Ï SEISMIC HAZARD AREAS (MICC 19.16.010)

Seismic Hazard areas are those areas subject to severe risk of damage as a result of earthquake -induced ground shaking, slope failure, settlement, soil liquefaction or surface faulting.

Seismic Seismic Hazard Area (Known or Suspect) Hazard For all other areas risk is unknown or limited to ground shaking

Supplemental Data Potential for seismically induced ground failures including settlement, cracking, SE 20th St lateral spreading, liquefaction due to ground shaking. Seismically hazardous areas include the following:

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h t Seattle ( 8 Uplift 8 ( ( ( ( Mercer Island falls within the Seattle fault zone and at least two stands of the Seattle fault cross the ( ( island. No direct evidence of surface fault rupture has yet been documented for Mercer Island (Troost and Wisher, 2006).

E SE 47 S th S t e SE 47th St The Seattle Fault Zone is the area where several parallel strands of the Seattle fault have either

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t broken the ground surface or caused deformation of geologic materials. Earthquakes of magnitude

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o M7 or greater have occurred on some of these fault strands within the Holocene (last 10,000 years) F and will likely occur again (Blakely, et al., 2002; Sherrod 2002, 2005). The Seattle Fault Zone is one ve SE SE 48th St A h of several active crustal faults zones in the Puget Lowland currently undergoing research. 6t 8 On Mercer Island, evidence for movement along these fault strands consists of exposures of deformed sedimentary strata and geophysical images of folded and faulted strata (Troost and Wisher, 2006; Stephenson et al., 2007). Elsewhere in the Puget Sound lowland, evidence for movement on ndle Li y the fault strands consists of uplifted beach deposits, down-dropped tidal marshes, offset strata, fault

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n E Bu h R scarps, and deformation such as sheared and tightly folded strata. Evidence of the Seattle fault zone S tte rt y r wo a e l v P in the subsurface consists of aeromagnetic, gravitational, and seismic reflection anomalies (Liberty W A rd r 3 ce h 5 er t W M 0 E and Pratt, 2008). 9 S SE 54th St East of Mercer Island, the Vasa Park fault and Newcastle Hills fault each have surface expression in the form of fault scarps and subsurface expression in the form of magnetic and seismic linear anomalies (Liberty and Pratt, 2008; Sherrod, 2002). The magnetic and seismic anomalies may be SE 56th St continuous with similar features to the west of Lake Washington, but those continuities are not firmly established (Liberty and Pratt, 2008). The locations of these faults are not well defined on Mercer Island (Pratt, 2009, pers. comm.) SE 58th St SE 58th St SE 59th St The Deformation Front is an east-west-trending, convex-upward fold in geologic strata, where those strata drape over the northern-most thrust fault in the Seattle Fault Zone. North of the Deformation SE 60th St SE 61s Front is the Seattle Basin, where strata lie nearly flat; south of the Deformation Front the strata dip t St SE 61s down toward the north beneath the Seattle Uplift (Pratt, 2009). The location of the Deformation Front t Pl E 9 S 2 was moved northward from previous interpretations (Brocher, et al, 2004) following detailed n SE 62nd St d e v SE 62nd St A evaluation of seismic lines by Pratt (2009). A v e

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E W Geologic materials were assessed then classified as either strongly, moderately, or poorly consolidated.

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e Degree of consolidation is a direct translation of geologic unit based on geologic history and predominant

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A 66 C th S t t lithology. Because considerable variability exists within each geologic unit, more detailed analysis is

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8 l needed for site-specific evaluations or to evaluate the degree of consolidation at a larger scale than

s I provided. Slope and degree of saturation also affect the degree of consolidation, but have not been SE 68th St

E factored into this map. This qualitative assessment should be used to evaluate and understand the

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S character of the island as a whole. These data should not be used for purposes of site-specific land-use

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t E planning or site-specific geologic evaluations. The classification shown on the map does not account for A

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7 h 8 d t 0 t H S r th l the built environment and impervious surfaces. P 6 t 3

o 9 l ly 8 SE 71st St H i ll D r SE 72nd St GENERAL NOTES FOR GEOLOGICAL HAZARDS MAPS nd St 72 E SE 72nd Pl W R S idg e Rd This map is one of a suite of revised Geological Hazard Maps for the City of Mercer Island. This suite includes maps showing Seismic Hazards, Landslide Hazards, and Erosion Hazards.

Other geological and/or natural hazards may exist and geological events may occur on Mercer Island

S that are not specifically identified on these maps. Examples of geologic hazards and hazardous events SE 76th St E 7 that are not identified on these maps include, but are not limited to, tsunamis and seiches in Lake 6th P l Washington.

7 8th St 9 E 7 t S h These maps are for the sole use of the staff of the City of Mercer Island’s Development Services Group A

v e 79th St (DSG) for the purposes of permit application evaluation. These maps provide DSG staff a general SE S E y a assessment of known or suspect geological hazard areas for which the City will require site and Lewis Ln W r e project-specific evaluation by a Washington State-licensed engineer, geologist or engineering geologist rc e M prior to issuing a permit for site development. All areas have not been specifically evaluated for St E 82nd SE geologic hazards and there may be locations that are not correctly represented on these maps. It is the responsibility of individual property owners and map users to evaluate the risk associated with their proposed development. No site-specific assessment of risk is implied or otherwise indicated by the r W n D M valo City of Mercer Island by these maps. e A rc e r

W a The City of Mercer Island is using guidance provided by the State of Washington regarding the y

l definition of geologically hazardous areas in accordance with WAC 365-190-080 and the Growth P o Management Act. “Geologically hazardous areas”, by State definition, “include areas susceptible to th o n e erosion, sliding, earthquake, or other geological events. They pose a threat to the health and safety of B citizens when incompatible commercial, residential, or industrial development is sited in areas of significant hazard.”

This new set of maps represents an update of the 2002 Geologic Hazard Map Series and is based on a review of Best Available Science for the Seattle Fault and related events, a new Geological Map of Mercer Island by Troost and Wisher (2006), and a geologic database of Mercer Island compiled by GeoMapNW at the University of Washington. Information about data used for the maps, references, and data limitations are all described in an associated “Read Me” document. The digital version of these maps is accompanied by a meta data file containing pertinent information about map construction. These data and maps are all available on the City of Mercer Island website.