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• T'hc planned building can be supported on spread footings. Allowable bearing pressures are <br /> presented in thc Shallow Foundation section of this report. Existing fill will nced to bc removed <br /> from below planned foundations. <br /> • The lowest level can be constructed as a slab•on-grade. <br /> . Wc rccommend an underslab dramage layer and wall drainage measures be incorporated into the <br /> design to capture perched groundwater seepage common in the glacial deposits and observed in <br /> the borings. <br /> • Excavations needcd to remove existing fill and/or contaminated soils that extend bclow the <br /> garagc slab should bc backfilled with structural fill. <br /> EARTM�UAKE ENGINEERING <br /> Seismieity <br /> Scismicity in wcstem Washington is primarily driven by the Cascadia Subduction Zone, which is thc <br /> zonc where the westward advancing North .Amcrican Ylate is ovcrriding the subducting Juan de Fuca <br /> Plate. Thrce potential seismic source zones arc gcnerally acecplcd for wcstern Washington: (1)shallow <br /> crustal carthquakcs associatcd with known and/or unknown faults; (2)Cascadia Subduction Zone <br /> intcrface carthquakes., which occur along the boundary located between thc luan de Fuca and North <br /> American plales; and (3)Cascadia Subduction Zone intraplate carthquakes, which occur within thc <br /> subducting Juan de fuca Platc. <br /> Shd1oN Crustal Earlhquakes. Shallow crustal carthquakes occur within the North American Plate to <br /> dcpths up to IS miles. Shallow earthquakcs in westem Washington are expected to havc durations <br /> ranging up to 60 seconds. Four magnitude 7 or grcater known earthquakes have occurred in the last <br /> 1,100years in Ihe Cascadia rcgion, two of thesc occurred on Vancouver Island and two in westem <br /> W ashington. <br /> Thc largcsl historic carlhquakc in wes�cm Washington occurrcd in 1872 in thc North Cascadcs and is <br /> estimated to havc had a magnimde of 7.4. T'his canhquake is believed tr have occurted at a depth of Iess <br /> than 10 miles. The othcr magniwdc 7+ westcm Washington earthquake occurred on the Scatde Fault <br /> approsimatcly I,l00ycars ago. <br /> Scveral olher surfacc faults havc bccn identificd in the Puget Sound region but have not been studied in <br /> dclail to dalc. Thc ncarcsl shallow faull mappeJ is the South Whidbey Tsland Fault,a northwest-southeast <br /> trcnding swcwrc dcwu-thrown on thc northcast sidc (USGS OPR 99-3R7, 1999). Thc fault is marked <br /> passing south of thc Gvcrctt arca Gom Whidbcy lsland through Mukiltco. <br /> Cascadia Subduction 7une Interfacc Earthquakes. Intcrfacc carthquakcs occur on the boundary <br /> bchvecn thc Juan de 1'vca and North Amcrican tectonic pla�es. The Cascadia Subduction Lone extends <br /> from Vancouver Island to Northern Califomia. Interfacc earthquakes on the Cascadia Subduction Zone <br /> arc anticipated to ha��c durations ranging up to 4 minates. <br /> Gvidcnc� of thc occurrcncc of largc (magnitudc S to 9+) carthquakcs occurting on thc Cascadia <br /> Subduction 2onc has reccntly hecn discovcrcd. The last large intcrface carthquake is believed to have <br /> occurred in thc ycar 1700. It is cstimated that ihe avcragc recurrence intcrval for inter(acc earthquakes on <br /> Ou Cascadia Subductirni 7.onc is about 400 to 600 ycars, howevcr, thc intcrval bctween carthquakcs is <br /> ittegular. <br /> Far,�'o. �;il:-nru-nn Puge 4 G[OEMCHrt[�f� <br /> ✓�me!.?OW' <br /> � <br />