3620 36TH PL NE 2018-01-02 MF Import

3.4.6 Sand Deposits The compressible fine-grained alluvial and estuarine deposits are underlain at depth by loose to medium dense, fine to medium sand with variable silt content. All but seven test pits were terminated at depth in coarser -grained alluvial sand deposits. The CPTs indicate that the sandy soils extend to the full depth of the explorations. The sand deposits contain interbeds of fine-grained deposits. 3.5 GROUNDWATER CONDITIONS Moderate to rapid groundwater seepage was observed in several of the test pits. Although not encountered in all of the test pits, we anticipate that groundwater levels at the site will be in response to water levels in Steamboat Slough and the Snohomish River. Rapid groundwater seepage was observed when the bottom of the fine-grained deposits.in some of the test pits was penetrated. For example, in TP-11, rapid groundwater seepage was observed at a depth of about 20 feet when the bottom of the fine- grained soils was penetrated. After about %2-hour, the water lev.ol in TP-11 rose 10 feet and to within j about 10 feet of the ground surface. We anticipate that the groundwater elevation will fluctuate as a function of the tides, surrounding surface water elevations, and other factors. Moderate to rapid groundwater seepage was observed in several of the test pits at depths ranging from about 3 to 5 feet. The groundwater at this elevation was observed perched on the underlying finer -grained and less permeable alluvial soils. We anticipate that the perched groundwater elevation will also fluctuate as a function of`precipitation, time of year, and possibly the tides and surrounding surface water elevations, and other factors. 3.6 SEISMICITY 3.6.1 General Seismicity in the ; iget Sound area is primarily driven by the Cascadia Subduction Zone, which is the zone where the westward advancing North American Plate is overriding the subducting Juan de Fuca Plate. Thr-e potential seiL.nic source zones are generally acknowledged for the Puget Sound area: (1) shallow crustal earthquakes associated with known and/or unknown faults; (2) Cascadia Subduction Zone interface earthquakes, whic'.1 occur along the boundary located between the Juan de Fuca and North American plates; and (3) Cascadia Subduction Zone intraplate earthquakes, which occur within the subducting Juan de Fuca Plate. 3.6.2 Shallow Crustal Earthquakes Shallow crustal earthquakes occur within the North American Plate to depths up to 15 miles. Shallow earthquakes in the Puget Sound region are expected to have durations ranging up to 60 seconds. Four magnitude 7 or greater known earthquakes have occurred in the last 1,100 years in the Cascadia region, two of these occurred on Vancouver Island and two in Western Washington. The largest historic earthquake in Western Washington occurred in 1872 in the North Cascades and is i estimated to have had a magnitude of 7.4. This earthquake is believed to have occurred at a depth of less than 10 miles. The other magnitude 7+ Western Washington earthquake occurred on the Seattle Fault 7 1 approximately 1,100 years ago. — N G e o E n g i n e e r s 6 File No. 10625-001-O21123003