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

Shallow crustal faults with ;mown or suspected Quaternary displacements within the general project area include the Southern Whidbey Island Fault. The Southern Whidbey Island Fault is a inorthwest -southeast trending structure located near Whidbey Island and extending as far southeast as Everett. Recent evidence suggests that the Southern Whidbey Island Fault is an active fault system with Quaternary displacements. This fault system is considered capable of magnitude 7 earthquakes and is located approximately 15 km from the site. 3.6.3 Interface Earthquakes Interface earthquakes occur on the boundary between the Juan de Fuca and North American tectonic plates. The Cascadia Subduction Zone extends from Vancouver Island to Northern California. Interface c:*thquakes on the Cascadia Subduction Zone are anticipated to have durations ranging up to 4 minutes. Evidence of the occurrence of large (magnitude 8 to 9+) earthquakes occurring on the Cascadia Subduction Zone has recently been discovered. The last large interface earthquake is believed to have occurred in the year 1700. It is estimated that the recurrence interval for interface earthquakes on the Cascadia Subduction Zone is about 400 to 600 years; however, the interval between earthquakes appears irregular. 3.6.4 Intraplate Earthquakes Cascadia Subduction Zone intraplate earthquakes occur within the subducting Juan de Fuca Plate at depths of 30 to 40 miles within the Puget Sound area. Intraplate earthquakes are expected to have durations ranging up to 30 seconds and magnitudes ranging up to 7.5. The Olympia 1949 (magnitude 7.1), the Seattle 1965 (magnitude 6.5), and the Nisqually 2001 (magnitude 6.8) were intraplate earthquakes. Other earthquakes that are considered to be intraplate events occurred in 1882, 1909 and 1939. 3.6.5 Liquefaction Potential Liquefaction refers to the condition where vibration or shaking of the ground, usually from I earthquake forces, results in the development of excess pore pressures in saturated soils with subsequent loss of strength in the deposit of soil so affected. In general, soils that are susceptible to liquefaction include very loose to medium dense, clean to silty sands that are situated below the water table. 1 The evaluation of liquefaction potential is a complex procedure and is dependent on numerous site parameters, including soil grain size, soil density, site geometry, static stresses, and the design ground acceleration. Typically, the liquefaction potential of a site is evaluated by comparing the cyclic shear stress ratio (the ratio of the cyclic shear stress to the initial effective overburden stress) induced by an earthquake to the cyclic shear stress ratio required to cause liquefaction. We evaluated the earthquake -induced cyclic shear stress ratio at this site using an empirical relationship developed by researchers for this purpose. A design earthquake with a magnitude of 6.5 and a peak horizontal acceleration of 0.27g (27 percent of the acceleration due to gravity) was used for our analysis. This analysis also assumes a level ground surface. The cyclic shear stress ratio required to cause liquefaction was estimated using an empirical procedure based on correlations from the cone penetration tests. This method relates the cyclic shear IZ G e o E n g i n e e r s 7 File No. 10625.OGI.O 123003