2202 100TH ST SW Geotech Report 2022-10-25

Subsurface Exploration, Geologic Hazard,and Stephens Everett Property Preliminary Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic, slope, and shallow groundwater conditions as observed and discussed herein. 5.0 LANDSLIDE HAZARDS AND MITIGATIONS Based on our site reconnaissance, review of the surveyed topography depicted on Figure 2, and review of Snohomish County's online mapping tool (PDS Map Portal), the slopes onsite do not meet geometric criteria for treatment as landslide hazard areas. It is our opinion that the risk of damage to the proposed structures by Iandsliding is low. This opinion is based on gentle slope inclinations and the presence of dense to very dense soils observed at relatively shallow depths beneath the surface of the site. 6.0 SEISMIC HAZARDS AND MITIGATIONS Earthquakes occur regularly in the Puget Lowland. Most of these events are small and are not felt by people. However, large earthquakes do occur, as evidenced by the 2001, 6.8-magnitude event; the 1965, 6.5-magnitude event; and the 1949, 7.2-magnitude event. The 1949 earthquake appears to have been the largest in this region during recorded history and was centered in the Olympia area. Evaluation of earthquake return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely within a given 20-year period. Generally, there are four types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture, 2) seismically induced landslides, 3) liquefaction, and 4)ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. 6.1 Surficial Ground Rupture The nearest known fault trace to the project site is the South Whidbey Island Fault Zone (SWIFZ). The project area is located approximately 0.5 miles northeast of the closest fault trace of the SWIFZ. A recent study by USGS (Sherrod et al., 2005, Holocene Fault Scarps and Shallow Magnetic Anomalies Along the Southern Whidbey Island Fault Zone Near Woodinville, Washington, Open-File Report 2005-1136, March 2005) indicates that "strong" evidence of prehistoric earthquake activity has been observed along associated fault strands thought to be part of the SWIFZ. The study suggests as many as nine earthquake events along the SWIFZ may have occurred within the last 16,400 years. The recognition of this fault splay is relatively new, March 29,2019 ASSOCIATED EARTH SCIENCES,INC. TG/ms-180567E001-2-Projects1201805671 KEI WP Page 7