5200 EVERGREEN WAY SEPA 017 - 010 Geotech Report 2025-04-23

Subsurface Exploration, Geologic Hazards, and Former Brien Ford 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 ground and surface water conditions, as observed and discussed herein. The discussion will be limited to slope, seismic, and erosion issues. 5.0 SLOPE HAZARDS AND MITIGATIONS The majority of the site is relatively flat. Based on aerial photos and visual reconnaissance of the site, it does not appear that any slopes onsite meet the City of Everett definition for treatment as steep slope or landslide hazard critical areas. No detailed assessment of slope stability was prepared as part of this report and none is warranted, in our opinion. 6.0 SEISMIC HAZARDS AND MITIGATIONS The following discussion is a general assessment of seismic hazards that is intended to be useful to the owner in terms of understanding seismic issues, and to the structural engineer for preliminary structural design. Earthquakes occur regularly in the Puget Lowland. The majority of these events are small and are usually not felt by people. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event; the 2001, 6.8-magnitude event; and the 1965, 6.5-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 Generally, the largest earthquakes that have occurred in the Puget Sound area are sub-crustal events with epicenters ranging from 50 to 70 kilometers in depth. Earthquakes that are generated at such depths usually do not result in fault rupture at the ground surface. However current research indicates that surficial ground rupture is possible in the South Whidbey Island Fault Zone. Due to the suspected long recurrence interval, the potential for surficial ground May 16,2017 ASSOCIATED EARTH SCIENCES,INC. AWR/Id-160594V003-2-Projectsl20160594\KV\WP Page 6