4730 COLBY AVE YMCA Geotech Report 2025-04-14

Subsurface Exploration, Geologic Hazard, and Snohomish County YMCA 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 conditions as observed and discussed herein. 5.0 SLOPE HAZARDS AND MITIGATIONS The site does not appear to contain slopes that constitute a slope stability hazard, in our opinion, and does not contain slopes that meet the definition for landslide hazard areas as contained in Everett Municipal Code (EMC) Section 19.37.080. No quantitative slope stability analysis was completed for this study, and none is warranted for the currently proposed project, in our opinion. 6.0 SEISMIC HAZARDS AND RECOMMENDED MITIGATION Earthquakes occur regularly in the Puget Sound Lowland. Many of these events are small and are not felt by people. However, large earthquakes do occur as evidenced by the most recent 6.8-magnitude event on February 28, 2001 near Olympia Washington, the 1965 6.5-magnitude event, and the 1949 7.2-magnitude event. The 1949 earthquake appears to have been the largest in this area during recorded history. Evaluation of 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 Based on review of the Washington State Department of Natural Resources (WADNR) website, suspected traces of the southeastward extension of the South Whidbey Island Fault Zone (SWIFZ) are about 4 miles southwest of the site. The recognition of this fault splay is relatively new, and data pertaining to it are limited with the studies still ongoing. The recurrence interval of movement along this fault system is still unknown, although it is hypothesized to be in excess of 1,000 years. Due to the suspected long recurrence interval, the potential for surficial ground rupture is considered to be low during the expected life of the new building, and no mitigation efforts beyond complying with the current (2015) International Building Code (IBC) are recommended. February 26,2018 ASSOCIATED EARTH SCIENCES,INC. AWR/ms-170671E001-2-Projects120170671 IKE‘WP Page 7