615 75TH ST SE Geotech Report 2022-10-25

Subsurface Exploration, Geologic Hazard, and Boardwalk Condominium, Building C Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations H. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic and ground water conditions as observed and discussed herein. 5.0 SLOPE STABILITY ASSESSMENT Based on Section 37.080 Part A.Ld of the Everett Municipal Code, the steep slope is classified as a landslide hazard area. The observed site conditions described previously, including the pattern of cracking in the foundation elements, apparent settlement of soils in the affected foundation areas, and the presence of loose soils, support the landslide hazard area designation. The geotechnical engineering recommendations for the affected building area are intended to provide support for the structure without increasing the landslide risks. The foundation improvements are not intended to support the rockery slope area. The owners must understand and accept that there is inherent risk associated with building near a designated landslide hazard area. 6.0 SEISMIC HAZARDS AND MITIGATION Earthquakes occur in the Puget Lowland with great regularity. The vast majority of these events are small and are usually not felt. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event; the 1965, 6.5-magnitude event; and the 2001, 6.8-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 6.0 and 7.0 is likely every 25 to 40 years in the Puget Sound area. 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 September 12, 2012 ASSOCIATED EARTH SCIENCES, INC. FSM/!d-EE120288A8-Projects1201202881EEMP Page 6