18E.90.020 Seismic Hazard Areas.
A. General. Seismic hazard areas are areas subject to severe risk of damage as a result of earthquake-induced landsliding, seismic ground shaking, dynamic settlement, fault rupture, soil liquefaction, or flooding caused by tsunamis and seiches.
B. Potential Seismic Hazard Areas. Potential seismic hazard areas, as depicted on the Critical Areas Atlas-Seismic Hazard Areas map, are those areas where the suspected risk of earthquake induced landsliding, dynamic settlement, fault rupture, ground deformation caused by soil liquefaction, or flooding is sufficient to require a further seismic hazard area review as set forth in PCC 18E.90.030. These potential seismic hazard areas are determined using the following criteria:
1. Earthquake Induced Landslide Hazard Areas. Areas identified as potential landslide hazard areas in PCC 18E.80.020.
2. Liquefaction and/or Dynamic Settlement Hazard Areas. Areas identified as high and moderate liquefaction and dynamic settlement hazard areas on the Washington Department of Natural Resources, Division of Geology and Earth Resources liquefaction and dynamic settlement hazard area table.
3. Fault Rupture Hazard Areas. [Reserved]
4. Tsunami and Seiche Hazard Areas. Areas that are adjacent to Puget Sound marine waters, lakes, and ponds that are designated as "A" or "V" zones as defined by FEMA and depicted on the FEMA maps or other maps adopted by Pierce County.
C. Seismic Hazard Area Categories.
1. Earthquake Induced Landslide Hazard Areas. Earthquake induced landslide hazard areas include slopes that can become unstable as a result of strong ground shaking, even though these areas may be stable under non-seismic conditions.
2. Liquefaction and/or Dynamic Settlement Hazard Areas.
a. Liquefaction hazard areas are areas underlain by unconsolidated (corrected Standard Penetration Test blow counts, [(N1)60 ] less than 30) sandy or silt soils (Unified Soil Classification System S or M soil-types) and a shallow groundwater table (static groundwater depth < 30 feet) capable of liquefying in response to earthquake shaking.
b. Dynamic settlement hazard areas are areas underlain by a significant thickness (more than 10 feet) of loose or soft soil not susceptible to liquefaction (e.g., peats or organic silts and clays, unsaturated loose sands or silts), but that could result in vertical settlement of the ground surface in response to earthquake shaking.
3. Fault Rupture Hazard Areas. Fault rupture hazard areas include:
a. Active fault rupture hazard areas are areas where displacement (movement up, down, or laterally) of the ground surface has occurred during past earthquake(s) in the Holocene Epoch; and
b. Areas adjacent to the active fault rupture hazard area that may be potentially subject to ground surface displacement in a future earthquake. (See Figure 18E.90-1 in Chapter 18E.120 PCC.)
4. Tsunami and Seiche Hazard Areas.
a. Tsunami hazard areas are areas of high-energy waves along a Puget Sound marine shoreline which may be generated by earthquakes, volcanic activity, or landslides (both submarine and sub-aerial).
b. Seiche hazard areas are areas of high wave action in a lake or pond which may be generated by earthquakes, volcanic activity, or landslides (both submarine and sub-aerial).