MEMORANDUM

The Department of Planning and Development (DPD) proposes to amend the geologic hazard designations of the Environmentally Critical Areas (ECA) regulations.

The proposed Council Bill addresses the issues raised in the Final Order and Decision of the Central Puget Sound Growth Management Hearings Board in the appeal of the City’s new ECA ordinance (CPSGMHB Case No. 06-3-0024 Seattle Audubon Society, Yes for Seattle, Heron Habitat Helpers and Eugene D. Hoglund v City of Seattle). The order and decision requires Seattle to designate the Seattle Fault, tsunami, and seiche and lahar inundation areas as geologic hazard areas.

The proposal is based on a review of the best available science for geologic hazard areas in Seattle. Key recommendations are to designate as geologic hazards the Seattle Fault Zone, areas at risk from tsunamis in Seattle’s marine waters and Lake Washington, areas at risk from lahars in the Duwamish River, and areas at risk from seiches in Seattle’s major waterbodies. Further regulation of properties or development in these areas is not proposed.

Attachment A: Director’s Report and Recommendation- Amendments to the Geologic Hazard Areas Designations of the Environmentally Critical Areas Regulations

Amendments to the Geologic Hazard Areas Designations of the Environmentally Critical Areas Regulations

January, 2007

The Department of Planning and Development (DPD) proposes to amend the geologic hazard designations of the Environmentally Critical Areas (ECA) regulations. This legislation addresses the issues raised in the Final Order and Decision of the Central Puget Sound Growth Management Hearings Board (GMHB) in the recent appeal of the new ECA ordinance (CPSGMHB Case No. 06-3-0024 Seattle Audubon Society, Yes for Seattle, Heron Habitat Helpers and Eugene D. Hoglund v City of Seattle). The order and decision requires Seattle to designate the Seattle Fault Zone, tsunami and seiche inundation areas and lahar inundation areas as geologic hazard areas.

Currently the ECA ordinance designates landslide-prone areas, steep slopes, and liquefaction-prone areas as geologically hazardous areas. Information on and analysis of the risks associated with these geologic hazard areas can be found in the ECA Ordinance (number 122050) Best Available Science Review (August 2005) and in the ECA Ordinance Director’s Report and Recommendation (August 2005).

The GMHB decision specifically cited the Seattle Fault Zone, tsunami and seiche inundation areas and lahar inundation areas as geologic hazard areas that Seattle must designate in the ECA ordinance. The following summarizes the findings of the Supplemental Best Available Science Report for Geologic Hazard Areas (Exhibit A) and recommends how best to incorporate the scientific findings into the ECA ordinance. The last subsection summarizes some of the on-going emergency management planning and implementation actions that are mitigating the risk from these hazards.

Seattle Fault Zone

A fault is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Not all earthquakes result in surface rupture, and not all surface rupture occurs along pre-existing faults.

Prior to the 1990’s, shallow crustal earthquakes had not been attributed to specific faults in the Puget Sound region, and no evidence of Holocene fault rupture (movement in the last 10,000 years) had been observed. In 1992, scientists discovered the first evidence that the Seattle Fault Zone is active and capable of producing earthquakes that may result in ground surface rupture - a magnitude 7.0 or greater earthquake approximately 1100 years ago resulted in as much as 7 meters of uplift at Restoration Point on Bainbridge Island, over 4 meters of uplift at Alki Point, and 1 to 1.5 meters of subsidence at West Point. The Supplemental Best Available Science Report for Geologic Hazard Areas (Exhibit A) provides a complete summary of the latest scientific information on the Seattle Fault Zone.

The lengthy estimated recurrence interval of a major Seattle Fault earthquake and the inability of current scientific studies to accurately identify the specific location of future fault rupture create a challenge for designating and mapping the Seattle Fault Zone. However, the science shows that there is risk of surface rupture and an increased risk from ground shaking associated with a Seattle Fault event. This risk is currently taken into account through the requirements of the Seattle Building Code with respect to ground movement, and may also do so with respect to faults if specific faults in the Seattle Fault Zone are located. The City has also developed emergency planning and educational measures and programs to decrease the risk. Scientific studies related to the Seattle Fault are ongoing.

Currently there is no performance standard that can be applied on a site by site basis to determine the risk from the Seattle Fault. Trenching is one possible approach, but not practical as there is no map showing the fault strands at a usable scale. Trenching will not necessarily reveal active strands, depending on the amount of non-native material or regrading at the site. The State of Washington, in collaboration with the U.S. Geological Survey, plans to develop a map of active faults in Washington in 2007. This work may lead to more definitive answers for how best to classify the risks from the Seattle Fault.

The best available science review indicates that the Seattle Fault zone represents a known or suspected risk as per WAC 365-190-080(4)(b)(i). As such it is recommended that the Seattle Fault Zone be classified as a geologic hazard area and mapped according to the best available science as presented in Exhibit A. The U.S. Geological Survey is the authoritative research organization regarding seismic hazards and is therefore relied upon as the source for designation and mapping.

A tsunami is a series of water waves of extremely long period and long wavelength (distance from crest to crest) caused by a sudden disturbance that vertically displaces water. Sudden offsets in the earth’s crust, such as during earthquakes, can cause a tsunami. Landslides and underwater volcanic eruptions can also generate tsunamis. Washington’s outer coast is vulnerable to tsunamis from distant sources (such as earthquakes in Alaska, Japan, or Chile) and from the adjacent Cascadia Subduction Zone (CSZ). Washington’s inland waters, such as those in the Puget Sound region, are also subject to tsunamis, particularly those generated by local crustal earthquakes or by surface and submarine landslides.

Tsunami risks are currently mitigated through emergency management techniques. Considering the high population density in the potential tsunami hazard area, education to a broad spectrum of the Seattle citizenry may be warranted regarding the potential for tsunami hazards.

The Supplemental Best Available Science Report for Geologic Hazard Areas (Exhibit A) provides a complete summary of the latest information on the tsunami risk in Seattle. This review, concludes that the mapping by Walsh et al. (2003) represents the most current delineation of the area of suspected tsunami hazard along Seattle’s marine shorelines. Although this map only considers a tsunami that may be generated by a major earthquake on the Seattle Fault Zone, this event is likely to be more severe than other potential tsunamis caused by local landslides or lateral spreading/flow slides into the Duwamish River. Hazard areas for tsunamis from these other sources are likely contained within the Walsh et al. (2003) map. Thus, this map represents a reasonable boundary for suspected tsunami risks on Seattle’s marine shorelines. As such, tsunamis represent a known or suspected risk to Seattle’s marine shorelines as per WAC 365-190-080(4)(b)(i).

In addition, the science points to a known risk from tsunamis in Lake Washington. However the risk to the shoreline and upland areas surrounding Lake Washington is unknown. In addition, there is no performance standard that can be applied on a site by site basis to determine the risk. Since there is no documented damage, areas adjacent to Lake Washington are recommended to be classified as having an unknown risk as per WAC 365-190-080(4)(b)(iii), as to both the likelihood of risk and its potential distance from the high water mark.

As additional scientific information becomes available it should be reviewed to determine whether these classifications should be adjusted and whether additional measures should be taken. The U.S. Geological Survey, the National Oceanic and Atmospheric Administration and the Washington State Department of Natural Resources are authoritative research organizations regarding tsunamis and are therefore relied upon as the source for designation and mapping.

Seiches are a series of standing waves contained in an enclosed or partially enclosed body of water and are analogous to the sloshing of water that occurs when a bowl of water is moved back and forth. Seiches can occur in harbors, bays, lakes, rivers, and canals. Locally, Lake Union, Lake Washington, and to a lesser extent, Elliott Bay hold potential for seiche activity.

Historical records do not document any damage to Seattle shorelines due to seiche activity, although the 1964 Alaska earthquake caused a seiche in the reservoir at Aberdeen, Washington that resulted in an embankment failure. Thus, impacts are clearly possible. Scientific studies on this subject also remain insufficient to characterize the potential impact of seiche activity on shorelines as they lack any analysis of land inundation. Since seiches are standing waves rather than moving water flows, potential inundation on the shore is considered to be a minimal risk.

The Supplemental Best Available Science Report for Geologic Hazard Areas (Exhibit A) provides a complete summary of the latest information on the seiche risk in Seattle. Based on this review, it is recommended that Lake Union, Lake Washington and Elliott Bay be classified as having a known seiche hazard risk as per WAC 365-190-080(4)(b)(i). As there is no documented damage above the high water mark in Seattle, areas adjacent to these waterbodies are recommended to be classified as having an unknown risk as per WAC 365-190-080(4)(b)(iii), as to both the likelihood of risk and its potential distance from the high water mark. In addition, there is no performance standard that can be applied on a site by site basis to determine the risk. Seiche hazards are best addressed though education and emergency management planning. As additional scientific information becomes available it should be reviewed to determine whether these classifications should be adjusted and whether additional areas can be designated for this risk.

Lahars
A lahar is a gravity-driven mixture of sediment and water that originates from the flanks of a volcano. Such flows are analogous to debris flows, but typically are very large in size due to the high elevations, steep slopes, and abundance of loose or hydrothermally weakened material associated with volcanoes. Lahars can initiate as a result of (1) melting of snow and ice by radiant heat or pyroclastic flows generated during an eruption, (2) collapse of the steep sides of a volcano, (3) heavy rainfall eroding volcanic deposits, (4) seismically induced landslides, (5) magmatic intrusion (magma rising to the surface and causing ice to melt) or (6) floods generated by lake or glacial outburst.

According to the best available science review, a Case M lahar could potentially travel to Seattle from Mount Rainier. Case M flows are low-probability, high-consequence lahars, such as the Osceola Mudflow, the largest lahar to occur at Mount Rainier in the past 10,000 years. Case I lahars (recurrence interval of 500 to 1000 years) and Case II lahars (recurrence interval of about 100 years) could lead to post-lahar sedimentation or some lahar deposition in Seattle. The Supplemental Best Available Science Report for Geologic Hazard Areas (Exhibit A) provides a complete summary of the latest information on the risk from lahars in Seattle.

Based on the best available science review, lahars represent a known or suspected risk to Seattle, as per WAC 365-190-080(4)(b)(i). As such, it is recommended that the mapping in Hoblitt et al., 1998, fully described in Exhibit A, be used to designate areas that could be at risk from lahars.

Lahar hazards in Seattle are best addressed though education and emergency management planning. A warning system combined with public education should provide citizens sufficient time to evacuate potential lahar hazard areas, limiting potential impacts. The U.S. Geological Survey is the authoritative research organization regarding volcanic hazards and is therefore reled upon as the source for designation and mapping.

For hazards where there may be updates from specific reliable sources, the proposed ordinance authorizes the Director to update the designations using sources set out in the ordinance. These updates will be by Director’s Rule. For hazards where the presence or absence of the hazard is currently classified as unknown, Section 4 of the ordinance directs the Director to bring new information to the City Council for consideration of whether to change the designation. Subsection 25.09.030A is proposed to be amended to distinguish between maps that are advisory and the new geologic hazard areas that are designated by map.

Emergency management techniques represent an effective way to manage risk from seismic and volcanic hazards. Seattle is largely built out and many areas that may be considered at risk from these hazards are already fully developed. The Seattle Office of Emergency Management (http://www.seattle.gov/emergency/) works to mitigate the risk from all types of hazards, including those being considered in this proposal. The Office has led the development of a number of studies, plans and actions that help to prepare citizens and to minimize the risk from hazards. The following is a summary of several of these efforts. The continuing work of this office and other branches of local, state and federal governments to continue to work to understand and mitigate the risk from seismic and volcanic is essential.

The purpose of this 2003 plan is to explain how the City would lead the response to a major disaster. The mission is to provide all of Seattle’s residents, property owners, businesses and institutions, government departments and commissions, and emergency support organizations with a comprehensive emergency management system. To view this plan go to: http://www.seattle.gov/emergency/library/Seattle_Disaster_Readiness_And_Response_Plan.pdf.

This 2004 document provides a narrative assessment of the history of hazards in Seattle and the city’s exposure to them. It is a tool that is being used to build an emergency plan around the most dangerous disasters the city faces, including seismic and volcanic hazards. The plan can be viewed by going to http://www.seattle.gov/emergency/library/SHIVA.pdf.

The 2004 Seattle All-Hazards Mitigation Plan builds off of the findings of the SHIVA and represents the city’s first comprehensive effort to describe mitigation efforts across city departments and to develop an integrated mitigation strategy. The plan emphasizes mitigation of city-owned and operated facilities and infrastructure. It also includes reference to mitigation efforts undertaken by related public, quasi-public, and private entities. Natural hazards, in particular, are emphasized in this plan. For example, the plan rates the risk from various hazards and presents methods for mitigating their impact (see Sections 2.4 and 2.5). The plan can be viewed by going to http://www.seattle.gov/emergency/library/Haz%20Mit%20Plan%20Feb%2004.pdf

Project Impact is a public-private partnership whose overall goal is to make our communities more resistant to the damaging effects of disasters. The Project encourages people to take action before a disaster occurs through initiatives promoting safer homes, schools, businesses, and better earthquake and landslide hazard mapping. More information about Project Impact can be found at http://www.seattle.gov/projectimpact.

The proposed amendments will promote the public interest by designating additional geologically hazardous areas in accordance with the Growth Management Act. This will help to inform the citizens of Seattle about the risks and hazards inherent in living and working here and meets the requirements of the Central Puget Sound Growth Management Hearings Board. DPD recommends that the proposed amendments be approved.

Department:	Contact Person/Phone:	DOF Analyst/Phone:
Dept. of Planning and Development	Miles Mayhew/ 615-1256	Amanda Allan/ 684-8894