AbstractDetection of active faults and seismic hazards in the Seattle area is

problematic, owing to thick surficial deposits and abundant vegetative cover. Aeromagnetic mapping techniques have given insight into this once enigmatic area. The Seattle fault zone is segmented and comprised of several east-trending, north-verging splays along a reverse-thrust system. Three distinct magnetic anomalies are associated with the fault zone, and in combination with regional stratigraphic records, help elucidate seismic history in the area. The fault has been active from 40 Ma to the present, with a major M 7 earthquake occurring 1,100 years ago. The Seattle area is prone to co-seismic ground failure such as that associated with the Olympia 1949 and Seattle-Tacoma 1983 earthquakes. In 2001, the Nisqually event yielded a M 6.5 tremor that was felt as far south as Salem. The Nisqually earthquake had a similar hypocenter to that of the 1949 Puget Sound event. The area has been associated with widespread liquefaction processes during past earthquakes. The expansion of urban development in the Seattle area over the past two decades has significantly increased the risk of widespread, catastrophic damage when the next seismic event occurs. This paper presents a summary of past seismic activity in the Seattle area and risk potential for catastrophic earthquakes in the future.

Aeromagnetic Survey of Seattle Fault Aerial survey using a special instrument that detects magnetic fields Not all rocks give of the same magnetic fields Differences in the magnetic field could indicate faulting Seattle fault zone has three east trending distinct magnetic anomalies (Fig .1)

Seattle Fault Details and History

Consist, of segmented east trending, north verging thrust faults (Fig. 2) Its being active since 40 MA to the present Mostly shallow crustal fault earthquakes 24 km or less Two large deep earthquakes in 1949 and 1965 There is an uplifted area to the south caused by a M 7 around 1,100 yrs ago Basin is filled with unconsolidated sediment

Conclusions

(1) Not all of the crustal faults are known.(2) The Seattle basin is a very active seismic zone.(3) The Seattle Fault Zone is segmented. (4) Liquefaction and co-seismic landslides are common and devastating.(5) There were two major recent seismic events one in 1949 the other in

1965.(6) Urban population growth has lead to an increase in the potential damage

during the next seismic event

References CitedSchuster, R. L., 2004, Earthquake-Induced Ground Failure in Western Washington: U.S. Geological Survey.

Blakley, R. J., 2002, Location, structure, and seismicity of the Seattle fault zone, Washington: Geological Society of America

Grant, W.P., 1996, Evaluation of Liquefaction Potential in Seattle, Washington

Chleborad, A. F., 1996, Ground Failure Hazards in Puget Sound: Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest

Regional Case Study I: Seismic Hazards in the Seattle Area

Brad Adams, ES473 Environmental Geology, Spring Term 2009

Figure 2. Map of the Seattle Fault Zone. Depicting sediment type and age, also location and type of known faults.

Figure 1. Aeromagnetic photograph of three magnetic anomalies in Seattle area. Anomaly A is a elongated, narrow magnetic high; B is broad magnetic low; C is a complex magnetic low.

Seismic Hazards in SeattleCo-seismic Mass-Wasting● Mostly landslides●Very common with Seattle Fault Zone●Largest occurred in the 1949 Olympia earthquake liquefaction● Unconsolidated soil partially liquefies during seismic events amplifying the ground shaking● Increases the damage to infrastructure●Different sediments have varying liquefaction potential

1949 Olympia Earthquake● M 7.1● Focal depth 0f 40 mi● Landslides occurred as far away as 110 mi● Total of 23 landslides but possibly more● The largest landslide occurred at the Tacoma narrows with a Volume around 650,000 yd 3

1965 Seattle-Tacoma Earthquake● M 6.5● 24 individual landslides within 60 miles● Assumed there was and under reporting of landslides

Potential Damage from a Seismic Event● In the ‘49 and ’65 earthquakes liquefaction caused $25 million and $12 million respectively in damage● Co-seismic landslides can block roads, and move buildings off foundations● Seattle population and infrastructure has grown since the ’49 and ’65 earthquakes● More people and more building means more damage or bigger risk

Figure 3. A map ofWashington and Oregonshowing the intensity ofthe 1949 and 1965 Earthquakes.

Table 1. Geologic deposits and the risk of liquefaction in Seattle area.

IntroductionThe Seattle area overlies a distinct fault zone with a history of past and

recent seismic activity. The fault locations remain a mystery due to abundant surficial cover obstructing geologists view of the ground surface. Seattle has the potential for moderate to severe damage to infrastructure and loss of lives if a large seismic event were to occur.