Prehistoric Earthquakes in the Puget Lowland, Washington

Abstract

Coastal marsh deposits and lidar topographic data show evidence for past earthquakes on at least eight fault zones in the Puget lowland. Three major fault zones, the Seattle fault zone, Tacoma fault, and the Southern Whidbey Island fault zone (SWIFZ), cut through the heavily populated portions of central Puget Sound. Faults in four other areas, namely the Darrington‐Devils Mountain fault zone, Olympia fault, the northern margin of the Olympic Mountains, and the southeastern Olympic Mountains, show that the area of active Holocene faulting extends over the entire Puget Sound lowlands. As recently as 1998, field evidence could confirm only one fault with evidence of past earthquake activity. Uplifted coastlines and surface ruptures are the field evidence for past Seattle fault earthquakes. Raised intertidal platforms along the Seattle fault zone show that regional uplift of as much as 7 meters accompanied a large earthquake about 1100 years ago. This earthquake also caused a tsunami, which inundated low‐lying coastal areas north of Seattle. All of the lidar scarps found in the Seattle fault zone are north‐side‐up, opposite the vergence suggested for the Seattle fault from regional geological studies. Excavations across these scarps reveal north‐dipping thrust faults that roughly follow bedding planes in bedrock and disrupt late Holocene soils. Soil stratigraphy and radiocarbon ages suggest as many as three surface‐rupturing earthquakes in the past 2500 years. Lidar mapping revealed several en echelon scarps along the trace of the Tacoma fault. Existence of the Tacoma fault was previously hypothesized on the basis of large‐amplitude gravity, aeromagnetic, and seismic‐velocity anomalies, shallow marine seismic reflection surveys, glaciolacustrine strandlines, and coastal marsh stratigraphy. Coastal marsh deposits and scarp excavations suggest that the scarps formed during an earthquake on the Tacoma fault ∼1100 years ago, possibly by folding above a buried reverse fault. Coastal marsh stratigraphy, lidar mapping, and fault scarp excavations help define recent activity along the Southern Whidbey Island fault zone (SWIFZ). Abrupt uplift of more than one meter at a coastal marsh on south‐central Whidbey Island suggests that a MW 6.5 – 7.0 earthquake on the SWIFZ shook the region between 3200 and 2800 years B.P. Subtle scarps on Pleistocene surfaces are visible on high‐resolution lidar topography at a number of locations in the mainland region, often closely associated with aeromagnetic lineaments. In the field, scarps exhibit northeast‐side‐up vertical relief of 1 to 5 m. Four excavations across two lidar scarps show that the SWIFZ produced at least four events since deglaciation about 16,400 years ago, the most recent after 2700 years ago. Lidar mapping on northern Whidbey Island revealed the location of a scarp along the broad Darrington‐Devils Mountain fault zone. Subsequent excavations across this scarp found evidence for late Holocene oblique left‐lateral offset with as much as 2 meters of both vertical and horizontal slip. At the north margin of the Olympic Mountains, scarps along the Little River fault extend over 30 km east‐west. At the southeast corner of the Olympic Mountains, lidar mapping shows that the previously recognized southeast‐side‐up scarps at Price Lake are longer and more numerous than previously thought. Southwest of Price Lake, a 10‐km‐long, en echelon scarp cut across Holocene alluvial fans. An excavation across this scarp showed one episode of movement on a normal fault in the last 3800 years. Further southwest, excavations across the Canyon River fault near Lake Wynoochee showed evidence for oblique left‐lateral displacement, with one event in the last 3500 years. The evidence for Holocene deformation across the entire Puget Sound lowlands is now very pervasive but still incomplete. Lidar scarps have been identified in several areas not associated with the eight zones noted here but have yet to be investigated. Lidar data covers about 70% of the Puget Sound basin, but key areas with suspected crustal faults in northwestern Washington have yet to be flown. Still, the combination of paleoseismological field investigations and lidar imaging allowed remarkable progress in understanding the Holocene earthquake history of greater Puget Sound in just 7 years. The new observations are an important addition to observations used to calculate the National Hazard Maps.