Holocene fault scarps near Tacoma, Washington, USA

Abstract

Airborne laser mapping confirms that Holocene active faults traverse the Puget Sound metropolitan area, northwestern continental United States. The mapping, which detects forest-floor relief of as little as 15 cm, reveals scarps along geophysical lineaments that separate areas of Holocene uplift and subsidence. Along one such line of scarps, we found that a fault warped the ground surface between A.D. 770 and 1160. This reverse fault, which projects through Tacoma, Washington, bounds the southern and western sides of the Seattle uplift. The northern flank of the Seattle uplift is bounded by a reverse fault beneath Seattle that broke in A.D. 900-930. Observations of tectonic scarps along the Tacoma fault demonstrate that active faulting with associated surface rupture and ground motions pose a significant hazard in the Puget Sound region.Figure 1. Tectonic setting of Cascadia subduction zone. Western Washington region (brown), between fixed North America and Oregon Coast Range, is undergoing transpression, which creates folds and reverse faults across Puget Sound. Bold arrows indicate motions of tectonic blocks inferred from geologic and geodetic data. Modified from Wells et al. (1998) and Wang et al. (2003).

High-Resolution Seismic Reflection Imaging of Growth Folding and Shallow Faults beneath the Southern Puget Lowland, Washington State

“…Growth folding within the kink band beneath Case Inlet is consistent with geomorphic and paleoseismic observations made on the Catfish Lake fold scarp immediately to the west (Sherrod et al, , 2004. A 30-m long trench across the fold scarp (Fig.…”

“…2c; Johnson et al, 2004). The warping of the water bottom seen on the seismic reflection profile is similar in amplitude and wavelength to that at the Catfish Lake fold scarp to the west (Sherrod et al, , 2004, and it lies at the south edge of the uplifted terrace. The data therefore are consistent with latest Pleistocene or Holocene growth of the kink band in response to motion on an underlying fault (Fig.…”

“…The lineaments are ∼80-m wide sloping surfaces that form scarplike linear features that we will refer to as fold scarps. A trench dug across one of these lineaments, the Catfish Lake fold scarp, showed a fold in the strata with more than 2 m of vertical relief, up to the north, but only minor faulting (< 30 cm; Sherrod et al, 2003Sherrod et al, , 2004. Their interpretation of the trench exposure is that the fold scarp is caused by warping of the postglacial surface above a blind fault tip.…”

“…Evidence for Holocene land-level changes along the Tacoma fault and Olympia structure suggest that there was a large earthquake about 1100 years ago in the southern Puget Lowland (Bucknam et al, 1992;Sherrod, 2001). Sherrod et al (2004) document an uplifted tidal flat (terrace) at Case Inlet and a series of east-trending, south-facing, en echelon topographic lineaments deforming the late Pleistocene to Holocene glacial deposits along the west part of the Tacoma fault zone (Fig. 2b).…”

LiDAR Data for Characterizing Linear and Planar Geomorphic Markers in Tectonic Geomorphology

“…Field investigation and analysis of other geological and geophysical data indicate that the linear features have a neotectonic origin, representing the surface expression of a seismically active fault. In areas with dense vegetation cover in the U.S. Pacific Northwest and Europe, 2-m resolution DEMs derived from LiDAR data have been successfully used for delineating earthquake surface ruptures [19][20][21]. Using 2-m and 3-m resolution DEMs derived from LiDAR data along portions of the Alpine and Hope faults in New Zealand, Langridge et al [22] found that the surface strike variations of the Alpine Fault are more variant than previously mapped, and that unprecedented views of the surface geomorphology of these active faults can be revealed by LiDAR data.…”