Brian L. Sherrod

High-resolution lidar topography of the Puget Lowland, Washington - A bonanza for earth science

More than 10,000 km2 of high-resolution, public-domain topography acquired by the Puget Sound Lidar Consortium is revolutionizing investigations of active faulting, continental glaciation, landslides, and surficial processes in the seismically active Puget Lowland. The Lowland—the population and economic center of the Pacific Northwest—presents special problems for hazards investigations, with its young glacial topography, dense forest cover, and urbanization. Lidar mapping during leaf-off conditions has led to a detailed digital model of the landscape beneath the forest canopy. The surface thus revealed contains a rich and diverse record of previously unknown surface-rupturing faults, deep-seated landslides, uplifted Holocene and Pleistocene beaches, and subglacial and periglacial features. More than half a dozen suspected postglacial fault scarps have been identified to date. Five scarps that have been trenched show evidence of large, Holocene, surfacerupturing earthquakes.

Surface Rupture and Slip Distribution of the Denali and Totschunda Faults in the 3 November 2002 M 7.9 Earthquake, Alaska

The 3 November 2002 Denali fault, Alaska, earthquake resulted in 341 km of surface rupture on the Susitna Glacier, Denali, and Totschunda faults. The rupture proceeded from west to east and began with a 48-km-long break on the previously unknown Susitna Glacier thrust fault. Slip on this thrust averaged about 4 m (Crone et al. , 2004). Next came the principal surface break, along 226 km of the Denali fault, with average right-lateral offsets of 4.5–5.1 m and a maximum offset of 8.8 m near its eastern end. The Denali fault trace is commonly left stepping and north side up. About 99 km of the fault ruptured through glacier ice, where the trace orientation was commonly influenced by local ice fabric. Finally, slip transferred southeastward onto the Totschunda fault and continued for another 66 km where dextral offsets average 1.6–1.8 m. The transition from the Denali fault to the Totschunda fault occurs over a complex 25-km-long transfer zone of right-slip and normal fault traces. Three methods of calculating average surface slip all yield a moment magnitude of M w 7.8, in very good agreement with the seismologically determined magnitude of M 7.9. A comparison of strong-motion inversions for moment release with our slip distribution shows they have a similar pattern. The locations of the two largest pulses of moment release correlate with the locations of increasing steps in the average values of observed slip. This suggests that slip-distribution data can be used to infer moment release along other active fault traces. Online Material : Descriptions and photographs of localities with offset measurements.

Holocene fault scarps near Tacoma, Washington, USA

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.

Late Holocene earthquakes on the Toe Jam Hill fault, Seattle fault zone, Bainbridge Island, Washington

Five trenches across a Holocene fault scarp yield the first radiocarbon-measured earthquake recurrence intervals for a crustal fault in western Washington. The scarp, the first to be revealed by laser imagery, marks the Toe Jam Hill fault, a north-dipping backthrust to the Seattle fault. Folded and faulted strata, liquefaction features, and forest soil A horizons buried by hanging-wall-collapse colluvium record three, or possibly four, earthquakes between 2500 and 1000 yr ago. The most recent earthquake is probably the 1050–1020 cal. (calibrated) yr B.P. (A.D. 900–930) earthquake that raised marine terraces and triggered a tsunami in Puget Sound. Vertical deformation estimated from stratigraphic and surface offsets at trench sites suggests late Holocene earthquake magnitudes near M7, corresponding to surface ruptures >36 km long. Deformation features recording poorly understood latest Pleistocene earthquakes suggest that they were smaller than late Holocene earthquakes. Postglacial earthquake recurrence intervals based on 97 radiocarbon ages, most on detrital charcoal, range from ∼12,000 yr to as little as a century or less; corresponding fault-slip rates are 0.2 mm/yr for the past 16,000 yr and 2 mm/yr for the past 2500 yr. Because the Toe Jam Hill fault is a backthrust to the Seattle fault, it may not have ruptured during every earthquake on the Seattle fault. But the earthquake history of the Toe Jam Hill fault is at least a partial proxy for the history of the rest of the Seattle fault zone.

Gradient analysis of diatom assemblages in a Puget Sound salt marsh: can such assemblages be used for quantitative paleoecological reconstructions?

Abstract Taphonomy is important to coastal paleoecologists because processes acting on diatom thanocoenoses tend to work towards obscuring original ecological relationships between diatom assemblages and the environment. The purpose of this paper is to briefly describe diatom taphonomy and present a method for quantitative reconstruction of environmental parameters from salt marsh diatom assemblages. The main hypothesis for this study is that major environmental and taphonomic processes (e.g., tides) act in predictable ways to distribute living and dead diatoms along environmental gradients. To test this hypothesis, a modern transect was established across a large salt marsh in southwestern Puget Sound for the purpose of determining modern species/environment gradients and calibrating species assemblages to environmental variables. Canonical correspondence analysis (CCA) relates modern species assemblages to environmental gradients, and weighted averaging calibration is used to develop transfer functions for predicting environmental information. CCA showed that the effect of salinity and elevation on the species distributions is significant, indicating that environmental processes control the distribution of sedimentary diatoms across the salt marsh surface in predictable ways. Salinity was strongly correlated with CCA Axis 1 and elevation with Axis 2. The calibration results indicate that, although mixing of allochthonous and autochthonous diatoms does occur, salt marsh diatom assemblages reflect major environmental gradients in Puget Sound salt marshes and can be effectively used for quantitative reconstructions of former environmental conditions.

Evidence for earthquake-induced subsidence about 1100 yr ago in coastal marshes of southern Puget Sound, Washington

Buried forest and high marsh soils indicate abrupt changes in relative sea level at four coastal localities in southern Puget Sound. At Little Skookum Inlet and Red Salmon Creek, Douglas fir stumps in growth position are buried by salt-marsh peat. At localities along McAllister Creek and the Nisqually River, high marsh soils are buried by tidal-flat mud. Localized liquefaction coincided with submergence of the high marsh soil at McAllister Creek. Dramatic changes in seed and diatom assemblages across these contacts confirm rapid submergence. At Little Skookum Inlet and Red Salmon Creek, salt-marsh peat immediately above a buried forest soil contains diatoms indicative of low marsh and tidal-flat environments. At McAllister Creek and Nisqually River, low-marsh and tidal-flat diatoms are abundant in laminated mud directly over high marsh peat. Inferences from modern analogs indicate at least 1 m of subsidence at each site and possibly up to 3 m at Skookum Inlet. Abrupt burial of lowland soils in southern Puget Sound is best explained by coseismic subsidence. Some of the submergence may be the result of coseismic compaction and postearthquake settlement. Widespread buried soils, large amounts of subsidence, coeval submergence across a wide area, and ground shaking at the time of subsidence all point to a large earthquake between 1150 and 1010 cal yr B.P. in southern Puget Sound as the most likely cause of subsidence.

Land-level changes from a late Holocene earthquake in the northern Puget Lowland, Washington

An earthquake, probably generated on the southern Whidbey Island fault zone, caused 1‐2 m of ground-surface uplift on central Whidbey Island ;2800‐3200 yr ago. The cause of the uplift is a fold that grew coseismically above a blind fault that was the earthquake source. Both the fault and the fold at the fault’s tip are imaged on multichannel seismic refection profiles in Puget Sound immediately east of the central Whidbey Island site. Uplift is documented through contrasting histories of relative sea level at two coastal marshes on either side of the fault. Late Holocene shallow-crustal earthquakes of Mw 5 6.5‐7 pose substantial seismic hazard to the northern Puget Lowland.

Evidence for Late Holocene Earthquakes on the Utsalady Point Fault, Northern Puget Lowland, Washington

Trenches across the Utsalady Point fault in the northern Puget Lowland of Washington reveal evidence of at least one and probably two late Holocene earthquakes. The “Teeka” and “Duffers” trenches were located along a 1.4-km-long, 1- to 4-m-high, northwest-trending, southwest-facing, topographic scarp recognized from Airborne Laser Swath Mapping. Glaciomarine drift exposed in the trenches reveals evidence of about 95 to 150 cm of vertical and 200 to 220 cm of left-lateral slip in the Teeka trench. Radiocarbon ages from a buried soil A horizon and overlying slope colluvium along with the historical record of earthquakes suggest that this faulting occurred 100 to 400 calendar years b.p. (a.d. 1550 to 1850). In the Duffers trench, 370 to 450 cm of vertical separation is accommodated by faulting (∼210 cm) and folding (∼160 to 240 cm), with probable but undetermined amounts of lateral slip. Stratigraphic relations and radiocarbon ages from buried soil, colluvium, and fissure fill in the hanging wall suggest the deformation at Duffers is most likely from two earthquakes that occurred between 100 to 500 and 1100 to 2200 calendar years b.p., but deformation during a single earthquake is also possible. For the two-earthquake hypothesis, deformation at Teeka trench in the first event involved folding but not faulting. Regional relations suggest that the earthquake(s) were M ≥ ∼6.7 and that offshore rupture may have produced tsunamis. Based on this investigation and related recent studies, the maximum recurrence interval for large ground-rupturing crustal-fault earthquakes in the Puget Lowland is about 400 to 600 years or less.

Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone

A key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources. For the Seattle fault zone, an upper plate structure within the Cascadia convergent margin, evaluating seismic hazard requires understanding how north-side-up, bedding-plane reverse faults, which generate late Holocene fault scarps, interact with the north-vergent master-ramp thrust and overlying backthrust of the fault zone. A regional uplift at A.D. 900–930 involved an earthquake that nucleated at depth and included slip on both the master-ramp thrust and the backthrust. This earthquake also included slip on some of the <6-km-deep north-side-up, bedding-plane reverse faults. At locales where the north-side-up reverse faults intersect the Puget Sound coast, an earthquake a few centuries earlier than the A.D. 900–930 regional uplift only uplifted areas within hundreds of meters north of the reverse faults. We infer that the bedding-plane reverse faults are seismogenic because shore platforms near the reverse faults have been abruptly uplifted during earthquakes when other shorelines in the Seattle fault zone were unaffected. Faults of the Seattle fault zone therefore can both produce regional uplift earthquakes, with or without surface displacement on the reverse faults, and produce earthquakes that rupture the bedding-plane reverse faults causing fault scarps and uplift localized to hundreds of meters north of these faults. This latter type of earthquake has occurred at least twice and perhaps three times in the late Holocene, and all these earthquakes preceded the regional coseismic uplift of A.D. 900–930. To account for the paleoseismic observations, we propose that the Seattle fault zone is a wedge thrust, with the leading edge being a fault-bend, wedge thrust fold. The active axial surface of the wedge thrust fold is pinned at the tip of the wedge, and a steeply north-dipping sequence of Tertiary sediment forms the south limb of the wedge thrust fold. Some of these steeply north-dipping, bedding-plane surfaces are seismogenic reverse faults that produce scarps. Earthquakes on the wedge thrust produce the regional coseismic uplift events, and earthquakes within the faultbend fold cause the local uplift earthquakes. Thus, bedding-plane faults can rupture during earthquakes when the wedge thrust does not rupture but instead continues to accumulate seismic energy.

Saddle Mountain fault deformation zone, Olympic Peninsula, Washington: Western boundary of the Seattle uplift

The Saddle Mountain fault, first recognized in the early 1970s, is now well mapped in the Hoodsport area, southeastern Olympic Peninsula (northwestern United States), on the basis of light detection and ranging (LIDAR) surveys, aerial photography, and trench excavations. Drowned trees and trench excavations demonstrate that the Saddle Mountain fault produced a M W 6.5–7.0 earthquake 1000–1300 yr ago, likely contemporaneous with the M W 7.5 Seattle fault earthquake 1100 yr ago and with a variety of other fault and landslide activity over a wide region of the Olympic Peninsula and Puget Lowland. This near synchroneity suggests that the Saddle Mountain and Seattle fault may be kinematically linked. Aeromagnetic anomalies and LIDAR topographic scarps define an en echelon sequence of faults along the southeastern Olympic Peninsula of Washington, all active in Holocene time. A detailed analysis of aeromagnetic data suggests that the Saddle Mountain fault extends at least 35 km, from 6 km southwest of Lake Cushman northward to the latitude of the Seattle fault. A magnetic survey over Price Lake using a nonmagnetic canoe illuminated two east-dipping reverse faults with 20 m of vertical offset at 30 m depth associated with 2–4 m of vertical displacement at the topographic surface. Analysis of regional aeromagnetic data indicates that the Seattle fault may extend westward across Hood Canal and into the Olympic Mountains, where it terminates near the northward terminus of the Saddle Mountain fault. The en echelon alignment of the Saddle Mountain and nearby Frigid Creek and Canyon River faults, all active in late Holocene time, reflects a >45-km-long zone of deformation that may accommodate the northward shortening of Puget Lowland crust inboard of the Olympic massif. In this view, the Seattle fault and Saddle Mountain deformation zone form the boundaries of the northward-advancing Seattle uplift.