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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 Keywords: reverse faults, fault-bend folds, Seattle fault zone, paleoseismology, relative sea level.

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 [M.sub.w] = 6.5-7 pose substantial seismic hazard to the northern Puget Lowland. Keywords: paleoseismology, relative sea level, blind faults, seismic reflection, Puget Sound.

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. Keywords: Holocene, scarp, Puget Sound, paleoseismology, fault.

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. Keywords: paleoseismology, earthquake recurrence, reverse faulting, Puget Lowland, fault trenching, earthquake hazards.

Holocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west-northwest-striking crustal faults herein named the North Olympic fault zone, which extends for ∼80  km along strike and includes the Lake Creek-Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west. This study focuses on the Sadie Creek fault, which extends for >14  km west-northwest from Lake Crescent. Airborne light detection and ranging (lidar) imagery reveals the trace of the Sadie Creek fault and offset postglacial landforms showing a history of Holocene surface-rupturing earthquakes dominated by dextral displacement along a steeply dipping fault zone. Paleoseismic trenches at two sites on the Sadie Creek fault reveal till and outwash overlain by progressively buried forest and wetland soils developed on scarp-derived colluvial wedges. Trench exposures of complex faulting with subhorizontal slickenlines indicate dextral displacement with lesser dip slip. Correlation of broadly constrained time intervals for earthquakes at the Sadie Creek sites and those to the east along the Lake Creek-Boundary Creek fault is consistent with rupture of much of the length of the North Olympic fault zone three to four times: at about 11, 7, 3, and 1 ka, with a shorter rupture at about 8.5 ka. Dated ruptures from trenches only partially coincide with coseismic landslides and megaturbidites in Lake Crescent, indicating that some earthquakes did not trigger megaturbidites, and some turbidites were unrelated to local fault rupture. Landform mapping suggests single-event dextral displacement of 4±1  m on the Sadie Creek fault. Inferred maximum rupture length and single-event slip imply earthquake magnitudes Mw 7.0-7.5. Dextral slip rates of 1.3-2.3  mm/yr and the ∼11,000  yr slip history suggest that the North Olympic fault zone is a prominent contributor to permanent strain in the northern Cascadia fore-arc. [ABSTRACT FROM AUTHOR]

The epicenter of the 16 July 1936 M 6 Milton-Freewater earthquake, also known as the State Line earthquake and the largest historical earthquake in northeastern Oregon or southeastern Washington, is uncertain. Various studies place the epicenter of the earthquake, which was widely felt in eastern Washington, northeastern Oregon, and northern Idaho, within 30 km of the intersection of the Hite and Wallula faults. In the absence of reported coseismic surface rupture for the earthquake, we sought to determine which epicentral location is most consistent with the intensity observations and with its aftershock sequence, which lasted for at least 27 months. An epicenter between Umapine and Milton-Freewater best matches the observations. This location falls within the region that experienced the highest intensities of VII and reported the largest number of aftershocks, compares favorably to the (2018) International Seismological Centre-Global Earthquake Model (ISC-GEM) epicenter, and is proximal to sites that experienced ground failure and groundwater effects. Modeling of aftershock rates is consistent with this suggested epicenter and with the estimated 10 km long subsurface rupture of the earthquake. This suggested epicenter lies at, or just west of, the intersection of the Hite and Wallula faults. The elongation of ground failure along the Wallula fault and the aftershock distribution appears more consistent with rupture of the Wallula fault or of a subparallel fault than with rupture of the Hite fault. Rupture on faults to the north or east of the Wallula-Hite fault intersection is inconsistent with most observations, including the perceived impulsivity of the mainshock. [ABSTRACT FROM AUTHOR]