Holocene earthquakes of magnitude 7 during westward escape of the Olympic Mountains, Washington

Abstract

The Lake Creek–Boundary Creek fault, previously mapped in Miocene bedrock as an oblique thrust on the north flank of the Olympic Mountains, poses a significant earthquake hazard. Mapping using 2015 light detection and ranging (lidar) confirms 2004 lidar mapping of postglacial ( &lt; 13 &#x2009;&#x2009; ka "> < 13 ka ) and Holocene fault scarps along the 22‐km‐long eastern section of the fault and documents Holocene scarps that extend &#x2265; 14 &#x2009;&#x2009; km "> ≥ 14 km along a splay fault, the Sadie Creek fault, west of Lake Crescent. Scarp morphology suggests repeated earthquake ruptures along the eastern section of the Lake Creek–Boundary Creek fault and the Sadie Creek fault since &#x223C; 13 &#x2009;&#x2009; ka "> ∼ 13 ka . Right‐lateral ( &#x223C; 11 &#x2013; 28 &#x2009;&#x2009; m "> ∼ 11 – 28 m ) and vertical (1–2 m) cumulative fault offsets suggest slip rates of &#x223C; 1 &#x2013; 2 &#x2009;&#x2009; mm / yr "> ∼ 1 – 2 mm / yr Stratigraphic and age‐model data from five trenches perpendicular to scarps at four sites on the eastern section of the fault show evidence of 3–5 surface‐rupturing earthquakes. Near‐vertical fault dips and upward‐branching fault patterns in trenches, abrupt changes in the thickness of stratigraphic units across faults, and variations in vertical displacement of successive stratigraphic units along fault traces also suggest a large lateral component of slip. Age models suggest two earthquakes date from 1.3 &#xB1; 0.8 "> 1.3 ± 0.8 and 2.9 &#xB1; 0.6 &#x2009;&#x2009; ka "> 2.9 ± 0.6 ka ; evidence and ages for 2–3 earlier earthquakes are less certain. Assuming 3–5 postglacial earthquakes, lateral and vertical cumulative fault offsets yield average slip per earthquake of &#x223C; 4.6 &#x2009;&#x2009; m "> ∼ 4.6 m , a lateral‐to‐vertical slip ratio of &#x223C; 10 : 1 "> ∼ 10 : 1 , and a recurrence interval of 3.5 &#xB1; 1.0 &#x2009;&#x2009; ka "> 3.5 ± 1.0 ka . Empirical relations yield moment magnitude estimates of M 7.2–7.5 (slip per earthquake) and 7.1–7.3 (56 km maximum rupture length). An apparent left‐lateral Miocene to right‐lateral Holocene slip reversal on the faults is probably related to overprinting of east‐directed, accretion‐dominated deformation in the eastern core of the Olympic Mountains by north‐directed, margin‐parallel shortening and westward escape of the mountains.