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Paleoseismic Investigations of Quaternary Active Faults in the Forearc and Backarc of the Central Pacific Northwest, u.s.a.

Description: 

Oblique northeast-directed subduction of the Juan de Fuca plate and northward impingement of the Sierra Nevada block drive clockwise rotation of the PNW region relative to stable North America. Our work has yielded new observations of surficial deformation across the central Cascade Arc in Oregon (OR) where the rates and timing of faulting during the past 100,000 years are largely unconstrained. We combine lidar and field-based mapping to identify Quaternary active structures, characterize their slip rates, and where possible, constrain the timing of prehistoric surface rupturing earthquakes. In eastern OR, on the Strawberry Mountain fault, we document extensional faulting in a region broadly inferred to be compressional - related to Yakima Fold and Thrust deformation. Here, we document 2 post-LGM earthquakes and calculate a slip rate of 0.18 ± 0.02 mm/yr since the late Pleistocene.

The central Cascade volcanic Range is defined by a lower relief ancestral Eocene to Miocene arc forming the western foothills and is offset relative to the active high Cascades arc, a result of ~ 1°/My clockwise rotation for the last 16 My. The westward escape of the forearc creates a space problem, earlier research shows that the arc creates new crust on the trailing edge, and we document structural accommodation of tectonic extension by normal faulting. We observe numerous N-S trending extensional faults through both the high and ancestral arc. On the Mt Hood fault zone we map glacial moraines from the most recent glaciation, 21.2 ka ± 3 based on our new He³ surface exposure ages, offset by 2.7 m high scarps. A trench across an uphill facing scarp revealed 2 post-LGM surface rupturing earthquakes, and 2.3 m of total vertical separation. In the OR Coast Range on the Gales Creek Fault, we map right laterally deflected drainages, aligned springs, and side-hill benches. A paleoseismic trench here revealed evidence of three surface rupturing earthquakes in the last ~8,800 yr. We speculate that the earthquake timing may be linked to large on-shore stress changes in the crust immediately following a megathrust subduction zone earthquake.

Session: Learning Across Geological, Geophysical & Model-Derived Observations to Constrain Earthquake Behavior - I

Type: Oral

Date: 5/1/2024

Presentation Time: 08:45 AM (local time)

Presenting Author: Ashley

Student Presenter: No

Invited Presentation: Yes

Authors