Abstract
A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems.
Highlights
Field investigations were undertaken in this study to test previous field reports, which stated that large-scale paleoliquefaction was absent in the latest Holocene deposits in the Willamette forearc valley of the central Cascadia convergent margin [1] [2]
Late Holocene sequences in exposed cutbanks were identified on the bases of 1) overbank mud thickness (≤4 m thickness), 2) absence of multiple paleosol soil-A horizons [30] in the overbank mud sequences, 3) relatively low values of unconfined compressive strength (1.5 ± 0.5 kg∙cm−2) measured in sandy clastic dikes and sills using a pocket penetrometer ASTM D2573, and 4) radiocarbon dating of hosting overbank mud deposits
Late Pleistocene Paleoliquefaction Late Pleistocene paleoliquefaction sites were identified in modern cutbank exposures in the lower and upper sections of the Willamette River (Figure 2 and Figure 3)
Summary
Field investigations were undertaken in this study to test previous field reports, which stated that large-scale paleoliquefaction was absent in the latest Holocene deposits in the Willamette forearc valley of the central Cascadia convergent margin [1] [2]. The Willamette Valley should be within reach of potential seismic shaking from megathrust ruptures of the Cascadia subduction zone [3]. The reported lack of large-scale clastic dikes and sills (≥10 cm in width) in the Willamette Valley has contributed to informal speculation that the Oregon portion of the subduction zone megathrust might be aseismic, weakly coupled, or highly segmented [14], thereby precluding Mw 8 ruptures. Historic earthquakes in the Puget forearc valley of western Washington (Figure 1) reached magnitudes of 7.1 in Site. Coastal Sites pm cs, cd [7] psm cs, sv [22] psm cs, cd [8] pm cd, cs [9]
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