Spatially limited mud turbidites on the Cascadia margin: segmented earthquake ruptures?

Abstract

Abstract. A series of 23 thin, mostly mud-silt turbidites are found interspersed between larger, well-dated and regionally correlated paleoseismic sandy turbidites that extend along most of the Cascadia margin, northwestern United States. Investigation of the structure, distribution, and sedimentology of these thin mud-silt units supports the interpretation of these units as turbidites originating on the continental slope. Interpretation of mud turbidites is inhibited by bioturbation and lower response to analytical and imaging techniques; nevertheless most of the 23 interpreted beds exhibit most of the characteristics of coarser turbidites. These characteristics include sharp bases, fining upward sequences, darker color, increased gamma and CT density and magnetic susceptibility relative to the hemipelagic background, sparse microfossils, high lithic content, and evidence of transport from marine sources on the continental slope. New core data from sites south of Rogue Apron indicate that sandy and muddy turbidites may be correlated at least 150 km south to Trinidad Plunge Pool for the period ~ 4800 yr BP to present. Many of the mud turbidites initially described at Rogue Apron coarsen southward, becoming sandy turbidites. High-resolution Chirp seismic profiles reveal that turbidite stratigraphy along the base of the southern Cascadia continental slope is continuous, with little variation for at least 240 km along strike. The Chirp data show that turbidites along the Cascadia base of slope are ubiquitous, and likely not sourced solely from submarine canyon mouths, but may also have been delivered to the proximal abyssal plain as sheet flows from the open continental slope and coalescing local sources. Regional stratigraphy reveals that hemipelagic sedimentation rates and total Holocene turbidite thickness and mass are similar at widely separated sites, yet the total thickness of the Holocene section is greater by a factor of two in southern Cascadia. This difference is primarily due to the presence of the 21 mud and two additional sandy turbidites. We conclude that the Cascadia mud turbidites are ubiquitous along southern Cascadia only, with only one likely example of a correlated turbidite limited to the northern margin. Eight onshore sites including three marsh sites and five lakes include potential seismogenic correlatives of the southern Cascadia turbidites. In all, the onshore sites may have recorded > 80% of the events attributed to plate boundary earthquakes offshore during the period 0–6000 yr ago. Slope stability calculations suggest that earthquakes of Mw = 7.0 or greater should generate ground accelerations sufficient to destabilize open slopes and canyon heads with or without excess pore fluid pressure. Estimates of Mw for segmented ruptures are in the range of 7.4–8.7, exceeding the slope stability criteria for typical slopes by at least a factor of ~ four.

Simulated tsunami inundation for a range of Cascadia megathrust earthquake scenarios at Bandon, Oregon, USA

“…Estimates of peak coseismic slip for the 2011 Tohoku-oki earthquake exceed 60 m near the Japan Trench (e.g., Ozawa et al, 2011;Wei et al, 2012;Yamazaki et al, 2011). If subduction zones store energy over long intervals that span several earthquake cycles, higher slip defi cits than inferred in this study might be achieved (i.e., supercycles; Sieh et al, 2008;Goldfi nger et al, 2013a).…”

“…1A). The along-strike continuity of the mud turbidites is supported by 29 new cores and high-resolution compressed high-intensity radar pulse (CHIRP) seismic-refl ection profi les that image stratigraphy between core sites (Goldfi nger et al, 2013b). Goldfi nger et al (2012) showed that each of the 41 turbidites in the 10,000 yr Cascadia record meets a variety of criteria that favor synchronous triggering by plate-boundary earthquakes.…”

A comparison of the sedimentary records of the 1960 and 2010 great Chilean earthquakes in 17 lakes: Implications for quantitative lacustrine palaeoseismology

“…Individual turbidites can only be mapped with high‐resolution seismic sources in specific cases (e.g. Goldfinger et al ., ) and/or when they are megaturbidtes (for example, this study; Schnellmann et al ., ). However, determining the source region of the turbidites that are not linked to MTDs from this type of data is complicated.…”

SedCT: MATLAB™ tools for standardized and quantitative processing of sediment core computed tomography (CT) data collected using a medical CT scanner

“…In recent years, medical computed tomography (CT) scanners have become common in the nondestructive analysis of sediment cores [St-Onge et al, 2007], including high-resolution reconstructions of relative or calibrated density for paleoenvironmental interpretation [Boespflug et al, 1995;Ashi, 1997;Lis e-Pronovost et al, 2009;St-Onge and Long, 2009;Støren et al, 2010;Davies et al, 2011;Dorfman et al, 2015], assessment of coring deformation [Ashi, 1997;Barletta et al, 2010;Walczak et al, 2017], and variations in sedimentological structures or components [Michaud et al, 2003;Gagnoud et al, 2009;Goldfinger et al, 2013;Mena et al, 2015;Patton et al, 2015]. Medical CT scanners measure the attenuation of X-rays, a function of density and atomic number, and store these values in pixels as relative gray scale values or Hounsfield units (HU), achieving resolution around 0.5 3 0.5 3 0.5 mm [Hounsfield, 1973].…”