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| Titel |
Spatially limited mud turbidites on the Cascadia margin: segmented earthquake ruptures? |
| VerfasserIn |
C. Goldfinger, A. E. Morey, B. Black, J. Beeson, C. H. Nelson, J. Patton |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Science ; 13, no. 8 ; Nr. 13, no. 8 (2013-08-23), S.2109-2146 |
| Datensatznummer |
250085504
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| Publikation (Nr.) |
 copernicus.org/nhess-13-2109-2013.pdf |
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| Zusammenfassung |
| 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. |
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