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| Titel |
Great earthquakes along the Western United States continental margin: implications for hazards, stratigraphy and turbidite lithology |
| VerfasserIn |
C. H. Nelson, J. Gutiérrez Pastor, C. Goldfinger, C. Escutia |
| 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 ; 12, no. 11 ; Nr. 12, no. 11 (2012-11-01), S.3191-3208 |
| Datensatznummer |
250011181
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| Publikation (Nr.) |
 copernicus.org/nhess-12-3191-2012.pdf |
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| Zusammenfassung |
We summarize the importance of great earthquakes (Mw ≳ 8) for
hazards, stratigraphy of basin floors, and turbidite lithology along the
active tectonic continental margins of the Cascadia subduction zone and the
northern San Andreas Transform Fault by utilizing studies of swath
bathymetry visual core descriptions, grain size analysis, X-ray radiographs
and physical properties. Recurrence times of Holocene turbidites as proxies
for earthquakes on the Cascadia and northern California margins are analyzed
using two methods: (1) radiometric dating (14C method), and (2) relative
dating, using hemipelagic sediment thickness and sedimentation rates (H
method). The H method provides (1) the best estimate of minimum recurrence
times, which are the most important for seismic hazards risk analysis, and
(2) the most complete dataset of recurrence times, which shows a normal
distribution pattern for paleoseismic turbidite frequencies. We observe that,
on these tectonically active continental margins, during the sea-level
highstand of Holocene time, triggering of turbidity currents is controlled
dominantly by earthquakes, and paleoseismic turbidites have an average
recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great
earthquakes are approximately 300 yr for the Cascadia subduction zone and
130 yr for the northern San Andreas Fault, which indicates both fault
systems are in (Cascadia) or very close (San Andreas) to the early window
for another great earthquake.
On active tectonic margins with great earthquakes, the volumes of mass
transport deposits (MTDs) are limited on basin floors along the margins. The
maximum run-out distances of MTD sheets across abyssal-basin floors along
active margins are an order of magnitude less (~100 km) than on
passive margins (~1000 km). The great earthquakes along the Cascadia
and northern California margins cause seismic strengthening of the sediment,
which results in a margin stratigraphy of minor MTDs compared to the
turbidite-system deposits. In contrast, the MTDs and turbidites are equally
intermixed on basin floors along passive margins with a mud-rich continental
slope, such as the northern Gulf of Mexico.
Great earthquakes also result in characteristic seismo-turbidite lithology.
Along the Cascadia margin, the number and character of multiple coarse
pulses for correlative individual turbidites generally remain constant both
upstream and downstream in different channel systems for 600 km along the
margin. This suggests that the earthquake shaking or aftershock signature is
normally preserved, for the stronger (Mw ≥ 9) Cascadia earthquakes. In
contrast, the generally weaker (Mw = or <8) California earthquakes result
in upstream simple fining-up turbidites in single tributary canyons and
channels; however, downstream mainly stacked turbidites result from
synchronously triggered multiple turbidity currents that deposit in channels
below confluences of the tributaries. Consequently, both downstream channel
confluences and the strongest (Mw ≥ 9) great earthquakes contribute to
multi-pulsed and stacked turbidites that are typical for seismo-turbidites
generated by a single great earthquake. Earthquake triggering and
multi-pulsed or stacked turbidites also become an alternative explanation
for amalgamated turbidite beds in active tectonic margins, in addition to
other classic explanations. The sedimentologic characteristics of turbidites
triggered by great earthquakes along the Cascadia and northern California
margins provide criteria to help distinguish seismo-turbidites in other
active tectonic margins. |
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