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Titel |
Protracted fluvial recovery from medieval earthquakes, Pokhara, Nepal |
VerfasserIn |
Amelie Stolle, Anne Bernhardt, Wolfgang Schwanghart, Christoff Andermann, Elisabeth Schönfeldt, Jan Seidemann, Basanta R. Adhikari, Silke Merchel, Georg Rugel, Monique Fort, Oliver Korup |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250127878
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Publikation (Nr.) |
EGU/EGU2016-7801.pdf |
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Zusammenfassung |
River response to strong earthquake shaking in mountainous terrain often entails the flushing
of sediments delivered by widespread co-seismic landsliding. Detailed mass-balance studies
following major earthquakes in China, Taiwan, and New Zealand suggest fluvial recovery
times ranging from several years to decades. We report a detailed chronology of
earthquake-induced valley fills in the Pokhara region of western-central Nepal, and
demonstrate that rivers continue to adjust to several large medieval earthquakes to the present
day, thus challenging the notion of transient fluvial response to seismic disturbance. The
Pokhara valley features one of the largest and most extensively dated sedimentary
records of earthquake-triggered sedimentation in the Himalayas, and independently
augments paleo-seismological archives obtained mainly from fault trenches and historic
documents.
New radiocarbon dates from the catastrophically deposited Pokhara Formation document
multiple phases of extremely high geomorphic activity between ∼700 and ∼1700 AD,
preserved in thick sequences of alternating fluvial conglomerates, massive mud and silt beds,
and cohesive debris-flow deposits. These dated fan-marginal slackwater sediments indicate
pronounced sediment pulses in the wake of at least three large medieval earthquakes in
∼1100, 1255, and 1344 AD. We combine these dates with digital elevation models,
geological maps, differential GPS data, and sediment logs to estimate the extent of these
three pulses that are characterized by sedimentation rates of ∼200 mm yr−1 and
peak rates as high as 1,000 mm yr−1. Some 5.5 to 9 km3 of material infilled the
pre-existing topography, and is now prone to ongoing fluvial dissection along major
canyons. Contemporary river incision into the Pokhara Formation is rapid (120-170
mm yr−1), triggering widespread bank erosion, channel changes, and very high
sediment yields of the order of 103 to 105 t km−2 yr−1, that by far outweigh bedrock
denudation rates inferred from cosmogenic 10Be inventories in river sands. The
rapid infill of about a dozen tributary valleys displaced river channels, and caused
them to re-incise into bedrock along steep epigenetic gorges. We conclude that the
Pokhara Formation offers a unique archive of medieval earthquakes as well as the
associated protracted fluvial response that may have been ongoing for up to 900 years. |
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