Earthquaketriggered increase in sediment delivery from an active mountain belt

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Earthquake-triggered increase in sediment
delivery from an active mountain belt Dadson,
Hovius, Chen, Dade, Lin, Hsu, Lin, Horng, Chen,
Milliman, Stark
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Study Location

• Tectonically active
• High precipitation

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Study Objectives/Questions

• How much sediment is generated by an earthquake
  of a given magnitude?
• How long does sediment take to travel from
  mountain hillslopes to rivers and the sea?
• What evidence of past earthquakes might be
  recorded in ocean deposits?

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Natural Experiment 1996-2001
August 1996 Typhoon Herb TWD 1.2
km3 September 1999 Chi-Chi Earthquake Mw
7.6 July 2001 Typhoon Toraji TWD 0.8 km3
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Earthquake- and rainfall-triggered landslides
Chi-Chi Earthquake - 20,000 landslides
- most within 0.2g contour - 56 reactivated
during typhoon Toraji Typhoon Toraji -
30,000 landslides - 80 on undisturbed land
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Earthquake- and rainfall-triggered landslides
Chi-Chi Earthquake -exponential decay away
from epicenter Typhoon Toraji -increase in
disturbed land with proximity to
fault Suggests substrate was preconditioned to
fail
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Earthquake- and rainfall-triggered landslides

• Highest probability of landslide area is below
  detection limit (3600 m2)
• Statistics are independent of triggering
  mechanism

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Landslide location and delivery of sediment to
rivers
Chenyoulan River Basin - close to Chi-Chi
epicenter - well instrumented
Typhoon Herb 289 landslides Chi-Chi Earthquake
584 landslides Typhoon Toraji 1,903 landslides
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Landslide location and delivery of sediment to
rivers

• 1 km2 is critical upslope area for channelized
  flow
• Area-slope scaling exponent of -0.42 is typical
  for river channels (Montgomery and
  Foufoula-Georgiou, 1993)

A drainage area of the most downstream point
reached by each landslide (m2) -Calculated using
D8 flow-routing algorithm in ARC/INFO
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Landslide location and delivery of sediment to
rivers
Chi-Chi Earthquake 8 delivered sediment to
rivers Typhoon Toraji 13 delivered sediment to
rivers Typhoon Herb 24 delivered sediment to
rivers
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Spatial pattern of river response to the Chi-Chi
Earthquake
C kQb C is sediment concentration (g/L) Q is
water discharge (m3/s) b is strength of relation
between Q and C, and is determined by process of
sediment mobilization (transport capacity or
sediment supply) k is unit sediment
concentration ?k kpost/kpre ?k should be
related to sediment supply by co-sesimic and
storm-triggered landslides
Choshui
Chenyoulan

• post earthquake
• pre earthquake

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Spatial pattern of river response to the Chi-Chi
Earthquake

• High ?k along Chelunpu fault
• ?k pattern correlates with PGA

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Spatial pattern of river response to the Chi-Chi
Earthquake
r2 0.86
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Temporal pattern of river response to the Chi-Chi
Earthquake

• Post earthquake erosion rates are lower in
  winter and years with no typhoons (eg. 1999)
• Channel sediments were depleted rapidly after the
  Chi-Chi earthquake
• Hillslopes deliver sediment to rivers during
  typhoons

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Hyperpycnal Flows
pw is the density of seawater pf is the density
of river plume pf gt pw
Salinty and water temperature require C of
36-40 g/L
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Hyperpycnal Flows
Stratigraphic indicator of increased tectonic
activity
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Conclusions

• Suspended sediment concentrations increased 4X
  following the Chi-Chi earthquake
• Typhoons mobilize stored co-seismic sediment on
  hillslopes. Travel time is dependent on typhoon
  frequency.
• Hyperpycnal flows can be stratigraphic indicators
  of tectonic activity

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