Outlines

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Outlines

• Earthquakes in the Manhattan area
• Big ones
• Small ones
• Earthquakes and earthquake hazard
• Causes of earthquakes
• Why Manhattan had earthquakes
• How serious is the situation?
• What should be done to address the problem?

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Earthquakes in NE Kansas

• Five notable ones occurred between 1867 and
  1929. Concentrated in time
• About 50 small ones were detected during
  1977-1989 by a KGS seismic network
• Along two zones bordering the MCR
• The locations of the two largest events cannot be
  accurately determined

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How Strong is an MMI 7 event?

• Answer similar to the shaking level at 50 km
  from a magnitude 7.1 earthquake

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• Earthquakes are the results of relative movement
  of crustal blocks along faults
• In the Manhattan area the relative motion is
  mostly up-and-down
• All the faults were the results of previous
  earthquakes
• Faster movement results to more frequent
  earthquake occurrence
• The maximum magnitude is limited by the length of
  a fault

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• Most earthquakes in the world occur along narrow
  zones, which are boundaries between individual
  crustal blocks (plates).
• Very few earthquakes occur inside plates. NE
  Kansas is one of those places where intra-plate
  earthquakes did occur and will occur

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The Strange River

• (1). The Blue river
• valley is too straight compared with a
  normal one
• (2). It makes a near-90 deg turn near
  Marysville
• (3). See next slide

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The ultimate cause ??

• The Midcontinental Rift (MCR) is a 2000-km long
  feature
• During a short period of time about 1.1 billion
  years ago, it was a zone of large earthquakes and
  many volcanoes, which erupted a layer of volcanic
  rocks that is about 20 km thick along the rift
  valley

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Gravity anomalies caused by the heavy volcanic
rocks
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Possible Earthquake Mechanism in NE Kansas

• The crust of the earth beneath the MCR and a wide
  adjacent area was anomalously thick, due to the
  closure of the MCR 1.1 billion year ago.
• Because rocks in the crust is lighter than those
  in the mantle, the surface of the Earth in this
  area has been moving upward since then.
• Most of the movement occur along the flanks of
  the rift, because the volcanic rocks along the
  rift axis are too heavy to move
• This vertical movement is along vertical faults
  such as the Humboldt fault and the Big Blue
  Fault
• Movement along faults cause earthquakes

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How Serious is the Threat?

• The largest earthquake ever-felt in Kansas
  occurred on April 24, 1867. It had an MMI of 7 or
  8, and was felt over an area of 300,000 square
  miles in Kansas, Nebraska, Missouri, Illinois,
  Indiana, and perhaps Ohio.
• Several people were injured. People rushed into
  the streets.
• Buildings swayed and walls of many buildings were
  cracked. A heavy wave about 2 feet high was
  observed in the Kansas River at Manhattan.
• (Merriam, 1956 Dubois and Wilson, 1978).

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The Problems

• At the present time nobody knows about
• (1). The occurrence frequency of Manhattan
  earthquakes
• (2). Their magnitude
• (3). The place they will most likely occur

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To Address the Problems

• The most effective way is to establish a
  multi-station seismic network in NE Kansas to
• (1). Locate small earthquakes which most likely
  occur along active faults
• (2). Image the underground structure to look for
  faults
• (3). Determine fault slip-rate and direction to
  determine occurrence interval

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A modern digital seismic station on Konza
established by the KSU Department of Geology. It
is currently the 2nd digital seismic station in
Kansas. Estimated cost 50,000
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Summary

• 1). The Manhattan area did have some damaging
  earthquakes in recent history, although their
  locations cannot be determined accurately until
  extensive (and expensive) geophysical studies are
  conducted

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• 2). There are some good reasons for them to occur
  again, although we do not know when and where
  they will occur
• 3). But the probability that they will cause
  significantly damage in the next 100 or so years
  is very low

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• 4). If the Tuttle Creek dam needs to be removed
  because of earthquakes, then all the other dams
  in the western U.S., the New Madrid area, SE US,
  and NE US should all be removed

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Thanks!