Transcript Earthquakes in NE Kansas (ppt format)

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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
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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 multistation 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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