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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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