Environmental Geology lecture outline -
Events associated with earthquakes and some case histories.
What happens during an earthquake that is of potential
- ground movement: up to .7
g well documented, length of shaking very important.
- aftershocks: exponential
decrease, damaging due to integrity loss from main shock.
- ground breakage/rupture:
meters vertical offset, 10s to 100s kms in length.
- triggering of mass wasting: great variety of types,
blocks rescue routes.
- liquefaction: wet sediments
- tsunamis: generated by submarine
crustal movements associated with EQ.
- seiche waves: sloshing of
Interplate earthquake case histories (so many!).
Kobe, Japan, January 17, 1995,
- 6.9 on Richter scale.
- 10 km deep hypocenter, local ground motion of 5 m, 40
km long rupture.
- 5,480 fatalities, 94,900 injured; $100 billion loss.
- tectonic setting -> arc-subduction complex.
San Fernando Valley Earthquakes:
Feb. 9, 1971 and Jan 17, 1994
- second one known as the Northridge earthquake.
- 6.5 and 6.8 on the Richter scale respectively.
- hypocenters at depths of 20 and 16 km respectively.
- San Fernando: 58 fatalities, $497 million in damage.
- Northridge; 56 fatalities, 25,000 dwellings uninhabitable,
$10 billion in damage.
- both occurred on thrust faults associated with the transpressive
bend in the San Andreas plate boundary in the LA area.
Intraplate earthquake case histories:
New Madrid, Missouri:
- 1811 Dec. 16th, 7.5; Jan. 23rd 1812, 7.3; Feb. 7th 1812,
- Mississippi river channel shifted, church bells in steeples
in Boston rang due to swaying.
- continued macro- and microseismicity.
- if such an earthquake occurred today?
Charleston, South Carolina
August, 31st 1886:
- 110 deaths and alot of building damage.
- continued seismicity to data, and continued concern.
Nebraska and the Humboldt fault zone?
Common traits of intraplate earthquakes? They are associated
with old buried fault zones that are part of continental rifts and they
have lower slip rates and longer recurrence intervals than interplate earthquakes.
Induced sesimicity case histories
1960s, Rocky Mountain Arsenal in Colorado:
- injection of toxic wastes into 3 km deep hole.
- temporal correlation between injection and earthquakes.
- 1967, 5.2 RM EQ, stopped injection.
Rangely anticline, CO and S-central NE, secondary recovery
of oil by injection of fluids, increasing pore pressure and flushing oil
Koyna Dam near Bombay, India;
- reservoir infilling 1962, 103 m high, 2.8 billion cubic
- built on Deccan basalt traps.
- 1967 6.4 RM EQ kills > 200, injures 2,300.
- EQs form N-S zone.
- additional dam built down stream - Warna dam, 1.3 billion
cubic meters capacity.
- > 150 EQs > or = to 4.0 R. M. between 1962 and
- Gupta, H. K., Radhakrishna, I. , Chadha, R. K., Kumpel, H. J. &
Greksch, G., 2000, Pore Pressure Studies Initiated in Area of Reservoir-induced
Earthquakes in India; EOS, 81, p. 145 & 151.
Lake Mead another example.
Fluid injection (or withdrawal) and reservoir infilling
can induce earthquakes.
The orange colored gravity anomaly depicted on the image below is a representation
of an old buried feature, the Keweenawan rift. The rift contains basaltic
rocks, which are denser than the surrounding rocks and hence cause a positive
gravity anomaly feature. The Humboldt fault zone in the SE corner of Nebraska
likely represents present-day seismic reactivation of this older feature.
H.D. Maher Jr., 3/2/98
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