Earthquakes

Earthquakes

Principle author Daniel Zacharias

Edited by:

Introduction.

Ever since the beginning of the earth there has been motion in some form or

another. It is the goal of this paper to address three main ideas, the first is to introduce

earthquakes. Then we will cover a how earthquakes have effected Yellowstone National

Park. As well as Geological hazards related to earthquakes in general, The Hebgen Lake

(figure 1-1) will serve as at least one example.

What are Earthquakes?

An effective description of an earthquake is to imagine two blocks pressing

against each other at opposing directions they are restricted in movement due to friction.

This friction has an elastic stretching effect on the two blocks, as pressure is applied it is

stored in the rocks which stretch under pressure. Another way to visualize this concept is

to take a pencil, and pull the ends down while pressing up towards the middle with your

thumbs. The rocks bends as time and pressure increases. However it is not usually visible

on the surface of the earth as the flexing itself is often hard to recognize. Just like the

pencil when the force applied exceeds the tensile strength the pencil snaps. The same is

true with the earth. This snapping action is called elastic rebound.(figure 1-2) Once the

rocks release their elastic energy, the two pieces of rock (blocks, pencil halves, etc.,) slide

past each other. This release of elastic energy is an earthquake.

When the energy is released seismic waves travel through the ground away from

the hypocenter, which is the location along the fault plane itself where the earthquake

occurred. The epicenter is the location on the surface of the earth above the hypocenter.

There are two types of seismic waves created during an earthquake, these are

what we feel and what causes the actual damage associated with earthquakes the two are,

primary (P) and surface (S) waves. Primary waves are compression waves that are the

faster moving of the two types, and can travel through liquid. Whereas surface waves

come in two categories depending upon the direction of elastic vibrations. Love waves

occur with horizontal movement where Rayleigh waves are vertical. Surface waves cause

the most damage to infrastructure and homes. And in the case of Yellowstone can alter

geothermal ventilation etc...

Earthquakes can obviously be triggered by slippage along faults. As well as by

volcanoes, reservoir changes, meteor impacts and even by bombs. (Nelson 2003)

There are many types of faults; for the most part there are three basic faults, normal,

strike-slip, and reverse faults. A fault consists of a fracture or zones of fractures along

which a displacement occurs each fault has a hanging wall and a footwall. (Yeats, Sieh,

Allen 1997) To aid in the distinction between a footwall and a hanging wall imagine that

you're digging a tunnel through the fault, the hanging wall is where you would hang the

lantern and the footwall is where you're standing. A normal fault is where the hanging

wall has slid down relative to the footwall. Strike-slip faulting is similarly caused by

horizontal compression, each side releases its energy by rock displacement in a

horizontal direction almost parallel to the compression force. The fault plane is

essentially vertical, and the relative slip is lateral along the plane. (Encyclopedia

Britannica 2010) And last but not least, is the reverse fault, where the hanging wall

moves upwards relative to the footwall. (Yeats, Sieh, Allen 1997) Earthquakes often

occur underground on the fault plane. The area where the earthquake occurred is known

as the hypocenter, and directly above the hypocenter on the surface is the epicenter.

Effects of Earthquakes on Yellowstone

Earthquakes and Yellowstone have always seemed to go hand in hand, or rather

the hotspot itself and earthquakes. My rationality for this idea is that as the continental

crust moved over the hotspot, the current seismic events had occurred with the hotspot

continuously along the Snake River Plain. Think about it, back when the hotspot was

under what is now South Central Idaho. it is entirely possible that following an eruption

the magma chamber collapsed and created a caldera. A supporting detail in regard to

earthquakes being linked to hotspots, look at Hawaii, it is in the center of the pacific

plate. And yet it has frequent earthquakes. Many of which are caused by magma flowing

into volcanic edifices and others are caused by the collapsing flanks of volcanoes. (Yeats,

Sieh, Allen 1997) Yellowstone experiences more than a thousand earthquakes each year.

During the month of May 2010 there were 118 earthquakes in the Yellowstone region.

(USGS 2010) Earlier this year the Yellowstone Region experienced a swarm of

earthquakes thought to have been triggered by the 7.0Mw earthquake that in Haiti. As of

April 6th, 2010 The Madison Plateau swarm reached 2,347 earthquakes. (USGS 2010)

It is not entirely farfetched to suppose that larger earthquakes can set off a swarm

of Yellowstone earthquakes. May 2002, seismic waves from the Denali Fault Earthquake,

over 2,000 miles away in Alaska reached Yellowstone and triggered a swarm of smaller

earthquakes,(UNews 2004) and altered hydrothermal frequencies in regard to existing

geysers. the following is the journal abstract;

"Following the 2002 M 7.9 Denali fault earthquake, clear changes in geyser activity and a series of local earthquake swarms were observed in the Yellowstone National Park area, despite the large distance of 3100 km from the epicenter. Several geysers altered their eruption frequency within hours after the arrival of large-amplitude surface waves from the Denali fault earthquake. In addition, earthquake swarms occurred close to major geyser basins. These swarms were unusual compared to past seismicity in that they occurred simultaneously at different geyser basins. We interpret these observations as being induced by dynamic stresses associated with the arrival of large-amplitude surface waves. We suggest that in a hydrothermal system dynamic stresses can locally alter permeability by unclogging existing fractures, thereby changing geyser activity. Furthermore, we suggest that earthquakes were triggered by the redistribution of hydrothermal fluids and locally increased pore pressures. Although changes in geyser activity and earthquake triggering have been documented elsewhere, here we present evidence for changes in a hydrothermal system induced by a large-magnitude event at a great distance, and evidence for the important role hydrothermal systems play in remotely triggering seismicity."

I wish I could have accessed that journal through the library (as of the time of this writing

the school has not received it via interlibrary loan.) As it is suggested in the abstract

earthquakes can trigger swarms within Yellowstone, They can also alter the frequency of

geysers, increase the likelihood of fumaroles expelling large quantities of steam, as well

as altering the piping and vent system in the geyser valleys. In the U of U press release

more details are available, such as the following statement;

"... Scientists do not know if the strong surface waves from the Denali quake independently triggered Yellowstone’s small quakes and changes in geyser activity. Smith suspects not. He believes the Denali quake’s waves affected the geysers by changing water pressure in underground conduits or “pipes” that feed the geysers. Such changes – which in some cases would have made hot water “flash” explosively into steam – would have altered the pressure on adjacent faults, triggering small earthquakes nearby. That would explain why the quakes were clustered around geyser basins..." (UNews 2004)

To have the water suddenly flash to steam would most definitely alter pressure and

trigger a swarm of earthquakes over time as the pressure balance is restored to its

norm. Earthquakes themselves have been triggered by other earthquakes, changes

in reservoir pressure, and of course by magma filling edifices or receding from the

magma chamber.

Geologic and other Earthquake related hazards.

Earthquakes in general don't directly cause millions of dollars in damage or kill

thousands of people. It is a combination of factors triggered by the effect of seismic

waves from the earthquake. For instance with the Haitian earthquake of January 2010,

most deaths were caused by collapsing buildings contrasted by the Chilean earthquake.

Infrastructure is usually destroyed leaving the regions susceptible to fires, diseases, etc...

Many areas are at risk of a condition known as liquefaction during an earthquake, as a

basic description damp soils spread and loose their structural capabilities. It is not

uncommon for homes to sink into the earth during an earthquake. One the USGS

earthquake F.A.Q.'s they try to debunk the supposed myth about the earth swallowing

people during earthquakes, but liquefaction has swallowed cars, homes, and quite

probably a person or two. (figure - ) In addition, flooding occurs, usually because a

landslide was triggered by the earthquake and blocks a river. Or possibly the seismic

waves weaken a dam, which eventually breaks and floods a valley. Tsunamis and Seiche

can cause flooding as well.

Conclusion:

Seiche, usually one wavelength equals twice the length of the lake.

figure 1-1

Seiche marks on the walls of a unit at Hilgard Lodge. These water marks indicate the height and number of waves which passed this particular site. Gallatin County, Montana. August 1959.http://earthquake.usgs.gov/earthquakes/states/events/images/1959_08_18_020_small.gif

Figure 1-2

The fence was built straight.

Elastic Strain Energy builds up.

Elastic Energy is released.

http://peer.berkeley.edu/course_modules/eqrd/EQDef/eqdef2.htm

http://www.digitalgeology.net/page16.html

http://www.digitalgeology.net/page14.html

Works Cited:Yeats, R.S., Sieh, K., Allen, C.R., 1997, The Geology of Earthquakes: New York, Oxford University Press, Glossary pages 486-499.

Yeats, R.S., Sieh, K., Allen, C.R., 1997, The Geology of Earthquakes: New York, Oxford University Press, Page 14 paragraph 3.

Witkind, I.J., 1964, The Hebgen Lake, Montana, Earthquake of August 17, 1959: USGS Professional Paper 435, Washinton, United States Government Printing Office.

The Science of Earthquakes. June 6th, 2010. http://earthquake.usgs.gov/learn/kids/eqscience.php

Types of Seismic Hazards. June 5th, 2010. http://peer.berkeley.edu/course_modules/eqrd/EQDef/eqdef1.htm

Tarbuck, E.J., Lutgens, F. K., Tasa, D., Digital Geology: Web based publication. http://www.digitalgeology.net/page1.html

Quake in Alaska changed Yellowstone Geysers. May 30th, 2010. http://www.unews.utah.edu/p/?r=030306-10

Researchers report Yellowstone earthquake swarm. May 30th, 2010. http://www.sltrib.com/news/ci_14221536

"Strike-slip fault." Encyclopedia Britannica. 2010. Encyclopedia Britannica Online. 08 Jun. 2010 http://www.britannica.com/EBchecked/topic/569119/strike-slip-fault.

DigitalGeology. June 6th, 2010. http://www.digitalgeology.net/page1.html

"USGS." Yellowstone Volcano Observatory Monthly Update. June 6th, 2010.http://volcanoes.usgs.gov/yvo/activity/

"USGS." Summary of 2010 Madison Plateau, Yellowstone Earth. June 6th, 2010. http://volcanoes.usgs.gov/yvo/publications/2010/10swarm.php

Husen, S., Taylor, R., Smith R. B., Healser, H. June 2004, Changes in geyser eruption behavior and remotely triggered seismicity in Yellowstone National Park produced by the 2002 M 7.9 Denali Fault earthquake, Alaska.Geology, Boulder, 32(6):537-540

Quake in Alaska changed Yellowstone Geysers. May 30th, 2010. http://www.unews.utah.edu/p/?r=030306-10

Types of Seismic Hazards. June 5th, 2010. http://peer.berkeley.edu/course_modules/eqrd/EQDef/eqdef1.htm