Denali Earthquake


Denali Seismic Activity #1
Denali Seismic Activity #1
Introduction-

The Denali earthquake struck midday November 3, 2002 with a magnitude of 7.9. It is ranked as one of the largest strike slip earthquakes since the San Francisco earthquake of 1906. This earthquake was reportedly based in the mountainous region of Alaska causing little harm to both human and structure because it hit an area of south-central Alaska with very little population. This seismic event gives much insight into how the San Andreas Fault will react once it finally decides to give way.

Earthquake Basics-

Earthquakes are caused by sudden slips along a fault line. Earthquakes are the equivalent of what happens when one tries to snap his or her fingers. The fingers are pressed against one another causing friction and when this friction reaches a breaking point it causes a sound that can be heard in the form of a snap.
A similar process goes on in an earthquake. As planet Earth is made up of smaller portions of free floating masses called plates we see to formation of what are called plate boundaries. These plate boundaries tend to collide causing friction in opposite directions. When this friction cannot be withstood any longer the plates "snap" sending shock waves through the ground and Earth alike that can be felt for miles around.
Earthquakes are measured within what is called the Richter scale of Magnitude. As an earthquakes energy is released it is measured on a seismometer which scientist use to indicate the severity of the earthquake. Along with earthquakes having different magnitudes they are also produced in different fashions. There are three types of plate boundaries; subduction, divergent, and convergent. Where convergent and subduction boundaries produce shaking the divergent plate boundary does not. Also, subduction zones where plate boundaries converge produce deep violent earthquakes whereas convergent boundaries that produce strike slip faults produce slightly smaller not so deep earthquakes.

What is a strike slip fault?

A slip strike fault happens where two plate boundaries meet and rub together rather than subduct or converge upon one another. The friction between the two plates causes what is known as elastic rebound. Meaning that the two sides of the fault are strained with both force and shift. The opposite sides of the fault will move and bend along side one another until they reach maximum internal strength causing the two sides to "snap" back into original position. This theory give sufficient evidence toward explaination of earthquakes and is easily seen when two sides of a fault share a road, fence, or some sort of straight line where a scientist can measure actual displacement between the two sides of a fault.
  • Fault rupture of the ground generates vibrations-or waves-in the rock that we fell as ground shaking.
  • Faults are weaknesses in the rock and therefore earthquakes tend to occur over and over along the same faults.
  • Most major faults in the United States, particularly in California, are what are known as strike-slip faults.
  • For strike-slip faults, the rupture is nearly vertical and during an earthquake one side slides past the other.
  • The San Andreas fault is a strike-slip fault formed where two parts of the earth’s crust (plates) slide past each other.

Examples of Strike Slip Faulting-

Real Example of Strike Slip Fault
Real Example of Strike Slip Fault
Strike Slip Faulting #2
Strike Slip Faulting #2


What caused this earthquake?

As with large events of the past we have seen triggers leading to much more devastating and horrific natural disasters, and the Denali earthquake of 2002 was not much different. The Denali earthquake was a triggered by an earthquake in Nenana Mountain with a magnitude of 6.7 and it happened to hit 10 days earlier than the Denali earthquake. This event is referred to as the Nenana Mountain earthquake, which was said to be felt from Anchorage to Fairbanks Alaska. This event occurred on the Denali fault line which is responsible for the majority of the earthquakes in the area. Although a fairly shallow earthquake it produced a series of events that lead to the Denali 7.9. The Denali fault is believed to produce much larger earthquakes, scientists believe the fault to be capable of earthquakes as large as magnitude 8, but none that large have been recorded in historically. As this fault is a right lateral strike slip fault it is a very strong indicator of how the San Andreas fault will react to similar seismic activity. The streams and glacial moraines crossing the fault have been offset which are great indicators of seismic rebound and are notches measuring the long term displacement of the fault. This small incident proved to be quite harmless in its remote location, this earthquake did not directly damage property or harm lives, in contrast the Northridge earthquake which was placed in the center of a highly populated area of California caused death and billions of dollars in damages.
Denali Seismic Activity #2
Denali Seismic Activity #2

Earthquake parameters for Nenana Mountain Earthquake:

Origin Time (UT): 10/23/2002 11:27:19.43
Latitude: 63.5144N
Longitude: 147.9116W
Depth: 4.2 km
Magnitude: mb 6.7, MS 6.7, MW 6.7

Earthquake parameters for Denali Earthquake:

Origin Time (UT): 11/03/2002 22:12:41.52
Latitude: 63.5141N
Longitude: 147.4529W
Depth: 4.20 km
Magnitude: MW 7.9, MS 8.5, ML 7.2, mb 7.0

Denali Earthquake Effects-

The Denali earthquake ruptured along a stretch of 209 miles worth of fault line. As this area is sparsely populated and extremely rugged no actual lives or property was every actually in danger of being harmed. Although the earthquake did slightly damage the Trans-Alaska Oil Pipeline it did not break or harm the actual contents within. As this pipeline carries approximately 17% of the US oil this would have lead to major economic and environmental issues.
As this earthquake produced violent heavy shaking it caused many steep mountain sides to slide. These landslides were compiled of land and ice which toppled from mountainsides filling in open valleys. The only form of infrastructure that seemed to be damaged in the earthquake was the runway of Northway Airport. This airport was located 40 miles away from the actual earthquake itself but it felt the shaking through damages to the runway in the form of lateral spreading and sand boils. These boils were caused by what is know as liquefaction where the ground shaking causes the soil to turn to a liquidfied form of itself. This same sort of liquefaction was also seen near other smaller rivers and what is also know as Tanana River Valley. Lastly the shock waves from this earthquake were felt as far as 2000 miles away causing for homes on the water near Seattle to fall off of there foundations. As well as creating a surge of seismic activity to flow into the Yellowstone area which is also known to be a caldera the most explosive and dangerous form of volcano.
Landslide from a 7,000 foot high peak in Alaska Range, less than 10 miles from the Trans-Alaska Oil Pipeline.  Triggered by the 2002 Denali Fault earthquake.
Landslide from a 7,000 foot high peak in Alaska Range, less than 10 miles from the Trans-Alaska Oil Pipeline. Triggered by the 2002 Denali Fault earthquake.


Conclusion-

The most important idea that can be pulled away from what has been learned from the Denali earthquake is most certainly how these sort of faults work. A greater understanding of how these smaller earthquakes trigger much larger events will provide great insight into how we can protect ourselves from the destructive forces of nature. If a greater emphasis is placed on understanding the reactions and actions of this sort of fault line we as Californians can be much more ready for when the San Andreas fault finally begins to shake.

References:

  1. http://abag.ca.gov/bayarea/epmaps/fixit/ch2/sld003.htm
  2. http://www.accessscience.com/abstract.aspx?id=YB051120&referURL=http%3a%2f%2fwww.accessscience.com%2fcontent.aspx%3fid%3dYB051120
  3. http://www.aeic.alaska.edu/Denali_Fault_2002/
  4. http://www.aeic.alaska.edu/Nenana_Mnt_2002/
  5. http://pubs.usgs.gov/fs/2003/fs014-03/
6."Rupture in South-Central Alaska-The Denali Fault Earthquake of 2002 | USGS Fact Sheet 014-03." USGS Publications Warehouse. Web. 18 Mar. 2010. <http://pubs.usgs.gov/fs/2003/fs014-03/>.