Science Spotlight

Station GRNR


Researcher: Jeff Freymueller
University of Alaska

I think I became a scientist because I never stopped asking questions about everything, never stopped being curious and wanting to figure things out.


The antenna for GRNR is in the foreground. Its upgraded replacement, site GRNX, is in the background.


Name: Garner
State: AK
Country: United States
Elevation: 596.1 m
Lat/Long:  63.8358 / -148.9783

2002 Denali Fault Earthquake

On November 3, 2002, the Mw 7.9 Denali Fault earthquake struck central Alaska (Figure 1). It ruptured for over 300 km. Although only one person was hurt, the shaking damaged roads and structures and was felt as far away as Seattle. The maximum measured offset was 8.8 meters.

There are three main interesting features in the data from GRNR (Figure 5). The most obvious one is the displacement from the Denali fault earthquake. The site moved southeast by 58 mm as a result of the earthquake. If you look closely at the time series, you can see two jumps. The first one is a smaller foreshock of magnitude 6.7 that occurred about two weeks before the mainshock.

After the earthquake, you can see a clear curvature of the time series, especially in the north component. For a few years after the earthquake, the site moved southward at a faster rate than it had before. This is a sign of postseismic deformation, or deformation of the Earth caused by transient processes that were triggered by the earthquake. By the most recent data, the postseismic deformation signal seems to be almost gone, and the rate of motion is close to what it was before the earthquake.

The third interesting thing about these data is in the characteristics of the noise (or scatter) in the data. In the north and vertical components, you can see that some days fall well below the general cluster of points, but there are almost no such points that fall well above the main cluster of points. (They look almost like strings of beads hanging down). I found that these outliers in the north component are the same days as the ones in the vertical component, and the two are usually proportional. This means that some days our position solution is biased by something, and the bias is almost always in the same direction in space, southward and down. I think this is due to a large change in the properties of the atmosphere north and south of the Alaska Range under certain weather conditions. GRNR is on the north side of the range, and the north side is quite dry. The south side is much wetter, and I think that these distinctive biases result from atmospheric models used in the GPS processing that are not good enough, and don't allow for such a dramatic change over a short distance. I hope to test that hypothesis in the future, which will require looking into the way we model the atmosphere more closely.

For more information on this earthquake, see the USGS Historical Earthquakes page.


Figure 1.Tectonic background of the Denali Fault earthquake. Arrows indicate motion in cm/yr. Location of GRNR is shown in cyan. The white line is the Trans Alaska Pipeline and the rupture is in gray. Colors associated with seismic hazard levels are shown in the legend. Original figure: Peter Haeussler.


Figure 3. Offset near the point of maximum displacement. The edge of the Jack Creek stream bank on the far (northern) side of the fault was offset 8 m to the right with respect to the edge of the stream bank in the foreground (between the geologist on the left and the small tree). The north side also was offset about 1.5 m upward with respect to the south side. (Credit: Wes Wallace).


Figure 5. Position changes for GRNR in a North American fixed reference frame. (For help interpreting the graphs, see the GPS Data page.)

 

Figure 2. The fault trace on the Richardson Highway, about 100 km from the epicenter. The horizontal offset is approximately 2 to 2.5 m. The Denali earthquake ruptured 300 km of the surface of the Earth. (Photo credit: Dr. Akihiko Ito)


Figure 4. Landslide triggered by the Denali Fault earthquake onto the Black Rapids glacier. (Credit: USGS)

Spotlight Questions

  • Why does the ground sometimes rupture far away from an earthquake's epicenter?
  • What direction did GRNR move during the earthquake? What does this tell you about the direction of stresses on the fault? Find information about the fault online to see if you were right.
  • What might be the cause of the "hanging beads" pattern in the GPS data? Explain how the atmosphere can affect GPS signals. Is this information just "noise," or can we learn something from it?

Last modified: 2019-12-26  16:24:56  America/Denver  

 

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