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U of U Seismograph Stations Research:

The Earthquake Clock on the Wasatch Fault




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The Wasatch Fault

The Wasatch Fault is the largest fault of its type in the world. Like all active faults in Utah, the Wasatch Fault was created and is still active because the Earth's crust is being stretched, or extended, in an east-west direction. For more information on why earthquakes occur in Utah, see the "Active Fault Information" page.

We know there have been many large earthquakes on the Wasatch Fault in the past. In many places there are fault cilffs. One place these cliffs are visible is the mouth of Little Cottonwood/Bells Canyon in southern Salt Lake County. The Utah State Geological Survey has mapped the exact location of the Wasatch Fault in Salt Lake, Davis, Weber, and Utah counties. Click on the county you are interested in to view these maps.

The earthquake clock, or time interval between earthquakes, for the Wasatch Fault is a very large unknown. The Wasatch Fault is unusual in that it rarely has earthquake activity, a condition seismologists call "aseismic" or without earthquakes. Earthquake monitoring at the University of Utah Seismograph Stations since 1960 shows that there is very little earthquake activity related to the location of the Wasatch Fault.

The reason for the lack of small to moderate earthquakes is not known. Some faults do have small earthquakes on a fairly regular basis. These frequent small earthquakes may be preventing the build up of large forces (strain) in the rock, thus preventing large earthquakes. There is concern that this is not happening on the Wasatch Fault and thus the fault is "locked" and that a significant amount of strain is building within the rocks which will ultimately result in large earthquakes.

The last earthquake which may have occurred on the Wasatch Fault is the magnitude 4.3 earthquake which occurred under Herriman, UT in 1992. This earthquake occurred at a shallow level and instrumentation limitations prevented the positive identification of this earthquake happening on the Wasatch Fault as opposed to a nearby "blind" fault. It is certainly possible that this earthquake occurred on the Wasatch Fault, but movement on another fault cannot be ruled out.

Pinpointing exact locations for earthquakes is a common problem throughout the world. When an earthquake happens, the location in the Earth's crust is determined by the triangulation method. (This link is a series of activities where you can learn how to locate an earthquake by using seismograms). The triangulation method has uncertainty associated with it that is reduced when a large number of seismograph machines are available to record an earthquake from several different directions. It is rare to have sufficient seismograph coverage to pinpoint the location of a single earthquake in the Earth's crust so that there is small uncertainty and so that the earthquake can be assigned to a particular fault with confidence. This type of seismograph coverage is expensive and is currently not available in Utah.

The best way of locating which fault has moved is to track a whole series of earthquakes that occur on the fault. For instance, after the magnitude 6.7 Northridge California earthquake, there were hundreds of aftershocks that followed the main shock. These aftershocks occurred for several months, but were most frequent in the days following the main shock. Seismologists recorded the swarm of earthquakes using portable instruments brought in for this purpose and then used the triangulation method to locate each one. When the locations of the entire set of earthquakes were plotted, the set outlined a planar feature in the Earth's crust which was the fault that had moved.

For a diagram of Northridge aftershocks outlining fault plane

http://www-socal.wr.usgs.gov/mori/north.html

For an animated view of the Northridge aftershocks (need JAVA)

http://www.scecdc.scec.org/Mpegs/smallavs.mpg

Because the Wasatch Fault has very few earthquakes and because the earthquakes that seem to occur on this fault have been small and isolated (single) events, it is very difficult to determine if the earthquake actually occurred on the Wasatch Fault or on a nearby "blind" fault. Seismologists and technicians at the University of Utah Seismograph Stations stand ready to rapidly deploy portable instruments in the case of a moderate to large earthquake in Utah, so that they can use the same technique used at Northridge to answer the question of "Which fault moved in the earthquake?"

So... What Is the Earthquake Clock for the Wasatch Fault?

There is no earthquake clock for small to moderate earthquakes on the Wasatch Fault. The lack of identified earthquake activity on the Wasatch Fault means that seismologists cannot determine the "earthquake clock" for small to moderate earthquakes on this particular fault. The danger from these earthquakes is real, but seismologists cannot accurately determine the risk. There is however, an earthquake clock for the entire Wasatch Front region that has been determined by various seismological data. There is a somewhat more precise clock for large magnitude (>6.5) earthquakes on the Wasatch Fault. This clock is determined by much different methods that are explained on the "Paleoseismic earthquake clock" page. This technique studies large prehistoric earthquakes in order to determine their recurrence interval and assess the future risk of large earthquakes on the Wasatch Fault

 



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