Science Spotlight

Station P227

Researcher: Jessica Murray
United States Geological Survey

It was very important to me for my work to have direct relevance to the public at large.

P227 is on the North American side of the North American - Pacific plate boundary.

State: CA
Country: United States
Elevation: 707.4 m
Lat/Long:  37.533 / -121.7896

Alum Rock Earthquake

Station P227 is located about 5 km northeast of the Calaveras Fault, and about 65 km southeast of San Francisco (Figure 1). A plate boundary is generally made up of many faults covering a broad zone that can be 100s of kilometers wide. In the San Francisco Bay Area, there are three major faults (the San Andreas, Hayward, and Calaveras) in addition to several smaller faults. There are even faults in eastern California, Nevada, and Utah that accommodate some of the relative motion between the Pacific and North American plates.

Most of the time, station P227 moves steadily relative to North America because it is within this plate boundary zone (Figures 1 and 2). However, on October 31, 2007, a Mw 5.4 earthquake occurred on the Calaveras Fault. An earthquake of this magnitude causes very small movements of the Earth's surface, but P227 and stations near it were in the right place at the right time to record it!

P227 jumped to the southeast due to fault motion (called coseismic fault slip) at the moment the earthquake occurred (Figures 3 and 4). Then the station continued to move slowly over the next couple of months because of continued fault motion called afterslip (Figures 3 and 5). Usually the amount of afterslip is small compared to the amount of fault slip during the earthquake, but in this case the GPS data show that the amounts were similar. Scientists are still working to understand what controls the amount of afterslip.

Using the GPS data, my colleagues and I estimated the location and amount of fault slip and how much this slip increased or decreased stress on nearby faults. Understanding how stress changes due to one earthquake might influence the likelihood of a future earthquake is an area of active research.

Figure 1. Map of the San Francisco Bay Area. The three major faults are shown by the red lines. Each blue arrow shows the velocity of a GPS station relative to the center of the U.S., which is on the North American plate outside the plate boundary zone. The black arrow indicates the velocity scale. Station P227 is shown with the black dot, and the yellow star marks the epicenter of the October 31, 2007 Mw 5.4 earthquake. SF marks the location of the city of San Francisco.

Figure 3. This figure again shows the position of P227 over time, but the linear trend has been removed from the data to make it easier to see the changes that happened at the time of the October 31, 2007 earthquake and in the four months following it. The green lines show the offset due to the coseismic fault slip, and the portion of the time series after the earthquake shows how the afterslip caused continued motion at P227. As you can see, the station moved much more in the north-south direction than the east-west.

Figure 5. This map shows how much the GPS stations near the epicenter moved due to afterslip during the four months following the earthquake. The relative magnitudes of displacement among these stations is different from the coseismic displacement shown in Figure 5, but the largest afterslip displacement at any station is still about 4 mm.


Figure 2. This figure shows the north, east, and vertical positions of station P227 since 2006 relative to the North American plate. Most of this time P227 has moved steadily, as is shown by the nearly linear change in position with time. (For help interpreting the graphs, see the GPS Data page.)

Figure 4. This map shows the direction and amount that GPS stations near the epicenter moved due to the coseismic fault slip (i.e., during the earthquake). The vector scale shows that the largest displacement at any station was only about 4 mm.

Spotlight Questions

  • What direction is P227 moving relative to North America? At what rate? What sense of relative motion would you expect on strike slip faults in the Bay Area?
  • How much did P227 move during the Alum Rock earthquake? In what direction? Is this direction what you would have predicted based on the sense of relative motion you answered in Question 1?
  • How much did P227 move during the four months following the earthquake? Did it move in the same direction as it did during the earthquake? Did it move at a constant rate?
  • Can you find examples of displacements due to coseismic slip and afterslip in time series from other PBO stations?

Last modified: 2019-12-26  16:24:52  America/Denver  


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