Science Spotlight

Station P612

Researcher: Sally McGill
Cal State University San Bernardino

P612 is located on the campus of California State University, San Bernardino.

State: CA
Country: United States
Elevation: 531.7 m
Lat/Long:  34.1874 / -117.3155

San Andreas Fault

P612 is about 460 meters southwest of the San Andreas Fault. A number of stream drainages are offset right-laterally by the San Andreas Fault in this area, as can be seen in Figure 1. Both the offset drainages and the GPS time series (Figure 2) show that P612 is moving north and west relative to the North American plate. Figure 3 shows the motion of P612 in the context of other continuously monitored GPS sites in southern California. The lengths of the arrows represent the speed of each site relative to the stable interior of the North American plate. In Figure 3, you can see that there is NOT an abrupt change in the speed of the sites as you cross the San Andreas Fault. Instead, the speed of the sites increases gradually as you cross the network of faults that makes up the boundary between the North American and Pacific plates. In part this is due to the broad width of this complex network of faults. But even if the San Andreas Fault were the only fault in the plate boundary, the arrows would still only change their length gradually in the vicinity of the fault rather than changing their length abruptly across the fault. The reason for this is that the San Andreas Fault has been locked during the entire time that these GPS stations have been operating (for the past 10-20 years). This means that, at the surface (and within the top 15 km or so of the Earth's crust), there has been no slippage along the San Andreas Fault since the last major earthquakes, more than 150 years ago.

At a broader scale, the Pacific plate has still been moving northwestward relative to the North American plate during this time, and this has caused both the Pacific and North American plates to bend in the vicinity of the San Andreas Fault (and of other faults within the plate boundary zone) (Figure 4). In addition, gradual creep along the deep part of the San Andreas Fault (below about 15 km depth) has probably been occurring as the hot rocks at those depths flow past each other along the fault. This also contributes to the bending at the surface of the Earth that we can observe by noting the gradual change in the lengths of the arrows across the plate boundary in Figure 3. GPS data from continuously operating sites can be supplemented with GPS data collected from portable GPS equipment that is set up temporarily and moved from site to site. These additional data give us a more detailed picture of how the surface of the Earth is deforming near the faults. CalState University San Bernardino geology majors and local high school science teachers and their students have helped to collect such GPS data in the vicinity of the San Bernardino Mountains over the past 10 years (Figure 5), contributing to our understanding of the San Andreas Fault in this region (Figure 6).

In addition to telling us about tectonic deformation around the San Andreas Fault, P612 is also being used to study vegetation growth; see this site for the vegetation-related records for P612.

Figure 1. LiDAR image of P612 and the adjacent San Andreas Fault zone. Several drainages that have been right-laterally offset by the San Andreas Fault are visible. Two secondary faults that are related to the San Andreas Fault are also visible in the mountainous region northeast of the San Andreas Fault. The red arrow shows the direction of movement of P612 relative to the North American plate.

Figure 3. Map of southern California showing active faults and motion of sites with continuous GPS monitoring. The length of each arrow represents the speed at which that site is moving relative to the North American plate. Site P612 is marked with a red star and red arrow. Other selected sites are marked with green circles. SBM = San Bernardino Mountains.

Figure 5. High school Environmental Science students set up GPS equipment temporarily over a survey benchmark in Riverside.

Figure 7.Example of how to calculate station velocities from time series.


Figure 2. North, east and vertical position of stations P612 and P610 in a North American fixed reference frame. See Figure 3 for locations of sites P612 and P610. The steeper slopes of the plots for P612 (black), located just southwest of the San Andreas fault, indicates that this station is moving much faster relative to the North American plate than is site P610 (blue), which is on the western edge of the North American plate. (For help interpreting the graphs, see the GPS Data page.)

Figure 4. Deformation across a strike-slip fault at various stages in the earthquake cycle. The process illustrated here is known as elastic rebound. During the time in between earthquakes (2a and 2b) the tectonic plates bend elastically near the fault. When an earthquake eventually occurs, the plates suddenly slip past each other (rebound).

Figure 6. Purple arrows show velocities of survey-mode GPS sites, where undergraduate geology majors and high school science teachers and their students have collected GPS data 4-5 days per year for the past 10 years. These data complement the information provided by continuous GPS stations (blue arrows), adding greater detail to our view of how the tectonic plates are bending in this region.

Spotlight Questions

  • Read about GPS time series and velocity vectors and then see if you can use Figure 2 above to calculate how fast P612 is moving in the north direction and in the east direction. For example, subtract the north position of P612 at the beginning of 2006 from the north position of P612 at the beginning of 2013, then divide by the time it took for P612 to move that far. Do the same thing for the position of P612 in the east direction.
  • Use a ruler to draw arrows in the north and east directions that are proportional in length to north and east velocities that you calculated for P612 from Figure 2. For example, if you estimated that P612 is moving 10 mm/yr in the north direction and -12 mm/yr in the east direction, draw an 10-mm-long arrow pointing to the top of your paper, and then from the tip of that arrow, draw a 12-mm-long arrow pointing to the left, which would represent the westward (or negative eastward) direction (Figure 7). Now draw a diagonal arrow connecting the tail of your north arrow to the tip of your westward pointing arrow. The length and direction of this arrow represent the speed and direction in which P612 is moving. Use your ruler to measure the length of this arrow. How closely does your measurement agree with the velocity for P612 that is given in Figure 3 (25.9 mm/yr)?

Last modified: 2019-12-26  16:24:51  America/Denver  


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