May 2, 2016
by Shelley Chestler
Earthquakes happen on big cracks in the ground called faults. There are all kinds of faults. The biggest faults in the world are those on the edges of tectonic plates. Our local, giant fault is the Cascadia Subduction zone where the oceanic Juan de Fuca plate dives beneath the continental North American plate. Another large plate-boundary fault is the San Andreas Fault, where two plates slide past each other rather that moving towards each other like in Cascadia.
Smaller faults occur within tectonic plates where the Earth is stretched or compressed. For example, if you take a bucket full of sand and push on the sides of the bucket, cracks will form in the sand. These are miniature faults. If you took the same bucket and pulled out on the sides, you would also get miniature faults. Like a rubber band, the Earth can only be stretched so far before it breaks. The Seattle fault, which runs from Bainbridge Island, across the Puget Sound, and through West Seattle and Bellevue, is an example of a fault formed as a result of the Earth horizontally compressing.
USGS scientist Brian Sherrod provided the fault locations that we now show on the PNSN map. How did he and his colleagues find the faults? Sometimes you can see faults at the Earth’s surface. Finding faults this way involves lots of fieldwork hiking around and looking for evidence of faulting. Another technique, called LiDAR (Light Detection and Ranging), involves shooting lasers at the ground to precisely measure surface elevation. By looking at maps made with LiDAR, scientists can identify faults that are not immediately obvious out in the field.
PNSN recent event page with faults displayed. To display faults, go to the “Control Panel” and check the “Display Faults” box. The Cascadia Subduction zone and Seattle Fault are labeled here, but are not labeled on the actual map.
Unfortunately, both looking for faults in the field and LiDAR surveys have limitations. Copious amounts of vegetation, such as we have in the Pacific Northwest, can cover up evidence of faulting. In addition, earthquakes often happen on faults that are buried underground and leave no expression at the surface. For example, the Nisqually earthquake in 2001 happened on a fault 30 miles deep. While we knew that Nisqually-type earthquakes had happened near there in the past, we did not know the location of the fault that the earthquake occurred on. Even now, we do not know the fault’s precise dimensions or where other similar faults are located in the region.
Image of a fault near Entiat, Washington. Here, scientists dug a 4-foot trench to look at the angle of the fault beneath the surface. Only the most significant faults are trenched—this fault is a candidate for the fault that caused a large earthquake in 1872. Photo courtesy of Brian Sherrod.
Another complication is that faults can look different at depth than they do on the surface. A fault that looks like a single crack at depth might break up into multiple faults near the surface. This is called a fault zone. In addition, faults can be tilted. Therefore the line drawn on the map from the surface expression may not represent where the fault is at depth.
Many of you are probably wondering why many of the earthquakes on the map do not lie on faults. This is a direct result of the problems discussed above—our knowledge of faults beneath the earth’s surface, where earthquakes occur, is limited. Plus, even if we knew where all the faults were, we still could not predict when each fault would produce an earthquake.
The new “Display Faults” tool is a great way for you to learn more about the geology of the Pacific Northwest, but before you start exploring the map, here are some final caveats:
1) The faults shown are the ones we know about, and not all the faults that exist,
2) Only the faults in Oregon and Washington are on the map, and
3) Because the earthquake locations are only approximate and fault locations are sometimes uncertain, we do not want you to over interpret how close your house might be to a fault, or try too hard to identify earthquakes with specific faults. Therefore the faults do not show up when the map is the most zoomed-in.
 Sherrod used several other fault compilations to help map the fault lines. These include the USGS Quaternary Fault and Fold Database, maps by the Washington Geology and Earth Resources Division, and maps by the Oregon Department of Geology and Mineral Industries. Mapping faults is difficult, and scientists often interpret where faults are differently. Therefore, don’t be surprised if these other fault maps look different than the one on the PNSN webpage.