I married in to a family of pilots. One of the perks of this is that once in while they’ll humor me and fly me around so that I can see Southern California from the air; more specifically, I can see the amazing faults and stunning geomorphology of Southern California.
Yesterday we flew along the Elsinore and San Jacinto Fault zones. These faults are some of the many faults (including the San Andreas) that are helping to accommodate motion between the Pacific and North American Plates.
I’ve roughly annotated the photos below (on my phone, while in the air) so that the fault location is more obvious for those who aren’t used to seeing such features in the landscape. It’s important to keep in mind that faults are rarely just one simple strand; they are fault zones. Fault zones are broad zones of deformation with many fault strands and areas of crushed and deformed rocks – some fault zones can be km wide. So my annotations are gross oversimplifications that are showing the rough location of the most prominent strand that I could identify from the air.
One more quick note – geologists locate faults by looking for particular things in the landscape. The faults I looked at were right lateral strike-slip faults, which means that one side of the fault is sliding horizontally past the other so that everything on the other side of the fault looks as if it moved to the right. The landscape features (or geomorphology) that I was looking for were offset streams, changes in the slope of the land, offset ridges, lines of vegetation (water moves easily through the broken rocks of the fault so trees often grow along faults) and other changes in the shape of the land that are associated with faulting. When you see many of these types of features and draw a line between them we call that the fault trace, or the place where the fault intersects the surface of the earth. Not all faults make it to the surface – these are called blind faults. The Northridge earthquake occurred on a blind thrust fault.
All of the maps are from the USGS Quaternary Fault and Fold Database. The lines are the faults; the different colors indicate the last known activity on that fault (more info available on the USGS website linked above). Solid lines means geologists are very confident that the fault is in that location, whereas dotted lines are places where they have inferred that the fault is likely to be.
The Elsinore Fault
A sag pond is a common feature found along strike-slip faults. When there are two strands of a strike-slip fault that are parallel to each other they will sometimes pull apart a little bit, forming a depression that fills with water – this is the sag pond. In other areas parallel (or sub-parallel) strands may squeeze together a bit, and this forms a pressure ridge (sometimes called a shutter ridge). A famous pressure ridge is Dragon’s Back along the San Andreas Fault.
The San Jacinto fault
When you talk about earthquakes and faults in Southern California you hear a lot about the famous San Andreas, but all of these faults (and many many more) are capable of producing damaging earthquakes. To learn more about how to prepare for earthquakes visit these great sites – Shakeout, Putting Down Roots in Earthquake Country, and Ready.
Thanks again to Fred and Scott for flying us around and giving me the opportunity to see these amazing features from the air! And thanks for reading!