Flying the faults of Southern California

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.

Many thanks to my husband Scott and father-in-law Fred for flying me around to see the faults.

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

Lake Elsinore is a sag pond that was formed by multiple strands of 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.

Lake Elsinore
Lake Elsinore
This is where the I15 crosses the Elsinore Fault.
The following images are of a more southern location of the Elsinore Fault near Palomar Mountain.
Lake Henshaw, just southeast of Palomar Mountain. This is another sag pond.
The Elsinore Fault at Lake Henshaw. The terribly drawn arrows show the relative direction of motion of each side of the fault (right lateral).
The Elsinore Fault
The fault is running through the valleys (which it helped to form).

The San Jacinto fault

The photos below were taken while we were southwest of the fault looking northeast.
San Jacinto Fault near Hemet
The San Jacinto fault near Hemet (the Hemet airport runway is in the foreground).
The following pictures were taken looking NW (over the left wing) from around the location of the blue dot as we flew northeast.
Do you see what looks like a line going across the center of the picture? That’s the fault. If you look closely you can see vegetation growing along the fault, a change in slope and offset channels.
A different view of the fault shown in the photo above
Such beautiful geology and geomorphology along the San Jacinto fault!
The next few photos are of the San Jacinto Fault near Borrego Sorings. As you can see from the map this area is very tectonically complex.
San Jacinto fault north of Borrego Springs
San Jacinto fault strands NE Borrego Springs.
San Jacinto fault
The following images were taken near Warner Springs. I think the fault is one of several fault strands that the USGS Fault and Fold database call the San Felipe fault (shown in black).
San Felipe fault
Lots of fault strands in this photo – I just annotated one.

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.

My father-in-law and his copilots

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!

7 thoughts on “Flying the faults of Southern California

  1. Lifelong S.D. County resident. This was fascinating. It will never be the same driving through these areas again.

    Liked by 1 person

  2. Eric Vander Velde August 31, 2019 — 11:11 pm

    Great stuff, great opportunity. Thank you for sharing.

    Liked by 1 person

  3. How about the faults that form Signal Hill? Also the fault that runs under the parking structure of Community Hospital in Long Beach.

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    1. Yes, the Newport-Inglewood fault lies below both of those locations (https://scedc.caltech.edu/significant/newport.html). We only had time to fly a small area of Socal and those locations are much farther to the north. You can find fault maps showing the location of various faults in Southern California (and beyond) here https://scedc.caltech.edu/significant/ and here https://local.scedc.caltech.edu/significant/losangeles.html

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  4. This was so informative. Thank you

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  5. Thank you so much for this fascinating tutorial!
    As a second gen native Californian, I grew up with a mother who taught us that earthquakes were part of living in our state. She was born & raised in Brawley & work on my grandparents’ ranch wasn’t interrupted just because of a little shaking. This was in the early 1900’s.

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