Ridgecrest California earthquakes have dominated many lives and much of the news over the Independence Day weekend of 2019. I own a home in Ridgecrest though I am currently in my residence in Alaska. I have been inundated with many phone calls, texts, and social media messages asking how our lives and property are faring. Too many to allow me to respond and have any time left in the day. I would like to take the time and opportunity to share what I have heard, and what I know. I hope this will make an interesting story for friends and neighbors. As a retired earth-science teacher I also want to share what I know, understand, and believe about the evolving geology of the region.
July 4, 2019
My phone rings. It is my friend who lives in Bakersfield, approximately 100 miles to the west of Ridgecrest across the Sierra Nevada Mountains. Mike tells me his house has been shaking for about 30 seconds and he has heard the epicenter of the earthquake was between Ridgecrest and Trona measuring 6.6 on the Richter scale. (Subsequently reevaluated at 6.4). An earthquake of such a magnitude is considered to be a strong quake and severe damage is anticipated. Emails and messages start coming in as I try to located local friends and neighbors who might be able to look in on the property. Most folks I know are away for an extended holiday weekend.
My main concern regards a 5000 gallon water tank on the property (we have 2.5 acres with a significant irrigation system). I want to make sure it is still standing and undamaged and that I have not developed waterfront property. I am thankful that prior to leaving for the season I shut off water and gas to the house. The house is a double-wide mobile home that has been remodeled to look like a house. When we did the remodel we refitted the “foundation” (or lack of) with jack stands that clamp onto the home’s i-beams and rest on secured skid plates. a new roof covered the unit with trusses.
I am able to locate a friend who has not left for the weekend yet and she graciously agrees to go give our property an inspection. No signs of water leaking anywhere. For the most part the house looks fine. Apparently the front door is unlocked (the wife and I are sure we have locked it) and a large cabinet near the door has fallen and largely blocked access through that door, but peaking inside, everything looks reasonable and the other doors are locked.
I am also able to locate a contiguous neighbor who is out of town but returning later that night and he will also give the property a check.
July 5, 2019
Neighbor Frank calls me in the morning as he walks the property and largely affirms earlier reports from friend June. He is able to text along a few pictures of the exterior and interior. I am surprised how much is left standing in the house. Looks like we will need to replace a door which has been damaged by the shaking. We suspect the shaking and impact of the falling cabinet jarred the front door loose, and Frank locks that up for us.
I receive another call from Frank later in the day. Apologies for the pun, but he is apparently very shaken. He exclaims that he just experienced another quake that was much stronger and lasted many minutes. Furnishing in his home that were left standing after the first earthquake were now flying and falling. Reports have come in that this new quake measures 7.1 on the Richter scale. This magnitude is considered to be major, and damage is anticipated to be widespread and heavy. (As a means of comparison, the Northridge earthquake of 1994 measured 6.7) This 7.1 temblor is 5 times the strength of the previous day’s 6.4 shake, which is now considered to be a fore-shock.
I hear reports of fires in town, power and gas outages. Pacific Gas & Electric sends me a recorded call advising me that if I had turned my gas off (yes!) do not turn it back on before allowing them to check systems. Frank gives the property an inspection the next morning and I am once again relieved. There is more on the floor, but less than I would have anticipated from what we can see. Structurally things seem to be intact.
Little damage reported
Now on to geology.
I am listening to the news on NPR, and reading reports on the internet. The general consensus is that given the strength of the earthquakes (and I have heard numbers of hundreds (>500?) of aftershocks (many around 5.5 magnitude)) there has been relatively little damage.
Ridgecrest California is a quiet, well mannered community of around 29,000 people. The town is located in the Indian Wells Valley in the Mojave desert, on the southern end of the eastern Sierra Nevada mountains. In the first place there is not a lot that can fall on you. We have very few tall structures. There are no underground parking lots. We have no major overpasses on crowded freeways.
The Indian Wells Valley is in the very southwest corner pocket of the Basin and Range geomorphic province. The ground of the valley is largely alluvium (granular sand largely composed of quartz and feldspar) shed from the granitic mountains of the Sierra Nevada. The water table is often hundreds of feet below the surface. There is little chance of liquefaction. Soil liquefaction occurs when a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake, in which material that is ordinarily a solid behaves like a liquid. To put that in (hopefully) simpler terms, the shaking and rolling of the recent Ridgecrest earthquakes took place in a somewhat competent and coherent fashion with everything moving in a relatively similar direction at a given time, rather than being torn apart by the ground moving every which way at once, which often occurs during liquefaction. A good example of such damage would be the marina in San Francisco during the Loma Prieta earthquake of 1989
Tectonics: a bigger picture
It has been jokingly said that Ridgecrest California has its faults. The Indian Wells Valley is riddled by faults. The Valley is wedged between the Garlock Fault to the south and the Sierra Nevada Frontal Fault to the west. The Garlock Fault is a left lateral strike slip fault that plays a significant role in the overall geology of Southern California. (More information on Digital Desert and Wikipedia.) The Sierra Nevada Frontal Fault is a normal fault (meaning the mountains rise and the valley sinks along the fault). Being located adjacent to these two major players, the fault map of the Indian Wells Valley resembles a shattered piece of safety glass. The valley has been majorly torqued over time. Given the frequency of minor earthquakes in the Valley and the physics that explain how seismic waves loose energy with each fault they cross, I was surprised at the magnitude of the recent quakes.
Ridgecrest has also been called the earthquake capital of world due to the frequency of relatively small seismic events that occurs regularly. This frequency and distribution led me to speculate years ago that the tectonic regime of California was (is) undergoing a significant change.
The San Andreas Fault is a transform fault. This means that it defines the boundary between the Pacific and North American tectonic plates. To give a very brief history of a long process, the North American and Pacific plates used to converge, traveling in opposite directions. About 30 million years ago, the North American Plate ran over the Pacific Plate’s spreading center resulting in a transform boundary. This transform boundary has been expressed in the strike-slip fault of the San Andreas Fault. Now both plates move in a westerly direction, albeit at different speeds and somewhat different trajectories.
Given the North American Plate’s continued movement to the west, the relative position of the relic spreading center (now transform fault) should be moving to the east. My observation on SCEC’s maps of a regular lineament of small scale seismic events along a long stretch of hitherto unidentified faults in southern California and up through the Indian Wells Valley suggests to me that the transform fault system may well be moving up into our valley and along the east side of the Sierras. A few years back Dr. Montesero, a geophysicist working at the China Lake Naval Weapons Center at Ridgecrest came to the same conclusion. Rather gratifying.
Poetic and prophetic?
Without getting into a lot of structural geology and geophysics, suffice it to say the tectonic regime of western North America is undergoing a dynamic shift over millions of years. (Geology happens). There is a new tectonic spreading center developing, and it is called the Gulf of California. This has caused the Baja peninsula to separate from the Mexico mainland. It is essentially an unzipping of continents as new plates evolve. The unzipping continues from south to north. This unzipping is aligned rather well with the lineament I have referred to that runs up southern California toward Indian Wells Valley. Will this new spreading center unzip through Indian Wells Valley? Geologists call these spreading centers “mid-ocean ridges.” They are also popularly known as …. “ridge crest’s.”