This is a two part article
Two large tectonic plates are engaged in a gigantic multi-million year struggle to slide past each other. As the North American Pacific Plate collides with the North American Plate along the San Andreas Fault, mountains are formed on both sides of this fault. On the Pacific side, the Santa Monica Mountains and San Gabriel Mountains are squeezed upwards, while on the North American side the Sierras and San Bernardino Mountains are also squeezed upwards, where the two plates collide. In fact, we ride north at a breakneck pace of 1 “-2” per year, past Palmdale. But it wasn’t a smooth ride as both horizontal and vertical plate movements occur in jerk and takeoff movements that we experience as periodic earthquakes. Earthquakes occur along fault lines because this is where the relative movement between different land masses takes place. In our area, earthquakes occur along both the long San Andreas Fault (think San Francisco 1906) and a variety of shorter faults (Newport Inglewood, Hollywood, Palos Verde, Puente Hills, etc.) that are essentially more or less parallel to the main fault act one.
How often do major tremors happen in Los Angeles?
With about a century of modern instrumentation and records, we can see that in Southern California, periodic large earthquakes occur irregularly about every 11 years:
- 1933 Long Beach – 6.4 (120 dead)
- 1952 Frazier Park – 7.5 (12 dead)
- 1971 San Fernando 6.7 (65 dead)
- 1992 Yucca Valley – 7.3 (3 dead)
- 1994 Northridge – 6.6 (57 dead)
- 1999 Joshua Tree 7.1 (no deaths)
- 2010 Delta (Baja) 7.2 (2 dead)
Also from this simplified graph, we can see that Santa Monica was extremely lucky in two ways. First, the larger (over 7.0) most dangerous earthquakes in Southern California have all occurred far from the heart of the Los Angeles metropolitan area, and second, we have not had a dangerous (6.0+) quake in the last 27 years since the Northridge earthquake the proximity that many of our readers have personally experienced. There are three additional considerations that can aid our understanding of seismics. First, the magnitude of earthquakes is logarithmic, since every increase of one represents a quake ten times stronger. For example, a quake with a rating of 7.0 is 100 times more powerful than a quake with a rating of 5.0. Second, the longer the fault, the more potential it has for a larger earthquake. And third, in general terms, if there are no perverse soil conditions, the distance reduces the risk. As you can see from the graph, the death rate from large quakes in sparsely populated areas is much lower than from weaker quakes in the vicinity. Even if a large earthquake (the Big One at 8+) occurred along the long San Andreas Fault in Palmdale (55 miles away), it may not do as much damage to Santa Monica as a narrower earthquake of 6.4 up the shorter Newport-Inglewood Fault in Long Beach, which is 25 miles away. For example, that mistake destroyed Santa Monica High School 88 years ago.
Eventually, new faults are constantly being discovered as engineers and geologists better understand the complex geology of the basin. For example, the Puente Hills fault was not identified until 1999. Hence, in our complicated geology, it is practically certain that it is somewhere in the LA Basin. And that such a new fault, depending on its discovered length, could have a potential earthquake of 6.5.
Yes it is our fault
Our next mistake is the Santa Monica Fault, which runs from Beverly Hills / Hollywood along Santa Monica Blvd and runs offshore to Malibu and beyond. It represents the geological boundary between the Santa Monica Mountains and the Los Angeles alluvial basin. The center of this error run is quite confused. Starting at Santa Monica Blvd and Centinela, it appears to split into a fan of three (or more) short fault segments to the west that have spread to a width of about half a mile when this fault complex joins the ocean at Santa Monica Canyon. Note that this “fan” covers a good portion of our core Mid-Wilshire with many large buildings. Fortunately, our two regional hospitals are a bit outside of this fan area, but certainly in the earthquake risk area.
The good news is that, as we currently understand, this Santa Monica Fault appears to be seldom active, moving only about every 1,000 to 8,000 years. Given this large probabilistic spread, it could be postponed tomorrow, but most likely it is less of a risk to our city compared to the many other repeat offenders nearby like the Newport-Inglewood Fault, which keep up a steady drum beat of daily small tremors that are coming soon this could result in a major quake.
Can we predict earthquakes?
Earthquake predictive science is not yet advanced enough to be able to tell exactly when a quake will occur at a particular fault, let alone those we know nothing about. But the historical records, rising over 200 years, as well as massive geological drilling, modeling and mapping lead us to believe with credibility that a quake of a certain size is certain to occur along a certain fault every X,000 years. Although we do not have a certain date as to when a certain error can be postponed, we do have a probabilistic certainty that helps us to assess the risk over a long period of time.
Paradoxically, as soon as an earthquake starts, we have predictability for nearby areas in one place, since earthquakes generally generate two types of seismic waves that travel at different speeds: P (primary waves) arrive first, while S (surface waves) each other move 1.7 times slower. The S waves are the ones that damage buildings because they travel across the surface of the earth like a wave on the ocean. The bottom surface actually moves up and down and horizontally like a wave in water. The state of California uses a network of sensors to pick up the first P waves of an earthquake, which can then tell your cell phone, for example, that S waves are on their way and will arrive around 40 seconds later. This response time enables people to dodge tall objects, exit buildings, turn off the street, step off ladders, and take other defensive measures. Again, the distance from the source of the earthquake disturbance provides security as it increases the time interval in which people must react defensively when the fully developed and deployed sensor system sends its urgent earthquake notification.
Given the probabilistic spread of earthquakes over thousands of years, there is no “overdue” event for a major earthquake due to a particular fault. For our current level of knowledge, the pattern is too random and irregular. It’s like trying to predict exactly when a particular car will break down. However, if there are enough cars around, you can be assured that around X cars will have to be towed every year. Likewise, based on our current knowledge, we in Southern California do not know when a particular fault will cause an earthquake. But the statistical aggregate of the dozen of faults in the basin with their respective histories lets us know with a high degree of certainty that we are “overdue” for a large (6+) somewhere in Southern California. In the past 77 years we have had 7 6+ earthquakes, or about one every 11 years. The smallest interval in our admittedly small sample between large quakes was 2 years and the largest was 21 years. The last big one was in 2010, and that was 11 years ago: exactly on the statistical average for the next one. Therefore we are now in the statistically highly probable zone in which we can expect a major earthquake soon.
Next week’s SMart article is about preparing for our next big quake.
By Mario Fonda-Bonardi for SMart Santa Monica Architects for a Responsible Tomorrow
Thane Roberts, Architect, Robert H. Taylor AIA, Ron Goldman FAIA, Architect, Dan Jansenson, Architect, Building and Fire Safety Commission, Samuel Tolkin Architect, Mario Fonda-Bonardi, AIA, Planning Commissioner,
Marc Verville MBA, CPA (inactive), Michael Jolly, AIR-CRE.
For previous articles see www.santamonicaarch.wordpress.com/writing