19.2: The Hazard of Place

Highway to the Danger Zone

There has always been an oddity about where populations developed in California. In this mostly arid state where one would imagine water accessibility would influence urbanization. But today where there is plentiful water–the northern regions–very few people reside, while in the parched deserts of the south, endless ranch house clones, festooned with backyard chlorinated swimming pools, stretch to the arid horizon.

California has an exquisitely beautiful coastline, but few people live within sight of the ocean. Some areas of California have exceptional air quality, but population growth is more concentrated in the Central Valley, which the American Lung Association has described as “the most dangerous place in the United States to breathe."

It’s almost as if California developed with no coherent, rational plan. That’s because California developed with no coherent, rational plan. No one is actually in charge. No single authority is empowered to declare, “Enough!” and veto development in inappropriate, unaesthetic, or hazardous places. So growth has been unmanaged, unplanned--and unsustainable.

This aspect of California is not new. California went from under a hundred thousand residents at the Gold Rush to almost four times as many just a decade later. By 1900, California had almost a million and a half residents; the subsequent two decades added two million more. The prewar population doubled by 1960, then doubled again by the 2010s. You would imagine that such a population explosion corresponded with necessary growth in transportation systems, public utility systems, increased numbers of schools and hospitals, and you would be wrong.

You might imagine that a regulatory agency is tasked with protecting residents by considering if growth should be allowed in seismically dangerous areas, and again you would be wrong.

Let’s examine this feature of California from the intersection of population and seismicity, and how it came to be that Californian urban centers tend to be in such dangerous locations.

Other states experience regular earthquakes. Alaska is very seismically active, but has a population of only 733,000. (San Francisco's population is 815,000.) Most of California’s 38 million residents are not found in the open deserts of the Mojave, or the barren lava flows of the Modoc, but rather are concentrated in three major clusters: the Sacramento-San Joaquin Valley area, the greater Los Angeles area, and the San Francisco Bay region (see Figure 19.2.1).

Figure \(\PageIndex{1}\): California population density map, from the USGS Interactive Map. "Population map" by USGS is in the public domain. Access a detailed description.

While Sacramento-San Joaquin Valley is relatively untouched by quakes, the greater Los Angeles area and the San Francisco Bay region sit directly on major earthquake clusters (see Figure \(\PageIndex{2}\)).

Quite simply, most Californians live in the most dangerous locations for earthquakes; moreover, many live in the most vulnerable types of buildings.

Figure \(\PageIndex{2}\): Seismicity of California, from the USGS Interactive Map. "Seismic map" by USGS is in the public domain. Access a detailed description

One place where the intersection of faults and urban population is particularly acute is the San Francisco Bay Area. The most worrisome fault in this region is the Hayward fault, as explained in this video:

Video \(\PageIndex{1}\): Video about the Hayward fault

Watch this video to learn more about the Hayward fault.

Proximity to Faults

Earthquake energy decreases rapidly with distance. In other words, if you are 1 km from an earthquake epicenter, your seismic experience will be markedly different from a person 10 km away, and much different still from someone 100 km away. When an earthquake pops, concentric shells of energy spread out at depth at about the speed of sound. One may envision this like a pebble being dropped into a pond, albeit a rather solid one when it comes to the lithosphere. Many factors can lessen, or attenuate, the quake energy, such as fracturing, friction heating, and comminution (pulverizing rock into smaller particles). While the mathematics of this are beyond the scope of this introductory book, suffice it to say that all else being equal, you want to be as far away from faults as possible, and that ground shaking increases exponentially the closer you are.

This phenomenon can be illustrated in shake maps which plot quake intensity. In 2019, a series of earthquakes occurred near Ridgecrest, a small town neither on a ridge nor on a crest, but rather in the middle of a wide basin in the far southern Owen’s Valley. The intensity of the largest quake (M_w 7.1, 1 death) is mapped in Figure \(\PageIndex{3}\). The color coding is that the light blue and light green show very little motion and are probably undetectable for most people; conversely, the red and orange zones show intense shaking. As this diagram shows, the area very close to the epicenter (indicated by a star) has bad shaking which very quickly diminishes even a few kilometers away. At 200 kilometers away, the shaking is mostly unfelt.

Figure \(\PageIndex{3}\). A shake map showing the intensity of the 2019 Ridgecrest quake. "Population map" by USGS is in the public domain. Access a detailed description.

Most major California cities have developed far too close to major faults. Let’s look at a few examples of the intersection of metropolitan areas and faulting.

In Figure Figure \(\PageIndex{4}\), we see the complex fault systems of Southern California. Faults are indicated with red lines. The San Andreas is shown diagonally, moving from the Salton Sea. Note also the great number of recognized faults that branch off the San Andreas.

Figure \(\PageIndex{4}\). Major fault map of Southern California, from the USGS Interactive Map. "Fault map" by USGS is in the public domain. Access a detailed description.

Now in Figure \(\PageIndex{5}\), we take that same fault base map and superimpose the population centers of Southern California. San Diego’s cluster of faults is now obscured by the dense housing. The LA region, from Palos Verdes to the San Gabriel Hills, is dense sprawl sitting atop numerous faults.

Figure \(\PageIndex{5}\). Major population concentrations superimposed on Southern California fault system. "Fault and population map" by USGS is in the public domain. Access a detailed description.

The Hidden Danger of Thrust Faults

Everyone has heard of the San Andreas fault, but how many are familiar with the Whittier Narrows fault? Charismatic, brand-name strike-slip faults get a lot of media attention. But we should also be concerned about lesser-known reverse faults that branch off from the major tectonic ruptures.

Many faults in California are thrust faults, a type of reverse fault where the angle of the fault plane is very low. Because of this low angle, the surface trace may be covered by the fault movement itself, creating a blind thrust fault. These are illustrated in Figures \(\PageIndex{6}\) and Figure \(\PageIndex{7}\).

Figure \(\PageIndex{6}\). A cartoon illustrating the San Andreas and associated thrust faults. "San Andreas thrust faults 1" by USGS is in the public domain. Access a detailed description.

Figure \(\PageIndex{7}\) A scale illustration of the San Andreas fault and associated thrust faults. "San Andreas thrust faults 2" by USGS is in the public domain. Access a detailed description.

Though thrust faults and blind thrust faults are less well known, that does not mean they are less dangerous. In fact, the proximity of such faults to large population centers means that they are more likely to cause damage locally than distant celebrity faults such as the San Andreas. Yet California legislation–most notably the Alquist-Priolo Act–does little to address their dangers.

Failures of the Alquist-Priolo Act

Pop quiz: Does the State of California legally protect residents from buying or living in buildings constructed in seismically dangerous areas?

If you answered, “Yes,” you might want to recalibrate your assumptions about whose interests governments serve. And you might want to read this section carefully.

The M_w6.6 1971 San Fernando earthquake (sometimes referred to as the Sylmar quake) killed 64 people and caused half a billion dollars in damage, including to hospitals and schools. It was the first substantial postwar quake in the modern greater Los Angeles area, where reckless and unplanned expansion grew virulently in the early twentieth century. The 1971 quake became a stress test of modern Southern California infrastructure–fragile highway overpasses, water mains, unstable dams–and the vulnerabilities exposed by this temblor were unsettling.

In response to the 1971 quake, the following year the state of California passed the Alquist-Priolo Earthquake Fault Zoning Act. (The official name has been unnecessarily rebranded numerous times since, so for clarity we will refer to this as the “AP Act.”). There is precedent to California enacting legislation only after a major disaster, rather than preventatively; the 1933 Long Beach quake severely damaged over two hundred schools (see Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{8}\): The collapsed John Muir School in Long Beach, destroyed in the 1933 Long Beach Quake. "1933 Long Beach" by W.L. Huber, USGS is in the public domain. Access a detailed description.

The fortuitous thing about the 1933 quake was its timing: it occurred at Beer O’Clock (late on a Friday afternoon) but had it happened just a few hours earlier, then possibly thousands of school children would have died or been injured in the collapsed schools. The 1933 quake led to the swift passage of the Field Act, which proscribed school building construction with unreinforced masonry.

The 1972 AP Act went much further than the 1933 Field Act. The AP Act mandated the creation of official regulatory maps detailing the locations of active faults, with restrictions on building within recognized Earthquake Fault Zones (EFZs). In theory, if real estate developers wanted to build within these restricted areas, they would have to establish the location of the fault (as certified by a licensed Professional Geologist, or P.G.) and allow for a setback, a safety buffer.

The AP Act focuses on ground surface rupture, and this fundamental limitation is the biggest failure of the AP Act. Big quakes don't always rupture the ground. The AP does not consider the degree of ground shaking in its mapping, though one would naively imagine this would be the most important thing to consider. Surface rupture, if it occurs at all, can be a very different thing than shaking intensity.

The AP Act also involves proximity to known faults. Common sense would suggest the benefit of being as far away as possible from active faults. However, the AP language reads:

“...the area within fifty (50) feet of such faults shall be presumed to be underlain by active branches of the fault unless proven otherwise”

This means that local jurisdictions can mandate as little as a 50 foot setback from an identified surface rupture and comply with the AP. But a major fault system such as the San Andreas is more than just a single crack exposed at the surface, but rather a network of multiple fractures across wide swaths. The AP Act is based on convenient magical thinking that the surface trace is all that matters.

The AP Act is concerned with “active” faults, which it defines as having movement during the Holocene (11.7 ka to today). However, this cutoff is completely arbitrary, ignores the potential of new faults emerging, and assumes that past Holocene activity will be indicative of future movement. Nothing at all prevents an older fracture from reactivating under changing stress conditions.

Because the AP Act involves officially recognized faults, it may ignore unknown or poorly-described faults. For example, the 1994 Northridge Quake (M_w6.7, 57 people killed, with many casualties occurring in soft-first story apartments) did significant damage in the San Fernando Valley from the movement of a poorly-understood blind thrust fault (see Figure \(\PageIndex{9}\)). The AP Act's focus on surface rupture does not provide any regulatory protections for residents facing such calamities.

Figure \(\PageIndex{9}\): Damage to a highway from the 1994 Northridge Quake. "1994 Northridge" by USGS is in the public domain. Access a detailed description.

The AP Act focuses on surface exposures of faults, but quakes don’t happen right at the surface, but rather many kilometers deep. Often faults curve in the crust, so that the surface exposure is not where the majority of the fault plane resides. Moreover, not every quake even disrupts the surface and offsets the ground. The AP Act’s limited focus on the surface is a conceptual blindness that leaves Californians without the protection that a full consideration of the science would bring.

Even more unsettling, the regulations of the AP Act include an incredible exemption: single-family homes. The type of home in which most everyone lives is not covered by the AP Act. The original AP Act language did cover single-family homes; however, within two years, an exemption was created at the behest of real estate lobbyists. Potential buyers of homes within an AP Earthquake Fault Zone will be notified of this status, but in the context of the blizzard of disclosures one signs when buying a home, this will hardly raise an eyebrow for eager purchasers who may naively assume that "the government" would not let them buy into a hazardous place.

In this multitude of ways, the California legislation intended to protect residents fails its job. Most Californians live in single-family homes, which aren’t restricted by the AP Act. Most Californians live uncomfortably near fault traces, and they are allowed to live and work in buildings that are close to known fault traces. Those are just the faults we know about, and history suggests that our knowledge is incomplete. The AP Act is limited to known faults with surface traces, rather than more salient question of unseen, lurking seismic dangers.

In answer to the pop quiz that began this section, the state of California fails in its responsibility to protect residents from living in dangerous buildings in dangerous areas. Though many resident assume that "the government" will protect them, the AP Act is inadequate for a state with such seismic hazards. Some local jurisdictions have more restrictive laws, but having a piecemeal approach to public safety guarantees that many people will be left less protected than they could be.

In a state as riven with tectonic dangers as California, if we were to adopt reasonable safety measures, such as expanding the no-build zone to several kilometers on either side of fault traces, and taking into consideration not just active fault traces but also suspected faults and also low-angle thrust faults, then there would be very few places where Californians could build at all. Given California's chronic affordable housing shortage, it seems unlikely that this situation will change any time soon.

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References

1. Rodriquez-Delgao, C. (2020, July 16). California has some of the worst air quality in the country. The problem is rooted in the San Joaquin Valley. Retrieved June 18, 2024, from https://www.pbs.org/newshour/nation/california-has-some-of-the-worst-air-quality-in-the-country-the-problem-is-rooted-in-the-san-joaquin-valley#:~:text=Today%2C%20the%20direct%20and%20second,pollution%20in%20the%20Fresno%20area
2. Bryant, W. A. (2010). History of the Alquist-Priolo Earthquake Fault Zoning Act, California, USA. Environmental & Engineering Geoscience, XVI(1). https://doi.org/10.2113/gseegeosci.16.1.7