Oakland Geology

The first two posts in this series were about the Hayward fault itself in Oakland and about the different types of ground in Oakland that earthquakes on the fault will affect. This post goes into some details about the kinds of earthquakes we can expect in Oakland. I’m going to try and ignore all the interesting complexities — the subject is full of rabbit holes to go down — and keep things really simple.

Earthquakes are a release of energy that is stored in the rocks along and near the fault surface. The energy comes from the movements of the great plates of rigid rock that form the outermost skin of the Earth. Oakland sits in the middle of a wide boundary between the Pacific plate on the west and the North America plate on the east. In the grossest terms, the Pacific plate is sliding quickly northwest and the North America plate is moving slowly west, pushing against it like a semi merging onto a busy freeway.

The input of energy into the fault from plate tectonics is extremely steady, while the output of energy from the fault — in earthquakes — is pretty much random. That’s the essential mystery at the heart of earthquake science.

At our latitude, that wide boundary between the Pacific and North America plates extends from the San Andreas fault, across the Bay, to the Concord and/or Calaveras faults over the hills. Each of those faults takes up part of the overall motion between the plates. They’re basically long, vertical cracks that extend downward about 13 kilometers — below that depth (8 miles), the rocks are too hot and soft to crack and instead they just deform like modeling clay.

^{From the USGS Earthquakes Map}

The energy that earthquakes release builds up steadily in the Earth’s crust along the two sides of the fault. Friction keeps the two sides from sliding past each other. We say that the fault is locked. Instead, the rocks slowly warp, exactly as if they were great blocks of rubber. And at some point the friction is overcome, the fault ruptures, the rocks spring back into shape, and that elastic energy is released as an earthquake.

(This is where the math kicks in! Various well-known laws of physics allow us to turn my word descriptions into actual equations, and science can exert its superpowers. Careful measurements and creative mathematics give us ever-better answers to ever-deeper questions, and seismology, the study of earthquakes, has progressed into a vibrant field of science with important problems to explore. This paper by Rundle and Donnellan on earthquake clusters is a fresh example of research on the leading edge.)

An earthquake’s magnitude is based on how much the ground moves back and forth (or up and down), as measured by seismographs. Each unit of magnitude represents a factor of ten: if a magnitude-4 shaker moves the ground by a millimeter — not much but definitely perceivable — a size-5 event would move it a centimeter. The same unit of magnitude represents a 32-fold difference in the total amount of energy released, because the geometry is different. (The U.S. Geological Survey has a page with more detail about magnitudes.)

The amount of energy in an earthquake depends on how big a patch of the fault gives way. The largest possible earthquake, then, would happen if the whole thing rips. And how big is the whole thing? We used to measure the Hayward fault as 120 kilometers long, from Point Pinole to the hills east of San Jose, enough to generate a magnitude 7.0 quake. In 2016 we learned that the fault is directly connected to the Rodgers Creek fault in the North Bay, which extends up to Santa Rosa. The combined fault is roughly twice the length of the Hayward fault alone, and it could produce a magnitude 7.4 quake. That’s getting close to the size of the 1906 San Francisco earthquake (about 7.7), and it would tear right through Oakland.

That’s the largest possible earthquake — and also the least likely. There’s a strong element of chance in the way that ruptures grow. A rupture starting out does not “know” how big it will become. The growth of a rupture is more like a run of luck at a casino. Our earthquakes generally don’t rupture the ground surface unless they’re larger than magnitude 6 or so. Those, we have a hope of detecting in the ancient sediments along the fault.

Studies made by digging trenches across the Hayward fault have given us a fair idea of its history of large earthquakes. There seem to have been six in the last 900 years and a dozen in the last 2000 years. Age estimates are fuzzy, but these events aren’t very rhythmic. Their average rate is one every 160 years, but the time between these major ground-breaking earthquakes has ranged from 95 to 225 years.

It’s been 152 years since the last one on 21 October 1868, which had a magnitude estimated at around 6.8. While that matches the average rate, these things aren’t like clockwork. Earthquakes aren’t scheduled. Therefore the next Big One is not “overdue.” Nevertheless, it appears that enough stress has accumulated on the fault since 1868 to power another event of about the same size. The scientific authorities say that a large earthquake (magnitude 6.7 is the size they picked) has a two-in-three chance of happening by 2043 somewhere in the Bay area. Half of that probability comes from the Hayward-Rodgers Creek fault.

What about smaller quakes, like magnitude 5 or greater? These would be strong enough to knock down things like chimneys and crack walls and windows. The area around the Hayward fault has had maybe 20 since 1850, as shown in the map below.

^{Magnitude 5 earthquakes since 1850 within the box including the Hayward fault, from the USGS catalog search}

But since 1889, only three were actually on the Hayward fault (5 September 1955, 13 June 1988 and Halloween 2007). Historically (aside from 1868), Oakland has suffered more from large earthquakes elsewhere in the Bay area than from homegrown ones. The most recent of those were in October 1989 (Loma Prieta, magnitude 6.9) and August 2014 (Napa, magnitude 6.0).

We’ve been lucky. But luck is a human concept, not a geological fact. There is a hint in the Bay area historical record (documented in this 2002 paper and elsewhere) that big quakes have been preceded by clusters of middle-sized ones. Or rather, big quakes appear to quiet down activity on Bay area faults for several decades.

Time keeps on ticking into the future. Our fault will reawaken. My next post will look more closely at that. In the meantime, I recommend that you bookmark Temblor.net in addition to the usual sites at the U.S. and California Geological Surveys. In a Web long plagued by armwavers, alarmists and frauds, these guys are quick on their feet, know what they’re talking about and know what to say.

This entry was posted on 28 September 2020 at 8:01 am and is filed under Earthquakes. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.