Oakland Geology

The last week (Oct 2021) has had its share of local earthquake news, even though there weren’t any earthquakes nearby. It all centered around the release of volume 3, the last part, of the massive HayWired Scenario report, conveniently timed for 21 October, anniversary of the 1868 Hayward earthquake (not to be confused with the 17 October earthquake of 1989).

HayWired is a virtual magnitude-7 earthquake, complete with aftershocks, that represents a typical Big One on the Hayward fault. Seismologists created it as accurately as their science permits, then asked emergency responders, social scientists, planning agencies, structural engineers and other specialists what they think would happen to the Bay area and how they would handle it. Volume 3, “Societal Consequences,” presents all their answers, as accurate as their expertise permits.

In brief, the consequences would be dreadful. Ace reporter Ron Lin of the Los Angeles Times wrote an able summary that I’ll just point you to rather than write my own. Besides, I covered some of the same ground a few months ago.

The East Bay Times, to its credit, also ran Lin’s story, and two days later it issued a wake-up editorial, “Prepare today for next major Bay Area earthquake,” aimed at goosing its readers into action against the threat. It’s a bit overdone, starting with the opening paragraph: “Gulp.” I don’t really mind that, but the editors went on to say something sloppy that I will focus on today:

“We know that the last major earthquake on the Hayward Fault occurred in 1868 — 153 years ago. We also know that, on average, dating back to the year 1134, the fault produces a major earthquake roughly every 150 years. So, yes, we’re overdue.”

No, we are not overdue. Scientists don’t use that word because it’s a deep error in thinking. Something that’s overdue is late, behind schedule, and earthquakes don’t follow a schedule. I don’t like scaring/motivating people with inaccurate statements.

Ron Lin, to his credit, stopped short of using the O-word:

“The Hayward fault is one of California’s fastest moving, and on average, it produces a major earthquake about once every 150 to 160 years, give or take seven or eight decades. It has been 153 years since the last major quake — a magnitude 6.8 — on the Hayward fault.”

Instead, he included the uncertainty around that irresistibly tempting “average.” That was helpful, but he didn’t come up with the best word.

Even the U.S. Geological Survey creeps close to the wrong word in its excellent Fact Sheet 2018-3052 titled “The Hayward fault — Is it due for a repeat of the powerful 1868 earthquake?” It sidles up to this D-word, and by implication the O-word, by saying that “the interval between successive quakes has varied from 95 to 183 years, averaging 150 years, and it is now more than 150 years since the 1868 earthquake….” and trails off with that coy ellipsis. The sentence leads with the uncertainty, which is good, but the conclusion it implies is not correct. A helpful graphic shows the raw numbers behind the average:

There’s a rhythm to this timeline, but not a good beat. Here’s a longer timeline, currently the best we have, that presents the uncertainty of the radiocarbon-based dates in it:

^{Source: USGS}

Those snappy stars are actually smeared into blurs. For instance, the date of that event “in the year 1134” that the newspaper cited is uncertain by over a hundred years.

Maybe I’ve made it clearer what frosts me (and most seismologists) about the O-word. Now the correct, best word for the situation on the Hayward fault is this: the fault is ready for a major earthquake. It’s primed, loaded, set to go. This is scientifically correct because we’ve measured the actual motions of the crust around the fault, and we know that since 1868 it has accumulated enough strain energy (the kind in a stretched rubber band) to be released in a HayWired-sized earthquake.

“Ready” is not as scary as “overdue,” but sit with it and the word is pretty motivating just the same. Are YOU ready?

The prospect of reading the whole HayWired Scenario report is intimidating. I recommend Chapter R as a good summary that will guide you to specific chapters where you can dive deeper.

This entry was posted on 25 October 2021 at 8:01 am and is filed under Earthquakes, Hayward fault. 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.