Stop saying “overdue”

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.

3 Responses to “Stop saying “overdue””

  1. Andrew Alden Says:

    Russell, sorry I didn’t address the question in your last paragraph!

    Today, we can measure the strain, and the relief on the accumulated strain, on the basis of radar/lidar imagery and land surveys before and after an earthquake. On well-characterized faults, the measurements are quite accurate. The problem is with past earthquakes. We do have a few survey measurements in the East Bay from the 1860s, and they’re useful. Strain release in the 1868 quake isn’t totally unknown (I haven’t checked the literature), but other than that small number of measurements we only have written records from scattered locations by untrained observers. We can learn more about this problem only as fast as nature sends us evidence.

  2. Melodie Says:

    Tbh, cocked and loaded and ready to go scares me more than overdue.

    After those last two earthquakes a few weeks ago I got the kick I needed to make sure my supplies are up to date.

  3. Russell Yee Says:

    Thanks Andrew, Mea Culpa, I will stop using the O-word!

    But it seems like we do need some way to distinguish shorter vs. longer timeframes.

    On one hand, strain does build up over time and leads to quake periodicity on a given fault. So average times mean something as an indicator of strain accumulation.

    On the other hand, every fault is always “ready” in that a next quake can happen anytime. I think of the back-to-back Ridgecrest quakes (July 4, 2019 M6.4 and then July 6, 2019 M7.1). Clearly that fault was “ready” immediately after the first shake.

    I guess the question is: how to distinguish the senses in which the Hayward fault is ready today vs. how it was ready, say, October 22, 1868. Can we measure how much strain accumulation a given shake does and does not relieve?


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