M4.0 Earthquake Strikes California’s Hayward Fault: 17 August 2015


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The location of the M4.0 earthquake of 17 August 2015

This is the location of the M4.0 earthquake of 17 August 2015. Image by USGS

It wasn’t the Big One (or even, in global terms, a particularly large one); and it wasn’t on the San Andreas Fault. But the earthquake of magnitude 4 (M4.0) which struck in California’s San Francisco Bay area won’t have passed unnoticed — the United States Geological Survey estimates that upwards of 5 million Californians will have felt some kind of earth movement.

For most of them, admittedly, this earthquake wasn’t much. Only around 30,000 people, in the immediate area of the epicentre, experienced shaking which the USGS describes as strong.

If you were one of the 3.8 million or so whose neighbourhood experienced only light shaking you might not even have woken up (the tremor struck just before 7am local time).

Nevertheless, with so many people experiencing movement associated with this tremor, it’s worth taking a look at the M4.0 California earthquake of 17 August 2015.

San Andreas and the Hayward Fault

There’s an understandable tendency in the media to refer to the San Andreas Fault as if it were a single entity, a line on the map that separates the Pacific and North American tectonic plates, and along which the two are sliding past one another.

Up to a point this is true, and such simplicity serves news headlines (and much of the population) perfectly well; but there’s inevitably more to it.

The boundary between the plates is a wide (from offshore to western Nevada) zone of deformation running pretty much the length of California and consisting of many, broadly parallel, faults of which the San Andreas is just one. The attached map of faults in part of the state (with all topographic markings stripped out) clearly shows both the pattern and the number of the faults.

The San Andreas Fault Zone comprises many different faults

The San Andreas Fault Zone comprises many different faults. Image by USGS.

The earthquake of 17 August occurred very close to the Hayward Fault, which, according to the USGS, “runs from San Pablo Bay in the north to Fremont in the south, passing through the cities of Berkeley, Oakland, Hayward, and Fremont.”

This section is not one which generates frequent large earthquakes; the largest in recent times was in 1868 (no known magnitude) and the last half century has seen only a handful of tremors of the order of that of 17 August.

By contrast, other sections of the fault zone, such as the Calaveras Fault which extends to the south and also immediately to the east of the Hayward Fault, has seen several larger tremors, the biggest in recent years being an M6.1 in 1984.

California Quakes: The Big One?

It’s the question everyone asks. When will the Big One come? But it’s also a question that no-one can (certainly at present) answer with any degree of accuracy.

We can be certain that earthquakes will continue along the SAFZ and that a major tremor, either on the San Andreas fault or close by, will occur and that it will have potentially serious economic and human consequences depending on where and when it occurs.

Map showing the earthquake probabilities for the San Francisco Bay area

This map shows the earthquake probabilities for the San Francisco Bay area. Image by USGS.

At present, the best seismologists can do is provide probability estimates — not predictions — of an earthquake of a particular magnitude occurring within a particular timescale. The USGS notes that: “The overall probability of a magnitude 6.7 or greater earthquake in the Greater Bay Area is 63%, about 2 out of 3” and goes on to add that: “The earthquake probability is highest for the Hayward-Rodgers Creek Fault system, 31%, or nearly 1 out of 3. The last damaging earthquake on the Hayward Fault was in 1868. The 140 years since 1868 is same length of time as the average interval between the past 5 large earthquakes on the southern Hayward Fault.

In other words, the probability forecasts suggest that a large earthquake on the Hayward Fault might reasonably be expected some time soon — and bear in mind that an M6.7 is very much smaller than the 1906 San Francisco earthquake, which had an estimated magnitude of between M7.7 and M8.3.

California Earthquake Reminders From a Little One

So the earthquake of 17 August was anything but a big one. That doesn’t mean it’s irrelevant. The sheer number of people shaken by a small tremor is a reminder, if any were needed, not just of the fact that California is highly vulnerable to earthquakes but also that potentially millions of people stand to be affected by them.

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