M6.1 Earthquake in Napa Valley Shakes San Francisco: 24 August 2014


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M6.1 San Francisco earthquake

Location of the M6.1 earthquake north of San Francisco, 24 August 2014. Image credit: USGS

In the early hours of Sunday, 24 August 2014 (0320 local time) a powerful earthquake centred in the Napa Valley shook a large area of northern California including the San Francisco metropolitan area.

Although details are sketchy at the time of writing, early information from the United States Geological Survey (USGS) indicates that the immediate area was subjected to very strong shaking with lighter shaking experienced in areas including San Francisco itself, Oakland and Sacramento.

At the time of writing there have been no reports of damage or injuries, although the USGS issued a warning of potential aftershocks, stating that “At this time (immediately after the mainshock) the probability of a strong and possibly damaging aftershock IN THE NEXT 7 DAYS is approximately 54 PERCENT” while 30-70 smaller shocks may also occur.

The USGS puts the probability of a larger tremor in the next week at around 5-10%.

California’s 24 August M6.1 Quake: Tectonic Setting

California is highly seismically active. The overall tectonic situation is that two of the Earth’s tectonic plates, the Pacific and North American, are sliding past one another, gearing friction which is released in periodic earthquakes.

Shake map for the 24 August California Quake

Shake map for the 24 August tremor. Image credit: USGS

The main boundary is defined by the notorious San Andreas fault, but the whole continental margin is subject to intense pressures and deformation, reflected in the series of broadly parallel, north-west to south east trending faults in the state with associated high levels of seismic hazard and earthquake probability.

These include the San Andreas Fault itself, which runs along the west of San Francisco Bay, the Hayward fault/Rodgers Creek Faults, which run along the east of the bay, and the Calaveras and Green Valley faults in the foothills of the Sierra Nevada.

The most recent earthquake occurred not on the San Andreas fault itself but between the Rodgers Creek and Green Valley faults. This location implies that it is associated with movement on a minor fault (possibly the Napa Valley fault) rather than at the plate interface itself.

San Francisco Earthquakes: How Significant is the M6.1 of 24 August?

Mention San Francisco and earthquakes in the same sentence and many people immediately think of of the so-called ‘Great’ San Francisco earthquake of 1906. Although no definite magnitude can be assigned to this tremor (which predated the development of an accurate measurement system), estimates place it at between M7.7 and M8.3.

San Francisco Quakes

Earthquakes of at least M6.0 in the San Francisco Bay area since 1906. Image credit: USGS

The magnitude scale is logarithmic. To place the most recent tremor in context, assuming that the 1906 tremor had a magnitude of M8.0 (in the middle of the range) this magnitude difference of 1.9 translates to a size difference of almost 80 – that is, the M6.1 is just one-eightieth the size of that of 1906.

The amount of energy released by the latter is a staggering 700,000 times larger.

This does not, however, mean that the most recent earthquake was insignificant – far from it. A search of the USGS earthquake archive shows just three other earthquakes of at least M6.0 in the San Francisco Bay region since 1906, although others of similar or larger magnitude have occurred in other areas of the state.

California’s High Seismic Hazard

So what’s in the future as far as northern California earthquakes? It’s worth observing that seismic hazard in the area is high, and large earthquakes are expected.

Yeats quotes figures of 21% probability of an earthquake of at least M6.7 occurring along the San Andreas Fault itself between 2000 and 2030; the overall probability for the Bay area as a whole is 70%.

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