Long Valley Caldera Earthquake Swarm: September-October 2014


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Long Valley caldera. Image credit: USGS

This is the Long Valley caldera. Image credit: USGS

So what’s happening in the Golden State?

In the past ten days there’s been a swarm of earthquakes in the east of the state, around the long-dormant volcanic caldera in Long Valley.

And the reaction in some parts of the Internet has been bordering on the apocalyptic with speculation of an upcoming eruption – and one headline even asking whether this might be ‘the end’ of California.

California Earthquake Swarm: Facts and Figures

The cause of all this brou-ha-ha is a swarm of earthquakes which has shaken part of California’s Long Valley (east of the Sierra Nevada) over the past week or so. Over the past 30 days the USGS real time earthquake map recorded around 1500 earth tremors in an area a few miles east of Mammoth Lakes.

The area is a large volcanic depression, or caldera (not dissimilar in style to Yellowstone) and the earthquakes occurred along the western part of the caldera.

Earthquakes in the area aren’t uncommon — most days will see a few of very small magnitude — but this month the number was unusually high – with over 1300 in the period 22 September through 3 October.

At the time of writing, however, the frequency is tailing off and it’s also worth noting that only nine of those tremors reached a magnitude of M3.0 or more; and the largest were just M3.5.

Is this abnormal? Well, no.

Earthquake swarms are common in this area — in fact, there was one in almost the same location in July,” the USGS’ Dr David Shelly told Decoded Science. “The recent swarm is more energetic (more larger magnitude earthquakes) than most swarms that we see, but still small in the context of what has been seen in this area of the past few decades.  For example, the 1997-1998 swarm in this area was much larger and longer lasting, with earthquakes up to M4.9

What’s Causing the California Tremors?

The Long Valley earthquake swarm 22 September-3 October. Image credit: USGS

The Long Valley earthquake swarm 22 September-3 October. Image credit: USGS

So should we worry? Do those earthquakes presage, as some suggest, an imminent eruption at Long Valley?

The short answer is, again, no.

The earthquakes are tectonic in origin, rather than generated by movement of magma (seismologists can differentiate between the two) and a glance at fault maps of the area indicate that they appear to be focussed along a margin known as the Hilton Creek Fault Zone.

In fact the detailed data from the earthquake swarm shows that movement along the Hilton Creek Zone is unlikely to be the source.

The Hilton Creek Fault is a normal fault, while the swarm activity … [is] dominantly ~N-striking left-lateral strike slip,” Dr Shelly told Decoded Science. “In the seismicity over the past decades, we see several structures with similar orientation (~N striking and steeply east dipping) to the one that activated in the recent swarm.  These may be old fault structures predating the caldera formation.”

Although seismologists aren’t yet certain as to the cause of such earthquake swarms, it seems that the most likely trigger for such movement is fluid injection — not dissimilar to the mechanism behind the ongoing swarm in Oklahoma except that in that case the fluid looks to have a human source whereas in Long Valley the origin is natural.

My interpretation (shared by many) is that this swarm and ones like it are triggered by fluids (probably dominantly water and CO2) moving up from the deep crust,” said Dr Shelly. “This movement is episodic, and when the fluids encounter a fault that is already stressed, they may trigger an earthquake swarm. In many of these swarms, we see propagation of the earthquakes in time in a manner consistent with fluid diffusion.”

The Long Valley Caldera

Long Valley caldera. Image credit: USGS

Long Valley caldera. Image credit: USGS

Long Valley, a volcanic caldera 17 km by 32 km in eastern California, has history. Formed by a super-eruption around 760,000 years ago, Long Valley remains geothermally active (a probable source for fluid triggering earthquakes) although the last eruption was as long ago as 50,000 years. Though it’s overshadowed by the better-known Yellowstone caldera, is is nevertheless potentially capable of a major eruption.

The USGS rates the threat potential for Long Valley as high, and this may well be a source of some of the online alarm.

In fact it’s a matter of definition, as Dr Shelly points out: “In 2005, the USGS Volcano Hazard Program conducted an analysis to rank the threat posed should an eruption occur at each US volcano,” he said. “Volcanic threat ranking is not a forecast of likelihood of eruption. The threat ranking is calculated on the basis of potential hazards should an eruption occur.

Goodbye California?

Defining the likelihood of an eruption is a different matter and there’s no current indication of a potential eruptive activity at Long Valley. Of course there are uncertainties — the processes driving earthquake swarms remain the subject of ongoing research and volcanologists are still well short of an accurate prediction method for eruptions — but in so far as we can say anything for certain in a science where there are still many unknowns, the earthquake swarm at Long Valley doesn’t mean Goodbye to California.

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