Earthquakes: ‘Foreshock,’ ‘mainshock,’ ‘aftershock,’ ‘pre-shock’ – what’s the difference?

CALTECH – Foreshock! Who knew?

For some Southern Californians, earthquakes have been all about, well, the earthquake – the main event, the big one (little b and little o). But Caltech scientists in the days since the July 4th 6.4 earthquake in Kern County and the 7.1 magnitude just a day later on a nearby second fault have a new lesson for us — or at least a reintroduction, of sorts, to the terms pre-shocks, foreshocks and mainshocks, and yes, the ubiquitous aftershocks – more than 3,000 of them, by the way, since Friday’s 7.1 magnitude temblor.

“It can be confusing the whole time,” said US Geological Survey seismologist Nicholas van der Elst.

But when it comes right down to it, the terminology seismologists use is about “relative timing and relative size,” he said.

In fact, looking at the history, not all foreshocks are within just a few days of the main shock. (Stand by for more on that).

In listening to scientists talk about the intense and frequent shaking in Kern County in recent days, the terms clearly help them describe the shaking – and prioritize it to themselves and to the public. In that sense, today’s mainshock could be tomorrow’s aftershock:

“So it began with the magnitude 4, which then turned out to be a foreshock of the magnitude 6, which got to enjoy being called the mainshock for a day,” Nicholas van der Elst said. “So that magnitude 6 involved the two faults, the main one down to the southwest and there’s a little extension to the northwest. The aftershocks filled in in that direction until the magnitude 7.1 exploded out of that little nest of aftershocks, and kind of finished the job to the northwest. I shouldn’t say finished, because there’s still a 1 in 30 chance that this one isn’t even the biggest to come.”

Got it?

In the Los Angeles area, high-profile – and devastating – earthquakes such as the Northridge Earthquake in 1994 did not have a foreshock – or at least one that Caltech scientists immediately knew of on Saturday. But half of all earthquakes have them, van der Elst said.

The 7.3 magnitude Landers Earthquake on the morning of June 28, 1992 in the Mojave desert had one – the 4.6 magnitude Joshua Tree earthquake on April 22, 1992, which, while in a relatively remote area, raised alarms because it was so close to the San Andreas fault, according to Caltech. At the time (that April) scientists pegged a 5 to 25% chance that an even larger earthquake would strike the mighty San Andreas within three days of the Joshua Tree shaker.

It never happened. But if you bet against a mainshock coming eventually, you lost the bet. Because, according to Caltech, two months and 6,000 aftershocks after Joshua Tree, the Landers quake “broke the surface of the Mojave in the largest quake to hit Southern California in 40 years, showing that the concern caused by the Joshua Tree earthquake was warranted, though not in quite the same way as anticipated,” according to a Caltech summary.

Bottom line: foreshocks and mainshocks aren’t always back to back, like in Ridgecrest this week. That said, the Little Lakes fault zone, where the Ridgecrest quakes occurred, tends to have sequences of shakers more close together, van der Elst said.

And by the way, when it’s that much earlier, you might also hear the term pre-shock, which van der Elst likened to a way early foreshock. Ultimately, van der Elst stressed that an earthquake is, plain and simply, an earthquake.

“If it happened before the largest one it’s a foreshock. If it happened after, it’s an aftershock. The largest is the mainshock. They are all just earthquakes. They get started. They grow as big as they like. Aftershocks,… they are just earthquakes. Each one of them can grow into something bigger than its parent.”