Foreshocks may warn that a big quake is coming

By Colin Barras

Advance warning is the ultimate prize for earthquake studies. Now, for the first time, one study offers tantalising evidence that it may be possible to build such a system to warn of some impending large quakes about an hour before they strike.

The finding comes from an analysis of the seismic record from the lead-up to a devastating earthquake that hit Turkey in 1999. This revealed that foreshocks rippled away from the source of the rupture in the 45 minutes before the quake – the first time that foreshocks have been conclusively linked to a major earthquake.

Theoretical models predict that the crust is unstable in the hours leading up to a major earthquake, but detecting that instability in the real world has proven a challenge.

Five foreshocks

Michel Bouchon at the University of Grenoble, France, and a team of French and Turkish geologists studied seismograms recorded before the Izmit earthquake, which killed some 17,000 people.

In one recording, the team saw five small shocks in the final 20 minutes before the event, each characterised by a signature sequence of two types of waves, called P-waves and S-waves. In each of the five small shocks, a P-wave was followed 2.4 seconds later by a higher-amplitude S-wave.

P-waves are sometimes known as primary waves, because they travel faster through the Earth and so are the first to arrive at monitoring stations.

“The difference in time between these two arrivals was always the same, implying that they came from the same spot on the fault,” says Bouchon.

The strongest of the shocks was also recorded at other nearby stations. By performing the same calculations on those records, the researchers could work out exactly where the shocks originated. This turned out to be within a few hundred metres of the focus of the Izmit quake itself, suggesting that the quake and the shocks were linked.

Warning, not prediction

Although the five foreshocks differed from each other in total magnitude, their waveforms were remarkably similar. Bouchon and colleagues hunted through the rest of the seismograms to see if they had missed other occurrences of that waveform. They found about 40 in total.

The team saw a pattern in the foreshocks: they became progressively stronger and more closely spaced leading up to the quake itself. The researchers say this is “encouraging” for the development of early warning systems, and plan to examine the records of other well-recorded earthquakes for similar signals.

Even if these findings lead to the development of an earthquake warning system, Ian Main, a seismologist at the University of Edinburgh, UK, warns that its usefulness will be limited. “Earthquake early warning is not the same as earthquake prediction,” he says. An early warning system could only give hours notice, not the days that would be needed to prepare an evacuation.

Main also says that the system is likely to be applicable only to some quake zones. The San Andreas fault zone that runs through California is the best-monitored in the world, yet analysis of the seismograms recorded in the build-up to the 2004 Parkfield earthquake there revealed no evidence of any foreshocks (Nature, DOI: 10.1038/nature04067).

The Parkfield quake was magnitude 6.0, compared to Izmit’s 7.6 and Bouchon thinks foreshocks may only occur in very large quakes. If that turns out to be true, his early warning system will only apply to the most severe events.

Journal reference: Science, DOI: 10.1126/science.1197341

https://www.newscientist.com/article/dn20148-foreshocks-may-warn-that-a-big-quake-is-coming/

A foreshock is an earthquake that occurs before a larger seismic event (the mainshock) and is related to it in both time and space. The designation of an earthquake as a foreshock, mainshock or aftershock is only possible after the full sequence of events has happened.

https://en.wikipedia.org/wiki/Foreshock

a foreshock is an earth tremor.It can or cannot be followed by an earth quake