After an earthquake, rescue workers may have to enter buildings that are in danger of collapsing because of an aftershock. Warnings must be given as early as possible to give them time to protect themselves.
13 February 2023
Alarm rangefinder pointing at an unstable building during rescue operations by Search and Rescue teams.

Alarm rangefinder pointing at an unstable building during rescue operations by Search and Rescue teams.

© BRGM - Samuel Auclair

Two violent earthquakes within hours of each other

On Monday 6 February 2023, two violent earthquakes of magnitudes 7.8 and 7.5 respectively hit southern Turkey and northern Syria within hours of each other. The immediate priority is assisting the affected population and rescuing those still trapped under the rubble.

In winter conditions and sub-zero temperatures, it has been a race against time. A race which places rescue workers in danger. This is why we have undertaken a study to develop a warning system to enable rescue workers to take shelter as quickly as possible in the event of aftershocks.

High-risk relief efforts

Initially carried out spontaneously by local residents and rescue teams, these "rescue and clearance" operations are now being carried out by specialist Rescue and Search teams which have come from all over the world in response to requests from the Turkish and Syrian authorities.

These interventions obviously take place in a degraded and hazardous environment. In this context, aftershocks are a major risk for teams who have to work in severely damaged buildings where parts may collapse, even if these tremors are relatively weak.

The period following a major earthquake is characterised by an increased probability of new, weaker but still dangerous tremors, called aftershocks. This is what happened in Turkey with more than 400 aftershocks of magnitude greater than 3.0 being recorded by the Euro-Mediterranean Seismological Centre in the first 24 hours.

Although this risk is well known to rescue teams, they currently have few ways of protecting themselves against it, with practices varying greatly from one team to another.

One of the most widely-used tools remains the "alarm telemeter", whereby a laser beam is pointed at an unstable building, and an audible alarm is emitted in case of a displacement of several millimetres of the structure, unfortunately often too late at the very moment of the collapse!

If not predict, then alert as early as possible

The principle of seismic early warning was conceived in California as early as 1868, with the idea of being able to warn San Francisco of the imminent arrival of destructive waves generated by earthquakes with epicentres a hundred kilometres from the city.

In principle, one only has to analyse the first few seconds of the primary-wave recordings ("P" waves, the weakest ones) to predict the strength of the earthquake and the severity of the quakes to come.

As this warning is then broadcast almost instantaneously, it is possible to inform people in areas that have not yet been affected by the most dangerous seismic waves: the further away from the epicentre, the greater the time interval between the arrival of the warning and the arrival of the "S" waves (as in "Second"), which are responsible for a large part of the damage, thus leaving more time to react.

The main complexity in implementing this principle is that the processing of seismic waves must be done automatically in just a few seconds. It took more than a century before this idea was applied for the first time in Japan in the late 1980s.

Alerting rescue workers seconds before destructive tremors

Noting on the one hand the high risk incurred by rescue teams in the event of aftershocks, and on the other hand the constant advances in the robustness of seismic early-warning systems, BRGM carried out a study, published in 2020, which explored the feasibility and interest of equipping rescue teams with such warning systems.

The aim was to be able to alert them a few seconds before the arrival of seismic tremors. This is very little time, but it can make a difference.

With expected delays of usually less than ten seconds between the moment the alert is received and the occurrence of the tremors, there are several advantages to these early warnings. They range from simple psychological preparation to reduce the surprise effect for alerts that leave hardly any time to react, to the possibility, with longer forewarning times, of reducing one's exposure to risk by putting oneself in a safe position, or even ceasing dangerous activities.

However, such an automated system is not 100% reliable, and false alarms are inevitable. One favourable point highlighted in the study is that, unlike other sectors of activity where the criticality of false alarms makes them unacceptable and the principle of early seismic warning virtually inoperative (e.g. for nuclear power stations), the impact of isolated false alarms on search and rescue activities seems relatively limited.

Although there is already such an aftershock early-warning system in Japan, it remains largely unknown and is consequently only used by a few Japanese teams. At a time when rescue efforts on an exceptional scale are continuing in Turkey and Syria in an attempt to save as many people as possible, it is undoubtedly appropriate to think about having this device tested by international rescue teams in order to be able to assess its potential usefulness.