From: Alice Elizabeth Taylor
Seismologists Say Large Aftershocks Unlikely, Government Should Retrofit Buildings for Safety

Following the magnitude 6.3 earthquake that struck Albania, there have been a number of aftershocks, some registering as high as 5.4 on the Richter scale. With 40 dead so far, over 650 injured and many still feared to be trapped under the rubble, search and rescue teams are racing to try and save them.

Many citizens have been made homeless and countless others are too scared to return home due to fear over aftershocks or further quakes of a similar magnitude. With fears running high and a risk of scaremongering and fake news triggering panic, Exit reached out to experts to get some answers.

Exit spoke to Dr Stephen Hicks, a research associate at the Faculty of Engineering, Department of Earth Science & Engineering at Imperial College London, Dr Wendy Bohon, a geologist with the Incorporated Research Institutions for Seismology, and Remy Bossu Secretary-General of the European-Mediterranean Seismological Centre (EMSC) and co-creator of the Last Quake application to find out more.

Following yesterday’s earthquake, what is the likelihood of a quake of a similar magnitude in the coming days and weeks?

Stephen: Following an earthquake, we expect many earthquakes to follow – we call these aftershocks – as stresses along the fault get redistributed. A rule-of-thumb that applies for most earthquakes is that the largest aftershock will be 1 unit of magnitude less – this is known as “Bath’s Law” – so in that case given the mainshock was magnitude 6.4 we can expect the largest aftershock to be magnitude 5.4. We have in fact already had an M5.4 aftershock in the area. However, sometimes, not all earthquake sequences follow this rule, and there can be larger aftershocks – although this scenario would be a little bit less likely.

The number of aftershocks should decrease with time following a well-known seismological law called Omori’s Law. The frequency of aftershocks should quickly decay away over time – i.e. whatever the probability of an aftershock is on the first day, the second day will have 1/2 the probability of the first day and the tenth day will have approximately 1/10 the probability of the first day.

Wendy:  It is very unlikely that there will be another quake of the same or larger magnitude. However, felt aftershocks will continue for quite some time. Aftershocks are just earthquakes that occur after a mainshock (or larger earthquake) and they can cause damage in their own right, especially to buildings or infrastructure that are already damaged.

Remy: In the vast majority of cases (>95%), we do not observe an earthquake as big or bigger. Each earthquake does generate its aftershocks and they will continue for days and weeks but the number of aftershocks in Albania has already decreased. A bigger shock is very unlikely but can never totally be excluded but it could take several months before shocks go back to ‘normal’ levels.

This is one of the largest quakes the area has seen in a long time, why is this? Could we expect to wait a long time for a similar size one?

Stephen: Unfortunately, we cannot yet predict the time and location of impending earthquakes. Generally, earthquakes happen when the tectonic stresses in the crust built up over 10s to 100s of years along a fault suddenly get released as the fault moves, generating vibrations through the ground. Some sequences in earthquakes can happen quite closely spaced in time (days to months to years), others may take decades to 100s of years between quakes. Without detailed mapping of the faults and monitoring of their motion over time, we just don’t know.

Wendy: This area experiences quakes of this magnitude fairly frequently. This is the 9th earthquake of M6.0+ to happen since 1905. This is because of tectonic compression of the crust that extends from Croatia south to Greece. When the crust is squeezed like this it produces it breaks and folds, causing earthquakes.

Remy: There have been other strong earthquakes in the past in Albania. For example a M7.1 in 1979. So this activity is normal for the region. Unfortunately, there is periodicity in earthquake occurrence, just average seismicity rate over a long period of times. So we -and no one- can predict when an earthquake will happen

Why is Albania experiencing a number of earthquakes recently?

Stephen: Earthquakes in Albania (and the eastern Adriatic region in general) result from rapidly-deforming tectonic plates – in particular, the northward collision of the Africa plate relative to the Eurasian tectonic plate. This collision is related to the gradual closure of the Adriatic Sea, resulting in sets “thrust” faults along the Adriatic coast. One of these faults probably hosted yesterday’s earthquake.

There is probably no statistically significant change in seismicity rate over time. Naturally, earthquakes have a degree of randomness, and we don’t always have the historical catalogues of earthquakes (instrumental records go back to the 1970s), so we are not always able to compare with past rates of seismicity for a given region.

Remy: There are aftershocks which are normal after the M6.4 earthquake. In this case, many of these aftershocks are close to inhabited regions meaning that even small magnitude ones can be widely felt. This is a situation which is very difficult to cope with from a psychological point of view and is very unnerving. This difficulty has to be fully recognized: all these felt aftershocks have a real psychological cost on citizens.

What measures do the government need to take to ensure they are prepared for such events in the future?

Stephen: Whilst large, felt earthquakes are generally quite rare in individual countries, they can result in a high impact – causing many deaths and injuries, as well as huge economic damage to regions. Therefore, it is important to provide the environment needed for in-depth research into the seismic hazard of an area before that earthquake happens to identify susceptible building types, faults that may rupture with a given magnitude and in a given time. Also, we often say the phrase: “earthquakes don’t kill people. Buildings do”, therefore, it is certainly worth regulating building codes and retrofitting buildings so that they can withstand the shaking in a likely earthquake scenario.

Wendy: The best things governments can do is require and enforce better building codes so that buildings do not collapse during earthquakes.

Remy: Preparation is not only down to the government, but it should be done also at an individual level as well. The government needs to enact and enforce building codes, preparation of the response etc. But people must prepare too: making sure there are no heavy objects on shelves, having a family plan, being sure to know how to react, and what to do in case of a strong earthquake.

It is important to note that the national seismological institute in Albania has been doing a tremendous job despite the tough conditions. We rely on their work to provide rapid earthquake information for the Last Quake app.

What should people do in the case of an earthquake?

Stephen: It depends on the building type. In places like the U.S., they recommend “drop, covering and hold on” – e.g. beneath a table or in a door frame – because many people get injured by falling masonry. But if the building you are in is susceptible to damage, and at risk of collapse, then it is probably better to be outside in the open, and away from potential falling debris.

I would recommend people in the area are prepared (e.g. having a spare torch, first-aid kit and food supplies in case a large earthquake hits). People should also listen to the advice of official earthquake monitoring organisations. Some charlatans on the internet often spread rumours about predictions of larger earthquakes, which can spread rapidly. These are wrong – we cannot predict earthquakes. It is better to remain prepared continuously in the long term.

Wendy: If you’re inside when you feel earthquake shaking drop down and take cover underneath a sturdy piece of furniture like a table. As soon as the shaking stops and it’s safe, exit the building. Move quickly but safely away from anything that can fall on you, like buildings, power lines and tree branches.

Remy: It is generally advised to stay inside, take cover during the shaking (e.g. under a sturdy table) and go in open space after. Why? Because trying to go out DURING shaking is dangerous: many objects can fall from the roof, balconies etc, and the chance of being hit by such an object is much higher than the chance of the building to collapse. 

Furthermore, if you hear someone saying they predicted an earthquake or a future one, ignore them as these predictions are false and just fuel anxiety. The best way to reduce anxiety is to know what to do in case of a future earthquake, be prepared, and remain informed.

All three interviewees expressed their solidarity and wishes with the Albanian people at this difficult time.