Sibylle Steimen: An earthquake of magnitude 5.8 was recorded in the region north of Modena on May 29. Previously, on May 20, the same region was hit by a magnitude 6.1 earthquake. In the aftermath of the event a number of aftershocks with magnitudes up to 5.1 were recorded. On May 22 an earthquake hit the region of Bulgaria's capital Sofia at magnitude 5.8, which was also followed by some aftershocks.
All these events are caused by the dynamics of the earth’s crust. Relative movements of the individual plates lead to friction at plate boundaries and at fault systems within the plates and from time to time sudden movements – this is when earthquakes happen.
Where should we expect the next earthquakes?
Steimen: Earthquakes cannot be predicted precisely. Of course, there are regions where they are more likely to occur than in others. But even locations away from tectonic plates can experience earthquakes. The area where the quakes now occurred in Italy, actually only has a "moderate" earthquake hazard, a quake of similar intensity occurred there some 700 years ago. What we currently see in Northern Italy is suspected to be part of a so called earthquake sequence. Events of very similar magnitude and earthquake mechanism – that is the way the earth moves during the quake. Over the next couple of weeks or months a number of further magnitude 4 to 5 events may happen.
Why does an earthquake with a similar magnitude create different amounts of damage?
Steimen: Magnitude is one factor, but many other factors also play a role. For example: at which depth did the earthquake occur? A shallow quake will create more damage than a quake that is very deep below the surface. On the other hand, even a shallow quake will not damage much, if it happens in an area with few inhabitants and little infrastructure, i.e. in a steppe. The quake on May 20 in Modena was shallow with approx. 5 kilometers below the surface, which led to substantial damage.
In Italy very significant damage occurred to world heritage buildings, why is that so?
Steimen: Cultural heritage buildings in Italy are usually built out of masonry or rubble stones. Such constructions cannot swing and absorb the induced earthquake energy but often behave brittle and are highly vulnerable to the earth shaking. Modern building codes as they have been in place for many years also in Italy, make sure that buildings can dissipate the induced shaking energy by having a certain amount of elastic behavior. Such earthquake-resistant buildings are built so that they will not collapse during an earthquake of a certain maximum strength.
However they can still be damaged heavily and need to be demolished afterwards. Thus, modern building codes help to save lives but do not always prevent the financial losses. In any case, it is important that construction rules are strictly applied. Furthermore, if a series of events hits the same area, pre-existing damages lead to a weakening of buildings and a further earthquake can aggravate the loss substantially.
Unfortunately, there have been some casualties. What other sort of damages are expected from the quakes?
Steimen: As always, it takes time to assess damages - also because areas often remain highly seismic for some time - like in Modena. Apart from damages to homes and buildings, daily life and business life get disrupted. Factories or businesses have delays in production or delivery. It is important that everyone takes good precautions for themselves, their buildings and their businesses. While states take over some protection, insurance companies also help with pro-active risk management and support people and businesses to get back to "normal" as quickly as possible.