‘Tremor that hit so close to Malta is rare but not the first’

Malta was hit by a tremor measuring 3.5 on the Richter scale late on Thursday, with its epicentre just 16 kilometres off Mellieħa.

As the tremor’s epicentre was recorded just off Malta’s coast, many people wondered whether this was the first tremor to occur so close to the island.

Speaking to Times of Malta, Agius, who works at the University of Malta’s Seismic Monitoring Centre, clarified that there have been tremors reported at a similar distance off Malta’s coast, especially off the island’s southern coast.

“The main difference now is that the magnitude was higher, so it was felt,” Agius said.

A map showing earthquake activity between 1995 and 2014 indicates that there were four tremors whose epicentres were around the same distance.

None of these recorded a magnitude higher than 3. Offhand, Agius was not sure of the tremors reported after 2014.

Agius added that the tremor is unlikely to produce any aftershocks. “I won’t exclude it, because it’s the nature of earthquakes, but, if there is an aftershock, it won’t have a high magnitude,” Agius said.

The behaviour of the fault in this area shows that this would usually be just a one-off earthquake, Agius added.

Between 1995 and 2014 there were four tremors whose epicentres were around the same distance

People were also awakened by a 4.7 magnitude tremor on Wednesday, when the epicentre was recorded off the coast of Catania. Agius said that the most recent earthquake has nothing to do with the one that happened the day before.

“Tectonically, they are different. They are even far from each other. For me, they are unrelated,” Agius said.

However, Agius added that readings show the most recent tremor was also recorded in Catania.

A map showing that the tremor was recorded in Sicily. Photo: University of Malta’s Seismic Monitoring Group.

A graph displaying the waveforms recorded at seismic stations in Sicily shows that the tremor was detected there after it was recorded in Malta. The waveforms are more prolonged and have a lower amplitude.

Although no damage seems to have been reported, Agius said that, if the tremor had recorded a magnitude of 4 or more, then some people might have experienced “minor superficial damage”.

“There could be cracks in the plaster, maybe broken windows, nothing major,” Agius said.

Historically, they have seen no evidence of a tremor with a magnitude of 4 in that area, Agius added. “I am not completely excluding it but the chances are low,” Agius said.

Many people reported hearing a “rumbling” sound. Agius explained, that when a tremor happens, it sends seismic waves causing vibrations, which are “sound waves travelling through the rocks”.

The rumbling sound, however, usually comes from things vibrating, such as doors, shelves, or anything around you. “People usually describe it like a truck passing through the streets,” Agius said.

Scientifically speaking, Agius said that this earthquake was different, as different stations in Malta reported a different reading of the tremor’s first pulse. “The first pulse, called the P wave, which is the cleanest pulse, was upwards in Gozo but downward in the south,” Agius said.

Since the tremors recorded in Malta happen further away, the stations in Malta usually record the same waveform as, relative to the tremor’s epicentre, they are so close to each other.

This time, since the tremor happened so close to Malta, the stations were more widely distributed relative to the earthquake. So, the different readings give them a better understanding of the tremor’s 3-D motion.