Friday, April 26

Mercalli – the earthquake man


The Volcanological Centre on the Isle of Vulcano.

I SEEM to be spending my twilight years visiting islands or living on islands, be they in the UK, Borneo or more recently, Sicily, where I stayed on the volcanic island of Vulcano.

This island is one of a group of volcanic isles in the Tyrrhenian Sea just north of Sicily. Whilst exploring these volcanoes, my schoolboy memories of Latin set books and later university studies came flooding back.

Why were the ancient Greeks, Romans and Italians so interested in documenting earthquakes, tremors and volcanic events? Wherever there are “volcanic events” from active and previously dormant volcanoes, folklore abounds, most of it relates to “the gods of the mountain.”

Mercalli Way – the route to the top of Vulcano Island.

The Greek writer Homer, in his famous book The Odyssey, suggested that the island of Vulcano and adjoining island of Vulcanello were the places where Charybdis and Scylla dwelt, fighting with each other to devour shipwrecked sailors.

Homer recorded eruptions on Mount Vesuvius, near Naples, on the Italian mainland 2,900 years before the present. Virgil, the Roman writer, very much later, also recorded another Vesuvius eruption.

However, Pliny the Younger gave the most vivid description of Vesuvius’s eruption in 79AD, when the Roman cities of Herculaneum and Pompeii were buried under ash and cinder and mudflows.

It has more recently been suggested that the island on which Prospero and his fair daughter Miranda landed from a shipwreck, in William Shakespeare’s play The Tempest, was none other than Vulcano!

19th and 20th century vulcanologists

Specimens collected of pumice and scoria volcanic bombs and obsidian (volcanic glass) from previous eruptions.

It was on a road leading to the tortuous footpath up to the summit of the Grand Crater of Mount Vulcano that I spotted a sign marked “Via Mercalli.”

This very name, Mercalli, sprung to the forefront of my mind, for he was the inventor of an earthquake scale of degrees of devastation.

Recalling my modest geological studies and teachings, Mercalli was the first to measure the impact of earthquakes upon mankind and man-made structures, followed by Charles Richter, a German-American.

Now in the 21st century, the Richter scale has been superseded by the Modified Mercalli Intensity Scale (MMIS) and also by the Moment of Magnitude scale (MM).

Most of the latter scales have developed since the invention of seismographs, which record both the time and intensity of an earthquake. Mercalli’s scale simply records the severity of earthquake damage that we can observe with our eyes.

Who was Mercalli?

Born in Milan in 1850, Giuseppe Mercalli lived a short but full life before he died in a fire accident in 1914. He was ordained as a Roman Catholic priest, yet his main interests lay in the natural sciences which he taught in a seminary in Milan.

Very soon he acquired Professorships in Natural Sciences in Calabria, Catania and finally in Naples, enthusing his students in volcanic studies.

In my recent visit to Sicily, I stayed in Catania and observed Mount Etna in all its glory, only 28 km away from the city, as it spewed out plumes of gases. Beyond Etna, Mercalli observed, at first hand, the eruptions of Stromboli and Vulcano.

In the latter’s last major eruption in the late 1880’s he described it as “Explosions sounding like a cannon at regular intervals…” Moreover, he photographed this eruption and its inevitable deposits.

In 1906 he was there on Mount Vesuvius to record and photograph the eruption there.

Mercalli scale of earthquake intensity

This scale can be used anywhere in the world without seismographic instruments to observe the intensity of an earthquake and its foreshocks and aftershocks. It is observational, as felt and seen by our eyes in terms of man-frightening and wall-smashing effects:

1. Not felt by anyone other than experienced observers at rest.

2. Noticed by only a few persons for it resembles vibrations from heavy traffic on a distant road.

3. Slight: felt by many people indoors: suspended objects sway: vibrations like those of a heavy lorry passing close by.

4. Moderate: felt by almost everyone indoors and some outdoors: dishes and ornaments rattle on shelves: floors and wall creak.

5. Generally felt and with alarm: furniture and beds shifted: pictures may fall from walls and dishes from shelves: church bells may ring.

6. Causes considerable alarm: awakening all who may be sound asleep: plaster may crack and fall: pendulum clocks stop; general ringing of bells.

7. Strong: alarm and panic: people rush outdoors: moveable objects overturn: plaster falls from ceilings: poorly built houses suffer severe damage.

8. Walls of well-built buildings crack: bricks, roof tiles and chimneys fall: monuments overturned in cemeteries: substantially built structures badly damaged.

9. Structures shifted from foundations: some buildings collapse: general panic: underground pipes broken as ground cracks appear.

10. Most masonry and frame structures destroyed: landslides on steep slopes: train rails bent.

11. Only buildings of reinforced concrete or steel-skeleton construction remain standing. Bridges collapse: all underground pipes and utilities out of service.

12. Maximum catastrophe: damage total.

Mercalli’s other legacies

It was through his observations that the various types of volcanic eruptions have been adopted by vulcanologists to produce a volcanic explosive index of central eruptions.

Mercalli is accredited to be the author of four out of seven types of eruption based on their explosive capacity and the contents raining down or pouring out from the central vent of a volcano.

These types of eruption are Strombolian, Vesuvian, Vulcanian and Plinian.

The very form of the shape of a volcano can be very variable in terms of the magma’s chemical composition below ground, the viscosity of the lava flow, the gaseous content and the pressure beneath the volcano.

Worldwide, there are few explosive eruptions but they still exist as in Iceland, Indonesia and, indeed, at Stromboli. Usually gaseous emissions are observed as pressure builds up beneath the crater chamber.

Sometimes, this pressure is released by blasting out the lavas that clog the vent, to create pyroclastic deposits on the cone’s slopes in the form of volcanic bombs.

On the island of Vulcano, a short, relatively recent blast ejected large quantities of material which fell back into the crater in the form of black obsidian (volcanic glass), pumice and other pyroclastic forms of all shapes and sizes.

A Plinian type of eruption, caused by a massive gas build up, will eject huge quantities of ash high into the atmosphere and can disrupt air-flights, as we have experienced in Iceland and in Indonesia, at Lombok, in very recent times.

Mercalli’s spirit survives

At the Volcanological Centre on Vulcano island, I met up with two final year geological students from the University of Bari in southern Italy.

Their mission was to educate the islanders and tourists on the history of the numerous volcanoes in the Aeolian Islands.

This volcanic information centre was dedicated to the late Professor of Geochemistry at the University of Palermo, Marcello Carapazza. He specialised in the first continuous monitoring of volcanic happenings on that island.

To see, live, the detail of recordings minute by minute on screens at that magnificent centre, was a joy to behold. Undoubtedly, the groundwork in observations of earthquakes and volcanic activity that Mercalli achieved in his 63 years of life has set a new 21st century group of Italian vulcanologists literally ablaze!

Yet more will be revealed next week concerning Vulcano island’s history. Surely this island must be included in anyone’s planned tour of Italy and Sicily.