Earthquakes in caves

23.01.2020.

Our curator Damir Lacković has prepared an exceptionally interesting topic for you!

The topic is “EARTHQUAKES IN CAVES”, and over the coming days, through four posts, he will explain exactly what it is about 🤓

Is it safe to be in caves during an earthquake?

As a rule, caves are safer during earthquakes than houses on the surface of the Earth. However, just as not all houses are equally safe, not all caves, nor different parts of the same cave, are equally safe.

First of all, the closer a cave is to the epicentre of an earthquake, the stronger the effect of the earthquake will naturally be. Furthermore, entrances and passages close to the surface are less safe than deeper passages.

The reason for this is that seismic waves are more destructive at the surface itself than deep inside the Earth, similar to sea waves that are stronger at the surface of the sea than in depth.

In addition, at the surface, rocks are affected by constant changes in temperature, humidity, bioerosion and other factors, which accelerates their weathering and makes them weaker than the same rocks in deeper parts with a stable cave climate.

Looking at the morphology of cave passages, smaller tube-shaped passages in solid bedrock, mostly limestone and dolomite in Croatia, that have no speleothems, cave decorations, are safer than large-diameter passages or large chambers rich in speleothems and other cave sediments.

Earthquake shaking will primarily bring down delicate speleothems such as stalactite tubes, popularly called “macaroni”, thinner stalagmites, helictites, and sometimes even whole curtains, massive stalagmites and columns.

All of these are only cave deposits which, during an earthquake, may collapse, crack or detach from the solid rocky base on which they were deposited.

But just as shaking and the falling of shelves, pictures and cupboards in apartments does not at the same time mean the collapse of ceilings and walls, so the falling of speleothems onto a passage floor is still very far from the complete collapse of entire passages and chambers.

The strength of the foundation of cave passages – their walls, floors and ceilings – depends primarily on the intensity of tectonic fracturing of the rock in which the passage is located, characteristics created by the complex geological development of the entire area.

Thus, passages formed within a system of numerous fractures will be more prone to damage than those formed along a single fracture or a smaller number of fractures.

Passages along faults – fractures along which rock displacement occurred during breaking, and chambers located at the intersections of several faults and fractures, will potentially be more prone to the breaking off and falling of fault-fractured parts of the rock.

The main passage of Veternica Cave on Medvednica extends beneath the steep slope below the Glavica mountain lodge and in this way very quickly moves away from the surface, and therefore from the effects of stronger surface seismic waves.

We can therefore assume a stronger earthquake impact only at the entrance and in parts of the cave that approach the surface.

Examples of delicate speleothems that can break or collapse during an earthquake:

Thin stalactite tubes can break even during a weak earthquake.

Photo: D. Lacković – in Tounj.

Thin helictites can also break even during a weak earthquake.

Photo: D. Lacković – in Tounj.

A thin tall stalagmite may crack or collapse completely as a result of earthquake shaking.

Photo: D. Lacković – at the location Kojina Jama.

Large thin curtains on the ceiling of a passage are also prone to cracking.

Photo: D. Lacković – at the location Kojina Jama.

A fault plane, paraclase, in the ceiling of a passage in Močiljska Cave near Dubrovnik. Traces of rock movement along the fault, so-called striations, are visible in the direction from the lower left corner of the image to the upper right.

In the right part of the image, traces of broken rock are also visible on the ceiling, probably detached during one of the past earthquakes.

Photo: D. Lacković – at the location Močiljska Cave.

Traces of earthquakes in caves

Caves are protected environments in which various events from the geological past of the Earth remain recorded and permanently preserved, including earthquakes.

On the floors of large passages and chambers, blocks of rock and entire speleothems or their parts are often found, most commonly the tips of stalactites, which fell from ceilings and walls or collapsed on the passage floor itself, stalagmites, during ancient earthquakes.

By determining the age of speleothems that continued to grow on fractures after an earthquake, it is possible to determine the age of the earthquakes themselves in the distant past.

Sometimes speleothems can collapse for other reasons, for example large stalactites thickened at the top can become too heavy for themselves and simply break, while thin stalactite tubes and helictites can be brought down even by strong air currents or by the weight of a bat.

Sudden major floods can cause large changes – knock down stalagmites, wash away the clayey base of speleothems and thus cause their collapse, and more.

It is interesting that some speleothems, such as cave palettes whose deposition begins with the formation of a crack in rock or in a speleothem, begin their growth precisely after an earthquake.

Through the circulation of water along the crack, thin calcite plates crystallise, which we call cave palettes, and subsequent water circulation on the outside can lead to their covering with flowstone and the growth of stalactites, as in the example of the cave palette from Lucinka Cave.

Probable traces of earthquakes:

A broken stalactite on the ceiling of a passage on which a new stalactite grew after the break, Špilja za Gromačkom vlakom.

Photo: D. Lacković – in Orašac, Croatia.

A stalactite that fell from the ceiling onto the passage floor, where it became fused with cave rimstone pools, Špilja za Gromačkom vlakom.

Photo: D. Lacković – in Orašac, Croatia.

IMAGE: Cave palette with stalactites formed on a crack in a column, Lucinka Cave.

Photo: D. Lacković – at the location Mala Paklenica.

DRAWING: Reconstruction of the formation of the cave palette and stalactites on a crack in a column in Lucinka Cave in Mala Paklenica, from left to right:

A – vertical movement of water along the column before the earthquake.
B – earthquake-caused crack in the column and the beginning of horizontal water movement along the crack.
C – formation, crystallisation, of calcite plates – the cave palette.
D – covering of the palette with flowstone due to external water circulation and formation of stalactites.

Earthquake-triggered formation of an unusual speleothem from the CNHM Speleothem Collection

This unusual speleothem was found during speleological research in the Neretva Valley area in 1998.

Its special feature is that, in the same speleothem, its upper part is a stalagmite and its lower part is a stalactite.

Since stalactites form on the ceiling of a passage and stalagmites on the floor, how is it possible that a floor stalagmite was deposited on the root of a ceiling stalactite?

The most likely answer is provided precisely by an earthquake!

An earthquake could have knocked a stalactite from the ceiling, which then became embedded in the clayey passage floor as it fell. Solution continued to drip from the ceiling and deposited a stalagmite on the root, or base, of the former stalactite, see image and drawings.

Detailed isotopic analyses of this unusual speleothem, using the ¹⁴C or U-Th method, could determine the time immediately after the earthquake, or the age of the earthquake.

From the CNHM Speleothem Collection: Stalagmite on stalactite, Jama na Plitvinama II. Size 75 x 7.5 x 7 cm. Inv. no. 600:ZAG; 9200:MP1.

Reconstruction of the formation of the stalagmite on stalactite in Jama na Plitvinama II:

A – Growth of the stalactite on the passage ceiling before the earthquake.