An earthquake is the sudden, violent shaking of the ground caused by the movement of tectonic plates, releasing stored energy in seismic waves. A sinkhole is a depression caused by the collapse of overlying material into a subterranean void. While these two phenomena often appear linked, earthquakes are not the primary cause of sinkholes. Seismic events act instead as a powerful trigger, leading to the rapid collapse of a cavity that was already formed and structurally unstable due to long-term geological processes.
Understanding Non-Seismic Sinkhole Formation
The vast majority of sinkholes form through dissolution, a gradual process requiring soluble bedrock. This process involves chemical weathering where rocks are slowly dissolved by circulating, slightly acidic groundwater. Soluble bedrock types include:
- Limestone
- Dolomite
- Gypsum
- Salt
This action creates a distinct topography known as karst, characterized by underground caves, channels, and voids.
Over thousands of years, this acidic water seeps through cracks, eroding the rock and creating a large underground cavity. The three main types of sinkholes are defined by how this void interacts with the surface material. A dissolution sinkhole forms where the soluble rock is exposed or thinly covered, creating a slow-developing depression on the surface.
Where soluble bedrock is covered by thick, unconsolidated sediment like sand or clay, the process is more dramatic. Cover-subsidence sinkholes occur when granular material washes downward into the cavity, leading to a gradual surface depression. The most hazardous type is the cover-collapse sinkhole, where a cohesive arch of soil forms over the underground cavity.
This arch remains stable until the void’s upward migration or a sudden external stress causes it to fail. The surface then collapses abruptly into the cavern below, often without warning. In all these cases, the underlying instability is created by the chemical action of water, which is the fundamental driver of sinkhole formation.
The Mechanisms of Seismic Triggering
When an earthquake strikes, its energy translates into two primary mechanisms that accelerate a potential collapse. The first involves the sudden application of dynamic stress and intense vibration to the subsurface. Where a subterranean void exists, seismic waves cause the ground to shake violently, delivering immense force to the cavity roof.
This rapid, cyclic loading can exceed the tensile strength of the arching soil or rock bridging the void. The cumulative stress from the shaking acts as the final tipping point, causing the weakened ceiling to fracture and drop, triggering an instantaneous cover-collapse sinkhole. This mechanism is effective on pre-existing, marginally stable cavities nearing their natural failure point.
The second mechanism is soil liquefaction, involving a rapid loss of strength in saturated, loose sediments. During an earthquake, intense shear strain waves cause water-saturated granular soil particles to temporarily lose contact. This increases water pressure within the soil, known as excess pore pressure, causing the ground to momentarily behave like a dense liquid.
When the soil above a subterranean void liquefies, it loses its structural integrity and cannot maintain the arch shape. The fluidized material flows rapidly downward into the void, or simply stops providing support, resulting in sudden ground failure. This mechanism explains why sinkholes can appear in areas with loose, water-filled sediments far from the earthquake’s epicenter.
Geological Requirements for Triggered Sinkholes
Earthquake-triggered sinkholes are rare, requiring a specific and relatively uncommon convergence of geological factors. Foremost is the existence of a robust karst system, meaning the bedrock must be soluble and contain established subterranean voids. Without this pre-existing structural weakness, seismic energy has nothing to collapse.
The location must be close enough to an active fault line to receive significant seismic energy, typically from a moderate to high magnitude earthquake. The ground motion must be strong enough to generate the intense vibration or liquefaction required for the mechanical trigger. For example, the 2020 Petrinja earthquake in Croatia triggered over 90 sinkholes in a region overlaying highly soluble karst rock.
A fluctuating water table also plays a significant role. Changes in groundwater flow or a sudden drop in the water table can reduce the buoyant support water provides to the cavity roof. An earthquake can then rapidly alter the subsurface hydrology, either by changing flow paths or by shaking out the supporting water, destabilizing the stressed ceiling and leading to collapse.
The effectiveness of the seismic trigger depends on three elements: an existing soluble rock structure, the presence of loose, saturated cover material, and proximity to a strong seismic source. Documented events, such as the sinkholes triggered by the 2009 L’Aquila earthquake in Italy, confirm these phenomena are confined to specific geological environments where instability has been building for centuries.