Salt Dissolution Causes German Earthquake

By: Cath Rueckeis

In the German city of Hamburg, the process of underground dissolving of salt, known as salt dissolution, has had serious consequences for the local population. In fact, a series of micro-earthquakes have been traced to the collapse of underground caves formed by salt dissolution.

When underground layers of salt dissolve, caves and domes develop, which are commonly referred to as salt diapirs.

salt diapir

These caves and salt domes can collapse when they are overburdened, resulting in sinkholes in an elliptical or circular shape, known as collapse dolines. Collapse dolines can be anywhere from several meters to a few kilometers wide.

collapse doline

In Hamburg, two types of sinkholes have been identified: dissolution sinkholes, which are characterized by slow and continuously subsiding zones, and dropout sinkholes, marked by abrupt and episodic collapse.

In this case, leading scientists at the Hamburg University linked a micro-earthquake in one of the city’s districts to the collapse of an underground salt dome. On the eve of April 8th, 2009, inhabitants of Flottbeck in a roughly 100m radius felt an unusual shock, though no damage or surface effects were recognized.

The shock was also recorded by three local seismic stations. At its strongest, the earthquake had a magnitude of 0.6 on the Richter scale. Thus, categorically this was a micro-earthquake. The maximum vertical ground displacement and velocity were measured and it was determined that the ground oscillated at 0.35mm/s over a height change of 0.03mm. Twenty possible aftershocks were detected by May 18th.

Professor Torsten Dahm of the Helmholtz Zentrum near Berlin worked with the Hamburg University on conducting calculations based on the observations of the local seismic stations. He argues that the earthquake was not caused by tectonics but by an impulsive origin at 100m depth. According to him,  crustal earthquakes are highly unlikely, because Flottbeck is located near the center of the Eurasian tectonic plate. Additionally, there exists a local salt diapir, which feeds the growing Flottbeck Markt sinkhole, a large, local dissolution sinkhole. The recorded pattern of seismic waves can also likely be explained by the collapse of such a cave. Thus, the superficial ground shocks are very likely related to sedimental cave collapse of a salt dome in the Flottbeck Markt sinkhole.

Indeed, similar earthquakes have been caused by gas recovery and reservoir depletion in other areas in Germany. There have between 25 reports of comparable ground shocks in Flottbeck within the past 250 years. Dahm claims that by harvesting oil from oil reservoirs near salt domes, humans cause diapirs to expand and become a hazard for urban societies.

New technologies, such as gravity surveys, have been developed for detecting underground salt domes and predicting potential collapses. Of course, sinkholes are now taken into account during urban planning in Hamburg. Additionally, oil reservoirs are no longer being tapped near urban civilizations.

References

Choi, Sungchan. “3-D Density Modelling of Underground Structures and Spatial Distribution of Salt Diapirism in the Dead Sea Basin.” GeoRef. Geophysical Journal International, Mar. 2011. Web. 1 Dec. 2014. DOI: http://dx.doi.org/10.1111/j.1365-246X.2011.04939.x

Dahm, Torsten. “A Seismological Study of Shallow Weak Micro-earthquakes in the Urban Area of Hamburg City, Germany, and Its Possible Relation to Salt Dissolution.” GeoRef. Journal of the International Society for the Prevention and Mitigation of Natural Hazards, Sept. 2011. Web. 1 Dec. 2014. DOI: http://dx.doi.org/10.1007/s11069-011-9716-9

 

writing in the natural sciences