After the Valais, the area around Basel is the second highest seismic hazard zone in Switzerland. Historically, earthquakes have been documented in Basel and its immediate surroundings since the 14th century, but there may have been a major earthquake near the Roman settlement of Augusta Raurica as far back as around AD 250. Basel was also the location of Switzerland's largest known earthquake to date. This took place in 1356 and had a magnitude of 6.6.
Due to its increased seismic hazard, population density and many particularly vulnerable buildings, the city of Basel has the highest earthquake risk in Switzerland. For example, if the magnitude-6.6 Basel earthquake of 1356 were to be repeated, the expected death toll in Switzerland would be around 3,000, with building damage totalling approximately CHF 45 billion. Such an earthquake can be expected in the Basel region about every 2,000 to 2,500 years. However, there is no way of predicting exactly when the next one will strike.
The largest natural earthquake to date in the canton of Basel-Stadt had a magnitude of 2.8 and occurred in April 1996 at a depth of around 12 km. The strongest quakes in the region have taken place north-west of Basel on the French side of the Upper Rhine Graben.
An earthquake sequence happened there near Sierentz in Alsace in July 1980, in which the main quake reached a magnitude of 4.4. This was followed by numerous aftershocks, with magnitudes of up to 4.0 and 3.8. In September 2022, a magnitude-4.7 earthquake struck the same region near Sierentz (France), around 15 km north of Basel. It took place at a depth of approximately 12 km, and the shaking could be clearly felt throughout northern Switzerland. Around ten minutes after the mainshock, at 18:07 (local time), a first aftershock, with a magnitude of 2.8, could also be felt near the epicentre. The earthquakes are tectonically related to the seismically active Rhine Graben, which stretches north-south between the Vosges and the Black Forest. Earthquakes are not uncommon in this region, although an earthquake of this magnitude only happens here every 10 to 20 years.
In May 2009, a magnitude-4.2 earthquake occurred north-east of Basel near the municipality of Steinen in the Black Forest, which shows that relatively large earthquakes can also hit the neighbouring Black Forest massif. The same is true of the Jura Mountains south-east of Basel, especially in the Fricktal region.
A characteristic feature of seismicity in the Basel area, and typical also of much of the northern Alpine foothills, is that earthquake depths span the entire thickness of the Earth's crust. This depth distribution runs counter to the usual rheological behaviour of the Earth's crust and is possibly related to the plate tectonic processes in the Central Alps referred to above.
|ML ≥ 2.0||326|
|ML ≥ 2.5||118|
|ML ≥ 3.0||30|
|ML ≥ 4.0||6|
On 18 October 1356, several violent earthquakes shook the city of Basel and the surrounding areas. The series of quakes began around noon, with the first damage caused by a stronger quake at approximately 6 p.m. It is assumed that most of the population moved outdoors during these tremors. Many people presumably stayed there because the ground continued to shake, sometimes severely. The largest earthquake in the series struck at around 10 p.m. With a magnitude of about 6.6, this was the strongest documented earthquake in Switzerland's history. Aftershocks continued to shake the city for many months.
The 1356 earthquake was the largest, but far from the only, quake to hit the Basel region in the historical past. There may even have been a major earthquake near the Roman settlement of Augusta Raurica in around AD 250.
The city of Basel is located at the southern end of the Upper Rhine Graben, bordering the Black Forest massif to the north-east and the Jura Mountains to the south. Deep boreholes have revealed that the local subsoil beneath the city of Basel consists of a sedimentary package approximately 2.5 km thick, made up of Cenozoic, Mesozoic and Permo-Carboniferous rock formations. This relatively thick sedimentary package, linked in part to the stretching of the Earth's crust during the formation of the Upper Rhine Graben, has a detrimental impact in the event of an earthquake, as it can lead to resonance effects and the amplification of seismic waves. Beneath this sedimentary layer lies the crystalline basement.
Earthquakes are caused by a sudden release of stress along faults in the Earth's crust. The continuous motion of tectonic plates causes a steady build-up of pressure in the rock strata on both sides of a fault until the stress is sufficiently great that it is released in a sudden, jerky movement. The resulting waves of seismic energy propagate through the ground and over its surface, causing the shaking we perceive as earthquakes.
The region around Basel was shaped by various tectonic processes in the geological past. The Upper Rhine Graben, which began to form 50 million years ago, is part of a European rift valley system stretching from the North Sea to southern France. During this geological era, the European continent was in a spreading phase. At the same time, crustal blocks were raised at the edges of the graben, forming the Vosges to the west of the graben and the Black Forest to the east. This process led to the formation of numerous faults in the region, running predominantly in a north-north-east to south-south-west direction. The second large-scale tectonic process was the last phase in the formation of the Alps and the folding of the Jura Mountains. It began around 10 million years ago and resulted in the formation of folds and thrusts in the Jura south of Basel.
The numerous fault systems created as a result – which also include faults from even earlier phases – are mostly reactivated as strike-slip faults in the present-day stress field around Basel. In a strike-slip fault, the two sides of a fault slip sideways past each other. Earthquakes occur regularly in the region due to this stress field, which is partly related to plate tectonic processes in the Earth's mantle under the Alps. Although today's deformations in the northern foothills of the Alps are comparatively small, tectonic stresses sufficient to trigger an earthquake like that of 1356 can build up over hundreds or thousands of years. Such an event can be expected to occur approximately once every 2,000 to 2,500 years.
This section on Basel's subsurface appeared in a slightly modified form in the 2022 Basler Stadtbuch dossier entitled Basel bebt (in German).
The Deep Heat Mining project in Basel was a ground-breaking Swiss energy research project that planned to build a pilot plant for a geothermal power station using an enhanced geothermal system (EGS). This technique involves forcing cold liquid, usually water, down into hot rock at great depths, where it heats up before being returned to the surface where it can be used to produce heat or electricity. In the medium term, the project was intended to generate environmentally-friendly power, harnessing Swiss energy sources to lower the country's reliance on imported energy. The planned location of the future geothermal power plant was the Industrielle Werke Basel site in Kleinhüningen. From there, the heat could have been injected into the extensively developed municipal district heating system. During the summer months, when there is low demand for district heating, the plant would mainly have generated power.
In December 2006, after an exploratory phase, water was due to be forced under high pressure into the crystalline basement rock over a two-week period. The aim of this procedure was to increase the rock's permeability at a depth of between 4,000 and 5,000 m and create a geothermal reservoir where the liquid would circulate and heat up. The geothermal injection process and the earthquakes it induced were recorded by a dense network of seismic monitoring stations.
The rate of injection (quantity of liquid over time) was gradually increased until, on the sixth day, the maximum rate was reached. Shortly after, an earthquake with a magnitude of 2.6 occurred, whereupon the rate of injection was first decreased and then stopped altogether a few hours later. Approximately five hours after that, an earthquake with a magnitude of 3.4 took place. Having an intensity of V, it was felt over a wide area and caused minor damage to buildings. By February 2007, there had been three more perceptible earthquakes with magnitudes greater than 3.0, and over 200 quakes with magnitudes of 0.9 or above had been recorded. The project management then suspended the project and finally ended it altogether in 2009 after a comprehensive risk analysis. The damage claims received totalled several million Swiss francs and related mainly to small cracks in buildings' plasterwork. Most resulted in the payment of compensation.
In recent years, the issue of seismicity in the canton of Basel-Stadt has been largely connected with the 2006 Deep Heat Mining project. However, these earthquakes, which occurred a short distance from the borehole, were relatively weak. The strong and damaging earthquakes seen in the past (and those potentially occurring in the future) are mainly attributable to the geological structure of the Rhine Graben, the southern end of which lies in the region. After the Valais, the area around Basel is the second highest seismic hazard zone in Switzerland. Major earthquakes are rare in Switzerland: a quake of magnitude 6 or more can be expected around every 50 to 150 years. This could occur anywhere in the country, at any time, and cannot be predicted.
This section on human-induced earthquakes in the Basel region appeared in a slightly modified form in the 2022 Basler Stadtbuch dossier entitled Basel bebt (in German).