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Earthquakes in Grisons

An earthquake causing at least minor damage to buildings occurs in the Canton of Grisons on average every 30 to 40 years. The main areas of heightened activity are Lower Engadine, the Val Müstair and central Grisons. By comparison, western Grisons experiences fairly low levels of seismic activity.

The strongest known earthquake in Grisons happened in 1295 in the Churwalden area (intensity VIII). If a similar earthquake were to happen today, it would cause around CHF 12 billion worth of damage across the canton. The most recent damaging quake occurred near Vaz in 1991, causing minor damage to buildings.

 

The region stretches from the Val Mesolcina and Vorderrhein (Anterior Rhine) valley in the west to the upper Vinschgau valley in the east, and from the Valtellina in the south to Bad Ragaz and Samnaun in the north. It therefore encompasses adjacent parts of Ticino to the south-west, of Glarus and St. Gallen to the north-west and of Italy and Austria to the south and east.

Seismic activity in Grisons

Over the past 40 years, 55 earthquakes with a magnitude of 3 or greater have occurred in Grisons and its immediate surroundings. All were felt by local residents. Eleven attained a magnitude of more than 4 and were felt over a wider area. The strongest earthquake in this period took place on 20 November 1991 in the region of Vaz. In the epicentral area around Vaz/Obervaz and Muldain, it caused cracks in the facades of several buildings. The main quake was followed by over 80 aftershocks, lasting until the spring of 1992.

Magnitude Number
ML ≥ 2.0 646
ML ≥ 2.5 182
ML ≥ 3.0 55
ML ≥ 4.0 11
Table: Magnitude frequency in and around Grisons between 1975 and 2014. ML is the local magnitude on the Richter scale.

All earthquakes of magnitude 3 and above, and some between 2.5 and 3, were felt by local residents.

Date Magnitude Location
17.07.1976 4.2 Filisur
31.07.1983 4.3 Val Venosta (I)
31.08.1983 4.0 S-Charl
08.06.1984 4.2 S-Charl
16.05.1990 4.0 Piz Tasna
20.11.1991 5.0 Vaz
29.12.1999 4.9 Bormio (I)
31.12.1999 4.4 Bormio (I)
06.04.2000 4.3 Bormio (I)
01.10.2001 4.3 Bormio (I)
21.01.2008 4.0 Paspels

Although the first seismographs in Switzerland began operating in the early 20th century, a uniform national seismic monitoring network has only existed since 1975. This has been continually expanded and adapted in line with the latest technological developments. As a result, its sensitivity has increased and its location accuracy has improved over time. The epicentres of instrumentally recorded earthquakes in and around Grisons between 1975 and 2014 differ significantly in their distribution (see epicentre map 1975 - 2014).

Historical records are the only information we have about earthquakes in times past. The strength of these quakes can be reconstructed based on the documented damage and quantified in the form of intensities. Although in principle it is possible to estimate a magnitude based on the observed intensities, this would be subject to considerable uncertainty.

In Grisons and surrounding areas, seven earthquakes with intensities between VI and VIII are known to have occurred since the end of the 13th century. All of these quakes caused minor to severe damage to buildings and in some cases injuries and fatalities. Nine earthquakes with intensities of at least VIII are known to have occurred in Switzerland from historical records. One of these happened in Grisons, namely the 1295 earthquake in Churwalden; this is the oldest historically recorded earthquake in Switzerland. While each individual historical information source for this event is only partly reliable on its own, taken together they paint a coherent picture of the location and date of the quake and of its effects. Thus, recorded damage to churches, monasteries and castles in the epicentral area can be attributed to this earthquake, but we can also conclude from a number of information sources that its impact extended to southern Germany, Austria and Italy. If the Churwalden earthquake were to occur today with the same strength, it would cause estimated building damage of around CHF 5 billion and total damage of around CHF 12 billion within Grisons.

List of known earthquakes with intensity = VI in and around Grisons

Date Intensity Location
03.09.1295 VIII Churwalden
03.08.1622 VII Ftan
27.08.1857 VI Tarasp
25.12.1905 VII Domat Ems
26.12.1905 VI Tamins
09.12.1917 VI Silvaplana
20.11.1991 VI Vaz

In the list of historically known damaging earthquakes in Grisons, it is striking that only two events are recorded between the late 13th and mid-19th centuries, while five such events have occurred in the last 160 or so years. This uneven distribution reflects the patchiness of data from earlier centuries. It is therefore difficult, on the basis of historically known earthquakes alone, to draw statistically reliable conclusions about the incidence of damaging earthquakes in Grisons. Nevertheless, if we assume that the last 200 or so years are more or less representative, an earthquake causing at least minor damage to buildings can be expected to occur in Grisons every 30 to 40 years. An earthquake causing major damage (intensity VIII) – such as can be expected in the Valais once every 100 years on average – has only occurred once in Grisons in the past 700 years.

Another notable thing about the list of damaging earthquakes is that the two quakes in December 1905, with intensities of VII and VI, occurred within a period of just 24 hours. The distance between the stated epicentres of these two earthquakes (Domat-Ems and Tamins) is only around 10 km, which is less than the margin of uncertainty regarding locations. The short time interval and distance between the two earthquakes and other smaller tremors that occurred before and afterwards in the same area suggest that these two events were part of a protracted swarm.

Figure: Epicentre map of historically known earthquakes in and around Grisons since 1200 with intensity >= VI (damaging earthquakes).

Earthquakes in Grisons often occur as a series of events which show up on earthquake maps as a cluster of epicentres. The duration of these series can vary considerably, with some lasting just a few days while others, such as the Bormio series, continue for several years. Typically, such series involve a large number of earthquakes in a localised area over an extended period of time, without a clear sequence of foreshocks, main quake and aftershocks. This is also known as an earthquake swarm. Swarm activity usually ends after a few weeks or months, but occasionally the quakes increase in strength and number over time. If the location of the individual foci in such swarms can be calculated with sufficient accuracy, it always defines one or more surface areas that correspond to the fracture surfaces activated by the earthquakes in the subsurface.

The Bormio earthquake series

A notable example is the Bormio earthquake series, which included four events with local magnitudes greater than 4. The epicentre of this swarm was in Italy, on the border with Val Mora, around 10 km south of the Ofen Pass. The main quake, with a local magnitude of 4.9, occurred on 29 December 1999. This was preceded by a magnitude 2.3 quake on 15 April and a magnitude 2.4 quake on 28 December. A total of almost 200 events were recorded in this swarm, lasting until the autumn of 2002.

The Paspels earthquake series

A particularly well-investigated example of such earthquake swarms in Grisons is the series of quakes that occurred in Paspels between 2007 and 2009. It began in early August 2007 and peaked on 21 January 2008 with a magnitude 4.0 quake. A total of 37 quakes assignable to this swarm were recorded through to late 2009. The seismic foci were located at a depth of approximately 8 km on a fracture surface running almost east-west with a horizontal length of at least 700 m and a vertical extent of around 500 m.

Essentially, earthquakes are caused by the sudden release of stresses in the earth's crust. These stresses are brought about by large-scale continental movements. If they are stronger than the rock, a sudden rupture occurs along existing areas of weakness in the subsurface, triggering the seismic waves felt on the earth's surface. Modern seismology uses seismic observations to draw conclusions about the causative stresses and so deduce regional differences in the stress field.

Grisons's predominant geological structures are huge stacks of superimposed rock strata testifying to the compression that formed the Alps. By contrast, most present-day earthquakes in Grisons are triggered by lateral spreading: this results from a superposition of stresses from the ongoing collision between Africa and Europe and of compensatory stresses caused by the topography of the Alps and their roots. Obviously, stresses in the earth's crust have changed over the course of millions of years.

Much of the Canton of Grisons faces a moderate seismic hazard, less than the Valais but greater than Ticino and the Swiss plateau. Earthquake-resistant residential or office buildings in Switzerland are designed to withstand shaking that is expected to occur where the building is situated once every 500 years on average. This is illustrated on the seismic zone map. The central and eastern part of the canton falls within zone Z2. In this zone, shaking with a maximum intensity of VII-VIII (moderate to severe building damage) is expected to occur once in 500 years. The western part of the canton, comprising the Hinterrhein valley and upper Vorderrhein valley, lies in zone Z1, where shaking with a maximum intensity of VI-VII (minor to moderate building damage) is expected to occur once in 500 years.

Amplification of seismic shaking by local subsoils

As well as the magnitude and epicentral distance, the local subsoil type has a major impact on the amount of damage caused by an earthquake. For example, unconsolidated river and lake deposits can strengthen seismic waves, meaning that the ground motion intensity on a valley floor may be up to 10 times greater than in the rocky substrate of the valley sides. In the Canton of Grisons, the impact of such site effects is particularly significant in the Rhine Valley between Chur and the municipalities of the Bündner Herrschaft.

Secondary natural hazards triggered by earthquakes

The impact of earthquakes is not limited to shaking and direct damage to buildings. Additional damage is often caused by other natural hazards triggered by earthquakes. The unstable slopes commonly found in mountainous regions may be suddenly weakened by earthquake-induced shaking, causing rockfalls or landslides into the valley below. These may destroy transport links and energy and communications infrastructure. The same is true of avalanches in winter. Studies of past earthquakes show that Grisons, with its steep mountainsides, is exposed to these additional earthquake-induced natural hazards. This must be taken into account when assessing the earthquake risk.