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.