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Earthquakes of 8 and 21 July 2010 in the area of Manosque and seismicity of Provence


Earthquakes of 8 and 21 July 2010 near Manosque

On 8 July 2010, at 20h 20 min UT, the Detection and Geophysics Laboratory of the CEA located an earthquake of magnitude 2.9 with an epicentre situated near Manosque (Alpes-de-Haute-Provence) and having a very shallow focal depth (less than 5 km). This earthquake occurred near the Middle Durance Fault. It was followed by a second event of magnitude 2.0 on 21 July 2010 at 04h39 UT.

Date 08/07/2010 21/07/2010
Onset time 20h 20min 51.61s UT 04h 39min 35.14s UT
Latitude 43.85 North 43.76 North
Longitude 5.82 East 5.73 East
Magnitude  2.9 (Ml)
2.0 (Ml)

Each year, on average, three earthquakes in Provence attain or exceed a magnitude of ML=2.9 (see figure 1, which shows the frequency of seismic events in the Provence region according to magnitude; this curve is established from the data base of seismicity recorded by the LDG since 1962 and the historical database SISFRANCE). In this context, the earthquake of 8 July 2010 is nothing exceptional, even if it was widely felt in Manosque because of its very shallow depth and proximity to the town.
In the current state of scientific knowledge, it is not possible to predict the time or the location of earthquakes. Very many earthquakes of small to moderate magnitude are not followed by a stronger earthquake. Certain large earthquakes are sometimes preceded by smaller seismic events (seismic precursors), but this is not always the case.

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 Figure 1: Frequency of historical and instrumental earthquakes up to 2009 as a function of their magnitude for the whole Provence region. For example, an earthquake of magnitude greater than 4.5 is observed, on average, once every 10 years.


Tectonic context and regional seismicity

The tectonic context of Provence is characterized by a compressive regime corresponding to the convergence of the African and European tectonic plates. In terms of regional tectonics, this results in several generations of faults including the Durance fault.
This latter fault, considered as seismically active, is one of the faults in metropolitan France that is best expressed in terms of past seismicity, with several important crises since the beginning of the 16th century. On 13 December 1509, the reported sed damage reached MSK intensity degree VIII in the area around Manosque (see glossary for the definition of MSK seismic intensity scale) [Translator’s note: there is no definition of the MSK scale in the CEA on-line glossary; MSK has now been superceded by the EMS scale]. A rather similar seismic crisis occurred in 1708. The main shock of 14 August 1708 (epicentral intensity VIII), although felt up to a hundred km away, caused damage only in one very restricted area. By 1812, the seismic activity had migrated about 15 km to the south-west (Beaumont de Pertuis), associated with an epicentral intensity of VII-VIII. The main earthquake of 20 March 1812 was followed by about sixty aftershocks that were only felt in the immediate vicinity of Beaumont. The last significant event dates back to 14 May 1913, occurring 15 km to the North-East of the seismic swarm of 1708 (area of Volx, epicentral intensity VII-VIII).
While several earthquakes of intensity lower than VII have also occurred in the area of Cavaillon (1731, 1763 and 1863), the Costes-Trévaresse system attracts our particular attention because it is associated with the occurrence of the only earthquake in Provence of intensity greater than VIII (Lambesc, 11 June 1909, epicentral intensity VIII-IX), which caused very serious damage in a radius of 10 km around its epicentre (maximum intensity IX).
More recently, the seismicity of Provence, as recorded by seismic sensors since 1962, has remained diffuse and of weak magnitude. The majority of the detected earthquakes have a local magnitude (ML) of less than 3.5, and only some seismic events have a local magnitude ranging between 3.5 and 4.4. For example, we can mention the following events: west of Avignon (25/02/1986) ML = 3.7; north-east of Marseilles (31/10/1973) ML = 3.6; south-east of Gardanne (19/02/1984) ML = 4.4.
This seismicity highlights the active character of a certain number of structures, such as the Durance fault or the Costes-Trévaresse system. Apart from localities situated within a few km of the major epicentres, the maximum catalogued intensities only seldom exceed degree V. The historical data for Provence, being more abundant than elsewhere in France, indicate that many catalogued seismic events probably correspond to earthquakes leading to very localized effects, which can be attributed to the shallow focal depths (5 km or less).