The seismic history of the area ? marked by the two great earthquakes of 1703. The two strong tremors of 14 January and 2 February 1703 occurred immediately to the north and south of the area affected by the sequence in progress. The magnitude of the two events? been estimated through macroseismic analysis and indicate a magnitude of 6.8 and 6.7 for the events of January 14 and February 2, respectively, magnitude significantly greater than the earthquake of April 6, 2009. The event of January 14 had intensity? maximum equal to the XI degree MCS in 4 locations? while the February 2 event reached X rank in several locations. Montereale and Cittareale suffered severe damage from these earthquakes. In particular, Cittareale had an XI degree and Montereale an X degree MCS. The seismic period of 1703 ? very complex with numerous replicas also important such as that of January 16th whose location? uncertain, what? if it falls within the area of the Basin of Norcia or ? moved to the south.
A few kilometers more northwest of the current sequence, in 1639 ? occurred an earthquake of magnitude 6.3 that hit the Amatrice area with intensity? maximum equal to the X degree of the MCS scale and which caused damage of the VII-VIII degree MCS in Montereale. The following figure shows the maximum effects of the tremors of 1703 with respect to the area of the seismic sequence in progress. Note that the area circled in red represents the area of maximum damage and does not indicate the extension of the seismogenic structures responsible for the two seismic events which could be more? long. In the literature, the faults of Monte Marine and the Norcia basin have been associated respectively with the event of February 2 and January 14, 1703. In the Abruzzo area, however, several active fault systems coexist and whose characterization? available on the recently updated Database of Individual Seismogenic Sources (see http://diss.rm.ingv.it/diss/).
Figure 1. Is seismicity represented? of the last two months between Cittareale and Montereale and the areas of maximum damage caused by the tremors of 14 January and 2 February 1703.
Seismicity? recent
The seismicity? in the crime area? started in the middle June 2009. Figure 2 shows the map of earthquakes from July 2009 to all of August 31, 2010, while Figure 3 shows only the earthquakes that occurred from July 1 to August 31, 2010. Note how starting from the end July 2010 seismicity? occurs mainly in the southern part of the area affected by the swarm and limitedly in the northern part.
Figure 2. Seismic map in the Rieti area from 1 July 2009 to 31 August 2010.
Figure 3. Seismic map in the Rieti area from 1 July 2010 to 31 August 2010. Note how the seismicity? concentrates mainly in the southern part of the seismic zone.
The earthquake swarm? located mainly in the area immediately west of the town of Montereale (Figure 4) and consists of about 300 earthquakes with local magnitude greater than or equal to 1.0. The depth? of earthquakes varies between a few kilometers from the surface to just over? of 15 km for the pi? deep.
Figure 4. Enlargement of the area affected by the seismic swarm The map represents the earthquakes starting from 1 January 2010 and updated at 12:06 UTC on 31 August, with the exclusion of earthquakes with a local magnitude lower than 1. The epicentral distance between the two events to the North West and the one with ML=3.6 ? of about 2.5 km.
Figure 5 shows the time trend for the entire sequence starting from June 2009. The initial period had hundreds of events per day and in any case the activity? ? always been present with some periods characterized by relative increases in seismicity? which have not exceeded the average number of 20 earthquakes per day.
Figure 6 shows the time trend of seismicity? only for the southern part of Fig. 4 and it can be seen how this alternates phases more? intense to others in which earthquakes are practically absent. Furthermore, at the turn of the first ten days of July the seismicity? increases both in the number of earthquakes and in the maximum magnitude recorded. During the month of August this behavior is accentuated first with the earthquake of ML=2.8 on 13 August and then, at the end of August, with three events of magnitude greater than 3.0 on 31 August.
Figure 5. Time distribution of seismicity? starting from June 2009 for the whole area between Montereale and Cittareale illustrated in figure 2.
Figure 6. Seismicity distribution? of the earthquake swarm in the southern part of the Rieti area starting from January 1, 2010.
Figure 7 shows the cumulative number of recorded events. It is highlighted how the process of seismic swarm alternates phases more? intense to periods of low activity? until the first ten days of July 2010. In the following weeks, albeit with daily variations, the number of aftershocks increases.
Figure 7. Cumulative representation of the number of events day by day from January 1 to August 31, 2010 for the southern area depicted in Figure 4.
Figure 8. Cumulative representation of the seismic moment released day by day by earthquakes in the period 1 January - 31 August 2010 for the southern area represented in Figure 4. The blue line indicates the sum of the single seismic moments related to each earthquake.
The cumulative sum of the seismic moments provides general indications on the progressive release of deformation in the area affected by the earthquakes. The continuous line in the graph of Figure 8 shows that on August 31st more than one year is released? of half of the deformation from the swarm. However, these are modest values of energy released.
focal mechanisms
The CNT routinely calculates the seismic moment tensors for earthquakes with magnitude greater than ML~3.4 (see http://earthquake.rm.ingv.it/tdmt.php). Figure 9 presents the solution obtained from the inversion of the waveform data recorded by the national seismic network for the seismic moment tensor. The solution shows a predominantly normal fault mechanism, but with a large right-hand lateral motion component. The moment magnitude ? equal to 3.5 and ? slightly lower than the local one that ? equal to 3.6. The focal mechanisms for the other two events with a magnitude greater than 3.0 on August 31, 2010 are also normal and exhibit similar right-handed transient components.
Figure 9. Solution of the seismic moment tensor for the ML=3.6 earthquake of August 31, 2010 at 07:12 UTC. The mechanism? predominantly normal fault with a right-handed transcurrent component.
Deformation
The information deducible from the geodetic analysis indicate that the area in question falls within an active deformation zone of the Apennine chain. In particular, it is observed that the speed? geodesic, in a Eurasian reference system, increase from the Tyrrhenian to the Adriatic of about 2-3 mm / year and that this increase in speed? it takes place precisely in the area of the Apennine chain. The deformation? of extensional type and the focal mechanisms of the earthquakes are generally in agreement with the ongoing deformation as shown in the previous figures.
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Information on Ml 3.6 of 31 August 2010