Page 260 - Numerical Analysis and Modelling in Geomechanics

F.PERGALANI, V.PETRINI, A.PUGLIESE AND T.SANÒ 241
Application

Some characteristics of The Umbria-Marche seismic
sequence
The seismic sequence started in the Umbria-Marche Apennine region on 4
September 1997 with an ML 4.4 earthquake located near the village of Colfiorito,
close to the boundary between the Marche and Umbria regions. Several
aftershocks with magnitude lower than 4 followed in the subsequent weeks. On
26 September at 00.33 GMT, an Ms 5.5 earthquake occurred with epicentre
located between the villages of Cesi and Colfiorito. It was followed by a stronger
earthquake at 09.40 GMT (Ms 5.9; Mw 6.0; Table 8.1), which represents the
largest earthquake of the entire seismic sequence and caused damage as large as
IX on the MCS macroseismic intensity scale. The epicentre was located north of
the previous one, between the villages of Colfiorito and Annifo. A few minutes
later a third shock occurred (ML 4.7) located more northward. Strong ground
motion accelerographs of the National Electric Company, recorded peak ground
accelerations as high as 0.5g at the
Table 8.1 Main earthquakes (ML≥ 5) of the Umbria-Marche sequence (Pergalani et al.,
1999)

village of Nocera Umbra in both the main earthquakes. In the following weeks
the seismic activity was very high with more than 2000 shocks between 26
September and 11 October with about 20 earthquakes exceeding magnitude 4 up
to 14 October. Table 8.1 gives a list of earthquakes with magnitude greater than
or equal to 5.
It is interesting to note the migration of the seismic activity during the entire
sequence. Up to 12 October, the seismic activity, which was initially
concentrated in the northern part of the area (Figure 8.12), migrated to the
southern part, between the villages of Sellano and Preci, where on 14 October, at
15.23 GMT an earthquake of Ms 5.5 occurred. Finally, in March and April 1998,
two earthquakes with magnitude greater than 5 occurred more than 20 km north
of the first sequence (Table 8.1). This activity seems to highlight the activation