What is the Lowest Magnitude Threshold at Which an Earthquake can be Felt or Heard, or Objects Thrown into the Air? - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 311

Fig. 6 General typical view of a rock pile ( upper left ) and three detail views near the heavily-
damaged village of Annifo following the Colfiorito earthquake. Most of the stones in the piles
(fragile marly limestone) were freshly fractured or broken (After Bouchon et al. 2000)
probably a long time ago to clear the land for farming (Fig. 6). Broken rocks and
stones were found everywhere throughout a zone which covers an area of about 1 km
by 1 km, and is located near the heavily damaged village of Annifo, where the max-
imum shaking intensity (IX) of the earthquake was registered (Camassi et al. 1997).
Freshness of cuts and fractures, visible in Figs. 5 and 6, and the consistency of the
observations for thousands of rocks and stones indicate that these rocks were tossed
into the air during the earthquake, with breakage occurring at the time of impact.
In several places, the old imprint of the stone in the soil was still visible. A similar
phenomenon, although not as extensive, occurred in a second area, located about
4 km away from the first zone, near the village of Colle-Croce, which was also
heavily damaged.
This earthquake, like most of the shocks in this sequence, had a normal-fault
mechanism typical of the extension regime that characterizes the present-day tec-
tonics of this region. The hypocentre was located at a depth of about 7 km near the
bottom of the aftershock zone that delineates the fault plane (Amato et al. 1998).
The fault dip was about 40 ◦ (Amato et al. 1998). The lack of surface ruptures
clearly associated with the earthquake fault plane (Cinti et al. 1999) and the near-
disappearance of seismicity at depths shallower than 2 km (Amato et al. 1998) sug-
gest that significant slip during the earthquake was confined to depths larger than
2 km. Satellite radar interferometry data of the area and local GPS measurements
(Stramondo et al. 1999) combined with the modelling of the rupture show that the
zones of upthrown rocks were located in the area where the largest vertical ground