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 310

Colfiorito, Italy, earthquake (Bouchon et al. 2000), the M = 6.6 2003 Bam, Iran,
earthquake (Jackson et al. 2006). One of the interests of these observations is that
they provide direct evidence that vertical ground acceleration locally exceeded grav-
ity during these earthquakes. Reports of the upthrow of man-made objects are some-
what more common but, as shown by Newmark (1973) and Bolt and Hansen (1977),
they do not necessarily entail vertical ground acceleration greater than gravity.
Recordings of vertical ground accelerations in excess of 1 g during earthquakes
are still sparse and uncommon. To date, only half a dozen such records have been
documented (Anderson 2006). Remarkably, the best recorded large earthquake to
date, the M = 7.6 1999 Chi-Chi earthquake, although it produced surface breaks
locally exceeding 7 m in height, generated vertical ground accelerations well be-
low 1 g at all the near-fault accelerometric stations (Lee et al. 2001). Furthermore,
although much field work was done following this earthquake, no observation of
upthrown rocks was reported.
The smallest-magnitude event for which the upthrow of rocks is well documented
is the M = 6.0 1997 Colfiorito, Italy, earthquake. This earthquake has been the
largest shock of a series of earthquakes that shook central Italy for several weeks in
the autumn of 1997. After this earthquake, it was observed that thousands of stones
and rocks, which are numerous in this region of smooth hills and scattered limestone
outcrops, had been freshly fractured and broken. Some of the broken stones were
lying isolated on soft detritic soil (Fig. 5) while others had been piled up together,
Fig. 5 Typical pictures of isolated stones (fragile marly limestone) found throughout a 1-km 2 zone
following the M = 6.0 Colfiorito earthquake. The two original stones on the left were broken into
several pieces while the one on the upper right was completely shattered. The rock on the lower
right had its top partly scaled (the white areas), likely at impact. (After Bouchon et al. 2000.)