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 312

displacement occurred. Vertical displacement inferred in the zones of upthrown
rocks is about 30 cm. The relatively moderate size of this event suggests that the
upthrow of rocks during earthquakes is a much more common phenomenon than is
usually thought.
5 Conclusions
Our study concludes that earthquakes much smaller than those commonly assumed,
and even with negative magnitudes, can be felt in the case of ultra-shallow earth-
quakes (those with a focus less than 1 km deep). It means that magnitudes for these
events should not be overestimated in historical-seismicity studies whenever such
testimonies are used. On the other side, we believe that reports of such phenomena—
whether in the past or at present time—should not be neglected. They pinpoint the
activity of local faults much more precisely than studies of large earthquakes with
complicated isoseismal curves. Felt events with negative magnitudes, usually below
the detection threshold of seismometers, finally demonstrate that the human being is
an instrument eventually much more sensitive—and perhaps cheaper to maintain—
than dense monitoring networks. Awfully, this fact reduces to populated areas the
places where the occurrence of such earthquakes can be asserted.
At the other end of remarkable effects, we showed that earthquakes of relatively
moderate size ( M = 6.0) associated with near-fault ground displacement of a few
tens of centimetres and no surface break can produce vertical ground accelerations
exceeding gravity, and toss objects and rocks into the air. Conversely, some great
earthquakes, such as the M = 7.6 Chi-Chi event which generated vertical ground
displacements more than 10 times higher and a 100-km-long surface break, do not
produce vertical ground accelerations exceeding gravity. Both sets of observations
are difficult to conciliate. They provide a formidable challenge to seismologists and
earthquake engineers for the years to come.
Acknowledgments Figures 1-4 of this article were drawn using the GMT software (Wessel and
Smith 1998). The Conseil general de l'Isere, the Delegation aux risques majeurs (French Min-
istry of the Environment), the Institut national des sciences de l'Univers (CNRS), and the Con-
seil regional Rh one-Alpes funded the Sismalp network. The French National Seimic Network,
the Grenoble Observatory, and several County Councils (Isere, Alpes-de-Haute-Provence, Haute-
Savoie, Ain, and Savoie) support its running costs. We thank Josep Batll o, Willie Lee, and Jacques
Dorel for their comments.
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