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 306

The 2002-3 earthquake swarm initiated at the beginning of December 2002 by
shocks perceived as explosions by the inhabitants of a 20-house hamlet close to

Clansayes. These abnormal sounds were not at once identified as earthquakes by the
inhabitants because local earthquakes are inexistent in the inter-swarm quiescence
periods, and—to our knowledge—the latter felt swarm dates back to 1933-6. A
temporary velocimetric station was installed in the basement of one of the houses
at the end of December 2002; thirteen more stations were installed later in January
after we identified the phenomenon as seismic.
Several scores of events could be located over a few weeks monitoring. Although
activity was maximum right beneath the hamlet, other shocks were detected along
a north-south-trending,

∼

7-km-long zone. Available geological maps identify no
corresponding fault. On several seismograms recorded by the station installed in
thehamlet,weobservedanS-Pintervalofonly45ms(Fig.2).Themassive
coral-limestone formation that outcrops in the vicinity can be assigned a velocity
of 5,000 m s − 1 . Consequently the corresponding focal depth for those ultra-shallow
earthquakes is 300 m at most (Jenatton et al. 2004).
Because of their small magnitude, most of these swarm earthquakes could not
be located by the permanent monitoring networks, although the Clansayes perma-
nent station could detect some of them. Only two events could be located (14 Dec.
2002, M L = 1.5 and 1 Jan. 2003, M L = 1.7), whereas in December explosions were

∼

SISMALP NETWORK 26.01.2003 21:06

Tricastin swarm earthquake

CLANZ

CLANN

CLANE

39 40 41 42 43

Fig. 2 Example of ultra-shallow swarm earthquake recorded in Tricastin by a temporary station
(vertical, N-S, and E-W components of a 2-Hz velocimeter; 200-Hz sampling rate). This 4-s
window shows P- and S-wave arrivals only 45 ms apart (S waves better observed on the E-W
component). Focal depth is about 300 m. Amplitude window for each component is ± 300 ms − 1