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 309

SISMALP NETWORK 28.03.2006 05:33

Conand aftershocks

CNDSZ

CNDSN

CNDSE

05:34

5 10 15 20 25

Fig. 4 Aftershock doublet felt at Conand ( M L = - 0.2 and - 0.7), 25-s time window, 200-Hz
sampling rate. Amplitude window for each component is ms − 1 . Note that the maximum

amplitude is here reached on the E-W component, whereas it is observed on the N-S component
for Fig. 3. It indicates either a slight difference in the position of the epicentre or a difference in
source mechanism

± 30

However, a very large uncertainty on magnitude values computed here is brought
by the conversion from M 0 to M L . Kanamori's (1977) relation does not apply here
because it addresses great earthquakes and involves the so-called moment magni-
tude. (Were it applied, it would provide a 1.3 value for the magnitude of the 10-Feb.
event.) Other empirical relations similar to Bakun's have been proposed, for instance
by Hainzl and Fischer (2002) in their study of an earthquake swarm with magnitudes
between

− .

0 5 and 3.2:

log 10 M 0 = 1 . 05 M L + 11 . 3 .

This relation would provide an M L = - 0.35 value for the 10-Feb. event, still smaller
than the 0.75 value computed with Bakun's relation. This conversion problem set
aside, it seems anyway rather clear that the two 28-Mar. events had very small, most
probably negative magnitudes.

4 The Upthrow of Rocks

Documented observations of upthrown rocks and boulders are relatively scarce.
They include the M = 6.9 1984 Western Nagano, Japan, earthquake (Umeda et al.
1987), the M = 7.8 1990 Philippine earthquake (Umeda 1992), the M = 6.0 1997