Review of Historical Earthquakes in the Lower Middle Ages: Earthquakes of the XIV and XV Centuries in Catalonia (NE Spain) - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 144

Imax Mw

IX 6 . 4 ± 0 . 53

VIII 5 . 8 ± 0 . 52

VII 5 . 3 ± 0 . 52

52
(b) Empirical relations giving magnitude from Io and focal depth (h).

VI 4 . 7 ± 0 .

One of the most used relations is that from Karnık (1969): Ms = 0 . 55Io + logh +

.

0 35 (Ms is surface wave magnitude). A more recent relation from Gutdeutsch

et al. (2002) obtained from earthquakes in Central and Southern Europe gives:
Ms

= 0 . 654Io + 1 . 868 log h − 1 . 682, with a standard deviation of ± 0 . 284.

(c) Empirical relations to obtain magnitude estimates from the radius of or area
delimited by isoseismal lines.
Levret et al. (1994), based on 140 events in France, proposed the following relation:
M 4, where

M is what the authors called macroseismic magnitude and I is the intensity of the
isoseismal at the mean focal distance R. One the one hand this relation has the
advantage of having included some events in or near our study area but, on the other
hand it may have some problems for intensities larger than VII as it was obtained
mainly with earthquakes of intensity III-VII; for larger intensities the use of this
equation is an extrapolation and the macroseismic magnitudes deduced from this
relation tends to be lower than instrumental magnitude.
Johnston (1996a; 1996b) obtained a relation between Mo and the area of the
isoseismal I, A I . The expression we have used for the isoseismal I

= 0 . 44I + 1 . 48 log R + 0 . 48, with a standard deviation =± 0 .

= VIII is:

00586 (A VIII ) 1 / 2 . Once the value of Mo
is obtained from the previous equation, Mw is estimated using Hanks and Kanamori
(1979) relation: Mw

Log Mo = 24 . 05 + 0 . 440 log A VIII + 0 .

= 2 / 3logMo − 10 . 7.

3 The 1373 Earthquakes: Felt Far Away from Epicentre but Few
Intensity Data Points Available

The study on the 1373 seismic series by Olivera et al. (1994) furnished new and
valuable data for the evaluation of seismic hazard. Historical contemporary sources
were thoroughly analyzed and then transcription errors in former catalogues were
detected. As a result of that research, the epicentral zone of this series, which
in many catalogues had been erroneously situated in Olot (southern slope of the
Eastern Pyrenees) was corrected, and was located 200 km to the west in the earl-
dom of Ribagor¸a (southern slope of the Central Pyrenees). Moreover, two earth-
quakes catalogued with intensities IX and VIII were considered to be false, and the
largest event, that of March 3, was identified and its epicentral intensity assessed as
VIII-IX (MSK).