Annual Meeting of the Seismological Society of America. Special Session: «Advances in Treating Macroseismic Intensity Data Quantitatively»

OGS was asked by the Seismological Society of America to organize and to chair one of the special sessions of his Annual Meeting. The session was entitled: "Advances in Treating Macroseismic Intensity Data Quantitatively".

(The three conveners of the session were: Livio Sirovich from Ogs, David Wald and James Dewey from the US Geological Survey).

OGS coordinated the discussion of the 61 authors and some hundreds participants on these topics:

Foreward. Intensity "I" (in a discrete and bounded scale, with inhomogeneous steps) is essential to study earthquakes of the pre-instrumental era. It is also successfully used in scenario and hazard studies, since (mostly, with the EMS98 scale) it is related to losses, specific damage and vulnerability. We addressed the following topics:

1) How can synthetic I values be calculated? As integer numbers? Real numbers? In the fuzzy-set logic?

2) We share the Musson, Gruenthal and Stucchi opinion: «Ideally, direct conversion between I scales should never be made» but, «I values are likely to vary more between two seismologists using the same scale than between two scales used by the same seismologist» and, thus, «relationship between major twelve-degree scales, such as MSK, MMI and MCS, and EMS-98 is more or less 1:1» (2006; First Eu. Conf. Earthq. Eng. and Seismology, Geneva). Is this widely accepted?

3) We encourage the use of I in hazard PSHA studies, and in rapid-response maps, but we propose that isoseismals are used in place of (or added to) the maps with soft-shaded colors; isoseismals follow the discrete nature of I and help understand the shape of the macroseismic field better. To draw isoseismals we prefer qualified contouring techniques (firstly, the non-filtering natural-neighbor interpolation).

4) I values observed in the past can be used for PSHA validation purposes.

5) Instrumentally-derived I.

6) Regional I patterns can be inverted for unknown fault sources, that generated destructive earthquakes prior to the advent of seismographs; this is mostly useful in regions with medium seismicity rates.

7) I and local seismic effects. A minimum number of surveyed buildings must be used to discriminate intensity values between different town blocks. Data banks with only one I datum for each town are sensitive mostly to sub-regional geotechnical and geophysical soil conditions.