Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology)
THE ESI 2007, THE INTENSITY ATTENUATION RELATIONSHIPS AND POSSIBLE
GAINS FOR SEISMIC HAZARD MAPS
102
I.D. Papanikolaou (1, 2)
(1) Laboratory of Mineralogy & Geology, Department of Geological Sciences and Atmospheric Environment, Agricultural University of
Athens, 75 Iera Odos Str., 118 55 Athens , GREECE.
(2) Aon‐Benfield‐UCL Hazard Research Centre, Department of Earth Sciences, University College London, Gower Str. WC1E6BT London, UK.
i.papanikolaou@ucl.ac.uk
Abstract: Fault slip‐rates are of decisive importance for the seismic hazard assessment. However, sensitivity analysis in geological fault slip‐
rate seismic hazard maps demonstrates that the uncertainty in the attenuation relationships is much higher than the implied uncertainty in
slip‐rates, so that even if a more accurate slip‐rate estimation is achieved, it would have little impact on the final outcomes. The recent
introduction of the Earthquake Environmental Intensity (ESI) 2007 promises to offer higher objectivity in the process of assessing macroseismic
intensities particularly in the epicentral area than traditional intensity scales that are influenced by human parameters. The ESI 2007 scale
follows the same criteria‐environmental effects for all events and can compare not only events from different settings, but also contemporary
and future earthquakes with historical events. As a result, a re‐appraisal of historical and recent earthquakes so as to constrain the ESI 2007
scale may prove beneficial for the seismic hazard assessment by reducing the uncertainty implied in the attenuation laws and eventually in the
seismic hazard maps.
Key words: intensity, seismic hazard assessment, active faults, earthquake environmental effects.
INTRODUCTION
The macroseismic intensity is not solely used for the
description of earthquake effects, but predominantly is a
major seismic hazard parameter, since it describes the
damage pattern. The new Environmental Seismic Intensity
Scale (ESI 2007), introduced by INQUA, incorporates the
advances and achievements of Palaeoseismology and
Earthquake Geology and evaluates earthquake size and
epicentre solely from the Earthquake Environmental
Effects (EEE) (Michetti et al., 2007). This paper: a)
demonstrates quantitatively how significant is the
uncertainty in the empirically based attenuation laws of
the existing traditional intensities for the seismic hazard
maps and b) shows how the ESI 2007 intensity scale could
prove beneficial for the seismic hazard assessment by
reducing the aforementioned uncertainty.
THE INTENSITY AS A HAZARD PARAMETER
Seismic shaking is usually expressed in terms of
macroseismic intensity or peak ground acceleration and
both parameters are widely used in seismic hazard maps.
However, peak acceleration values can be a poor guide to
the shaking and damage expected pattern, partly because
the overall damage may be more closely correlated to the
total duration of the strong‐motion than to any particular
peak on the record and it is mainly introduced for civil and
construction engineering purposes (e.g. Bolt, 1999,
Coburn and Spence, 2002). Therefore, intensity is the
direct measure of damage.
INTENSITY ATTENUATION RELATIONSHIPS
Isoseismals are lines that separate different intensity
values and represent the macroseismic information
obtained by the quantification of the effects and damage
produced by an earthquake. The isoseismal maps are
used to derive empirical relations for the decrease of
intensity with distance, which then are incorporated into
the attenuation laws and used to assess the seismic
hazard. Therefore, in order to define the seismic hazard
at a given site, it is necessary to know the expected
attenuation of intensity with epicentral distance. These
attenuation curves are compiled based on the statistical
elaboration of historical and instrumental data (Fig.1).
Fig. 1: Attenuation law derived from the statistical elaboration
of historical and instrumental data for the Apennines in Italy
(Modified from Grandori et al. 1991). Earthquakes with
epicentral intensity IX (I0=9) have a mean radius of 6‐7 km for the
intensity IX isoseismal (dark grey), whereas events with
epicentral intensity X (I0=10) have a mean radius of 20‐21 km for
the isoseismal IX (light grey).
However, there is a large variation in the data, which adds
uncertainty in the seismic hazard assessment. This
variation is nicely portrayed in an empirical magnitude‐
intensity database compiled by D'Amico et al. (1999),
which is presented in Table 1. This database is extracted
from instrumental catalogues ranging from 1880 to 1980,
covering the whole Mediterranean region. Table 1 for
example, shows that epicentral intensity X has been