Interevent and Interstation Variability Computed for the Italian Accelerometric Archive (ITACA)

The interevent and interstation ground-motion variability of the updated
Italian strong-motion database (Italian Accelerometric Archive [ITACA]) has been explored
through the development of new empirical ground-motion prediction equations
(GMPEs) for Italy. The regressions have been performed on 241 three-component
waveforms from 27 earthquakes with moment magnitudes ranging from 4.8 to
6.9, recorded by 146 stations at distances up to 200 km. The site classification follows
the schemes previously proposed for Italy, in which two soil classes are defined, considering
both shear-wave velocity and deposit thickness. The regression analysis uses
the values of the explanatory variables (magnitude, fault distance, site class, and style
of faulting) recently revised in the framework of a project funded by the Italian
Department of Civil Protection. The equations have been derived for peak ground
acceleration, peak ground velocity, and 5% damped spectral accelerations at 18 periods
from 0.03 to 2 sec.
The residual variance has been decomposed into interevent, interstation, and
record-to-record components by applying a random effect regression scheme.
The interevent and interstation error distributions have been analyzed as function
of periods to detect sites and events for which predicted values significantly deviate
from observations. For periods up to 0.35 sec, the interstation is the dominant component
of variance, indicating that an improvement in the site classification could
lead to a refinement of the GMPEs. For longer periods, the three components of
variance provide similar contributions, indicating that a reduction of the uncertainty
can be achieved by reducing the epistemic uncertainty affecting the physical model.
The interevent error highlights the peculiarity of few earthquakes, suggesting that
the evaluation of regional GMPEs can be important when specific scenario studies
should be carried out. The interstation variability allows us to detect stations with
peculiar site response and to assess the goodness of the considered site classification
scheme.
Introduction
In Italy, strong-motion recordings have been available
since the early 1970s. Because different institutions managed
the Italian strong-motion network (Rete accelerometrica
italiana [RAN]), both waveforms and metadata needed to
qualify the recordings are hardly available to end-users, except
for few cases, such as the 1997–1998 Umbria-Marche
sequence and the 2002 Molise earthquakes (see the Data and
Resources section). The Italian Dipartimento di Protezione
Civile (Italian Department for Civil Protection [DPC]) after
an agreement with the Istituto Nazionale di Geofisica e Vulcanologia
(Italian Institute for Geophysics and Volcanology
[INGV]) funded in 2004 the project called “Database dei dati
accelerometrici italiani relativi al periodo 1972–2004”
(Italian strong-motion database relevant to the period
1972–2004, hereinafter referred to as project S6,