Revision of the
theoretical and observational grounds of the seismic hazard estimates at a
national scale
Coordinator: Paolo Gasperini
Introduction
We would like to point out that this re-proposed
project received the present year funds only around the half of January 2003.
For this reason this report will reflect activities formally carried out in a
time period definitely shorter than one year. In particular, for the peculiar
nature of the collaboration with the private research company SGA, the planned
activities in the ambit of Task 1 are still at a very early stage and will be
presented here in a preliminary form. We thus reserve ourselves to complete the
reporting at a successive time. On the contrary the activities concerning the
other research lines were pursued almost exactly according to the initial
program, due to the voluntary effort of the contributors, although with some
difficulties.
The objectives of the project, as also described in
the re-proposition proposal, have been resized with relation to the referees'
comments and to the amount of assigned funds. The corresponding activities
affects only 4 of the tasks initially proposed. This strongly reduced the
unitariness and coherency of the project that originally was thought to have,
as final product, the revision of hazard estimates at national scale.
We tried however to develop a proposal that, starting
from the activities that the Evaluation Commission have judged new and
innovative, was able to give results directly usable by other GNDT project. In
this ambit, as also suggested by the Commission, we tried to integrate our
project with the one coordinated by A. Amato through information exchanges, in
order to avoid superposition or duplication of objectives. For this reason some
researches we proposed have been thought to give products that can be easily
used by anyone. Among the others we want to mention the "Boxer" code to analyze
macroseismic data and the database of Earthquake Mechanism of Mediterranean
Area (EMMA). These are tools designed for a free usage by Italian and foreigner
investigators and for which we also prepared a reference manual.
In the following we synthetically report, for each of
the active tasks, the planned activities, the expected results in the two years
(second and third) of the re-proposition and those effectively achieved in the
second year.
Task 1 HISTORICAL
SEISMIC CATALOG (Responsible: Gasperini, Participants: Albarello, Bernardini,
Camassi, Castelli, Ercolani, Lolli, Vannucci e SGA Working Group)
The planned activities concern:
i)
Improvement
of Boxer code by the introduction of the bilinear attenuation law (Gasperini,
2001).
ii)
Development
of new methods to compute the epicenter for offshore earthquakes.
iii)
Development
of robust techniques to estimate the depth of the source from macroseismic
data.
iv)
Application
of the Fuzzy Sets algorithm for intensity estimates to some strong Italian
earthquakes.
Expected results for
the second year:
·
New release of
Boxer code for the computation of the location, the magnitude and the
orientation of the source of historical earthquakes from macroseismic data
using the bilinear attenuation law.
·
Application of
the Fuzzy sets algorithm to compute the intensity field of the 1930 Irpinia
earthquake.
Expected results for
the third year:
·
New release of
Boxer code allowing the location of offshore epicenters and the computation of
depth.
·
Application of
the Fuzzy sets algorithm to other strong Italian earthquakes.
Results achieved in
the second year:
We implemented a new "experimental"
version of the Boxer code using the attenuation law proposed by Gasperini
(2001). This was tested on Ligurian earthquakes of July 19, 1963 and some
earthquakes of the Middle East.
Figure 1. Macroseismic field of Ligurian
earthquakes of July 17, 1963. The instrumental epicenters (ISS data) of the two
main shocks, occurred at a minute one from the other, are indicated with dark
blue circles. The macroseismic epicenter (regarding the effects induced by both
shocks) computed by the previous version of Boxer code (3.2) as the barycentre
of localities with maximum intensity (VI) is indicated in green. With light
blue and red circles we reported instead the epicenters computed with the new
version of Boxer code (4.0) using
two different approaches: the first one using all data within a radius of 350
km, the second one the maximum distance is reduced to 180 and are only included
localities where the expected intensity is larger or equal to IV (see
Gasperini, 2001). The moment magnitude is 6.1 and the epicentral intensity
(theoretical) is IX. It was not possible instead to evaluate the orientation of
the source due to the lack of data
in the epicentral area.
In Fig. 1 we can see
how the modified algorithm, different from the one previously used by Boxer
code (version 3.2), is able to correctly locate the earthquake in the sea.
Although the correspondence with the instrumental epicenters is not
particularly good, the result appears promising for future developments of the
method.
By the technique developed by Ferrari et al. (1995)
and Vannucci et al (2000), making use of Fuzzy Sets, we encoded and elaborated
the data of observed effects for the July 23 1930 Irpinia earthquake, to obtain
objective and reproducible estimates of the intensities. The comparison with
other events previously studied through this method (S.Sofia 1918, Mugello 1919
e Garfagnana 1920) indicates a typology of information rather different. In
fact if we use the empirical membership functions deduced from the other events
("invertite") we obtain macroseismic fields very different from the one
obtained by the membership function deduced from the event itself ("proprie")
as well as from the very similar one evaluated by a macroseismic "expert". This
discrepancy is particular evident comparing the results of the application of
the Boxer code on the different macroseismic fields (Fig. 2).
Figure 2. Application
of the Boxer code (version 3.2) to the macroseismic fields of the July 23, 1930
Irpinia earthquake, as resulting from the Fuzzy algorithm estimates (in yellow)
and from the estimates made by a macroseismic "expert" (in dark blue). The "invertite" boxes concern
elaboration made using Fuzzy membership functions deduced from the data of
other earthquakes (S.Sofia, 1918,
Mugello, 1919 and Garfagnana, 1920). The "proprie" and "unite" boxes
refer instead to the evaluations made using membership functions deduced from
the 1930 earthquake itself and from the union of the data of all earthquakes
respectively.
Task 6C, INTENSITY TOMOGRAPHY AND SITE EFFECTS (Responsible:
Gasperini, Collaborators: Albarello, Bernardini, Camassi, D'Amico, Ercolani,
Lolli, Mucciarelli, Vannucci)
The planned activities concern:
i)
The
detailed comparison of locality empirical residual with the lithological and
topographical characteristics of sites in order to verify whether they are
actually linked to specific local properties of the sites rather than to the
uncertainties in the determination of intensity.
ii)
Introduction
of the seismic source spatial orientation in the bilinear attenuation equation
in order to make more realistic and precise the computation of distances. This
can be done both using the Boxer technique (Gasperini et al., 1999) or computing,
simultaneously to the tomographic inversion, the orientation of sources that
minimize the equation residuals.
iii)
Extension
of the intensity dataset by the inclusion of the new data coming from the INGV
macroseismic Bulletin of years 1993 to 1997.
iv)
Continuation
of the study (Boccaletti et at., 2001) on the lateral variations of intensity felt
in the historical center of Florence on occasion of the 1895, Impruneta
earthquakes and comparison with results of 1D simulation of ground motion and
with soil amplification measures
Expected results for
the second year:
§
Integrated
macroseismic database including the data from the INGV macroseismic bulletin
for all the events with Imax>V.
§
Preliminary
tomographic inversion using the updated database.
§
Introduction of
the spatial extension of the sources in attenuation computations.
§
Study of the
lateral variations of the intensity in Florence for the earthquakes of 1895
compared with 1D simulations of ground motion.
Expected results for
the third year:
§
Database of the
lithological and topographical characteristics of the localities having felt
reports for more than 10 different earthquakes, basing on data used for the new
tomographic inversion.
§
Definitive
tomographic inversion using the updated methods.
§
Comparison of the
lateral variations of the intensity in Florence with measures of soil
amplification.
§
Study of the
lateral variation of intensity for other historical centers.
Results achieved in
the second year:
We prepared a
preliminary version of the macroseismic database, integrating the data coming
from DOM (Monachesi and Stucchi, 1997) and CFTI( Boschi et al. 1995, 1997,
2000) on the basis of the choice
made in the CPTI (CPTIWG, 1999), with data from the INGV Macroseismic Bulletin
of events with Imax>V from 1993 to 1996. We also substituted the data coming
from the Version 2 of CFTI (used by CPTI) with those of version 3 (Boschi et
al., 2000) also adding the earthquakes that were not present in the previous
version. The database now includes in all 61638 intensity data (of which 7102
added) relative to 2622 earthquakes (of which 142 added). We also approached
the reordering and standardization of the catalog of localities that have
intensity data. This was including initially 26450 localities, due to the
different encoding of DOM CFTI and INGV Bulletin. After the reorganization the
distinct localities are only 12267, although further tests and controls are
necessary to eliminate possible further duplications.
On the basis of such
integrated dataset, we made a new tomographic inversion of macroseismic intensity attenuation.
With respect to the previous one
(Carletti e Gasperini, 2003) presently in course of revision on an
international journal, the database was extended, besides the data after 1992,
even to ones, previously excluded, from 1600 to 1800. The total number of
macroseismic observation used thus augmented from 19997 to 25864, with a
relative improvement of about 30%. The inversion results (Fig. 3) are very
similar to ones obtained previously and only slightly extend the covered area.
The main improvement concerns instead the definition of locality residuals that
can be used to evaluate site effects. The number of usable localities (with 8
intensity estimates at least) almost duplicates going from 285 to 559. The
reason of such a large increase can be attributed to the larger frequency of
localities that are selected by the INGV for sending the macroseismic
questionnaires.
Figure
3. Results of
tomographic inversion of seismic intensity attenuation for the new integrated
dataset. On the left the differences with respect to average for the slope of
the first trait (distance<45
km) of the bilinear attenuation model (Gasperini, 2001). On the right the same
representation for the second trait (from 45 to 180 km). The areas in red are
characterized by attenuation larger than average, while the blue ones by
attenuation lower than average.
Just on the basis of
the list of locality empirical residuals obtained by tomographic inversion, we
faced up an analysis to characterize the sites from the point of view of the
surface lithology and topography. Using the available geological maps (mainly
obtained through the internet server managed by the Siena University), we
classified 149 localities among those for which we computed an average residual
using more than 10 macroseismic observations. We used a slightly modified
version of the classification schema prepared by the Rovelli (INGV) group
participating to the Amato GNDT Project.
Basing on such
schema the surface geology is divided in A = Consolidated Lithoid character, B
= Consolidated Semilithoid character, C = Unconsolidated (thickness<20m),
D=Unconsolidated (thickness>20m).
The unconsolidated
classes are further subdivided on the basis of the substratum: a=Lithoid
character, b=Semilithoid character, c=Mixed character, d=unknown.
Finally, on the basis
of the topography, the sites are evaluated in relation to the location of the
majority of the buildings. The categories are: Coast, Crest, Plain, Valley,
Slope.
In practice some of
these classes resulted difficult (when not impossible) to evaluate on the basis
of the cartography only. In particular it was never possible to infer the
presence of layers thinner than 20 m and thus the C class resulted empty.
However this investigation, that is presently at still a preliminary stage,
allowed to identify al least two quite clear situations. These regards the
combination of class A (Consolidated lithoid) in Plain, that significantly
attenuate with respect to average (about 0.4 intensity degrees), and class B
(consolidated semilithoid) in Valley that significantly amplify (about 0.2
intensity degrees). Further refinements are in course relatively to the
extension of the number of classified localities and to the study of the time
behavior of the residual for given localities.
Figure
4 Map of
seismic zonation of the urban area of Florence in terms of EMS92 intensity,
based on the analysis of effect on buildings of earthquakes of may-june 1895.
The sites where we measured spectral rates (HVSR) are indicated with blue
stars. A magenta star indicates where S waves velocity measurements are also
made (some of the investigated point are external to the figure).
In the ambit of a
seismic micro-zonation analysis of the city Florence we had carried out in the
first year of the project, we made measures of background noise spectral rates
in 15 sites located within the urban area as well as S-waves velocity profiles
in 7 of these sites. The results of these investigations, that are still in
course of elaboration (Albarello et al. 2003), will be compared with the
lateral variation of EMS92 intensity deduced for the 1895 earthquakes (Fig. 4)
and will also be used to constrain 1D simulations of ground motion.
Task 5 STATISTICS OF SEISMIC SOURCES AND
CATALOG COMPLETENESS (Responsible: Marzocchi, Collaborators: Albarello, Dal
Forno, D'Amico, Faenza, Gasperini, Lolli, Mucciarelli, Sandri, Selva, Vannucci)
The aim of this Task
is to analyze the available seismological data (seismic catalogs, focal
mechanisms, macroseismic fields, etc.) and to design specific experiments to
verify the various hypotheses and theories on earthquake occurrence that has
been proposed in the literature, as well as the ones that could issue from the
research itself. We intend to study the modalities of the earthquake occurrence
i)
in
space (seismogenic model)
ii)
in
time (statistical occurrence models)
iii)
in
energy (scaling law)
iv)
in
ground motion amplitude at site
(attenuation law and local effects)
Expected results for
the second year:
None (not planned)
Expected results for
the third year:
§
Spatial seismic
occurrence model relative to both the historical (long time-scale) and
instrumental (short time-scale) seismicity.
§
Statistical time
occurrence model and its calibration on instrumental data.
§
Verification of
the magnitude scaling law for the instrumental catalog.
§
Verification of
the seismic attenuation law currently in use.
§
Statistical model
for the time series of the intensity felt at the site.
Results achieved in
the second year:
We analyzed the
spatio-temporal distribution of large earthquakes by a new non-parametric
multivariate model. The method presents several advantages compared to other
more traditional approaches. In particular, it allows to test straightforwardly
a variety of hypothesis, such as any kind of time dependence (i.e., seismic
gap,cluster, and Poisson hypothesis). Moreover, it may account for
tectonics/physics parameters that can potentially influence the spatio-temporal
variability, and to test their relative importance. The method has been applied
to the Italian seismicity of the last four centuries (CPTIWG, 1999). The
results show that large earthquakes in Italy tend to cluster; the instantaneous
probability of occurrence is higher immediately after an event and decreases
until to reach, in few years, a constant value. The results also indicate that
the clustering is independent of the magnitudes of the earthquakes. A map of
the probability of occurrence for the next large earthquakes in Italy in the
next 10 years was provided (Fig. 5). This work is currently submitted to an
international journal (Faenza and Marzocchi, 2003).
We also continued the activities in the field
of the statistical modeling of earthquake occurrence properties. A first paper
(Lolli and Gasperini, 2003a) regarding the forecast of seismic aftershocks in
Italy, making use of the Reasenberg and Jones (1989) model is currently in
press. Other two studies are at an advanced state of preparation. They regards
the validation (Lolli e Gasperini, 2003b) of the results of the first paper by
the comparison with the data from 1997 to 2002, recently made available at the
INGV web site and the evolution of the aftershock occurrence model Gasperini e
Lolli, 2003).
Figure 5. Map of the probability of occurrence for
the next large earthquake in Italy in the next 10 years.
The comparison between the aftershocks average
properties for the two datasets (Fig.6) shows a good agreement for the time
decay rate while the absolute rates result about twice after 1996 with respect
to the period before. This discrepancy could be due to a residual calibration
bias among the magnitude estimates in the two periods. We also observed that at
significant fraction of the sequences occurred after 1996 (like for example the
Umbria-Marche one of 1997-1998) shows a "swarm" behavior with many shocks with
similar energy rather than the classical mainshock-aftershock behavior. In
these cases the model used until now is clearly inadequate.
To overcome this limit we tackled the problem of
the formulation of a new occurrence model, introducing the epidemic principle
(ETAS, Ogata, 1988) according to which every shock of the sequence is on its
turn a source of further aftershocks.
In parallel we studied the role of the parameters of the Reasenberg e
Jones (1989) model by a correlation analysis among the values estimated in
different regions of the World (California, New Zealand, Italy). The model we
proposed as the most suitable to represent the time behavior of the shocks rate
even for complex sequences is the following
where the parameters
appearing in the summations (a1, p, c, b1 e b)
are not correlated among each other and thus can be considered as
representative of specific physical properties. Parameter m0
is the background seismicity rate, while c is a time shift that slow down the decay in the first hours after the
shock. Parameters a1
e b1 are the
index of the productivity independent of and depending on the mainshock
magnitude respectively. Finally L(t) is the number of generating events preceding the time t. The parameters are estimated maximizing, by
non-linear multiparameter optimization techniques, the log-likelihood function
of the process
Figure 6 Comparison
between the modeled aftershocks rates for sequences from 1997 to 2002 with
M>Mmax-1 and those predicted on the basis of parameters estimated by Lolli e
Gasperini (2003a) using sequences occurred between 1960 and 1996. The colored
bands in light orange and green indicate, for the sequences from 1960 to 1996,
the intervals between the first interquartile and the median and between the
median and the third interquartile of the distribution of rates. The dashed
line shows the value predicted by the model using the medians of parameters.
The first results of the application of this
model to Italian sequences show a significant improvement of the fit (Fig. 7)
with respect to the simple Reasenberg and
Jones (1989) model.
Figura
7 Comparison between rates
observed (red dots) and expected (lines) on the basis of ETAS model, for
different minimum magnitude thresholds for the generation of subsequences up to
the maximum (M-6.0) corresponding to the Reasenberg e Jones (1989) model.
Task 7, FOCAL MECHANISMS (Responsible: Gasperini,
Collaborators: Dal Forno, Lolli, Morelli, Pondrelli, Vannucci)
The activities mainly concern the development and the
updating of the database of first motion mechanism of the Mediterranean area
(Vannucci and Gasperini, 2002; Gasperini and Vannucci, 2002) and the
computation of regional CMT solutions (Pondrelli et al., 2002).
i)
Insertion
in the database of further mechanisms coming from other published papers.
ii)
Improvements
of the database management software, including some new procedures in the
MS-ACCESS application like that the plot of the mechanism and the in-line
checking of inserted mechanisms.
iii)
Availability
on the web of a reduced version of the first motion solution database.
iv)
Computation
of new RCMT mechanisms.
v)
Analysis
of cumulative moment tensor (Kostrov, 1974) and of compatibility of stress
directions (Gephart & Forsyth, 1984) for various zones of the Mediterranean
area.
Expected results for
the second year:
·
New preliminary release of the database of Earthquake
Mechanisms of the Mediterranean Area (EMMA)
Expected results for
the third year:
·
First
public release of the database of Earthquake Mechanisms of the Mediterranean Area
(EMMA)
Figura
8. Summary an
plot mask of the database of Earthquake Mechanisms of Mediterranean Area
(EMMA). (Vannucci e Gasperini, 2003).
Results achieved in
the second year:
We have verified
that more than 40% of the fault plane solutions published in the literature
contains inaccuracies, misprints and in some cases true computation errors. In
many cases we found the focal planes or the deformation axis not perpendicular
or inconsistent among each other. The database, based on MS-ACCESS platform,
allows to select the mechanisms (in all about 5000 excluding the CMT catalogs),
to examine their parameters and in the last version even to display the
"beach-ball" plot (Fig. 8). A paper (Vannucci e Gasperini, 2003) is currently
in press on an international journal.
To make the
necessary tests before inserting new mechanisms in the database, we wrote a
package of Fortran subroutines allowing to make the principal computations
related to focal mechanisms (axes from planew and viceversa, a plane form the
other one, planes and axes from moment tensor and viceversa). We carefully
verified all procedures, by comparing their computations with data reported by
Harvard CMT catalog. The coincidence was found to be within reasonable
tolerances (3 degrees) for more than 99% of the mechanisms. Only for less than
1% we observed larger discrepancies, however addressable in all cases to the
limited number of significant digits given by the CMT catalog. This paper also
is currently in press (Gasperini e Vannucci, 2003).
Conclusion
The distinctive trait of most of the researches
proposed by our project is the focussing on the analysis of macroseismic data.
These are a set of information completely independent of the instrumental ones
that can represent for these last a useful control dataset.
An particularly significant example in this sense is
represented by the tomographic inversion of seismic intensity attenuation. It
gives in fact the possibility to "calibrate" the instrumental attenuation
investigation made in some areas of the Italian territory (i.e. in the ambit of Amato
Project) with a globally homogeneous result. An accessory product of such work
is the list of locality empirical residuals. This allows to verify, on a very
large database of localities (more than 500), the site effects estimates made
by several GNDT projects, basing on both theoretical/geological (simulations of
ground motion) and experimental
methods (absolute amplification or spectral rates measures).
Just in the field of site effects, the detailed
zonation of the urban area of Florence represents a unique occasion to directly compare the 1D modeling
techniques and instrumental measures with the distribution of effects really
observed in the city historical center after an earthquake (the shocks of
May-June, 1895).
Finally, in the ambit of the study of the seismic
sources statistics we faced on
some fundamental problems for the development of new hazard model that
appear instead somehow neglected by other GNDT projects. Our interest in this
case id particularly focussed at the spatio-temporal clustering properties of
earthquakes that usually are almost ignored by current hazard models. In many
cases in fact the aftershocks are even removed from catalogs while the
experience of recent Italian seismic sequences (Umbria-Marche, Campobasso)
clearly evidenced that aftershocks can induce further damages and a significant
extension of the area requiring emergency services
We also showed that the earthquakes time-space
clustering is not confined to the first times after the main shock and to the
area initially struck but even at spatio-temporal scales of the order of years
and hundreds of km. On the other hand it was already shown in the literature
that simple recurrence models, even those adopted by hazard estimates in deeply
studied areas (California, Japan), are not suitable to represent the real
occurrence of earthquakes (Kagan and Jackson, 1991, 1995, Mulargia and
Gasperini, 1995).
In this research field an essential condition is
represented by the availability of a homogeneous seismic catalog as most
complete as possible. This is the reason why, even if this kind of studies had
not been considered relevant by the Commission, some propaedeutical activities
on this problem will be carried out anyhow, at least until the revision of
historical and instrumental databases by the Amato Project will be completed.
Notwithstanding the difficulties due to the retarded
activation of the funding with respect to other projects, most of the first
year objectives of this project have been achieved an in some cases even
exceeded.
In most cases our results correspond to significant
improvements of the knowledge of some phenomena that are of interest for hazard
estimates (attenuation and site effects properties, lateral variation of site
effects in urban centers, statistical properties of eartquakes) as well as of
the tools required to infer such estimates (Boxer code, instrumental catalog,
focal mechanisms database, macroseismic database). The continuation of the
project according to the established program should allow in the third year to
further improve both.
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