Revision of the theoretical and observational grounds of the seismic hazard estimates at a national scale

 

Coordinator: Paolo Gasperini

 

 

Introduction

 

Due to the successive variations of the approval state of the project (initially annual with a further biannual re-proposition) its objectives have in turn changed with time. In its initial formulation the project was aiming at a complete revision of seismic hazard estimates at a national scale by the use of classical techniques and the development of innovative ones. After the significant resizing of the founding, it has been decided, even on the basis of referees' suggestions, to limit the objectives to few specific investigation fields which were propaedeutical to hazard assessment. This restricted the active tasks to only four of the fifteen, which were originally planned in the original project. These are:

Task 1A Historical seismic catalog (Resp. Gasperini/Guidoboni)

Task 6C Intensity attenuation tomography and site effects (Resp. Gasperini)

Task 5 Statistics of seismic sources and catalog completeness (Resp. Marzocchi)

Task 7 Focal mechanisms (Resp. Gasperini)

In the first year of the project were also active:

Task 3 Seismogenic model (Resp. Valensise)

Task 6A Seismic wave velocity tomography (Resp. Morelli)

Task 6B Seismic wave attenuation tomography (Resp. Mele)

Task 9A Geodetic measurements (Resp. Riguzzi)

During the course of the project (particularly in the first and last years) some other tasks has been partially active: Instrumental seismicity (2A) and Magnitude (2B) whose results, together with those provided by tasks 1A, 6C and 7, have been widely used by the recent INGV initiative for the revision of the seismic zonation of Italy following the ordinance PDC 3274 of march 20, 2003. After the reformulation, the project focused on the development of research tools, which can be helpful to improve classic and innovative hazard evaluations. In particular we had a special attention to the revision and validation of seismic data (historical and instrumental), the formulation and verification of earthquake occurrence statistical models, the collection of earthquake focal mechanisms, and the modeling of the regional (tomography) and local (site effects) variations of seismic intensity proagation properties.

 

 

Description of research activities

 

In the following, we synthetically report, for each of the tasks, being active during the three years of the project, the original and reformulated objectives, the activities actually carried on, the expected results planned in previous years with an indication of the state of completion as well as a description of the final products provided by the project.

 

 

Task 1 HISTORICAL SEISMIC CATALOG (Resp: Gasperini/Guidoboni)

Objectives. After the re-proposition of the project and its reformulation, this task was devoted to the development of computational tools able to estimate historical earthquakes parameters and to make seismic intensity assessment objective and reproducible. During the last year, under the solicitation of the cited INGV initiative for the revision of the Italian seismic zonation (Seismic Zonation Working Group, 2004), the objective of this task partially moved back to is original formulation, that was the revision of the Italian seismic catalog according to the results of new investigations made available after the issuing of the Catalogo Parametrico dei Terremoti Italiani (CPTI) in 1999 (CPTI Working Group, 1999). This slightly delayed the full completion of some of previously planned activities.

Activities. We developed the Boxer code for the computation of the epicenter, the magnitude and the fault strike of strong historical earthquakes (Gasperini et al, 1999) by introducing an alternative method of location and sizing based on the use of a bilinear attenuation law (Gasperini, 2001). The epicenter location is computed by minimizing the intensity residual sum of squares while the magnitude is deduced from empirical regressions with the average intensity computed at the epicenter. Different to the previous method used in Boxer, that simply computed the barycenter of strongest intensities, this strategy also allows to locate epicenters in sea-covered areas where intensity observations are not available. This method should also allow in the future to make estimates of the depth of the source, although some further efforts are needed.

A development of Boxer code was also involved in the general revision of the CPTI catalog made in the ambit of the INGV initiative for the revision of the Italian seismic zonation. We refined the method of computing the magnitude by adopting an improved weighting scheme that even takes into account the number of available observations. We also developed a new Fortran code (BoxCoeff) that, basing on a set of known instrumental magnitudes associated to macroseismic data, determines the relevant empirical coefficients to compute magnitude from the intensity distribution. The result of this procedure is a modified parameter input file that can be used with Boxer to compute magnitude according to the new relations.

By the technique developed by Ferrari et al. (1995) and Vannucci et al (2000), 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. This technique uses the Fuzzy Sets approach to make intensity estimates that are based on contradictory evidences. We used an empirical approach where the membership function of each coded effect, to each degree of the intensity scale, is deduced from data. The comparison of the Irpinia earthquake 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 other events we obtain macroseismic fields very different from the one obtained by the membership function inferred from the event itself. This discrepancy is particular evident comparing the results of the application of the Boxer code on the different macroseismic fields (Fig. 1). To investigate on this discrepancy we are now implementing the coding of effects for other Italian earthquakes.

Coordination with other tasks. This task was strictly coordinated with tasks 2A and 2B as these provided the necessary instrumental information regarding the time interval from 1981 to present.

Coordination with other projects. In the first and second years, this objective was pursued completely within the present project, while during the third one a collaboration has been activated with the GNDT project coordinated by Amato and Selvaggi for the revision of the earthquake catalog. The development of Boxer version 4.0, regarding the earthquakes of Western Liguria, was done in collaboration with the GNDT project coordinated by Lagomarsino,

Innovative aspects: Historical and instrumental data have been strictly integrated in a unique catalog. The attempt made to make the intensity estimate more objective and reliable was rarely approached in the literature.

 

Expected results (as previously planned)

 

First year.

None

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. (completed)

·      Application of the Fuzzy sets algorithm to compute the intensity field of the 1930 Irpinia earthquake. (completed)

Third Year.

·      New release of Boxer code allowing the location of offshore epicenters and the computation of depth. (not completed yet)

·      Application of the Fuzzy sets algorithm to other strong Italian earthquakes. (not completed yet)

 

Products:

1)    Parametric catalog of Italian earthquakes, version 2004 (CPTI04). Description. An integrated historical/instrumental seismic catalog updated to 2002 with magnitude homogeneously recomputed and appropriately suited to be used with different attenuation laws (Fig. 2). The portion of the catalog after year 1980 (with extension to 2002) has been completely revised on the basis of updated instrumental and historical data. The data coming from a revised version of the Catalogo sismico strumentale dei terremoti italiani dal 1981 al 1996 (CSTI) (CSTI Working Group, 2001) and from the INGV instrumental seismic bulletins from 1997 to 2002 are taken as reference. Also, the INGV macroseismic bulletin from 1992 to 1999, the third version of the "Catalogue of Strong Italian Earthquakes from 461 B.C. to 1997" (CFTI3, Boschi et al., 2001), the French and Swiss macroseismic databases SISFRANCE and ECOS, and a number of macroseismic investigation appeared in the literature in the last years were also considered and merged with instrumental data. Availability. Available at web address http://emidius.mi.ingv.it/CPTI/. Utility for DPC. This product represents the reference seismic catalog for present hazard assessments in Italy.

2)    Integrated macroseismic database. Description. It consists of all of the Italian macroseismic intensity observations used to compute the parameters of CPTI04. Presently it includes about 70,000 independent intensity estimates mainly coming from CFTI3 and DOM4.1 (Monachesi and Stucchi, 1997) databases. Availability. The final version will be available in the first months of 2005, but a preliminary one can be furnished upon request. Utility for DPC. This database is the prerequisite for the compilation of the Italian historic catalog and for the estimate of seismic intensity attenuation relations.

3)    New releases of Boxer code. Description. Boxer code is a computer tool to compute source parameters of historical earthquakes. Version 3.3 has been implemented to improve the computation of magnitudes, while version 4.0, allows the location of offshore earthquakes. The latter is still under test although some interesting results have already been obtained. Availability. Both versions will be made available before the end of the project at the web address: http://ibogfs.df.unibo.it/user2/paolo/www/boxer/boxer.html. Utility for DPC. Version 3.3 has been already used in the recent activities for the revision of seismic zonation. Version 4.0 could help to better define the seismic sources in some critical sea areas (Western Liguria and Northern Sicily).

 

Task 2A, INSTRUMENTAL SEISMIC CATALOG (Resp.:Monachesi/Gasperini)

Objectives. The revision of the Catalogo sismico strumentale dei terremoti italiani dal 1981 al 1996 (CSTI) published in 2001 (CSTI Working group, 2001). This task was reactivated in the last year of the project, in the framework of the INGV initiative for the revision of the Italian seismic zonation,

Activities. According to experience of using the previous version of CSTI (1.0), a new one (1.1) was prepared, where the location of events located close to the seismic network boundaries, and the local magnitudes of the earthquakes were revised. In particular we introduced in the location scheme a further attempt with depth fixed at 10 km and we adopted it, independently on the goodness of the fit with data, when both the angular gap and the distance to the closest station are larger than reasonable limits, thus indicating a location outside the network boundaries. Also, we discarded all amplitude magnitude estimates that were associated with very high time residuals. With these simple changes we significantly reduced the number of hypocenters with unreasonably high depth or magnitude due to bad conditioning of the hypocentral solution or to seismic phase association mistakes respectively (CPTI2 Working Group, 2004).

Coordination with other tasks. This task was strictly coordinated with tasks 1 and 2B. The former received and used the results, while the latter provided the revised true and synthetic Ml estimates used to homogenize magnitude.

Coordination with other projects. In the last year, we collaborated with the GNDT project coordinated by Amato and Selvaggi, for the revision of seismic bulletin data from 1997 to 2002.

Innovative aspects: None

 

 

Expected results (as previously planned)

 

First year.

None

Second Year.

None

Third Year.

None

 

Products:

1)    Instrumental seismic catalog of Italian earthquakes from 1981 to 1996 (CSTI version 1.0 and 1.1). Description. A database of more than 750,000 arrival times merging the data of the INGV national network with four regional networks. A total of about 45,000 earthquakes located, more than 35,000 of which having a homogenous Ml magnitude estimate. Availability. Version 1.0 is available on CD-ROM and on the web. Version 1.1 will be available before the end of the project at the same address of version 1.0 http://ibogfs.df.unibo.it/user2/paolo/www/gndt.html. Utility for DPC. These data have been used as reference for the revision of the seismic catalog used in last hazard estimates.

 

 

Task 2B, MAGNITUDE (Resp.: Gasperini)

Objectives. The homogeneous estimation of the magnitude of Italian earthquakes. Even this task was reactivated in last year, in the framework of the INGV initiative for the revision of the Italian seismic zonation,

Activities. We analyzed Ml estimates based on the data of two Wood-Anderson (WA) instruments operating in Italy from 1972 to 1989 and of synthetic WA traces from broad-band instruments operating from 1990 to present. We computed the average attenuation function for Italy and we found that it almost coincides with the one original deduced by Richter (1935) for Southern California (Gasperini, 2002). As attenuation in Italy is strongly heterogeneous we also attempted to deduce different attenuation functions for Northeastern (Adria and Po Valley) and Southwestern Italy (Apennines and Tyrrhenian). We found (Fig. 3) that while the latter is slightly more attenuating than the Richter one the former is definitely less attenuating than that (Gasperini et al., 2002). This would imply that Ml estimates made in such area using the Richter relation at the maximum distance of 600 km could overestimate the real magnitude up to one degree. However the inverted attenuation function is still not enough well constrained to be adopted in common computations and thus further effort is required to increase the ML estimate database.

In the ambit of the cited activities for the revision of the seismic hazard map of Italy, we built a magnitude database including, for each earthquake, various kinds of magnitude estimates (Mw, Ms, mb, Ml) available from the literature and from international organizations. We only considered estimates for which the modality of determination is known with reasonable certainty, particularly for Ml from WA. In all we collected 615 Ms, 2242 mb, 2256 Ml and 218 Mw. Of the 4175 earthquakes included in this database 862 have more than one estimate and thus can be used to derive empirical regressions by least squares (CPTI Working Group, 2004).

Coordination with other tasks. This task was strictly coordinated with tasks 1, 2A and 7. The former two receive and use the results, while the latter provided moment magnitude estimates from regional CMT mechanisms.

Coordination with other projects. None

Innovative aspects: Use of regionalized attenuation functions for magnitude computation.

 

Expected results (as previously planned)

 

First year.

None

Second Year.

None

Third Year.

None

 

 

Products:

1)    Database of Ml estimates for Italy and surrounding regions from 1975 to 1998. Description. Open file containing about 2200 Ml estimates from true WA instruments or synthetic WA traces. Availability. Upon request to the project coordinator. Utility for DPC. This is the reference database used to calibrate magnitudes in CSTI catalog.

2)    Database of magnitudes for Italian earthquakes. Description. Integrated dataset containing magnitude estimates coming from various sources and regarding about 4000 earthquakes occurred in Italy and surrounding regions from 1905 to 2003. Availability. Upon request to the project coordinator. Utility for DPC. This is the reference database used to calibrate relations among magnitudes and to assign instrumental magnitude to the earthquakes of the Italian catalog.

3)    Set of empirical relations among various magnitude definitions for Italy. Description. The empirical regressions were computed by least squares using the integrated magnitude database. More than 100 observations were used for each regression. Availability. Published on the web (in Italian) at address http://zonesismiche.mi.ingv.it/documenti/App1.pdf. Utility for DPC. Allow to compute homogeneous magnitude estimates to be used in seismic hazard computations.

 

 

Task 3 SEISMOGENIC MODEL (Resp.: Valensise)

Objectives. The improving of the knowledge on Italian seismic sources and the formulation of a new scheme of seismogenic zoning, constrained by evidence from global tectonics and strain rates from geodetic and field observations.

Activities. Due to the financial limitations imposed by the severe budget reduction, the activities of this Task proceeded essentially within other research projects. The first release (1.0) of the "Database of potential sources for earthquakes larger than magnitude 5.5 in Italy" (DISS) was prepared and a few sample copies distributed through Cds. This database collects all geological and geophysical information on seismic sources of significant Italian earthquakes within the framework of a Geographic Information System. During the project new data have been acquired and included in the Database as well as the management software has also been improved. A second public version was published on a special issue of Annali di Geofisica (Valensise and Pantosti, 2001).

Coordination with other tasks. Task 1 provided source parameters of historical earthquakes while task 7 the mechanisms of instrumental ones and moment tensor sums.

Coordination with other projects. In the first year this task was shared with the GNDT project coordinated by Amato and Selvaggi. Successively it proceeded essentially within such project.

Innovative aspects: Geo-referencing of all the available information on Italian seismic sources.

 

Expected results (as previously planned)

 

First year.

·      New release of DISS. (completed)

Second Year.

None

Third Year.

None

 

Products:

1)    Database of potential sources for earthquakes larger than magnitude 5.5 in Italy" (DISS) The description of this product can be found in the Amato/Selvaggi report.

 

 

Task 5 - STATISTIC OF SEISMIC SOURCES AND CATALOG COMPLETENESS (Resp.: Marzocchi)

Objectives. 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 or that could raise from the research itself.

Activities. The attention is focused in particular on the evaluation of the completeness and homogeneity of the magnitude estimates for the Italian instrumental seismicity from 1960 to present, on the analysis of the properties of aftershocks, on the evaluation of the statistical properties of strong shocks e and the estimate of their probability of occurrence.

For the joint catalog resulting from the combination of the CSTI with the "Catalogo del Progetto Finalizzato Geodinamica (PFG)" (Postpischl, 1985), we verified (Lolli and Gasperini, 2003a) that the event rate is strongly variable with time because of an incorrect calibration of the magnitude before year 1980 (Fig. 4). We also found that the application of empirical magnitude shifts allows to obtain a catalog whose annual rate is almost constant, excluding the aftershock of major earthquakes, over the entire period from 1960 to 1996.

Regarding the modeling and forecasting of seismic aftershocks in Italy, we have published a first paper (Lolli and Gasperini, 2003a) making using of the simple Reasenberg and Jones (1989) model where the average model parameters for whole Italy and six sub-regions are computed from the data of a set of 30 sequences occurred in Italy from 1960 to 1996. Using such parameters we plotted a set of nomograms (Fig. 5) allowing to expeditiously evaluate the number of aftershocks and the probability of strong shocks after a main shock with given magnitude. We have successively approached the problems of validating the forecasting ability of such model using the data from 1997 to 2002 (Lolli and Gasperini, 2003b) as well as of the improvement of the occurrence model with the introduction of the epidemic principle (ETAS) (Ogata, 1988) and the modification of the productivity parameter according to the result of a correlation analysis (Gasperini and Lolli, submitted).

The spatio-temporal distribution of large earthquakes was analyzed by a new non-parametric multivariate model (Faenza et al, 2003). 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 (Faenza et al., 2003). The method has been applied to the Italian seismicity of the last four centuries from the CPTI catalog. 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. The empirical model can be used to compute the map of the probability of occurrence for large earthquakes in Italy in the next 10 years (Fig. 6).

Coordination with other tasks. This task is strictly coordinated with tasks 1 and 2A, which provided the revised instrumental and historical catalogs respectively.

Coordination with other projects. None.

Innovative aspects: Application of a forecasting model of strong shocks including all possible hypotheses: Poisson, gap, and clustering.

 

Expected results (as previously planned)

 

First year.

None

Second Year.

None

Third Year.

§       Spatial seismic occurrence model relative to both the historical (long time-scale) and instrumental (short time-scale) seismicity (completed).

§       Statistical time occurrence model and its calibration on instrumental data (completed).

§       Verification of the magnitude scaling law for the instrumental catalog (not completed).

§       Verification of the seismic attenuation law currently in use (not completed).

§       Statistical model for the time series of the felt intensity at site (abandoned).

 

Products:

1)    Forecasting of the temporal behavior of aftershocks in Italy. Description. On the basis of a simple occurrence model combining the Gutenberg-Richter and the modified Omori law the average parameters, useful to forecast seismic behavior in Italy and in some subregions have been computed. Availability. The first paper was published on an international journal (Lolli and Gasperini, 200a). One further study was submitted (Gasperini and Lolli, submitted) and a third one is still in preparation. Utility for DPC. The aftershocks hazard after a strong main shock can be expeditiously estimated by some nomograms reporting the number of aftershocks and the probability of strong shocks as a function of main shock magnitude.

2)    Map of the probability of occurrence for strong earthquakes in Italy at short and medium term. Description. It reports the probability of occurrence for 1 and 10 years deduced on the basis of a time-dependent hazard model fitted on past seismicity. Availability. Published on an international journal (Faenza et al., 2003). Utility for DPC. The use of a more accurate occurrence model should permit more reliable hazard estimates. Moreover the suggested preference for the temporal clustering could require to modify the emergency measures after an earthquake.

 

 

Task 6C, SEISMIC INTENSITY TOMOGRAPHY AND SITE EFFECTS (Resp.: Gasperini)

Objectives. The accurate modeling of seismic strong motion in term of seismic intensity at global (tomography) and local (site effects) scale.

Activities. On the basis of the data of the integrated macroseismic database (product 2 of Task 1), we analyzed the attenuation properties of seismic intensity using both homogeneous and heterogeneous (tomographic) models. In a first study (Carletti and Gasperini, 2003) we considered a simple bi-linear model that proved to fit data better than other simple laws (Gasperini, 2001). The observed spatial variation of anelastic dissipation coefficients (Fig. 7) well agrees with surface heat flow map. This indicates that intensity, although based on qualitative information, is a reliable indicator of seismic waves propagation properties and particularly of strong motion. Therefore it can be compared with regional variation of attenuation laws in term of instrumental parameters (PGA, spectral ordinates, etc.). In a successive development we also included in the attenuation model a logarithmic term with distance to account for the geometrical spreading and a corrective term for epicentral intensity. The former makes the model more physically consistent, while the latter significantly improve the fit with data. The lateral variation of linear coefficient however does not change significantly thus confirming the validity of previous approach.

We also analyzed the intensity residuals, coming from the previous study, at the localities having a sufficient number of observations and correlated them with lithological and topographical characteristics of sites. We used a slightly modified version of the classification schema, prepared by the Rovelli (INGV) group participating to the Amato GNDT Project, to classify sites on the basis of the available geological maps (mainly obtained through the internet server managed by the Siena University).

We also studied the behavior of such residuals with time in order to determine the variations in the modality of seismic intensity determination in different centuries. Although this study is still at a preliminary stage we found a significant amplification before 1650 with respect to present time, which might be related to changes in the typology of sources after such date.

Moreover, we continued a macroseismic micro-zonation analysis of the city Florence, carried out in the first year (Boccaletti et al., 2001), were we used the reports compiled by the City of Florence Technical Office after the earthquakes of May 18 and June 6, 1895 to estimate, by a computer-aided methodology, the spatial variation of EMS seismic intensity (Fig. 8). We made measures of background noise spectral rates in several sites located within the urban area as well as S-waves velocity profiles in some of them. We are now completing a comparison study among different techniques (Albarello et al, 2004) that should allow to validate the reliability of various methods as well as to draw a reliable micro-zoning map of the Florence city center.

Coordination with other tasks. Tasks 1 giving the updated macroseismic database.

Coordination with other projects. We use the same site classification schema proposed by the Amato/Selvaggi project.

Innovative aspects. The introduction of spatial variability in seismic attenuation studies at a national scale.

 

Expected results (as previously planned)

 

First year.

§       Refinement of the bi-dimensional attenuation model. (completed)

§       Analysis of locality residuals in order to correlate them with lithological and morphological site properties. (completed)

Second Year.

§       Integrated macroseismic database including the data from the INGV macroseismic bulletin for all the events with Imax>V. (completed)

§       Preliminary tomographic inversion using the updated database. (completed)

§       Introduction of the spatial extension of the sources in attenuation computations. (abandoned)

§       Study of the lateral variations of the intensity in Florence for the earthquakes of 1895 compared with 1D simulations of ground motion. (almost completed)

Third Year.

§       Database of 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. (completed)

§       Definitive tomographic inversion using the updated methods. (almost completed)

§       Comparison of the lateral variations of the intensity in Florence with measures of soil amplification. (almost completed)

§       Study of the lateral variation of intensity for other historical centers. (abandoned)

 

Products:

 

1)    Study of attenuation properties of seismic intensity in Italy. Description. We analyzed the Italian macroseismic database to infer the average decay of intensity with distance and its spatial variability. The preferred attenuation law with distance includes a bi-linear term accounting for anelastic dissipation at near (< 45 km) and large distances (< 180 km), a logarithmic term considering the geometrical spreading, and a corrective term for epicentral intensity. Availability. A first study on seismic intensity attenuation, not including the spreading and epicentral intensity term has been published on an international journal while a new improved paper is currently in preparation. Utility for DPC. The knowledge of the attenuation properties of seismic intensity can be directly used to improve seismic hazard assessment in term of such parameter but it can also be used to calibrate the regional variation of attenuation laws even in term of instrumental parameters.

2)    Study of the correlation of seismic intensity residuals with lithological and topographical characteristics of sites. Description. We classified about 200 localities and compared their lithology and topography with average intensity residual. The correlation with single lithological or topographical characteristics is found to be poor in general. However, we found the combination of consolidated lithoid terrains in plain to significantly attenuate with respect to average (about 0.4 intensity degrees), and the combination of consolidated semilithoid terrain in valleys to significantly amplify (about 0.2 intensity degrees). Availability. A paper is still in course of preparation. Utility for DPC. This results could clarify the role of lithology and topography of a site on observed ground motion.

3)    Comparative study of the seismic response in the historical city center of Florence through instrumental measurements, numerical simulations and macroseismic data. Description. We compared the distribution of seismic intensity in the city center, following the 1895 earthquakes, with measures of background noise spectral rates and with 1D simulations of ground motion based on a digital representation of the top surface and of the bedrock topographies an on a series of S wave velocity profiles we have made. Availability. A paper is still in course of preparation. Utility for DPC. The comparison of different micro-zonation techniques allows to calibrate them and to estimate a reliable micro-zonation of the Florence city-center.

 

 

Task 7, FOCAL MECHANISMS (Resp.: Gasperini/Morelli)

Objectives. The development and the updating of a database of first motion mechanisms and the computation of regional CMT solutions for the Mediterranean area.

Activities. In the last few years we have collected and checked the focal mechanisms published in the literature for the Mediterranean and surrounding regions (more than 5000). To manage them, we developed a database (EMMA), on MS-ACCESS platform (Vannucci e Gasperini, 2003), also allowing to import the data of CMT catalogs, to select the mechanisms, to examine their parameters and to display the focal sphere plot (Fig. 9). To make the necessary tests before inserting the mechanisms in the database as well as to correct them when defective (we found that more than 40% of published data present misprints or inconsistencies), we wrote a package of Fortran subroutines (Gasperini e Vannucci, 2003) allowing to perform all relevant computations. In the last year we continued the insertion of further mechanisms and the improving of the management software. A new release (with enclosed CD-ROM) including in total more than 6000 mechanisms will be published on a special issue of Annals of Geophysics available in August 2004 (Vannucci and Gasperini, 2004).

The computation of new RCMT mechanisms was also continued by the INGV seismology group (Pondrelli et al., 2002; 2004). Recently, a retrospective analysis of available data, also allowed the computation of about 70 new mechanisms, not previously determined, of earthquakes with magnitude between 4.5 and 5.0 occurred from 1977 to 1996.

All the data collected in this task (EMMA and RCMT) and even those provided by the Harvard and ETH on-line CMT catalogs have been elaborated the Kostrov (1974) method to produce maps of cumulative moment tensors in Italy and in other Mediterranean regions. We plotted equivalent mechanisms (Fig. 10) and horizontal projection of P and T axes for different regions and on a grid with mesh of one degree, for the entire Mediterranean, and of half a degree for some smaller regions like Italy and the Aegean. For each cell we also computed a summary card (Fig. 11), which can be easily accessed through sensitive web pages, describing the relative contribution and coherency of literature data (EMMA) and CMT catalogs. These map have been described and commented together with the main geologic and tectonic features of the various Mediterranean regions on a paper (Vannucci et al., 2004) which will appear on the cited special issue of Annals of Geophysics.

Coordination with other tasks. Tasks 1 uses scalar moment estimates to assign moment magnitudes to the earthquakes. Task 2B uses the same data to compute regressions with other magnitude types.

Coordination with other projects. The RCMT catalog is mainly supported, outside this GNDT project, by the INGV itself and the relative data are available over the network for every investigator.

Innovative aspects: EMMA database is the most complete collection of earthquake mechanisms for the Mediterranean area.

 

 

Expected results (as previously planned)

 

First year.

None

Second Year.

·      New preliminary release of the database of Earthquake Mechanisms of the Mediterranean Area (EMMA) (completed)

Third Year.

·      First public release of the database of Earthquake Mechanisms of the Mediterranean Area (EMMA) (completed)

 

Products:

1)    Database of Earthquake Mechanisms of Mediterranean Area (EMMA). Description. It is an archive of focal solutions, taken from the literature also allowing the importing of on-line CMT databases (Harvard, ETH and INGV). Initially, it was designed to collect only Italian data but successively the area of interest has been extended to include the entire Mediterranean (from mid-Atlantic ridge to Iran). It presently includes about 6000 mechanisms relative to about 3800 earthquakes. The parameter consistency of all of the mechanisms has been checked and, in case of mistakes, the parameters have been corrected when possible. Availability. The first version was published on an international journal (Vannucci and Gasperini, 2003; Gasperini and Vannucci, 2003). A new release (with enclosed CD-ROM) will be published on a special issue of Annals of Geophysics that will be published in August 2004. Utility for DPC. The included data allow to extend back in time and down in magnitude the knowledge on the tectonic styles of the seismic release in Italy and surrounding regions. The first version was already used by the Seismic Zonation Task (Seismic Zonation Working Group, 2004) operating in the framework of the recent revaluation of seismic hazard of Italy.

2)    Database of regional CMT for the Mediterranean. Description. It is an archive of moment tensor solutions computed using regional broad-band recordings (mainly coming from the MEDNET Network of INGV). Presently it Includes earthquakes occurred between 1997 and 2002, with moment magnitude between 4.5 and 5.5, usually not computed by the Harvard seismology team. Recently, by a retrospective analysis of available data, the computation of mechanisms of earthquakes occurred before 1997 has been carried out. With the last updades the total number of mechanisms will exceed 500. Availability. The data from 1997 to 2002 were already published on international journals (Pondrelli et al., 2002; 2004) and are also available on-line at web address http://www.ingv.it/seismoglo/RCMT/. A paper including the data of earthquakes occurred before 1997 is in preparation. An updated version of the database will be included in the CD-ROM enclosed in the cited special issue of Annals of Geophysics. Utility for DPC. Even these data allow to extend down in magnitude the knowledge on the tectonic styles of seismic release in Italy.

3)    Atlas of moment tensor summations for the Mediterranean area. Description. This is the result of the application of the Kostrov (1974) method to the integrated database including mechanisms from the literature (product 1) and CMT mechanisms from Harvard, INGV (product 2) and ETH. The maps, representative of the seismicity distribution and of the cumulative moment tensor in Italy and in other six Mediterranean regions, are critically described to give a comprehensive framework of seismic deformation. Availability. It will be published on the cited special issue of Annals of Geophysics and probably even on the web. Utility for DPC. It gives a complete description of the state of seismic strain in Italy and surrounding region. Preliminary versions of these maps were already used by the Seismic Zonation Task (Seismic Zonation Working Group, 2004) operating in the framework of the recent revaluation of seismic hazard of Italy.

 

 

Task 9A, GEODETIC MEASUREMENTS (Resp.: Riguzzi)

Objectives. The original objective was the building of a semi-permanent network to monitor inter-seismic deformation along the most active seismic belts of Italian peninsula. Due to the funds reduction, this objective was first restricted to an area going from the surrounding of Aquila to Pollino Massif (instead of from Umbria to Calabria) and after first year completely abandoned. Activities. A compilation of the permanent, semi-permanent and temporary sites that operated in Italy in last years has been completed. Three new sites were occupied by semi-permanently (Madonna di Cristo e Le Serre) or permanently (Roseto degli Abruzzi) by GPS receivers in the weakest part of the network (coast of Lazio, southern Tuscany and Abruzzo), where the strengthening of the network is most urgent. GPS observation analysis, coming from campaigns performed in the framework of other projects, showed with high significance the horizontal relative velocities of the southernmost part of the network; its estimation is about 4 mm/year and the deformation field has the main component of extensional style, normal to the Apennine chain.

Coordination with other tasks. None.

Coordination with other projects. The installed stations are at disposition of other projects.

Innovative aspects: Use of GPS measurements to estimate inter-seismic deformation along seismic belts.

 

Expected results (as previously planned)

 

First year.

·      Materialization of new sites of the GPS network. (completed)

Second Year.

None

Third Year.

None

 

 

Results:

 

None

 

Conclusions

 

The distinctive trait of most of the researches proposed by our project is the focusing on the analysis of macroseismic data. These are information completely independent with respect to the instrumental ones, and can represent a useful alternative when instrumental information are scarce. A particularly significant example in this sense is represented by the tomographic inversion of seismic intensity attenuation we made. As we have found that this istrictly correlates with physical properties like the heat flow, it could be used to "calibrate" the instrumental attenuation relations found for some areas of the Italian territory in order to provide relation even for not investigated areas.

An accessory product of seismic intensity tomography is the list of locality empirical residuals. This could allow to compare, using a very large number of localities (more than 500), the site effects estimated by others, 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 micro-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 is particularly focused at the spatial-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 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, like Poisson, time-predictable and characteristc, often adopted by hazard estimates even in deeply studied areas like California and Japan, are not suitable to represent the real occurrence of earthquakes.

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 we have continued to carry on some activities on this argument during the project. In the last year our choice demonstrated to be correct as most of the results we obtained were widely used within the cited INGV initiative for the revision of seismic zonation of Italy

Notwithstanding the difficulties due to the delayed activation of the funding with respect to other projects, most of the 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 earthquakes) as well as of the tools required to make such estimates (Boxer code, instrumental catalog, focal mechanisms database, macroseismic database).

 

 

 

Scientific production of the project

 

 

Peer-reviewed papers

 

Boccaletti M., Corti G., Gasperini P., Piccardi L., Vannucci G. e Clemente S. (2001) Active tectonics and seismic zonation of the urban area of Florence, Italy. Pageoph, 158, 2313-2332.

Carletti F. e Gasperini P. (2003) Lateral variations of macroseismic intensity attenuation in Italy, Geophys. J. Int., 155, 839-856.

Faenza L. e Marzocchi W., and Boschi E. (2003) A non-parametric hazard model to characterize the spatio-temporal occurrence of large earthquakes; an application to the Italian catalog, Geophys J. Int. 155, 521-531.

Gasperini P., (2001). The attenuation of seismic intensity in Italy: a bilinear shape might indicates the dominance of deep phases at epicentral distances longer than 45 km, Bull. Seism. Soc. Am., 91, 826-841.

Gasperini P., (2002). Local Magnitude revaluation for recent Italian earthquakes (1981-1996), J. Seismology, 6, 503-524.

Gasperini, P. and Vannucci, G., (2003), FPSPACK: a package of simple FORTRAN subroutines to manage earthquake focal mechanism data, Computers & Geosciences, 29, 893-901.

Lolli B. e Gasperini P. (2003a) Aftershocks prediction in Italy Part I: Estimation of time-magnitude distribution model parameters and computation of probabilities of occurrence, J. Seismol, 7, 235-257.

Pondrelli, S., Morelli, A., Ekström, G., Mazza, S., Boschi, E. and Dziewonski, A. M., (2002), European- Mediterranean regional centroid-moment tensors: 1997-2000, Phys. Earth Planet. Int., 130, 71-101.

Pondrelli, S., Morelli, A., and Ekström, G., (2004) European- Mediterranean regional centroid-moment tensors: 2001 and 2002, Phys. Earth Planet. Int., 145, 127-147.

Vannucci, G., and Gasperini, P. (2003), A database of revised fault plane solutions for Italy and surrounding regions, Computers & Geosciences, 29, 903-909.

 

Technical reports

 

CPTI Working Group, (2004) App.1 catalogo dei terremoti CPTI2, in "Redazione della nuova mappa di pericolosita' sismica", Available at web address http://zonesismiche.mi.ingv.it/documenti/App1.pdf.

Seismic Zonation Working Group, (2004) Rapporto conclusivo sulla Redazione della nuova mappa di pericolosita' sismica, Available at web address http://zonesismiche.mi.ingv.it/documenti/rapporto_conclusivo.pdf.

 

 

Conference presentations (only for results not already been published)

 

Albarello D., Baliva. F., Boccaletti M., D'Amico V., Gasperini P., Picozzi M., Vannucci G. (2003), Deterministic interpretation of EMS intensity lateral variation for the Impruneta earthquake in the urban area of Florence (Italy), EGS-AGU-EUG Joint Assembly, Nice, SM10-1WE2P-0133.

Faenza, L.; Marzocchi, W.; Cinti, F.; Montone, P. (2003) A nonparametric hazard model to characterize the spatio-temporal occurrence of large earthquakes; an application to the Italian catalog, EGS-AGU-EUG Joint Assembly, Nice, TS19-1FR1O-008.

Gasperini, P., Lolli, B., Filippucci, M., and De Simoni, B. (2002) The ml magnitude scale in Italy, EGS XXVII General Assembly, Nice, SE5.04-1TH5P-150.

Lolli, B., Gasperini, P., (2003b) Aftershocks hazard in Italy: validation and improvement of the forecasting model, EGS-AGU-EUG Joint Assembly, Nice, NH10-1WE2O-002

Pondrelli, S., Salimbeni, S., Ekstrom, G., Gasperini, P., Morelli, A., and Vannucci, G. The Italian Moment Tensor Catalog from 1977 to Present, I EGU General Assembly, Nice, EGU04-A-03169.

Vannucci, G., Pondrelli, S., Boschi, E., Gasperini, P., and Morelli, A. (2004) Moment Tensor Summation in the Mediterranean Area Revisited: Comparison Between CMT and non-CMT Mechanisms Catalogs, I EGU General Assembly, Nice, EGU04-A-03263.

 

 

Papers submitted and in press

 

Gasperini P. and Lolli B., Correlation between parameters of aftershock rate equation: implications for the forecasting of future sequences behavior, Bull Seism Soc Am., submitted.

Vannucci G., Pondrelli S., Argnani A., Morelli A., Gasperini P., and Boschi E., An atlas of Mediterranean seismicity, Annals of Geophysics, In press.

Vannucci, G., and Gasperini P., The new release for Earthquake mechanisms of the Mediterranean Area (EMMA Version 2), Annals of Geophysics, In press.

 

 

Other references

 

 

Boschi E., Guidoboni E., Ferrari G., Mariotti D., Valensise G. e Gasperini P. (2000), Catalogue of Strong Italian Earthquakes from 461 B.C. to 1997, Annali di Geofisica, 43, n. 4, and enclosed CDROM.

CPTI Working Group (1999), Catalogo Parametrico dei terremoti Italiani, Ed. Compositori, Bologna, Italy, 88 pp.

CSTI Working Group, (2001) Catalogo strumentale dei terremoti italiani dal 1981 al 1996, Versione 1.0. ISBN 88-491-1734-5, Clueb Bologna, CDROM. Also available at http://ibogfs.df.unibo.it/user2/paolo/www/gndt.html.

Ferrari, G., Gasperini, P., and Guidoboni, E., (1995), Macroseismic intensity evaluation with the "Fuzzy Sets Logic", Annali di Geofisica, 38, 811-826.

Gasperini, P., Bernardini, F., Valensise, G. and Boschi, E., (1999), Defining seismogenic sources from historical earthquake felt reports, Bull. Seism., Soc., Am., 89, 94-110.

Kostrov V.V. (1974) Seismic moment and energy of earthquakes and seismic flow of rocks. Izv. Earth Phys, 1, 23-40.

Monachesi, G. & Stucchi, M., 1997. DOM4.1, un database di osservazioni macrosismiche di terremoti di area italiana al di sopra della soglia del danno, GNDT, Open File Report, Milano-Macerata, available at http://emidius.itim.mi.cnr.it/DOM/home.html.

Ogata, Y., (1988) Statistical models for earthquake occurrences and residual analysis for point processes, Journal of the American Statistical Association 83, 9-27.

Postpischl, D. (Edited by) (1985) Catalogo dei terremoti italiani dall'anno 1000 al 1980. Quaderni della Ricerca Scientifica 114 2B, pp. 239.

Reasenberg, P.A. and Jones, L.M., 1989, Earthquake hazard after a mainshock in California, Science 243, 1173-1176.

Richter, C.F., (1935) An instrumental earthquake magnitude scale, Bull. Seism. Soc. Am., 25, 1-31.

Seismic Zonation Working Group, (2004) App.2 zonazione sismogenetica ZS9, in "Redazione della nuova mappa di pericolosita' sismica", Available at web address http://zonesismiche.mi.ingv.it/documenti/App2.pdf.

Valensise G., and Pantosti, D., (2001), Database of potential sources for earthquakes larger than M 5.5 in Italy, Annali di Geofisica, Supplement to Vol. 44.

Vannucci, G., Gasperini, P., Ferrari, G. and Guidoboni, E., (1999), Encoding and computer analysis of macroseismic effects, Physics and Chemistry of the Earth, 24, 505-510.

 


Figures

 

 

Figure 1. 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.

 

Figure 2. Distribution of earthquakes in CPTI04 with indication of the origin of data.


Figure 3. Magnitude attenuation curves and corresponding 95% confidence intervals for Northeastern (blue) and Southwestern Italy (red). The Richter standard attenuation curve for Southern California is reported in black.

 

Figure 4. Cumulative number of events in Italy with M³4.0 from the PFG catalog (from 1960 to 1980) and from the CSTI Catalog (from 1981 to 1996). The quotes indicate the annual rate in the different periods (excluding the aftershocks of major events).


 

Figure 5. Nomograms for expeditious prediction of aftershock occurrence in Italy. In panel a) probability of strong aftershocks (with M³Mm-1) as a function of time after the main shock for different interval of time following the present time. In panel b) the same probability curves but for a larger main shock (M³Mm). In c) number of aftershocks in the following day as a function of time after the main shock, for different magnitude thresholds.


 

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Figure 6. Map of the probability of occurrence for the next large earthquake in Italy in the next 10 years.

 

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Figure 7. 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.


 

 

Figure 8. 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).

 

File written by Adobe Photoshop® 5.0

 

Figura 9. Summary an plot mask of the database of Earthquake Mechanisms of Mediterranean Area (EMMA). (Vannucci e Gasperini, 2003).


Figure 10. Sum of moment tensors on a regular grid with mesh of half a degree, for earthquakes with depth < 50 km. The equivalent focal mechanism plots are scaled with magnitude and located in the barycenter of the epicenters distribution weighted with magnitude.


 

 

Figure 25. Example of summary cards allowing to compare equivalent mechanisms resulting from the sum of the moment tensors (without replicates) available from EMMA database (green), from the CMT catalogs (blue) and from the combination of both EMMA database and CMT catalogs (red). See text for more details.