Regional macroseismic field of the 1980 Irpinia earthquake

This paper sets forth an analysis of the macroseismic field of the Irpinia ear thquake of 1980, which, by its magnitude and extent in the affected area, makes it the largest seismic event happening in Italy in the last 50 years. The data was collected by means either of direct inquiry or by macroseimic card, or by both these methods, and allows us to give a definition of seismic intensity, in altogether 1286 inhabited centres, in 13 different regions. The field, when compared to Blake's model (Y= 5.0), gives results compatible with the following focal parameters: I 0 = X° MSK ; <p0 = = 40.86 N ; Xu = 15.25 E ; Ioc = 9.99 ± 0.5 MSK ; hn = 15 Km. The anisotropy of the field was analysed by means of a reckoning of the azimuthal attenuation to be of the intensity of ( a z ) , the extreme values of which give the result 2.0 • 10~ and 3.9 • 10", in directions respectively NNW and SW. The mesoseismic area showed itself to be principally characterised by structural domains, brought out by the shadow method, following either the direction of the Apennines (NW-SE), going across the Apennines, or taking a meridional direction (N-S). A model for the seismic source has been elaborated along Shebalin's methodology. The relative values of the parameters came out thus: lxt = 1 4 Km ; Az =128° E lx = 5 6 Km : = 61° (dip SW) lz = 13.5 Km ; S„ =782 Km REGIONAL MACROSEISMIC FIELD, ECC. 2 9 The azimuth of the longitudinal section of the source (128° E) turns out to be very close to the strike azimuth of the fault plane proposed by various authors searching for a solution to the focal mechanism; the longitudinal extension and the relatively modest vertical development of the source are justifiable in the context of the geo-seismotectonic scheme of the Irpinia region. The angle of dip (61° SW), on the other hand, is in contrast to that generally proposed in research into the solution for the focal mechanism.

The azimuth of the longitudinal section of the source (128° E) turns out to be very close to the strike azimuth of the fault plane proposed by various authors searching for a solution to the focal mechanism; the longitudinal extension and the relatively modest vertical development of the source are justifiable in the context of the geo-seismotectonic scheme of the Irpinia region. The angle of dip (61° SW), on the other hand, is in contrast to that generally proposed in research into the solution for the focal mechanism.
1. -INTRODUCTION At 1930 hours local time on the evening of 23rd November 1980 a period of intense seismic activity began with a catastrophic main shock of magnitude 6.5, the intensity of which has been estimated as X o MSK. The worst hit area was the one lying between the localities of Castelnuovo di Conza, Conza della Campania, Laviano, Lioni, Sant'Angelo dei Lombardi and Santomenna. The first reports received from the region were both dramatic and contradictory.
The Istituto Nazionale di Geofisica decided upon the immediate despatch of microseismic and macroseismic survey teams to the area in order to install a local array to record seismic events and to carry out on-the-spot investigation and measurement.
A coordination centre of the Italian Geodynamical Project -CNR (P.F.G. -CNR) was rapidly set up in Naples with the task of coordinating collaboration between the various researchers that had been called in. In view of the enormous size of the disaster area it was decided to give each of the specially created geological, technical and macroseismic survey teams a well-defined zone to work in.
The macroseismic survey team of the Istituto Nazionale di Geofisica worked in cooperation with the Operational Unit of the Istituto di Geofisica of the University of Messina and the Istituto Internazionale di Vulcanologia -CNR of Catania. The ING was given the task of carrying out its investigation in the megaseismic area, i.e. in the localities of Guardia dei Lombardi, Sant'Angelo dei Lombardi, Morra de Sanctis, Lioni, Teora, Conza della Campania, Sant'Andrea di Conza, Castelnuovo di Conza, Santomenna, Laviano, Caposele and Materdomini, as well as in the epicentre area, i.e. in the vicinity of Foggia, Barletta, Lacedonia, Bisaccia, Vallata and Andretta. The Messina University -Catania CNR group was assigned the macroseismic survey of the upper Sele valley in the eastern Cilento, southern Basilicata, Calabria and Sicily. The above localities were visited and studied by the authors also for seismic microzoning purposes. The data on which to base a more precise macroseismic study of the phenomenon were extracted from the information gathered during the surveys.
In order to trace out as accurate as possible a macroseismic field of this tragic event, the survey was completed by means of the analytical processing of some 1,300 questionnaires (*) received by the Istituto Nazionale di Geofísica from all over Italy, as well as of the data collected by the macroseismic group of the Italian Geodynamical Project as a whole.
The present study may be considered of particular interest owing to the great mass of data used and the direct and indirect acquisition techniques used by the authors. Together with the fact the earthquake of 23-11-80 was the greatest seismic event as far as magnitude and area affected to have occurred in Italy in the last fifty years, these elements make the study of great significance. The huge Irpinia disaster area affected by the period of seismic activity beginning on 23-11-1980 is much the same as that which had been the scene of numerous terrible earthquakes in previous centuries. In view of the large number of earthquakes involved only those with I a 3= VIII o have been considered.
The epicentral data and the dates of the earthquakes prior to 1000 were taken from the Seismic Cataloque and Macroseismic Atlas of the ING (CSN -National Seismic Catalogue). For those after the year 1000 some were taken from the ENEL Seismic Catalogue (1975), others from our own surveys. Maps have been made ( fig. 1) for all major events (see tab. 1) causing damage, death or injury ii^ localities hit by the same earthquake inside the area lying within a radius of 80 km from a central point located at (*) Compiled by the operators of Arma Generale dei Carabinieri in accordance with the 1979 agreement with Istituto Nazionale di Geofisica, Rome. For the purpose of a more accurate interpretation of the event under examination within the framework of the seismicity of the Irpinia area, the macroseismic fields of three historical earthquakes have also been given. These are of great interest for the purposes of comparison, also in view of the recent critical review made of them by authors (figs. 2, 3, 4).

-Representation criteria
The total data acquired either directly and/or from macroseismic questionnaires have allowed intensity to be defined for a total of 1286 localities. The criteria used in assigning the grade of macroseismic intensity are set out below. The intensity is not expressed in the form of a range of values but as a single value. It thus follows that plotting the isoseismals these values may be taken as the lower, median or upper limit of the grade in question. In our view the second criterion makes it possible to plot the isoseismals using the intermediate values which, if specifically observed during the survey, have been plotted in the macroseismic field as intermediate grades (e.g. expressed as 2-3, IV°-V°). On the other hand, this is consistent with the use the same authors (Prochazkova and Karnik, 1978) propo- sed for the MSK scale. Although maintaining the set of points with the intensity observed as initial data, in plotting the isoseismals it is attempted to give a representation of the regional macroseismic field (BOTTARI and Lo GIUDICE, 1982). In other words, the points having a lower «weight» and those related to «restricted» anomalies are neglected (BOTTARI and Lo GIUDICE, 1981) and the isoseismals are smoothed to characterize the regional pattern, the anisotropy of which is mainly dependent on the geologico-structural characteristics governing the propagation of seismic waves. Any «local» anomalies appear in the plot as « islands » or « apophysis ».
In accordance with the above criteria the overall regional field has been plotted in fig. 5 (base scale 1 : 1,000,000). The same field (base scale 1 : 500,000) is shown in fig. 6  where the mean radii of the isoseismals (equivalent circumferences) over the range IV° =£ I VIII° were used to calculate h n \ the values of the area S shown in parentheses refer to the isoseismals whose plot lies partly in the sea.
After correction for an isotropic field the aforegoing can be extended to the rather anisotropic regional field. In this case the azimuthal attenuation a. is calculated, together with its reciprocal «macroseismic conductivity» p. in the more significant directions using the following relation (BOTTARI et al., 1982): The macroseismic conductivity values calculated for 16 azimuthal sectors having a width of 22°.5 are shown in fig. 8 and again in table 2, together with those of the attenuation.
The macroseismic field is characterized from X° to 111° MSK ( fig. 5). The X° and IX° isoseismals are affected by the source and are seen in particular to run along the Apennines (NW-SE). The VIII° and VII° isoseismals run to a lesser extent in this direction and to a greater extent in a NNW direction. The direction of maximum propagation of the intensity is apparent as far as the VI 0 isoseismal along the Apennines chain. Starting from this isoseismal, which is the last complete one, a preferential propagation towards the N appears (about 15° W), which can be followed as far as isoseismal IV°. Towards the S, despite the incompleteness of the isoseismals plot, the preferential propagation follows the Calabro-Peloritano arc. In the same direction it is possible to define also the limit of the 111° area in Sicily, while it cannot be plotted to the N owing to the inadequacy and inconsistency of the available data. The macrosismic field characteristics in the Calabro-Peloritano area allow the correlation between the macroseismic azimuthal attenuation and the field of structural patterns to be verified (BOTTARI et al., 1982;BOTTARI et al., in press).
Overall, the macroseismic field displays no significant anomalous areas, except for three small zones -two of which with a negative anomaly in the upper Calore river valley in the VIII° area ( fig. 7), and the third with a positive anomaly inside the VP area on the Sorrento peninsula ( fig. 6).  In order to define the size of the mesoseismic area in the broad sense the criterion adopted was that of Shebalin (1974) in which the local effect on the field related to the vertical development of the source is analysed. This effect, which is clearly revealed by the plot of the SJS l + l ratio, is restricted to the intensity X° and IX° areas ( fig. 9, fig. 13).
The mesoseismic area shows how the two rose diagrams ( 1979), is characterized mainly by structural domains with Apennine (NW-SE), anti-Apennine and southern (N-S) orientation. That the Apennine type domains with regional development predominate is confirmed by the Landsat image survey carried out by us ( fig. 11). In reading the relevant rose diagram account must be taken of the fact that the solar illumination (El. 36°, Az. 136°) heavily penalizes the iso-orientated angular sector (of about 30°), and enhances the sectors transversal to the normal one. The domain at N 45 u W must thus be considered a first-order maximum. Shebalin's method (1974) was used for the purposes of source modelling, except in the calculation of h and Y, for which it was preferred to follow the procedure developed by BOTTARI et al. (1979).
The barycentres calculated for each isoseismal area is shown in fig. 12. The overall result is a low dispersion which indirectly confirms the reliability of the assigned intensities and the relative isoseismal plots. In particular, the good agreement of the barycentres in both the X o and IX o and the VIII o and VII o intensity areas, which are defined by isoseismals governed wholly by observed intensities, allows a reliable determination to be made f°r ^on and E ol . As can be seen in fig. 13, they can both be associated with two well-defined plots of the function /(A). The focal depth of the surface portion of the source (SHEBALIN, 1974) calculated for the isoseismals of intensities X o and IX o using the method previously used for the normal depth is - classifies the seismic event as a «normal earthquake».

-FINAL CONSIDERATIONS
The parameters of the regional macroseismic field presented herein confirm those obtained experimentally (BOTTARI et al., 1981). Also confirmed are the correlations with the structural geological framework reported in previous works (BOTTARI et al., 1981;BOTTARI et al., 1982). As shown by the macroseismic fields of the historical earthquakes, particularly in the Campano-Lucanian region there is a recurrent correlation between the extension of the mesoseismic area in an Apenninic direction (NW-SE) and preferential propagation in the external field. For some earthquakes (1732, 1851 and 1930) there is also a secondary preferential propagation  in an anti-Apenninic direction (SW-NE)*. This is confirmed in greater detail by the mascroseismic field of the present Irpinia earthquake, which reveals a coincidence between the direction in which the mesoseismic area extends (/" = X°) and the direction of * The recurrence of this anomaly and particularly of the sickle shaped isoseismals with the concave side pointing towards the west points to the determining role played by the anti-Apennine structures on the Tyrrhenian side as opposed to what is found on the Adriatic side. The source characterization completes the picture of our knowledge of the earthquake and lends itself to several considerations. The azimuth of the longitudinal section of the source (N 128° E) has been found to be very close to that of the strike of the fault plane proposed by various authors as the solution to the focal mechanism (Seismometric working group of 23-11-1980CUSCITO and PANZA, 1981;GASPARINI C., 1981). The longitudinal extension of the source and its comparatively small vertical development seem quite justified by the geoseismotectonic situation prevailing in the Irpinia region. In particular the lower limit (18.5 -20 Km) of the source studied coincides with the limit of the known depths of the earthquakes occurring in the same region (DE Vivo et al., 1979) and with the transition from the upper crust (COLOMBI et al., 1979).
The angle of dip (61° SW) instead is in disagreement with the generally proposed value of approximately 65° NE. However, considering the great structural complexity of the area, the variety of the effects produced on the ground in relation to the possible dynamics of the deep structures and the presence of both dips (NE and SW) for the emergent faults, this disagreement does not seem to be a significant one. It is however stressed how the solution based on microseismic data provides a satisfactory answer to the lack of a single choice of main fault plane often implied in focal mechanism calculation methods.
Also significant is the fact that the distribution of the aftershocks (figs 15, 16) is compatible with the macroseismic field and with the source model described.
On the whole, the proposed source model, including also the dip direction, appears to be in good agreement with the structural and neotectonic characteristics that emerge from the majority of studies carried out on this region (DE VIVO et al., 1979;COLOMBI et al., 1979;PESCATORE, 1981;CINQUE et al., 1981;ORTOLANI and TORRE, 1981;CIARANFI et al., [in press)).