Irpinia Earthquake Research Articles

Applying the relative hypocentre location approach: where was the 1980 November 23 Irpinia earthquake?

The purpose of this work is to evaluate under what conditions it is feasible and with what accuracy it is possible to locate the nucleation point of a large earthquake, given the availability of aftershocks located with high precision by the deployment of a local network. We experiment with several approaches and apply them lo the location of the epicentre of the 1980 November 23 Irpinia earthquake (Mw= 6.9). First we use local Pg phases selected to optimize the azimuthal coverage, obtaining a well-constrained location with a small statistical error, which typically underestimates the true hypocentre uncertainty. We then exploit the relative location technique, obtaining stable, almost coincident solutions under three conditions: (1) using multiple independent master events to derive an average epicentre; (2) fitting simultaneously the larger data set for all available master events, using a forward approach; (3) conducting an a priori evaluation of the statistics of station and master events to separate model uncertainties and improve the statistical accuracy of the relative locations. Moreover, only by introducing station statistics can we achieve the desired accuracy of ≅ 1 km in constraining the rupture nucleation point of this large earthquake, and we show that the application of the relative location technique to uncleaned, unweighted data for a single master event provides only a crude epicentre with a confidence ellipse deceivingly smaller than the true hypocentre uncertainty. The revised epicentre for the 1980 November 23 Irpinia earthquake (48.803 °N-15.302°E) validates the class of multidisciplinary reconstructions of the source process such as the model of Valensise et al. (1989), based on the hypocentre of Westaway & Jackson (1987), and is shifted by almost 13 km to the NW of the epicentre recently proposed by Westaway (1992).

Mitigation of seismic hazard of a megacity: the case of Naples

The seismic ground motion of a test area in the eastern district of Naples was computed with a hybrid technique based on the rnode surnrnation and the finite difference methods. This technique allowed the realistic modelling of source and propagation effects, including local soil conditions. In the modelling, as seismic source we considered the 1980 Irpinia earthquake, a good example of strong shaking for the area of Naples, located about 90 km from the source. Along a profile through Naples, trencling N86°W, the subsoil is mainly formed by alluvial (ash, stratified sand and peat) and pyroclastic materials overlying a pyroclastic rock (yellow Neapolitan tuff) representing the Neapolitan bedrock. The detailed information available on the subsoil mechanical properties and its geometry warrants the application of the sophisticated hybrid technique. For SH waves, a comparison was made between a realistic 2-D seismic response and a standard I-D response, based on the vertical propagation of waves in a plane layered structure. As expected the sedimentary cover caused an increase in the signal's amplitudes and duration. If a thin uniform peat layer is present, the amplification effects are reduced, and the peak ground accelerations are similar to those observed for the bedrock model. This can be explained by the backscattering of wave energy at such a laqer. The discrepancies evidenced between the l -D and the 2-D seismic response suggest that serious caution must be taken in the formulation of seismic regulations. This is particularly true in the presence of the thin peat laqer where the misinatch between the l -D and the 2-D amplification functions is particularly evident in correspondence of the dominant peak and of the second significant peak.

A study of rupture characteristics of the 40 s subevent of the 1980 Irpinia earthquake

The Irpinia project, as carried out by ISMES under a commission from ENEL, had as objectives the developement of a general methodology to simulate broad-band seismic ground motion at near-source and regional distances, and the application of this methodology to the 1980 Irpinia earthquake. Within this general framework, one goal was the comparison of four previously published models for this earthquake in order to arrive at a plausible description of the source process. The comparative study was cast as an inverse problem: that of inferring the spatial extent and temporal behaviour of the rupture process, from geodetic measurements of surface deformation and near-source recordings of ground velocity. This study was complicated by the fact th the Irpinia earthquake was a complex event, involving at least three distinct rupture episodes in a time span of 40 s. However, this same complexity offers the opportunity of illustrating the use of inversion methodologies to 1) infer the spatial slip distribution on a multiple fault system; 2) address the problem of determining the accuracy of the inferred slip models, and 3) use information describing the static characteristics of an earthquakes as an aid in understanding the kinematics of the rupture. This last point is illustrated for the 40 s subevent through the results of a forward modeling study of high-frequency acceleration waveforms using a rupture model based on the inversion results.

The Architectural Heritage of Campania: Protection from and Prevention of Seismic Hazards

This paper concerns the rehabilitation of historical buildings located in seismic areas, and is organized in two parts. The first part considers some of the new trends in the field of research in preservation and rehabilitation. These are elaborated in the following five-point summary: 1) Maintenance of historical buildings as an efficient means of minimizing damage from an earthquake. 2) Reduction of consolidation. The present-day policy is that of limiting “strengthening work” intervention in favor of “small and timely repairs.” 3) In-depth knowledge as a necessary premise to plan a rehabilitation project. 4) Re-evaluation of traditional techniques by incorporating modern materials 5) Preservation of the function and character of monuments in their changing life (use) and new technological needs. The second part of the paper describes examples of rehabilitation undertaken in the aftermath of the 1980 Irpinia earthquake.

Teleseismici observation of the November 23 1980, Irpinia earthquake

Teleseismici observation of the November 23 1980, Irpinia earthquake

Damage versus ground acceleration correlations during the November 23 1980, Irpinia earthquake

Damage versus ground acceleration correlations during the November 23 1980, Irpinia earthquake

Summary of contributions on the 23 November 1980, Irpinia earthquake

Summary of contributions on the 23 November 1980, Irpinia earthquake

Paleoseismological evidence of repeated large earthquakes along the 1980 Irpinia earthquake fault

Paleoseismological evidence of repeated large earthquakes along the 1980 Irpinia earthquake fault

Neotectonic setting of the Benevento area: comparison with the epicentral zone of the Irpinia earthquake

Neotectonic setting of the Benevento area: comparison with the epicentral zone of the Irpinia earthquake

The November 23, 1980 Irpinia earthquake: an analysis with the new procedure of intensity evaluation

The November 23, 1980 Irpinia earthquake: an analysis with the new procedure of intensity evaluation

Source geometry and long term behavior of the 1980, Irpinia earthquake fault based on field geologic observations

Source geometry and long term behavior of the 1980, Irpinia earthquake fault based on field geologic observations

Modeling of the 1980 Irpinia earthquake source : constrains from geodetic data

Modeling of the 1980 Irpinia earthquake source : constrains from geodetic data

Aftershock location and P-velocity structure in the epicentral region of the 1980 Irpinia earthquake

Aftershock location and P-velocity structure in the epicentral region of the 1980 Irpinia earthquake

Fault rupture geometry for the 1980 Irpinia earthquake: a working hypothesis

Fault rupture geometry for the 1980 Irpinia earthquake: a working hypothesis

Fault Mechanism of the 40 Seconds Subevent of the 1980 Irpinia (Southern Italy) Earthquake from Levelling Data

A model for the 40 s subevent of the 1980 Irpinia earthquake is obtained from levelling data. An inverse method has been used to compute fault parameters, based on the computation of the probability density on strike and dip angles and midpoint fault location. The highest probability model consists of a fault almost parallel to the one associated to the first subevent. This fault dips about 80°SW, forming a small graben like structure with the main fault. Fault strike and dip parameters appear well constrained by levelling data, to within 10° (at 95% confidence level). It is noteworthy that seismological and accelerometric data were not able to well constrain the fracture mechanism of this subevent, which was the most controversial one in literature (Crosson et al., 1986; Westaway, 1987; Crosson et al., 1987; Westaway and Jackson, 1987; Bernard and Zollo, 1989).

Paleoseismology along the 1980 surface rupture of the Irpinia Fault: Implications for earthquake recurrence in the southern Apennines, Italy

The Irpinia fault was the source of the Ms 6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. Trenching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre‐1980 paleoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average 61 cm. The net vertical displacement of 2.12–2.36 m since 8600 cal year B.P. observed in the trenches gives a vertical slip rate of 0.25–0.35 mm/yr, a dip slip rate of 0.29–0.40 mm/yr, and an extension rate of 0.14–0.20 mm/yr. Although fault behavior data are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude (M7) earthquakes and (2) the Holocene rate of extension across the Apennines is ≤1 mm/yr. The 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation.

The rupture process of the 1980 Irpinia, Italy, earthquake from the inversion of strong motion waveforms

The distribution of slip and the behaviour of rupture propagation during the 23rd November, 1980, Irpinia earthquake ( M S = 6.9) was investigated by analysing strong motion accelerograms. The rupture was assumed to develop on a two-dimensional, purely normal fault whose geometry was constrained by geologic and geodetic data and by the aftershock space distribution. Theoretical seismograms have been computed using the isochron approach with a spatially variable slip intensity. The asymptotic ray theory is used to evaluate the far-field Green functions. The rupture model was updated following a trial-and-error approach. The fit of observed waveforms with synthetic seismograms allowed several peculiarities in the rupture history to be pointed out. The source model resulting from the solution of the forward waveform modelling was then used as starting model for the inversion of ground velocities. Therefore, we performed a linearized inversion for slip velocity and rupture times on the fault by iteratively perturbing the assumed starting model. The inverse problem is solved by means of a tomographic back-projection technique. The inferred source model shows a bilateral rupture which does not propagate uniformly away from the hypocentre. The rupture velocity and the slip distribution on the fault plane are rather variable. The solution confirms the shallow dislocation gap in the central part of the fault suggested by the behaviour of the surface rupture. The results indicate that the rupture propagated northwestward along a further fault fragment for which no surface evidence is observed. The aftershocks occurred mostly inside of or near to the edges of the source areas of maximum co-seismic slip.

An algorithm for the parametrization of real macroseismic fields

A modified Blake's law is proposed. It accounts for some of the anisotropies present in experimental macroseismic fields and yields isoseismal lines similar to the experimentally observed ones. An algorithm which inverts the modified Blake's law and fits it to experimental data also is described. The algorithm is applied to the macroseismic field of the Irpinia earthquake of 23 November 1980. The isoseismal lines obtained by inserting the best-fit values of the parameters into the modified Blake's law accommodate about 98% of the experimental data. The good quality of the results makes promising the application of our algorithm to the parametrization of real macroseismic fields.

Three-dimensional P-velocity structure in the region of the MS = 6.9 Irpinia, Italy, normal faulting earthquake

We have calculated the three-dimensional velocity structure in the epicentral region of the 1980 Irpinia (southern Italy) earthquake using Thurber's inversion technique for hypocenter and velocity parameters. Particular care was given to the evaluation of the resolution of the models, because of the poor reading accuracy of the analog stations operating at that time. We analyzed the averaging vector for each node of the model, in order to estimate the degree of smearing within the final tomographic image. This approach is suitable for determining the best parametrization to use (i.e. the minimum grid spacing) and for evaluating the region where the computed velocities are meaningful. The results show that the main rupture occurred along a sharp NW-SE trending velocity contrast, which probably reflects a crustal discontinuity. The velocity variations along the fault, probably due to lithological heterogeneities, account for the complex character of this earthquake. Based on the velocity distribution at depth, the secondary rupture that started about 40 s after the mainshock is interpreted to be the result of reactivation of an old thrust as a normal fault.

Local lithospheric stress distribution deduced by means of the total inversion algorithm and an objective classification method

Classification of populations of focal mechanisms are commonly used as a step in retrieving states of stress in the lithosphere. Although such classifications are usually made according to subjective criteria, an objective one based on a classification algorithm is presented. The procedure is based on two steps. First, each mechanism is characterized by the components of its principal tensional and compressional axes and then objectively classified accordingly. Second, each mechanism is characterized by its geographic coordinates (latitude and longitude) after which each group obtained in the first step is divided into new groups according to geographic coordinates. The classification algorithm, together with a standard inversion algorithm, is applied to data from aftershocks belonging to the Irpinia (southern Italy) Earthquake of November 23, 1980. A few incompatible groups of focal mechanisms obtained by the classification algorithm show a spatial overlap. Nevertheless, the new methodology allows us to present a more accurate description of a local state of stress for an aftershock area than the stress pattern obtained from subjective classifications. The new stress pattern is compared with principal stress axes deduced from the main event and two local states of stress deduced previously from the aftershock mechanisms.