Note on the seismicity of northeastern Italy (Friuli area)

The kinematics and mechanics of the seismicity of the Friuli area in northeastern Italy are discussed, especially the focal mechanism of the catastrophic earthquake of May 6, 1976. Alternative solutions are analyzed. A N-S section, down to 60 km below sea-level, across the Alps from the Bavarian foreland to the Adriatic Sea illustrates these possibilities.     

An attempt to reconstruct some features of the May 6, 1976, Friuli earthquake by means of aftershocks

The goal of the present work is to reconstruct some features of the May 6, 1976, Friuli earthquake in different sites of the area involved. The starting data for the elaboration are: seismograms of aftershocks at various sites; very detailed geological knowledge of the sites; accelerographs of the main shocks at Tolmezzo. Only aftershocks having magnitudes 2.5–4 and epicentres located within the most damaged area were considered. Fourier and response spectra for the events considered at the sites were computed. The problem is to establish the differences between, on the one hand, the Fourier and response spectra of the main shock, and on the other hand the aftershocks. Since one of these sites (Peonis) has about the same conditions (topography, geology, epicentral distance for the May 6 shock) as Tolmezzo, similarity in shaking and the shape of the spectra were assumed at the two sites for the main shock. On the basis of the known mean spectral ratios (computed on the aftershocks) between Peonis and the other sites, and the same ratio between the main shock and the aftershocks at Peonis, the spectral features of the May 6 shock at all the other sites were computed.     

Some medium properties at friuli (Italy) from amplitude spectrum analysis: A possible change in time or in space

We have studied the relation log M₀+ versus M_L in Friuli (Italy) by considering the amplitude spectra of local earthquakes. We show a possible time variation of the apparent average stress in the focal region between the two periods of May and November, 1976. Furthermore, the quality factor Q_p obtained from the same spectra indicates a spatial variation which could be correlated to the difference in the seismicity level.     

Statistical relations between source parameters for low and intermediate magnitudes (Friuli, 1976)

A data set of three-component short-period digital seismograms recorded in Friuli after the strong earthquake of 6th May 1976, allowed the local magnitude M_L and the seismic moment M₀ to be estimated in the range 0 < M_L < 2. The data set including the same parameters for the higher-magnitude Friuli events (M_L 5) shows two different slopes for the relation Log M₀ = CM_L + d for the two different ranges of M_L. One finds C ~ 1.0 (for 0 < M_L < 2) and C ~1.5 (for5 M_L 6.2), respectively.This implies that apparent stress release increases at low magnitudes, while it appears to be comparatively independent of the magnitude and to have an average value of about 100 bar for higher-magnitude earthquakes. Conversely, the fault dimensions do not appear to be magnitude-dependent for M_L < 2; for higher-magnitude events the linear fault dimensions range from about 1 km at M_L ~ 5 to about 12 km for the strong earthquake of 6th May 1976 (M_L = 6.2).     

Tilt-strain measurements in the NE Italy seismic area: Precursor analysis and atmospheric noise effects

Tilt-strain measurements have been collected since 1977 by several underground stations in the Friuli seismic area (NE Italy) with the aim of detecting seismotectonic deformations. In the work, meteorological effects are initially considered as being able to obscure possible tectonic signals. Two middle-term anomalous signals (tilt and areal strain), not correlated with atmospheric changes, are successively selected for the analysis. They are the strongest and more evident signals recorded in the area after the 1976 destructive seismic events and also are the only two cases of epicentres very near to the recording stations (M = 4.1 and M = 3.9 at 1.8 and 2.9 km, respectively). Evaluations on the basis of micro-cracking and fault creep are carried out. Estimates based on trivial rheological models furnishes crustal viscosity values in agreement with those obtained in the analysis of the silent earthquakes recorded in the same area before the 1976–1977 seismic events. Our data support the hypothesis that prominent precursory signals can be detected only within a distance few times the dimensions of the source area.     

The Idrija earthquake of March 26, 1511 —a reconstruction of some seismological parameters

Two earthquakes of March 26, 1511 with different positions of foci represent some of the strongest seismic phenomena which occurred in the past in the central part of Europe. In particular, both events were the beginning of an increasing seismic activity in this part of Europe, which was some centuries later again evidenced in a series of strong earthquakes in Friuli.Some characteristics of these shocks and the following aftershocks series show a pattern which is very similar to that of the Friuli earthquakes of 1976. However, the extent of the damage caused by the earthquakes in 1511 and the dimensions of the pleistoseistic region itself provide a basis for determination of some basic and only qualitative data about these shocks.     

Sources of Mw 5+ earthquakes in northeastern Italy and western Slovenia: An updated view based on geological and seismological evidence

We present an overview of the seismogenic sources of northeastern Italy and western Slovenia, included in the last version of the Database of Individual Seismogenic Sources (DISS 3.0.2) and a new definition of the geometry of the Montello Source that will be included in the next release of the database. The seismogenic sources included in DISS are active faults capable of generating M_w > 5.5 earthquakes. We describe the method and the data used for their identification and characterization, discuss some implications for the seismic hazard and underline controversial points and open issues.In the Veneto–Friuli area (NE Italy), destructive earthquakes up to M_w 6.6 are generated by thrust faulting along N-dipping structures of the Eastern Southalpine Chain. Thrusting along the mountain front responds to about 2 mm/a of regional convergence, and it is associated with growing anticlines, tilted and uplifted Quaternary palaeolandsurfaces and forced drainage anomalies. In western Slovenia, dextral strike–slip faulting along the NW–SE trending structures of the Idrija Fault System dominates the seismic release. Activity and style of faulting are defined by recent earthquakes (e.g. the M_s 5.7, 1998 Bovec–Krn Mt. and the M_w 5.2, 2004 Kobarid earthquakes), while the related recent morphotectonic imprint is still a debated matter.We reinterpreted a large set of tectonic data and developed a segmentation model for the outermost Eastern Southalpine Chain thrust front. We also proposed the association of the four major shocks of the 1976 Friuli earthquake sequence with individual segments of three major thrust fronts. Although several sub-parallel active strike–slip strands exist in western Slovenia, we were able to positively identify only two segments of the Idrija Fault System. A comparison of the regional GPS velocity with long-term geological slip-rates of the seismogenic sources included in DISS shows that from a quarter to half of the deformation is absorbed along the external alignment of thrust faults in Veneto and western Friuli. The partitioning of the deformation in western Slovenia among the different strike–slip strands could not be quantified.     

Local source tomography: applications to Italian areas

We describe three study cases in which we used local earthquake and shot travel-time residuals to investigate the upper crustal structure of three regions in Italy. We inverted for velocity and hypocentral parameters using a damped least-squares technique making use of parameter (velocity and hypocentre) separation. The three studied regions are in Italy, namely (a) the Vulsinian Volcanic Complex (Latium), where there is an active geothermal field; (b) the Irpinia (Campania–Lucania) region, in the Southern Appennines, site of the strongest earthquake in Italy for at least 65 years (November 1980, M_s= 6.9); (c) the Friuli region, in Northeastern Italy, where another strong earthquake (M_s= 6.5) occurred in 1976. The computed shallow velocity models generally correspond with surface geological structures. For the three studied areas, the main results are, respectively: (a) A low-velocity anomaly detected in the centre of the Vulsinian Volcanic Complex at a depth of 5–8 km, probably due to anomalous heat flow caused by a partially molten or cooling intrusive body; (b) the identification of a deep (10 km) discontinuity in the crust beneath the Irpinia fault zone, approximately corresponding with the fault extension at depth; (c) the detection of a wedge of high-velocity, high density material at seismogenic depth (5–10 km) beneath the Friuli region, interpreted as a buried thrust of the metamorphic basement.     

Chaetognaths in lower Narragansett Bay

A field study of the chaetognaths of lower Narragansett Bay was conducted from March 1975 through May 1976.Sagitta elegans Verrill 1873 was the predominant species, present in all months except August.S. elegans was most abundant in April and May in both 1975 and 1976. Maximum densities exceeded 100 m^?3 in May 1976. It appeared thatS. elegans was replenished in Narragansett Bay by influxes of chaetognaths from Rhode Island Sound. This species was restricted to the sound when bay temperatures exceeded 15 °C. In the bay, maturing chaetognaths were found deeper in the water column than juveniles. This phenomenon may have limited the frequency of older animals in bay plankton samples by concentrating them extremely close to the bottom, out of the reach of conventional plankton nets. The presence of chaetognath eggs in samples taken in Rhode Island Sound, and their absence from bay samples, indicated that breeding took place primarily offshore. Size-frequency analysis showed that peak breeding activity occurred in January, April, and July.     

Constraints on the location and mechanism of the 1511 Western-Slovenia earthquake from active tectonics and modeling of macroseismic data

The 1511 Western Slovenia earthquake (M = 6.9) is the largest event occurred so far in the region of the Alps–Dinarides junction. Though it strongly influences the regional seismic hazard assessment, the epicenter and mechanism are still under debate. The complexity of the active tectonics of the Alps–Dinarides junction is reflected by the presence of both compressional and transpressional deformations. This complexity is witnessed by the recent occurrence of three main earthquake sequences, the 1976 Friuli thrust faulting events, the 1998 Bovec–Krn Mountain and the 2004 Kobarid strike-slip events. The epicenters of the 1998 and 2004 strike-slip earthquakes (M_s = 5.7 and M_s = 4.9, respectively) lie only 50 km far from the 1976 thrust earthquake (M_s = 6.5).We use the available macroseismic data and recent active tectonics studies, to assess a possible epicenter and mechanism for the 1511 earthquake and causative fault. According with previous works reported in the literature, we analyze both a two-and a single-event case, defining several input fault models. We compute synthetic seismograms up to 1 Hz in an extended-source approximation, testing different rupture propagations and applying a uniform seismic moment distribution on the fault segments. We extract the maximum horizontal velocities from the synthetics and we convert them into intensities by means of an empirical relation. A rounded-to-integer misfit between observed and computed intensities is performed, considering both a minimized and a maximized databases, built to avoid the use of half-degree macroseismic intensity data points. Our results are consistent with a 6.9 magnitude single event rupturing 50 km of the Idrija right-lateral strike-slip fault with bilateral rupture propagation.     

Macroseismic fields of the main Friuli shocks of 1976

Isosieismal maps of the main Friuli shock and of three large aftershocks were compiled on the scale 1 : 1 million as a joint effort of seismologists from countries shaken by the earthquakes. All macroseismic fields are irregular and display different damping in different directions. Depths of foci and attenuation coefficients are calculated.     

The Town Effect: Dynamic Interaction between a Group of Structures and Waves in the Ground

In a conventional approach, the mechanical behaviour of a structure subjected to seismic or blast waves is treated separately from its surroundings, and in many cases, the dynamic coupling effect between multiple structures and the waves propagating in the ground is disregarded. However, if many structures are built densely in a developed urban area, this dynamic interaction may not become negligible. The first purpose of this contribution is to briefly show the effect of multiple interactions between waves and surface buildings in a town. The analysis is based on a recently developed, fully coupled, rigorous mathematical study, and for simplicity, each building in the town is represented by a rigid foundation, a mass at the top and an elastic spring that connects the foundation and mass. The buildings stand at regular spatial intervals on a linear elastic half-space and are subjected to two-dimensional anti-plane vibrations. It is found that the buildings in this model significantly interact with each other through the elastic ground, and the resonant (eigen) frequencies of the collective system (buildings or town) become lower than that of a single building with the same rigid foundation. This phenomenon may be called the “town effect” or “city effect.” Then, second, it is shown that the actual, unique structural damage pattern caused by the 1976 Friuli, Italy, earthquake may better be explained by this “town effect,” rather than by investigating the seismic performance of each damaged building individually. The results suggest that it may also be possible to evaluate the physical characteristics of incident seismic/blast waves “inversely” from the damage patterns induced to structures by the waves.     

On the possibilities of premonitory swarms for three sequences of earthquakes of the Burma—Szechwan region

Short term spatial and temporal variations in seismicity prior to the three sequences of earthquakes of m_b 5.8 of the Burma—Szechwan region are studied. Six years (1971–1976) of ISC seismicity data, as reported in the Regional Catalogue of Earthquakes, are considered. During the period, six earthquakes of body wave magnitude m_b 5.8 occurred in four sequences. Of these, three sequences are preceded by swarm activity in the epicentral regions. Evison (1977b) suggested that the swarm before the sequences of large shocks is a possible long-term precursor. He derived the conclusion by analyzing earthquakes in New Zealand and California. The analysis of the seismicity data for the region under investigation supports Evison's view and suggests that a relation between swarms and sequences of large events exists. The precursory time period (i.e. the time from beginning of the swarm to the main shock) for the Szechwan earthquakes of m_b = 5.9 (Feb. 6, 1973) and m_b = 5.8 (May 10, 1974) and the Burma earthquake of m_b = 6.2 (Aug. 12, 1976) are 305, 317 and 440 days, respectively.     

The Belchatow, Poland, earthquakes of 1979 and 1980 induced by surface mining

Five seismic events occurred between August 1979 and May 1980 in the Belchatow trench area in central Poland, where large brown-coal deposits have been surface-mined since 1976. The three largest shocks had a local magnitudeM_L = 3.5 , seismic momentM_o = 1.5 · 10¹⁴ N-m and source radiusr = 350m , approximately. The tremors had maximum intensity between 5 and 6, and from the magnitude—intensity—depth relation, the focal depth was estimated to be between 1 and 5 km.

The Belchatow tremors are the manifestation of an unusual type of induced seismicity, i.e., seismicity connected with surface mining. The removal of overburden and extensive ground-water withdrawal by the mining operations seem to be direct factors responsible for stress concentration, although preexisting tectonic stress might be the most important indirect factor responsible for the origin of these events.     

Reassessing intensity of some Friuli earthquakes at the turn of the eighteenth century

During 1770–1820 Northeastern Italy was hit by a series of high intensity earthquakes affecting the Piedmont area of Friuli from Maniago to Tolmezzo. Greater knowledge of these events, which seem to be extremely circumscribed and to have damaged only a small number of localities (1776, 1789, 1794: Tramonti; 1788, 1790: Tolmezzo; 1812: Cavasso), could make a significant contribution towards defining better the potential seismic hazard in Northern Friuli. A review of these shocks has been undertaken within the framework of activities organized by the macroseismic working group of the National Group for Protection against Earthquakes (GNDT). The critical revision of the information gathered by the programme ‘analysis through the compilations’, has stressed the need for a new interpretative method and for great caution to be exercised when assessing reliable intensity degrees, to avoid possible inconsistencies in their values. This preliminary investigation allowed us to identify both doubtful and some misestimated shocks. A rough macroseismic intensity distribution pattern of each event, showing that the earthquakes were felt over an area (including Italian, Slovene and Austrian territories) coherent with the epicentre intensity, has been also delineated.     

Age and growth,movements and distribution of the cownose ray,Rhinoptera bonasus,in Chesapeake Bay

Ages were estimated for 115 of 899 cownose rays,Rhinoptera bonasus, collected primarily from commercial fishing gear, in lower Chesapeake Bay and vicinity from May through October, 1976–78. Age determinations were made using sectioned vertebral centra and estimates of von Bertalanffy parameters were for males DW_∞=119.2, K=0.126, and t₀=?3.699, and for females DW_∞=125.0, K=0.119, and t₀=?3.764. Females attained a larger adult size and the oldest specimen aged was a female 13 years old and 107 cm disc width. Both sexes mature after reaching about 70% of their maximum size and ages at maturity were estimated at 5 to 6 years for males and 7 to 8 years for females. In spring migrating rays schooled by size; they arrived along the North Carolina coast by April and entered Chesapeake Bay by early May. Rays were abundant in the major Virginia tributaries of Chesapeake Bay throughout summer and occurred in salinities as low as 8‰ and at water temperatures between 15–29 °C. Size segregation continued during summer and adults schooled by sex. Most rays left Chesapeake Bay by early October.     

Seismic Hazard Mapping for Administrative Purposes

Local soil conditions, roughlysummarised by considering a reference soil for eachmunicipality of the Friuli – Venezia Giulia region inNE Italy, are introduced into probabilistic seismichazard estimates: the subsequent improvement ischecked by comparing these new results and the maximumobserved intensities in each municipality toinvestigate if the major differences betweenprobabilistic estimates and actually observed data canbe explained by local site effects and/or by thegeometry of the seismogenic zones used in thecomputation. In addition, a comparison between the newprobabilistic hazard results, and the standard onesreferred to rock is made for the present and theproposed Italian seismic zonation. The resultsunderline the influence of the seismogenic model used,but are not determinant on the role of site effects.     

Hydrogeochemical investigations and groundwater provinces of the Friuli Venezia Giulia Plain aquifers,northeastern Italy

Water resources are a key factor, particularly for the planning of the sustainable regional development of agriculture, as well as for socio-economic development in general. A hydrochemical investigation was conducted in the Friuli Venezia Giulia aquifer systems to identify groundwater evolution, recharge and extent of pollution. Temperature, pH, electric conductivity, total dissolved solids, alkalinity, total hardness, SAR, Ca²⁺, Na⁺, K⁺, Mg²⁺, Cl⁻, SO₄ ²⁻, NO₃ ⁻, HCO₃ ⁻ _, water quality and type, saturation indexes and the environmental stable isotope δ¹⁸O were determined in 149 sampling stations. The pattern of geochemical and oxygen stable isotope variations suggests that the sub-surface groundwater (from phreatic and shallow confined aquifers) is being recharged by modern precipitations and local river infiltrations. Four hydrogeological provinces have been recognised and mapped in the Friuli Venezia Giulia Plain having similar geochemical signatures. These provinces have different degrees of vulnerability to contamination. The deep confined groundwater samples are significantly less impacted by surface activities; and it appears that these important water resources have very low recharge rates and would, therefore, be severely impacted by overabstraction.     

TheUrquiola oil spill,La Coruña,Spain: Impact and reaction on beaches and rocky coasts

The supertankerUrquiola grounded, exploded, and burned at the entrance to La Coruña harbor (Spain) on May 12, 1976. A total of 100,000 tons of Persian Gulf crude oil was lost, of which about 30,000 tons washed onto shoreline environments. From May 17 to June 10, 1976, the impact and interaction of oil on fine-sand, coarse-sand, and gravel beaches and on sheltered and exposed rocky coasts was monitored in detail. At 32 localities, the beach was profiled, trenched, extensively sampled, and photographed. Another 67 stations were examined for surficial oil coverage and distribution. The surficial distribution of oil on the beaches was influenced primarily by wave activity, tidal stage, and oil quantity. Heaviest accumulations formed along the high-tide swash line. Within beach sediments, oil was present at distinct oiled sediment layers, which were often deeply buried. The depth of burial was related to wave energy and sediment type. Deepest burial (1 m) was on a high-energy, coarse-sand beach (M_z=0.82φ). Burial on fine-sand beaches was less than 30 cm. The thickness of oiled sediment depended on sedimentary characteristics, the quantity of oil present, wave action, and capillary forces. Oil-soaked sediment, as much as 65-cm thick, occurred on coarse-grained beaches. On fine-sand beaches, oiled sediment was limited to thicknesses of 10 cm or less. On rocky shores, oil distribution was determined primarily by wave energy. Along high-energy, cliffed, or steeply dipping rocky areas, wave reflection kept the oil approximately 5 m offshore and contamination was minimal. In low-energy, sheltered areas, oil readily accumulated, causing apparent environmental damage.     

Evidence for variations of mechanical properties in the Friuli seismic area

Tidal strains of the earth were extracted from three horizontal strainmeter records (1979–1986) in the Friuli seismic area by the “filter method”. The areal strain factor, ignoring the tidal loading effects, shows that its time variation is significant: their amplitudes in 1986 are about 50% of those in 1979.

The modifications of mechanical properties, estimated in terms of the local shear and bulk modulus variations, were obtained considering the envelope of the tidal strain signals. The seismic wave velocity determined by the simultaneous inversion of the arrival time data of the local seismometric network displayed comparable changes.

A major change both in the seismic velocities and the elastic parameters started in March 1982, which was about 11 months before an earthquake of magnitude 4.1, the largest event from 1979 to 1986, which occurred within the seismic network on Feb. 10, 1983.