seismic history and consistent seismicity: evidence from southern italy

11
NaturalHazards 14:11-21, 1996. 11 © 1996 KluwerAeademie Publishers. Printed in the Netherlands. Seismic History and Consistent Seismicity: Evidence from Southern Italy A. MARTURANO and V. RINALDIS Osservatorio Vesuviano- 249, Via ManzonL Naples, Italy (Received: 23 August 1994; in final form: 20 June 1996) Abstract. In this paper a method is proposed to evaluate the seismicity level of an area in a given historical period, based on records of seismic events, source characteristics and intensity attenuation with distance. Also considered is the seismic activity recorded in southern Italy during the 10th and l lth centuries, 'seismic records' being obtained from all available sources. To determine the level of seismicity, a key role is played by source characteristics, i.e. recording modalities and activity periods of recording centers. In addition, models of intensity attenuation with distance allow the assessment of the size of the area under investigation. This paper identifies the areas, in the 10th and 1lth centuries, where major earthquakes (M > 6.5) did not occur during periods of silence of sources, as well as those where such events cannot be excluded. For each area, different levels of probability were determined by applying the Cox linear logistic model to historical seismic data. The completeness analysis, in terms of area and time-span coverage, is a valuable tool to assess seismicity in seismogenetic areas. The reproducibility of the model for lower magnitude earthquakes (M < 6.5) is reliable. Key words: historical seismicity, linear logistic model, catalog completeness, southern Italy. 1. Introduction To determine the seismicity of an area, past seismic activity data are crucial. Gener- ally, historical data are summarized and arranged in catalogues. The homogeneity, accuracy and exhaustiveness of catalogues have a major bearing on the analysis of the seismic hazard (Mulargia and Tinti, 1985; Tinti and Mulargia, 1985; Fahmi et al., 1989; Kiykoo and Sellevoll, 1992; and for general references: McGuire, 1993; Giardini and Basham, 1993). Multidisciplinary investigations carried out over the past few years with a view to improving the historical record of earthquakes, produced interesting findings with some unexpected breakthroughs. Investigations on historical seismicity are based on data which, in terms of their characteristics, differ from those usually employed in geophysical trials, and hardly meet the requirements of conciseness set by those investigators who use the end-products of historical researches to make estimates of seismic hazard. This ended up by adversely affecting the robustness of findings, making their further use less reliable. The attempt at extracting weighted and independent information from historical seismicity data is, indeed, challenging. The user could, thus, rely on a familiar

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NaturalHazards 14:11-21, 1996. 11 © 1996 KluwerAeademie Publishers. Printed in the Netherlands.

Seismic History and Consistent Seismicity: Evidence from Southern Italy

A. MARTURANO and V. RINALDIS Osservatorio Vesuviano- 249, Via ManzonL Naples, Italy

(Received: 23 August 1994; in final form: 20 June 1996)

Abstract. In this paper a method is proposed to evaluate the seismicity level of an area in a given historical period, based on records of seismic events, source characteristics and intensity attenuation with distance. Also considered is the seismic activity recorded in southern Italy during the 10th and l lth centuries, 'seismic records' being obtained from all available sources. To determine the level of seismicity, a key role is played by source characteristics, i.e. recording modalities and activity periods of recording centers. In addition, models of intensity attenuation with distance allow the assessment of the size of the area under investigation. This paper identifies the areas, in the 10th and 1 lth centuries, where major earthquakes (M > 6.5) did not occur during periods of silence of sources, as well as those where such events cannot be excluded. For each area, different levels of probability were determined by applying the Cox linear logistic model to historical seismic data. The completeness analysis, in terms of area and time-span coverage, is a valuable tool to assess seismicity in seismogenetic areas. The reproducibility of the model for lower magnitude earthquakes (M < 6.5) is reliable.

Key words: historical seismicity, linear logistic model, catalog completeness, southern Italy.

1. Introduction

To determine the seismicity o f an area, past seismic activity data are crucial. Gener- ally, historical data are summarized and arranged in catalogues. The homogeneity, accuracy and exhaustiveness o f catalogues have a major bearing on the analysis o f the seismic hazard (Mulargia and Tinti, 1985; Tinti and Mulargia, 1985; Fahmi et

al., 1989; Kiykoo and Sellevoll, 1992; and for general references: McGuire, 1993; Giardini and Basham, 1993).

Multidisciplinary investigations carried out over the past few years with a view to improving the historical record of earthquakes, produced interesting findings with some unexpected breakthroughs. Investigations on historical seismicity are based on data which, in terms of their characteristics, differ from those usually employed in geophysical trials, and hardly meet the requirements o f conciseness set by those investigators who use the end-products o f historical researches to make estimates o f seismic hazard. This ended up by adversely affecting the robustness o f findings, making their further use less reliable.

The attempt at extracting weighted and independent information from historical seismicity data is, indeed, challenging. The user could, thus, rely on a familiar

12 A. MARTURANO AND V. RINALDIS

Abbey of Montecassino Abbey of S. Vincenzo al Volturno

Abbey of S. Sofia di Benevento Abbey of S. Clemente a Casauria

Cathedral of Bari

VIII

I I I J j "1 I I ~

"!"~:2"?"?~?~::?'::?"/'?i"'.':7

XII century

Figure 1. Recording centres and their time-span coverage. The dotted area shows the contin- uous activity of centres during the 10th and 1 lth centuries.

tool and a better use would be made of the syntheses obtained. The purpose of this paper is to provide a guideline to the use of historical seismic data, valuing their inherent characteristics. However, the use and interpretation of such sources cannot disregard their specific characteristics, in that they appear to be always very keen in recording even the smallest events occurring in the region where they are centered, while paying little attention to events occurring at a greater distance. The sources themselves suggest, that they should be considered and analyzed as local centers of continuously operating documentation. To achieve this end, the choice was made to tackle the problem radically, considering that the past seismic activity of any area is crucial to evaluate the seismic characteristics of the area itself. In order to more accurately outline the seismic behaviour of an area over a given time span, the ways in which seismicity was recorded and information preserved and transmitted over time must be known. If that is known, or can be estimated, the catalogue of recorded events can be supplemented probably by missing seismicity records. Therefore, for each area and historical period under review, information must be available on:

• the centers recording seismic events; • the characteristics of records; • the preservation and transmission of the records taken by centers.

Using these basic data in a model of intensity attenuation with distance, it is possible to estimate:

• the area monitored by each recording center; • the seismicity level which is consistent with the recording center.

2. The Area and the Tested Period

Figliuolo and Marturano (1994) analyzed the seismic activity of southern Italy during the 10th and 1 lth centuries. Their analysis suits the purpose of this paper very well, in that information is now available for those periods and meets the requirements set by our test, namely:

SEISMIC HISTORY AND CONSISTENT SEISMICITY: EVIDENCE FROM SOUTHERN ITALY 13

• the centers recording the seismic events are known and can be accurately sited. They are the Abbeys of Montecassino, orS. Vincenzo al Volturno, orS. Sofia di Benevento, orS. Clemente a Casauria and the Cathedral of the city of Bari (see Figures 5 and 6).

• the recording modalities and the activity periods of such centers have been clearly identified - in practice they have had ongoing activity throughout the 10th and 1 lth centuries (Figure 1 shows the centers' time-span coverage).

• the possibility of information gaps has also been considered. However, it appeared to be virtually null since for our analysis, reliance was made on original records (Cassino) or copies of the original texts which were who- ley tracked and reconstructed. In fact, the sources considered are in writing beginning from the second half of the 1 lth century; they rely, however, on earlier memories and documents which date back to the very origins of those centers. In the case of Montecassino, in addition to the collection of chroni- cles started by Leone Ostiense from the second half of the 1 lth century and continued by Pietro Diacono till the second decade of the 12th century, also the earlier narrative tradition on which it relies has survived to date. Further- more, it must be pointed out that if, for the towns of Benevento and Bari, the local narrative traditions were assembled and transcribed, the chronicle-like feature of the accounts for the Abbeys of S. Vincenzo and S. Clemente remain as background information and give way to a mere collection of public and private documents and deeds drawn up by notaries, concerning property rights and abbatial prerogatives.

Therefore, a reasonable assumption is that, for the period under investigation, only information provided by such sources could be actually recorded. This pro- vides the opportunity to estimate the seismicity level in the neighbourhood of the recording station throughout its activity period, once a minimum reporting threshold has been determined for one type of source.

In order to make such estimates, it is, first and foremost, necessary to know how the perception of seismic activity changes as a function of distance. Also important are, of course, the evaluation of a site's relative response as a function of the source characteristics and of the direction wherefrom the disturbance originated. Here, however, emphasis is laid on the fact that for given areas, over a given span of time, it is possible to determine the actual seismicity level without using predetermined models. In order to do this, the choice was made to estimate the extent to which the expected value at the location site is likely to be related to the occurrence of a major earthquake, at a given distance.

3. Attenuation of Intensity with Distance

The function of attenuation of intensity with distance can be represented in a probabilistic form by linear logistic models (Mucciarelli et al., 1990). These models (Cox, 1970) are generally utilized in the analysis of binary data, i.e. data in which

14 A. MARTURANO AND V. RINALDIS

Table I. Major earthquakes which occurred in southern Italy over the last four centuries and which are energetically comparable with the earthquake of 23 November 1980. Io is the epicentral intensity.

Earthq. of Lat. Long. Io Reference

08.09.1694 40.52 15.36 X Serva, 1985 14.03.1702 41.07 14.57 X Molin and Serva, 1985 29.11.1732 41.05 15.07 X Spadea et al., 1985 26.07.1805 41.32 14.31 X Esposito et al., 1987 23.07.1930 41.04 15.20 X Spadeaetal., 1985 23.11.1980 40.48 15.22 X Postpischl et al., 1985

an observation, represented for example by a random variable Y/, takes the form of either:

or

success:

failure:

E(Y/) = prob (Y/= 1) = Oi

E(I~) = prob (I,~ = 0) = 1 - qi.

The focus of this paper has been that of studying the stimulus/binary-response relation of these models, i.e. the relation that gives the binary response probability (success or failure) as a function of a continuous variable (stimulus).

It is, thus, possible to assess the probability (0i) that the intensity is less than the threshold value set (success) as a function of the epicentral distance xi (continuous variable):

Oi = exp(a + b. xi) (1 + exp(a+ b . x i ) ) '

where 'a ' and 'b' are the unknowns. Table I shows the major earthquakes, chosen for the test, which occurred in

southern Italy over the last four centuries and which are energetically comparable with the earthquake of 23 November 1980. The sample consists of 1748 MCS intensity values; Figure 2 shows the frequency distribution versus the natural log of the epicentral distance.

The regression technique generally utilized in the logistic models is based on the study of the maximum log likelihood function that, in simplified form (as modified by MucciareUi et al., 1990) reads:

n

L(a, b) - (a . Ta) + (b. Tb) -- E log(1 + exp(a + b. xi)), i=l

Ta = N ' a ,

SEISMIC HISTORY AND CONSISTENT SEISMICITY: EVIDENCE FROM SOUTHERN ITALY 15

[] 80

"~ Dn [] 70. m

50- "~ oOo

40. = Z

30. an OE]

20.

10. i [ ~ %

0 ' ' ~ I . . . . ! . . . . ! . . . . ! . . . . ! . . . . I . . . . !

0 1 2 3 4 5 6 7 Ln epicentral distance (Kin)

Figure 2. Frequency distribution of intensity observations versus the natural log (In) of epicentral distance. 1748 MCS intensity values (from I = X to I = III) were utilized.

n

Tb = Y ~ b . Y~, i=l

where N is the total number of observations at the distance xi from the epicentre; Y/ is the number of observations at the distance xi from the epicentre whose intensity value is less than X, IX, VIII, VII, VI and V; xi is the epicentral distance.

The parameters that give the best fit of experimental data are obtained by maximizing the likelihood function. Table II shows the evaluated parameters while, in brackets, confidence intervals obtained by the 95% t test are reported.

Figure 3 shows the fit of the experimental data for intensity values less than X, XI, VIII, VII, VI and V versus the natural log of epicentral distance.

4 . E a r t h q u a k e s i n S o u t h e r n I t a l y

All in all, for the 10th and 1 lth centuries, five earthquakes were recorded in the centers considered (Figliuolo and Marturano, 1994):

16 A. MARTURANO AND V. RINALDIS

Table II. Parameters "a" and "b" obtained by maximizing the likelihood function. In brackets confidence intervals obtained by 95% t test.

Intensity a b

I < X -4.76(-4-0.79) 2.00(-4-0.32) I < IX -8.16(4-0.93) 2.65(4-0.30) I < VIII - 11.64(4-1.45) 3.09(-4-0.38) I < VII -17.23(4-2,05) 4.01(4-0.47) I < VI -29,54(4-2.21) 6.27(4-0.51) I < V -29.84(4-2,45) 5.81(4-0.48)

4~

.Q 0 i._ r~

d

0 1 2 3 4 5 6 7

Ln d is tance (Km)

Figure 3. Probability that the intensity value of major earthquakes in southern Italy is respec- tively less than X, IX, VIII, VII, VI, V versus epicentral distance (natural logarithm). Symbols show experimental observations; lines show the theoretical trends.

SEISMIC HISTORY AND CONSISTENT SEISMICITY: EVIDENCE FROM SOUTHERN ITALY 17

oo Ooo Oo 3 o o ~* ~ o o o , ~ ,* . / : A t r _ . . g

* * ~ , "{ -',qr / i .%4-

" " " J ' - ' X o 9 . o o ~ 0 0 0 • •

, \ O o . >,~ )o ,,,o . * I

o Ooo.- .

I * \ °°'° * , f m *~ g o"_ /!0,,,>~, I

S 0 5 0<..M<6 5 0 40 80 Km ~ , s / ' 1 ~ , ~ • I f t ( , - , . . . . . ~ n ~ I * M~s.0 I

Figure 4. Seismicity (Ml > 4.0) in southern Italy since 1900. Two major earthquakes in the Irpinia area occurred in this period: on July 23, 1930 and November 23, 1980.

- 25 October 989; - 25 December 1004--31 August 1005; - 1 April 1019; - 19 April 1044; - 19 September 1087

With the exception of the epicenter of the 1087 earthquake which was estimated to be more eastwards, probably in the Adriatic Sea, the epicentral areas of such earthquakes are situated within the Appennines seismogenetic belt. Of these, only the 989 earthquake exceeded the magnitude threshold considered here (Macroseis- mic Magnitude: M > 6.5). Although it would be beyond the scope of this paper to try and compare the different ways of determining magnitude with the macroseis- mic intensities, it must be stressed that the category which is generically denoted by M > 6.5 conventionally includes those earthquakes which are energetically comparable to the earthquake that occurred in the Irpinia region on 23 November 1980 (Ms = 6.9; M1 = 6.5) based on the characteristics of the macroseismic field and, more specifically, on the size of the areas affected by a given intensity (Branno et al., 1986).

Figure 4 shows the activity recorded in southern Italy from 1900 to 1994 for earthquakes with a magnitude o f M l > 4. Based on the Karnik (1969) relation used in the Italian Catalogue (Postpischl, 1985), M1 -- 4 is matched by an epicentral intensity o f I = VI - an estimate basically confirmed by Tinti et al. (1986).

18 A, MARTURANO AND V. RINALDIS

In addition, it is pointed out that, during the three-hundred years which preceded the Irpinian earthquake of 23 November 1980, in the area under examination the major earthquakes succeeded at a rate of 2.6/Century (Marturano et al. 1988).

This paper attempts to answer such questions as: 'since in the period under examination, the seismicity rate of major earthquakes is less than was expected, can we assume that some earthquakes were missed by recording centers and, if so, what were the epicentraI areas involved and their associated probability? As a first approximation, each site is said to have recorded the event if this was perceived at an MCS intensity of at least VI. This is, partly, a prudential estimate, since major earthquakes are accounted for in chronicles regardless of the fact that they were directly perceived at an intensity of less than VI; but, it is also a standard estimate since the reasons for their being recorded differed from one location to another. Hence, although it should be better articulated, it fits the purpose of this study. However, it must be stressed that major earthquakes are often accounted for in the chronicles regardless of the fact that they were directly experienced by the writer; in such cases, only an accurate critical analysis can avoid glaring mistakes. With reference to the time span under examination, Figliuolo and Marturano (1994) have totally overcome interpretation problems.

Once the perception threshold is set, the previously raised question can be answered by estimating an area where, based on the computed probability, no major earthquake event was recorded throughout periods of silence of sources. In Figure 5, the isoprobabilities are shown. They are calculated for the recording center of Beneven to - i.e. the probability that if a major quake occurs in the neighborhood of the station, this event can be perceived with an intensity of less than VI and, therefore, go unreported. The area under examination is shown in grey in Figure 5 - 12 ° to 18 ° of North Longitude and 40 ° to 43 ° East Latitude - and is subdivided into grids of 10 km wide. As is shown in Figure 5, in the area inside the first curve, the probability that an earthquake may have occurred and remained unrecorded is negligible. In Figure 6, the probabilities are assessed for all the recording centers which were active in southern Italy during the 10th and 1 lth centuries. For each cell, the highest calculated value of probability was chosen. As can be seen in Figure 6, for a large portion of the tested area, the probability that a major event may have occurred is negligible.

Based on this methodology, the probability of occurrence of an earthquake of a given energy can be assessed:

• for a given area; • for a period where the characteristics of sources can be evaluated as shown

above; • for periods where no earthquakes were recorded.

SEISMIC HISTORY AND CONSISTENT SEISMICITY: EVIDENCE FROM SOUTHERN ITALY 19

Figure 5. Recording centre of Benevento. Isolines give the probability that, if a major event occurred in the shaded area around the center, it can be felt with I<VI degree and, therefore, remain unreported, as assumed.

Figure 6. Active recording centres of southern Italy in the 10th and l lth centuries. Isolines give the probability that, i fa major event occurred in the shaded area around the centers, it can be felt with I<VI degree and, therefore, remain unreported as assumed.

20 A. MARTURANO AND V. RINALDIS

5. Conclusions

Information drawn from the recorded seismic history of an area can provide a much better contribution than data from catalogues. In fact, historical data p e r se

are arranged differently in time and space; in catalogues, therefore, they hardly lend themselves to be synthesized in parametric form, nor can they provide critical insights.

With a view to taking better advantage of the characteristics of historical sources, a probability method is proposed here which allows the assessment of the space- time exhaustiveness o f existing seismic information. A test was made for southern Italy using information from the 10th and 1 l th centuries. Data were also provided on the consistent seismicity of the relevant areas, based on probability calculations.

Estimating the probability of the occurrence of lower magnitude earthquakes, as well as restricting the analysis to smaller areas, are the benefits of this method which, thus, proves to be very flexible and encourages better target-setting and use o f basic research.

Acknowledgements

The authors wish to express their gratitude to Ms Silvana Siciliano for the support she provided in translating the paper, and to the referees for their valuable sug- gestions. Special thanks are due to Prof. B. Figliuolo, Humanities Faculty, Udine University, Italy - for fruitful discussions.

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