EC project "Review of Historical Seismicity in Europe" (RHISE) 1989-1993

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• Foreword
• Introduction
• Earthquake duplications
• Inhomogeneity and forced parameterisation
• Conclusions
• Acknowledgements
• References

Introduction
The compilation of parametric earthquake catalogues covering areas, and/or time-windows, broader than those covered by two, or more, existing catalogues, is a problem well known to many investigators; some aspects are discussed for instance by Van Gils (1988), Stucchi (1994) and others.
The compilation is generally performed converting the input catalogues into a comprehensive format, merging them into a single file and then sorting out possible duplications: this operation is usually accomplished with the help of almost automatic tools. To follow this procedure implies the following, hidden assumptions:
- parameters of input catalogues are homogeneous; that is, each parameter called in the same way represent the same quantity and is expressed in the same unit;
- parameters are derived from primary data with homogeneous procedures;
- compilers are confident that their authomatic routines are so highly performing to be able to master possible duplication or similar problems.
Unfortunately, these assumptions seldom meet the reality. It is well known to most compilers, and it is discussed also in this volume, that parametric catalogues are far from being homogeneous and transparent. Therefore, the compilation of comprehensive, parametric catalogues often results in propagation and amplification of errors and in increasing internal inhomogeneity of the final product. The most frequent problems are:

- earthquake duplications (or losses)
- inhomogeneity and forced parameterisations.

It is to be stressed that most of these problems are also met inside national catalogues, as they are often compiled following the same procedures.

Earthquake duplications
The most common problem is duplication, that is, the presence of double or multiple entries representing the same event, carrying different set of parameters. The sources of duplication cover a wide range: they go from simple mistakes in the compilation of some parameters to different compilation procedures. But the most frequent source is the use of different sets of primary data and/or errors in historical interpretation.
Duplication problems derived from the latest source are well known as they are discussed for instance by Alexandre (1991), Bellettati et al. (1993) and many others. The examples which follow deal with cases covering also the instrumental period and even the most recent one.

The "Italian" catalogues
The Postpischl (1985) catalogue put together many entries of varied origin. The main contributing sets are parametric catalogues, such as: Enel file (1978; about 20600 entries), Carrozzo et al. (1973; about 435), Dell'Olio and Molin (1980; about 5500), Bernardis et al. (1977; about 1800), Carrozzo et al. (1975; about 7600). While the first two sets cover the whole territory, the other sets cover limited areas (Central Italy, Eastern Alps, Calabria and Sicily).
After merging the input catalogues, Postpischl sorted out many problems authomatically; but the huge task and the limited time did not allow avoiding some pitfalls, to be mastered with careful analysis only. Therefore, a limited, but significant number of duplications survived; an example follows.

1939.02.11. The two following entries are compiled on the basis of Bernardis et al. (1977) and Enel (1978) respectively (Fig. 1a):

Date	Time	Lat	Lon	M	Io	Ep.Area
1939 02 11	11 16	46 06	13 06		65	FAGAGNA	(B)
1939 02 11	11 16 54	44 04	11 39	52	70	MARRADI	(E)

Raimondi initially reported two entries; in their final table, however, only the second one, the origin of which is unknown, survived. Finally, Bernardis et al. (1977), dealing only with the Eastern Alps area, preserved the second entry which, later on, was assumed also by Postpischl (1985).

Neighbouring countries catalogues: Italy and Slovenia

1889.09.02. The following entries are taken from Ribaric (1982) and Postpischl (1985) respectively (Fig. 1):

Date	Time	Lat	Lon	M	Io	Ep. Area
1899 09 02		45 87	13 98		30	BUDANJE	(*)
1899 09 02	10 10	45 37	11 15		50	VALDAGNO	(o)

The search for the roots of the second record leads to Cancani (1900). The root of the Ribaric (1982) record are Mojsisovics (1900) and Bernardis et al. (1977). Strangely enough, Bernardis et al. (1977) quote an unpublished Slovenian catalogue by Ribaric as a source! It must be said that cross- references between these two catalogues are not an unfrequent case.

The Van Gils and Leydecker (VGL) catalogue
As already remembered, the compilation of this catalogue did not follow the recommendations which are found in the introduction to the catalogue itself (Van Gils, 1988). With respect to duplications, the compilers had the unpleasant task to merge national catalogues without having access to the supporting datasets: therefore, some duplications escaped their control.

The 1295, Central Alps earthquake. This earthquake, still a key one for the assessment of seismicity and seismic hazard in Central Alps, is represented in the VGL catalogue by two entries (Fig. 2), which come from the Swiss and the Postpischl catalogues respectively.

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
CH	1295 09 04		46.83	09.53		8.0	REICHENAU	(*)
ITA	1295 09 17		45.75	09.50		7.0	CAPRINO B.	(+)

Æt between the two events is 14 days, while the distance between the two epicentres is about 150 km: such a distance usually escape authomatic investigation. The background of the duplication is found in historical interpretation mistakes: Fig. 3 shows the roots of the two entries.
Actually, coeval sources mentioning Monza (Obituary, XIII-XIV) and Chur (Chronicon Osterhoviense, XIII-XIV) agree that an earthquake was felt "die sabbati tertio die mensis septembris" ("on Saturday, the third of September"), or "in octava Sancti Augustini", which means "the eighth day after the feast of St. Augustin" (which falls on August 28), that is September 4 (Albini et al., 1994).
On the contrary, a later Italian author, Corio (1503), made a mistake: he probably interpreted "Saturday, the third of September" as "the third Saturday of September", that is September 17.
Bonito (1691), using sources from the two "trees", list two earthquakes. Baratta (1901) is aware that the two dates correspond to one earthquake only, but he relies mostly on Corio (1503).

For these reasons the Italian catalogue compiled a second record; this was also favoured by the fact that the Italian sources mention Italian localities, while German sources mention localities North of the Alps only. The two records then survived the "duplication inspections" performed by VGL.

The duplication of the first entry with the third one was analysed by Albini et al. (1988). The most striking fact is not that both entries refer to the same event, but that the event itself is a fake quake; actually, what happened in the location proposed by the Swiss catalogue was a landslide. The situation is complicated by the second entry, the reliability of which is questionable.

1542

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
CH	1542 11 08		47.82	10.05		6.0	WIGGENSBACH
D	1542 11 08		47.83	10.00		6.0	BAYERN

Very probable duplication.

1616 and 1623

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
CH	1616 02 19	04 30 00	46.47	08.52		5.0	VAL BEDRETTO
CH	1616 02 29		46.83	08.40		8.0	ENGELBERG
CH	1623 02 20		46.30	09.78		6.0	CHIESA
ITA	1623 03 01	01 00 00	46.22	10.15		5.0	BIANZONE

The first of these possible duplications originated inside the Swiss catalogue, the other one originated when merging the Swiss and the Italian catalogues. They are probably due to different calendars (Julian and Gregorian) used by the sources.

1644

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
ITA	1644 02 15		44.00	06.75	5.7	9.0	VAL VESUBIE
FRA	1644 02 15	04 30 00	43.88	07.30		8.5	ALPES-MARIT.

Duplication originated from different epicentral locations, performed from the same dataset, but following different parameterisation procedures.

1670

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
ITA	1670 07 16	15	45.43	10.97	4.4	6.5	VERONA
A	1670 07 17	02 00 00	47.30	11.50	5.3	8.0	TIROL

Well known example, discussed for instance by Guidoboni and Stucchi (1993). The reason for this duplication is the use of different time-systems by the sources, and uncorrect interpretation by the compilers.

1690

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
D	1690 11 24		48.90	10.50		6.0	BAYERN
A	1690 12 04	15 45 00	46.60	13.80	6.2	9.0	KAERNTEN
ITA	1690 12 04	18	46.60	13.85	5.7	9.0

While the first entry is a possible duplication, still under investigation (Eisinger and Gutdeutsch, 1994), the Austrian and Italian entries are just unsorted duplications.

1743

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
ITA	1743 02 20	22	45.55	11.55	3.9	5.5	VICENZA
GR	1743 02 20		39. 0	20.30	6.9	9.0	CORFU

Classical example of duplication due to the use of a limited set of sources (a local compiler, Piovene (1888), from Vicenza, Italy), without data cross-checking from surrounding areas.
The earthquake was actually felt in Vicenza: the distance between the two epicentres is more than 500 km.

1778

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
A	1778 01 28		47.20	09.60	2.5	6.0	VORARLBERG
D	1778 01 28	02 30	47.90	07.60		6.5	BADEN-WUERT.
CH	1778 01 29	00 30 00	47.20	09.82		5.0	GR WALSERT.

Possible duplication.

Lost earthquakes

Exactly in the symmetrical way as for duplications, earthquakes may get lost from parametric catalogues. This can happen when a catalogue covering a large area is partly used because, for many reasons, another catalogue, covering a smaller area, inside the previous one, is preferred for the small area itself.

For instance, in the catalogue of the Balkan region (Shebalin et al., 1974) an entry describes, without parameters, an earthquake of 1556 which, according to Hoff (1840) and his sources, affected "Baiern, Österreich, Windischmark, Ungarn, Croatien, Dalmatien und Mähren" (Fig. 4), damaging 26 localities.
This entry today survives in a Croatian earthquake file only: the epicentre is located in the present Austrian territory and Io is 10! Right or not these parameters, it is a matter of fact that the Austrian part of the VGL catalogue ignores it, while no trace of this earthquake is found in the territories of the neighbouring countries, according to their catalogues.

Wrong puzzle solutions
It can also happen that only some "pieces of earthquake" get lost or, simply, that they float around looking for they parent earthquakes and then they stick to the nearest one coming. This might be due to the fact that earthquake parameters are often derived from different sets of data - this is mostly the case with magnitudes - and put together taking origin time as a guide.
But sometimes things go wrong. For instance, a recent, oustanding magnitude re-evaluation of earthquakes of Italy and sorrounding areas (Margottini and Screpanti, 1991), produced a final catalogue attaching the new evaluated magnitudes to the Postpischl (1985) parameters (To, lo, jo, etc). In this way, two large magnitude earthquakes, with very small Io, appeared in nearly aseismic zones (Fig. 5). In this case the VGL catalogue turns out useful, providing the solution:

Cat	Date	Time	Lat	Lon	M	Io	Ep. Area
ITA	1943 05 28	00 25	46 04	9 15		40	MENAGGIO
D	1943 05 28	01 24 08	48 16	8 58		80	BADEN-WUERTTEMB.
ITA	1955 11 23	06 24 40	45 54	11 00		40	ROVERETO
FRA	1955 11 23	06 39 12	47 24	5 58		60	HAUTE-SAONE

The two magnitudes are to be attached to two well assessed earthquakes in Germany and France respectively, of which the Italian entries are just duplications. Unfortunately, neither Postpischl, nor VGL, nor even Margottini and Screpanti sorted these duplications out.

Inhomogeneity and forced parameterisation
If the assumptions formulated in the introduction are not met by data, the comprehensive catalogue may result formally homogeneous, but its content may still be deeply inhomogeneous.
For instance, the VGL catalogue amalgamated national catalogue which might be internally homogeneous, but which are surely not homogeneous one to the other with special reference to: time-window, historical sources, intensity scales, parameterisation procedures, magnitudes, completeness.

More in detail, the VGL catalogue amalgamated epicentral intensities expressed in different macroseismic scales and assessed by varied "hands", including also figures like 6.3, and so on.
In the same way, magnitudes of different nature, both instrumental and macroseismic, were taken from the input catalogues and put all together in the same column, without qualification or warning for the users. For instance, Fig. 6 clearly indicates the presence of macroseismic magnitudes mixed together with instrumental ones in the Italian data.

On the contrary, Fig. 7 shows that, in the data for Spain, magnitudes cover a limited time-window and they are nearly only instrumental.

One result of this situation is that, when a user, unaware of such problems, plots magnitudes against epicentral intensities, he gets the picture shown in Fig. 8. With the current regression procedures a M = M(Io) correlation can certainly be obtained: but, what value will it have?

The depth column shows even more puzzling problems. Fig. 9 shows that, for the Italian data, many depths are "fixed", according to current procedures. Even more striking is that, for the Greek entries, depth is constant at 70 km for most entries before 1960 (Fig. 10). Asked about the reason, the Greek colleagues said that the material handled to Van Gils contained only the information that "most Greek events are intermediate".

Apparently, this information was turned into "70 km" for all events which did not carry a depth, probably because the information did not fit in the space available in the depth column.
Now the problem is that, if an user wants to investigate earthquake depths in Greece, he might be encouraged to assume the VGL data as an evidence of a very large, active layer, at a depth of about 70 km, covering all Greece and sorrounding areas.

Conclusions
The problems discussed show that handling those useful tools called parametric catalogues with little care, might result in a further step of earthquake data spoiling, after the main step represented by the parameterisation of historical records.
Problems of duplication appear in some way relatively easier to detect, with respect to homogeneity problems. For mastering all these problems properly, it is necessary to retrieve and process the primary data: playing with parameters usually increases the confusion.

Finally, it is to be stressed that a further problem is represented by the use which can be made of catalogues. Though this problem is not compiler's responsibility, however, compilers should be aware of it; it must be admitted that compilers sometimes do not help in this direction.

Acknowledgements
Working hard on this topic, authors are well aware of how time-consuming and unrewarding the catalogue compilation may be; they want to express respect and appreciation for the work of the colleagues.

References
Albini, P., Morelli, G. and Stucchi, M., 1994. Alcuni terremoti importanti per l'Alta Valtellina. In: Studi sismici in Alta Valtellina, ISMES-IRRS (CNR), pp. 24-50.
Albini, P., Bellani, A. and Stucchi, M., 1988. Terremoti e frane nelle Alpi Centrali. Atti del 7° Convegno Annuale del Gruppo Nazionale di Geofisica della Terra Solida, Roma, I, pp. 129-146.
Alexandre, P., 1991. The Seismic Cataclism of 29 March 1000: Genesis of a Mistake. Tectonophysics, 193: 45-52.
Baratta, M., 1901. I Terremoti d'Italia. Saggio di Storia, Geografia e Bibliografia Sismica Italiana con 136 sismocartogrammi. Torino, 950 pp.
Bellettati, D., Camassi, R. and Molin, D., 1993. Fake quakes in Italy through parametric catalogues and seismological compilations: case histories and typologies. Terra Nova, 5: 488-495.
Bernardis, G., Giorgetti, F., Nieto, D. and Slejko, D., 1977. Earthquakes Catalogue for Eastern Alps Region. OGS Trieste [computer printout].
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Malaroda, R. and Raimondi, C., 1957. Linee di dislocazione e sismicità in Italia. Bollettino di Geodesia e Scienze Affini, 16, 3: 273-323.
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Piovene C., 1888. Cronaca dei terremoti a Vicenza. Annali dell'Ufficio Centrale di Meteorologia e Geodinamica, s. II, VIII: 46-57.
Postpischl, D. (Editor), 1985. Catalogo dei terremoti italiani dall'anno 1000 al 1980. Quad. Ric. Scient., 114, 2B, Bologna, 239 pp.
Ribaric, V., 1982. Seismicity of Slovenia. Catalogue of Earthquakes (792 A.D.-1981). Ljubljana, 649 pp.
Shebalin, N.V., Karnik, V. and Hadzievski D. (Editors), 1974. Catalogue of earthquakes. UNDP/UNESCO Survey of the seismicity of the Balkan Region. Skopje, 600 pp.
Stucchi, M., 1994. Recommendations for the compilation of a European parametric earthquake catalogue, with special reference to historical records. This volume.
Van Gils, J.M., 1988. Catalogue of European earthquakes and an atlas of European seismic maps. CEC, Report EUR 11344 EN.
Van Gils, J.M. and Leydecker, G., 1991. Catalogue of European Earthquakes with intensities higher than 4. CEC, Nuclear Science and Technology, Report EUR 13406 EN.

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