EC project "Review of Historical Seismicity
in Europe" (RHISE) 1989-1993
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Foreword
Though the scope of the RHISE project was not
to review parametric catalogues, these recommendations partly follow from
the results of the project. They apply to earthquake data derived from written
accounts (historical records), regardless if produced 10 or 1000 years ago.
However, as 20th century macroseismic and instrumental data are to be dealt
with together, earthquakes before 1900 are here the main concern.
Parametric catalogues and historical
records
Users assume parametric catalogues as primary
data. It is useful to recall that, in their turn, parametric catalogues are
the results of elaboration performed on other data. This elaboration, well
known in the instrumental case where waveforms are the primary data, is similar
in the macroseismic case, where primary data are historical records or
macroseismic questionnaires. Their processing can be divided into four steps
(Fig. 1):
a
historical sources are investigated: earthquake records (historical observations)
are found and located in time and space;
b
the investigator decides which records belong to the same event and "builds
up" an earthquake: To is assessed;
c
earthquake records are interpreted in terms of macroseismic intensity, producing
intensity data points (macroseismic observations);
d
focal parameters
(lo,
fo, ho, Io, Mo, etc.) are evaluated,
according to some rules, and catalogue records are compiled.
For macroseismic questionnaires, in principle
step a does not exist and step b does not introduce many
errors.
The real difference between macroseismic and
instrumental data is that waveforms (step a) are more easily available
and that procedures by which instrumental data are processed, from step
b to d, are known and standard, at least to a certain extent:
instrumental catalogue records are therefore comparable and reliability can
be assessed. On the contrary, in the case of macroseismic data these steps
are not performed according to standard procedures: moreover, procedures
are often just unknown. Under such circumstances, catalogue records corresponding
to different datasets and procedures cannot be easily compared.
Current parametric catalogues
A comprehensive analysis of parametric catalogues
in Europe is out of the scope of this paper: moreover, it appears difficult,
as many of them are not published and come only as computer printout, without
explanations.
It is worth remembering that parametric catalogues
were developed in the current format of files of records around the end of
the 60's. This format was shaped to instrumental data, and was designed to
be a suitable input for computer routines. At that time, computer facilities
did not allow handling of many data, and synthesis were necessary: catalogue
records represent therefore synthesis of earthquake data, referred to the
"centre of mass".
The genesis of current historical catalogues
is clear. When parametric catalogues developed, catalogue compilers started
massive, sometimes careless, parameterisations of written seismological
compilations, directly in terms of focal parameters; most of these
parameterisations were performed according to personal, unreported procedures.
The weak points and the negative consequences
of these operations have been analysed by many authors; however, it is a
matter of fact that a great part of current historical catalogues still follow
from those operations.
A further problem is represented by 20th century
earthquakes, for which both instrumental and macroseismic data are available;
for them, compilers usually provide only one catalogue record, either
instrumental and macroseismic or, commonly, a mix of both.
In recent times, "reviewing" parametric catalogues
became a general trend. "Reviewing" means partially revising the content,
updating some records with reference to recent studies, re-evaluating some
parameters according to new procedures and scales. This trend follows from
the need for updating the dataset with the limited resources usually available.
Nevertheless, partial revisions are likely to enhance the inhomogeneity of
a catalogue, mostly when they are not performed by the same "hand". For instance,
a very detailed investigation of a single earthquake may improve the knowledge
on that earthquake, but also lower the global homogeneity of the
catalogue.
Current parametric catalogues are today the
output of several rounds of remake, the base of which is often unknown: in
many cases, duplications or other mistakes (very frequent, for instance,
in the case of magnitude evaluation from macroseismic data, or vice versa)
have been expanded through the various versions of the "same" catalogue.
The European catalogue by Van Gils and Leydecker
The only parametric catalogue covering a large
part of Europe and a large time-window is the one by Van Gils and Leydecker
(1991), now in expansion towards the East. It is indeed a useful tool, also
because it allows the understanding of many problems, still to be solved.
The introduction to the first version (Van Gils, 1988) carried clear views
about problems connected with the compilation of a European catalogue, with
respect to historical data:
II. 3 - The national
catalogues.
The source for the data base in most cases
are the national catalogues. Aside their state of more or less completeness,
they are different in outlook from one country to another and don't contain
the same seismic parameters. Some catalogues express the time of occurrence
in local time, others do it in Universal time. In other catalogues two events
are reported where only one should be listed, their dates making exactly
the difference between the Julian and the Gregorian calendar.
When epicentres are located in the immediate
vicinity of the borderline separating two countries, some of these epicentres
may reveal in a "double nationality" and therefore can be taken up in two
catalogues. Such failures should be rid of on a bilateral basis in order
to agree on the proper location of the epicentre.
Another default consists in the fact that
the applied intensity-scale is not the same everywhere; there are still countries
that make reference to the Modified Mercalli scale while others use the MSK-64
scale.
[ ... ]
Another thing to stress on is the disparity
of the data base as it is distributed over several countries, the consequence
being its inhomogeneity.
Aside, written documentation may appear
sufficient but not necessary reliable. Several catalogues of historical
earthquakes have been compiled but all of them are different for what concerns
their contents.
II.5 - Improvements to be considered for
the data base.
In order to improve the actually available
data base, the following points should be considered and realized when
possible.
5.1. First of all, macroseismic data should
be collected systematically using the same terms of reference; i.e. unique
"intensity-scale" and an identical "questionnaire" in conformity with the
adopted intensity scale.
5.2. All data relevant to historical events
should be gathered and then confronted among each other in order to eliminate
misinterpretations, wrong evaluations, duplicates, a.s.o. ...When the event
struck more than one country this should be done on a multi-lateral base
...
These recommendations are completely acceptable:
they wait only to be adopted. The only, minor disagreement concerns the
statement
"the compilation of a comprehensive catalogue,
i.e. one of the objectives of the present publication, may serve for improving
the data base."
It seems more opportune to change this statement
as follows:
"the compilation of a database is a condition
for the compilation of a comprehensive catalogue".
It is not clear to what extent the recommendations
were followed during the compilation of the catalogue. Apparently very little,
as the catalogue looks like a simple merge of national catalogues, with little
multi-lateral based effort; most of the problems above mentioned have not
been solved.
To merge or to re-compile
?
Each parametric catalogue derives from a dataset,
although it may be sometimes unknown. As recalled, national catalogues are
compiled according to varied criteria with reference to the main steps described
before: type of sources, intensity assessment, earthquake parameters assessment.
They also vary in the time-window, with respect
both to the source potential, and to the historical investigation. Furthermore,
some catalogues do not consider earthquakes with epicentres outside national
boundaries, some others do: in these cases, it may occur that such earthquakes
are treated in different ways by the two sides.
The most common problem is duplications
which, by the way, are widely found also inside national catalogues.
Many examples are available: duplications derive mostly from careless
interpretation of historical sources, and many of them escape any search
by means of automatic 'windows'.
Therefore, though the easiest way to compile
a European catalogue, starting from national ones, would be to merge them
carefully (Fig. 2, A), such attempt would produce unsatisfactory results.
The only way to sort out the problems is to go back to sources and historical
observations, reviewing carefully how earthquake parameters were assessed.
For that purpose it is recommended that all sets of primary information,
including those concerning the same earthquakes, are carefully merged into
a single dataset, from which a more reliable parametric catalogue can be
derived (Fig. 2, B).
To compile an historical, parametric catalogue,
some general standards are to be established. A good starting point is to
agree that an earthquake can be included in a parametric catalogue when
some observations are available: it can be requested that they accomplish
some standard level.
In general, the main requirement for compiling
a parametric historical catalogue is that observations are to be available
for each earthquake, no matter whether only a few, or even just one,
as it happens in many cases.
The recommendations which follow are organised
according to steps a, b, c and d described above.
Historical records: investigation and
processing
As discussed in the previous paragraph, observations
are represented in our case by historical records or questionnaires. The
problem of how historical records are retrieved, interpreted, assembled -
and how their reliability is evaluated - is of primary importance for the
quality and homogeneity of the catalogue. It is well known, for instance,
that presumed seismic gaps can be explained by historical source gaps and
that poor information can be simply due to non-exhaustive investigation.
It has already been recalled that most historical
parametric catalogues currently in use still follow from massive parameterisation
of seismological compilations which are, in their turn, the result of historical
investigations performed according to very different criteria: their reliability
is also very varied. Many papers pointed out, in recent times, wrong
interpretations or fake quakes supplied by the seismological compilations.
In general it can be agreed that, as a general trend, their reliability decreases
with time, reaching the bottom level in the period between the 20's and the
40's of this century.
Today, historical seismology makes use of the
historical method, adopted from nearly 15 years on. There exist some
recommendations on this subject and, mostly, a well established set of
case-histories, including the output of the RHISE project; therefore, this
topic will not be dealt with here in detail.
As the historical method requires the use of
"primary" historical sources, to compile a new catalogue requires in principle
starting a new, global round of historical investigation; the goal being
to have, for each earthquake, detailed records coming from
reliable sources, interpreted by professional historians,
aware of seismological problems.
Nevertheless, to start such a project would
require a large amount of time, funds and expertise, which is unrealistic
in many cases: lower standards might then be accepted, considering that the
homogeneity of a catalogue requires uniform level of investigation for all
entries.
It is recommendable that any re-interpretation
pointing out mistakes made by previous compilers is performed in such a way
to provide users with strong evidence about disappearing or heavy entries
modification in the existing parametric catalogues.
The database of macroseismic observations
To be suitable for seismological elaborations,
historical records need to be located in space and time and interpreted in
terms of macroseismic intensity; this operation produces the so-called intensity
data points, which can be assumed as macroseismic observations. To be included
in a database, intensity data points need to be homogeneous with respect
to:
- timing criteria, locality denominations and coordinates;
- intensity scale and intensity assessment
procedure.
Timing.
Timings carried by historical records are often expressed according to varied
calendars and time-systems: conversion to a uniform time-system is useful
if mastered with care. Timings of records related to the same earthquake
can be scattered along a large time-span. Assigning them the same To
is a decision of the investigator: therefore, it is recommendable that intensity
data points carry both the original and the given time.
Locality
directory. Same problems hold in
principle for locality denominations: the problem can be very complicated
by the changing of names and coordinates in time and by inexplicit
interpretations. Therefore, it is suggested to express locality names and
coordinates according to ad-hoc directories; it is also suggested to keep
trace of the original input.
Intensity
assessment. As for the second point,
it is to be considered that intensity assessment is still an important reason
for inhomogeneity. This topic is well known and will not be discussed in
detail here: however, the publication of the new EMS-92 intensity scale
(Grünthal, 1993) seems to be a good opportunity for reconsidering this
problem with care.
Existing
data. As a first step, it is
recommendable to inventory, analyse and, if necessary, re-compile according
to the same standard all the existing compilations which provide intensity
data points. Isoseismal maps without data points are not recommendable for
this purpose, as they are elaborations, not observations. When data related
to the same earthquake, but coming from different compilations (for instance,
partial, national investigations of transfrontier earthquakes), are available,
these data are to be carefully considered and merged only if they are
homogeneous.
Some criteria are also needed in order to evaluate
the available information (for instance: type of sources, number and distribution
of observations, reliability of intensity assessment, and so on). Such evaluation
will point out whether the data accomplish the required standard or, on the
contrary, further investigation is needed.
The database of primary observations will
enable:
- to re-assess earthquake parameters (To,
lo,
fo, Io etc.) according to homogeneous
criteria;
- to draw isoseismal maps (if needed) according
to homogeneous criteria;
- to evaluate, or to calibrate, seismic hazard
estimates at the sites.
Assessing earthquake parameters
Current catalogues are very inhomogeneous with
respect to how earthquake parameters have been assessed from macroseismic
data. This holds from one catalogue to another and, in some cases, even inside
the same catalogue. The only way to improve the situation is to re-assess
earthquake parameters from primary observations according to standard procedures.
Even in case that no historical investigation
can be performed, the quality of a catalogue can improve by assessing earthquake
parameters according to standard procedures from a dataset of macroseismic
observations.
Origin
time. To is the leading parameter
which characterises the earthquake: it is assigned by the historical
investigation. In principle, To proposed by the database does not
need further elaboration. It can be observed that precision of seconds is
of little sense for macroseismic data, even in the 20th century.
Epicentral
parameters.
lo,
fo and Io are to be evaluated following rigorous
procedures. No inner closed isoseismal is available in many cases, such as
offshore earthquakes or events with a poor number of observations.
lo,
fo and Io can be evaluated according to different
procedures, depending whether they are intended to be used for seismotectonic
or hazard assessment: it is important that the same procedure is adopted
for all earthquakes or, at least, that different procedures are evidenced.
It is worth recalling that instrumentally derived parameters usually come
from routines which allow to drop data when they do not fit in the required
standard. Furthermore, earthquakes which do not accomplish the adopted standard
can be sometimes simply discarded; on the contrary, applying similar criteria
to macroseismic data would require to discard most of the historical
content.
Depth.
Individual depth determinations are very unstable and show acceptable reliability
only in a statistical way.
Magnitude.
Regressions from Io have been often discussed in the literature: they are
strongly influenced by Io instability and other factors. On the other hand,
only a few regressions from isoseismals or intensity data points are available,
but they are all determined from limited samples and very unstable. Investigators
willing to test regressions on a regional basis are recommended to use
calibration magnitudes together with their uncertainties. Users must be ready
to accept large uncertainties associated to pre-instrumental magnitudes.
Aftershocks.
It is to be considered that, in many cases of aftershock sequences, macroseismic
observations may cumulate effects which are by no means separable. Aftershock
parameters other than To can be assessable using far-field observations in
a few cases only: on the contrary, epicentral intensities of 8 MSK for
aftershocks following a few hours, or even a few days, a main shock of, say,
Io = 10 MSK, are of little reliability, if assessed on the basis of damage
reports coming from the epicentral area only.
Attenuation.
It is strongly recommended that parameters of intensity attenuation relations
are determined by the same set of data used to determine earthquake
parameters.
Other parameters.
As epicentral intensity may differ
from maximum observed intensity, both parameters can be useful. A catalogue
record should also indicate which set of intensity data and which
parameterisation procedures were used. Quality factors are useful, provided
that they are coherent with the nature of the data: for instance, the number
of available observations (intensity data points) is a good, self-explaining
indicator.
For 20th century, catalogues usually provide
only one epicentral location associated with both macroseismic (Io)
and instrumental sizes (Ml, Mb, Ms, and so on). It is
a common feeling that instrumental and macroseismic data should not be mixed;
it seems more suitable to provide both sets of parameters, macroseismic and
instrumental, leaving the choice to users.
Catalogue completeness
The completeness of a historical catalogue is
mainly determined by:
- historical factors, influencing existence
and distribution of 'recorders', scattering and preservation of the
records;
- investigation factors, such as the strategy
and the skill of the investigator.
Therefore, current methods for assessing
completeness looking inside a catalogue should be replaced, for historical
catalogues, by historiographic analysis of source potential and by the evaluation
of the sources used for the catalogue compilation. It should also be considered
that the completeness of a catalogue for a given threshold is influenced
by the assessment of earthquake parameters, the uncertainty of which can
be very high in many cases.
It can be recommended to plot historical catalogues
in different time-windows and to compare these plots with the most recent
instrumental data; the problem whether time-space variations of seismicity
are real or should be ascribed to historical source gaps must be tackled
with care.
Conclusions
The knowledge of long-term seismicity of Europe
is still rather uneven, because it has not been investigated in a systematic
way at a European level.
The compilation of a European catalogue is a
major need for assessing seismicity and seismic hazard, but it will really
help if the following points are taken into account:
- national catalogues are compiled according
to different criteria with respect to supporting datasets and procedures,
with special reference to: historical investigation (types of sources,
time-windows, range of investigation), intensity assessment, earthquake
parameters assessment. With reference to boundary problems it may occur that
transfrontier earthquakes are treated in very different ways by the two sides.
Therefore, to merge national catalogues will not provide a reliable tool;
- though a systematic historical investigation is a primary need, a good
improvement with respect to the present situation is possible; actually,
many data are available but they are scattered in many places and compiled
in many ways. The only requirement is to re-compile and review these data
according to uniform, rigorous procedures;
- the preparation of a database of macroseismic
observations is today a reasonable goal. In order to prepare the database
it is desirable to inventory, homogenize and evaluate the existing intensity
data which are today a good number and of good quality;
- the compilation of a European parametric
catalogue, requires the assessment of earthquake parameters according to
standard procedures. It should follow the preparation of the dataset or,
at least, go along with it: existing parametric catalogues, including the
one by Van Gils and Leydecker (1991), can serve as a side tool;
- the synthesis represented by catalogues has
revealed unsatisfactory; many users prefer to make their own way from primary
data. Moreover, computer facilities today allow handling large amounts of
data without problems. Therefore, the current trend is to build up primary
data banks, from which users may extract data and synthesis - such as parametric
catalogues - according to their choice.
As a final point, it is recommended to keep
in mind the qualitative nature of historical records, which can be spoiled
by forced parameterisation. In many cases historical records are hardly
interpretable in terms of intensity: therefore, such information may not
fit in a catalogue or an intensity database. However, this is not a good
reason to ignore it: alternative elaborations can be investigated.
Milano, May 1993
References
Grünhtal, G., (Editor), 1993. European
Macroseismic Scale 1992 (updated MSK scale). Cahiers du Centre Européen
de Géodynamique et de Séismologie, 7, Luxembourg.
Stucchi, M. and Albini, P., 1991. New
developments in macroseismic investigation. Proc. Mexico-EC Workshop "Seismology
and Earthquake Engineering", Mexico City, 22-26 April 1991, pp.
47-70.
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.
Van Gils, J.M., 1988. Catalogue of European
earthquakes and an atlas of European seismic maps. CEC, Report EUR 11344
EN.
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