48,06,2005misc 923 ANNALS OF GEOPHYSICS, VOL. 48, N. 6, December 2005 Key words historical seismicity – velocity propaga- tion model – Joint Hypocentral Determination 1. Introduction In this paper I will show that when enough seismic recordings exist it is possible to compute an instrumental location of major earthquakes. The computation is carried out on an area of mid- high seismicity (Garfagnana, Northern Tuscany, box with dashes in fig. 3) and could be conduct- ed on any other area of similar characteristics; the results are used for a discussion on the role of the instrumental data of historical earthquakes. The role of instrumental versus macroseismic locations for earthquakes of the last century: a discussion based on the seismicity of the North-Western Apennines (Italy) Stefano Solarino INGV-CNT c/o Dipartimento Studio del Territorio e delle sue Risorse (DipTeRis), Università degli Studi di Genova, Italy Abstract Many seismological observatories began to record and store seismic events in the early years of the twentieth cen- tury, contributing to the compilation of very valued databases of both phase pickings and waveforms. However, de- spite the availability of the instrumental data for some of the events of the last century, an instrumental location for these earthquakes is not always computed; moreover, when available, the macroseismic location is strongly pre- ferred even if the number of points that have been used for it is low or the spatial distribution of the observations is not optimal or homogeneous. In this work I show how I computed an instrumental location for 19 events which oc- curred in the Garfagnana-Lunigiana region (Northern Tuscany, Italy) beginning from 1902. The location routine is based on a Joint Hypocentral Determination in which, starting from a group of master events, the systematic errors that may affect the data are summed up in the corrective factors complementing the velocity propagation model. All non-systematic errors are carefully checked and possibly discarded by going back to the original data, if necessary. The location is then performed using the classic approach of the inverse problem and solved iteratively. The ob- tained locations are then compared to those already available from other macroseismic studies with the aim to check the role to be attributed to the instrumental locations. The study shows that in most cases the locations match, in par- ticular when considering the different significance of the location parameters, especially for the strongest events: the instrumental location provides the point where the rupture begins, while the macroseismic one is an estimate of the area where the earthquake possibly took place. This paper is not meant to discuss the importance and the necessity of macroseismic data; instead, the aim is to show that instrumental data can be used to obtain locations even for old- er seismic events, without any intention to define which location is better or more reliable. Mailing address: Dr. Stefano Solarino, INGV-CNT c/o Dipartimento Studio del Territorio e delle sue Risorse (Dip- TeRis), Università degli Studi di Genova, V.le Benedetto XV 5, 16132 Genova, Italy; e-mail: peter@dipteris.unige.it 924 Stefano Solarino Many efforts have been made in the recent past to recover the historical seismicity of the Italian Peninsula with the aim to distinguish haz- ardous areas through the appropriate algorithm. The goal of designing a map of hazard for the Italy has been an impressive incentive in recent decades and has led to the compilation of sever- al parametric catalogues (Camassi and Stucchi, 1997; Gruppo di Lavoro CPTI, 1999; Boschi et al., 2000; Gruppo di lavoro CPTI, 2004). These catalogues differ for both period of coverage and number of events included, but they are especially distinct in the philosophy that guided their compilation. CFT (Boschi et al., 2000) is based on macroseismic data and locations only, contains 605 seismic events se- lected on the basis of the minimum intensity threshold of VII and contains earthquakes which occurred in the period 461 B.C. to 1997. Conversely, NT4.1.1, (Camassi and Stucchi, 1997) contains all earthquakes exceeding Io = = 5/6 or Ms = 4.0 in the period 1000-1980, and this sums up to 2421 records. The compilation was then enlarged to cover the period 1981- 1992. The catalogue is mainly based on macro- seismic data, a few of which are novel, but it al- so contains several locations whose parameters have been previously published in other cata- logues; the macroseismic data for these events are not always available. In particular, one cata- logue that has been extensively used for merging data in NT4.1.1 is that published by Postpischl (1985), which also contained instrumental loca- tions. NT4.1.1 is probably the most flexible and widely used historical catalogue in Italy and is thus adopted in this work as a reference. In this frame, it is especially important to emphasize that the records extracted from other catalogues and included in NT4.1.1 are indicated as CP: this code actually means that it is no straightforward to determine where the parameters come from (macroseismic or instrumental location) and es- pecially that there is no direct information of the quality of these parameters. In the next para- graphs I will show that this lack biases the com- parison, since more than 50% of the resulting in- strumental locations that do not match with the existing ones are marked CP. Finally, CPTI is the result of a joint project to merge and recompute part of the mismatch- ing locations of the mentioned (CFT and NT) catalogues: it has been recently updated to ver- sion CPTI04 (Gruppo di Lavoro CPTI, 2004). All these inventories cover almost the whole of human history and, being updated to very re- cent years, they overlap the period in which many seismological instruments were already operat- ing. As an example, in fig. 1 I show the number of installed instruments versus years in the period 1900-1950 as derived from the results of the TROMOS (http://storing.ingv.it/tromos) project. The histogram does not show two important fea- tures: stations were almost homogeneously spread over the Italian Peninsula and a few of them never interrupted they recording activity, even if performed by updated and more modern instruments. Figure 3, even if only showing the position of the stations used in this work, gives a frame on the distribution of historical observato- ries. More information can be retrieved from the TROMOS web pages. The existence of several seismic instru- ments provided good waveforms for the main earthquakes of the century; however, despite Fig. 1. Stations operating in Italy in the period 1900- 1950 (source TROMOS). Since the project made an inventory of observatories established before 1940, the histogram is updated to that time. The last bars of the plot takes into account only the stations that still existed in 1950 and does not include those established in the decade 1940-1950. The role of instrumental versus macroseismic locations for the seismicity of the last century in the NW Apennines the richness of the database, an instrumental lo- cation for these earthquakes is not always com- puted and when a macroseismic one is available it is strongly preferred, even when based on a poor and or not optimal distribution of the lo- calities. In theory, it is more correct to input, in whatever computation, data obtained with the same means or obtained applying similar methodology, that is in this case to always use macroseismic data for homogeneity. But cata- logues are already «biased» by a few instru- mental locations as discussed above, so it does not seem so difficult to use alternative locations especially if they prove to be of acceptable quality. The study is not meant to discuss the acknowledged importance and the necessity of macroseismic location; instead, the aim is to show that instrumental data can be used to ob- tain locations even for older seismic events, without any intention to define which location is better or more reliable. In fact, throughout the paper the corresponding macroseismic loca- tions will be used as a reference, and the discus- sion will be based on the differences between epicentral position rather than on quality of da- ta and result. In particular, I introduce the basics of the method, I make a short description of the main problems encountered in assembling the data and finally I discuss and compare the obtained locations to try to find out which is the role of instrumental data for less recent earthquakes. 2. Towards an instrumental location: Joint Hypocentral Determination The methodology applied in this work (So- larino et al., 1996) has been extensively de- scribed in Solarino (2002). It is based on the concept that it is possible to account for all sys- tematic errors that may interfere with the loca- tion of an earthquake by computing corrective factors in conjunction with the model used for locations; all non-systematic errors may instead be discarded by checking the single datum. For example, site effects or non perfect knowledge of the position of the instrument can be includ- ed in the correction factor, which will bias all data with the same importance, while a picking error can be avoided by a careful check of the single datum, especially if original waveforms are still available. This part of the work proves to be the most delicate, due to the difficulties in finding original seismic bulletins, digitized or on paper waveforms, original scripts by seis- mologists running the instruments. In a few cases data that would have represented a poten- tial source of error were finally recovered by going back to the original seismogram. One of the most important aspects in this frame is that of the time synchronization. It must be taken into account that the technology of clocks was very similar for all seismic obser- vatories, although provided by different brands, as happens today. It is then reasonable to be- lieve that the performances (and thus the errors) of these timing device were very similar. Data synchronized with these machineries can then be considered comparable. It is also well known that in the early days of seismology scientists used mechanical clocks, the drift of which was corrected at established intervals using more sophisticated radio timing systems, and ensuring a more accurate absolute time. In case of major earthquakes, the phase picking was corrected to take into account the drift. Sometimes this correction was directly an- notated on the seismogram, sometimes it was annotated apart, but in general the published and shared data was already corrected for the drift. However, in some cases, different phase pick- ings are found on diverse bulletins, perhaps re- porting corrected and non-corrected data respec- tively: in such cases either it is possible to real- ize which is the correct timing or the reading must be discarded. Finally, since many location programs use S-P times, for this kind of data the correctness of absolute time does not really mat- ter provided that no malfunctioning occurs dur- ing the recording of the event. While random errors can be avoided by careful control of the data, systematic errors can be included in the computation by applying a Joint Hypocentral Determination (JHD there- inafter) technique, the aim of which, as stated above, is to sum up all errors in a corrective fac- tor. As known, the JHD uses a master event to check the compatibility of its location with the rest of the data. In this case, the JHD technique 925 926 Stefano Solarino has been based on a group of very recent and well located events that acted as masters in suc- cessive steps. The starting point of the whole routine con- sisted in the computation of a very reliable ve- locity propagation model using a dataset of re- cent events (01/1999-12/2001, fig. 2); the com- putation of this propagation model, later indi- cated as «reference model», has been widely described by Ellsworth (1977) and Kissling (1988). The 1D model results from a combined, simultaneous inversion where the inverse prob- lem is solved by the full inversion of the damped least squares matrix ATA + λ. As the in- verse problem is non-linear, the solution is ob- tained iteratively, where one iteration consists of solving both the complete forward problem and complete inverse problem once. VELEST (Kissling et al., 1995), the code that performs the computation, can also be run in single event mode when only the location problem is carried out. In this case, an additional singular value decomposition of the symmetric matrix ATA is performed to calculate the eigenvalues. As known, this way of treating the inverse problem provides supplementary information on the res- olution: they will not be discussed in detail in this paper, but supplemental information can be found in Solarino (2002). At the end of the computation, different models with about the same residual variance and location precision can usually be obtained from the same data set. The model that coincides best with the surface geology and a priori information on the near- surface structure has to be chosen as the final model, and will be called «minimum» (Kis- Fig. 2. Earthquakes used for the computation of the reference model applied in this work. They cover the pe- riod 1999-2001. The role of instrumental versus macroseismic locations for the seismicity of the last century in the NW Apennines sling, 1988) to indicate that it provides mini- mum average (rms) values for all earthquakes used in the inversion. Additionally, relocation of known sources (shots, blasts) facilitates the choice of the best model and gives hints on its reliability. The 1D model resulting from the computa- tion with VELEST (fig. 3, left panel) is accompa- nied by P and S station corrections that are first calculated only for the stations that are still oper- ating and that contributed to the location of the master events, while those relative to seismic ob- 927 Fig. 3. Velocity propagation model (left panel) and position of stations used for this study. For each station, the correction factor is reported (circle for negative values, triangle for positive; size is proportional to the amount) as after the computation process resumed in table I. The distribution of delays is close to the optimal, being very nicely distinct the positive and negative areas (Kissling et al., 1995). The box with dashes shows the approxi- mate extension of the Lunigiana-Garfagnana area. Table I. Evolution of the computation of delays for the seismic observatories the readings of which are used in this work. At increasing steps the number of known stations corrections (in percentage with respect to the total number) expands, and even observatories that no longer operate can be taken into account. Step 1973 1972 1968 1965 1961 1959 1951 1939 1934 1931 1928 1928 1927 1926 1921 1920 1902 1902 1 72% 75% 50% 40% 50% 50% 50% 40% 42% 62% 28% 20% 20% 40% 25% 20% 40% 40% 2 100% 100% 66% 60% 75% 75% 66% 40% 42% 62% 28% 20% 20% 40% 50% 33% 40% 40% 3 100% 100% 100%100%100% 75% 83% 40% 50% 62% 28% 20% 20% 40% 75% 33% 40% 40% 4 100% 100% 100%100%100%100% 100% 40% 58% 62% 28% 20% 30% 40% 75% 33% 60% 40% 5 100% 100% 100%100%100%100% 100% 100% 58% 62% 58% 20% 40% 60% 75% 44% 60% 40% 6 100% 100% 100%100%100%100% 100% 100%100% 100% 72% 40% 80% 100% 75% 66% 60% 40% 7 100% 100% 100%100%100%100% 100% 100%100% 100%100%100% 90% 100% 75% 77% 80% 60% 928 Stefano Solarino servatories that are no longer operating or have run for a limited amount of time are adjoined and adjusted in the following steps. Basically, each step consists of a selection of older events, of the location with data for which station corrections are known and eventually of letting unknown sta- tion corrections free to float to compute relative delays. In detail, the latter operation consists in keeping the computed propagation model and fix the master locations (whose number is increased at each run with more events) and hence let the program compute, sum up and attribute delays to stations for which readings are present but have not been used for location (Kissling et al., 1995). In practice, station corrections are always com- puted with respect to master events, and the number of known station corrections increases at each step, being computed also for older or no longer operating stations. Table I reports in a very schematic way the evolution of station de- lays the readings of which are included in each event discussed in this paper. At the beginning of the process, almost nothing is known for some events. The successive steps are used to compute the diverse station delays until a correct location can be made. At the end of this routine, the model is com- plete with all station delays (fig. 3) and can be used for the location of the «historical» events of the Garfagnana area. 3. Data and results Apart from the limitation of the availability of instrumental data, in theory no constraints act in selecting the data to be relocated. In practice there are several limitations that must be taken into ac- count. First, the events must be included in some other (macroseismic) catalogue for comparison. This is not going to be necessary in a future appli- cation, but it is of paramount importance to test the results, and cannot be neglected at this stage. Second, events must have at least 4 usable phase pickings that in some cases may mean 6 to 7 read- ings to be conservative enough to consider mis- takes, picking errors and non-matching informa- tion. Third, there must exist more than one source for the data to cross-check the entries, as men- tioned in the last paragraph. By combining all these aspects, it turned out that the best selection should take into account events which occurred later than 1900, that are included in the NT4.1.1 Catalogue (the most complete) and whose data are reported on two or more of the sources quot- ed in Appendix A. It is noteworthy that some of the references reported in Appendix A are compi- lations, and should then be comprehensive of all data available for an event as collected from the various bulletins and observatories. The careful cross checking between these compilations and some other original data showed that in a few cas- es the compilations contain inconsistencies and differences, and this is another important issue to- wards the use of more sources. The initial selection of 56 seismic events (as extracted from NT4.1.1 for the area 43.6-44.6 and 9.1-10.5) was dramatically reduced to 30 when looking for the multiplicity of sources but the number dropped to 19 when considering the minimum number of readings required for loca- tions upon careful selection of the blunder-free data (the compiled phase readings for these da- ta are reported in Appendix B). Of course, this aspect diminishes the potential of the instru- mental locations versus the macroseismic ones, it being possible in this case only for fewer than 40% of the events, and will be discussed in the final paragraph. The characteristics of the se- lected events are reported in table II (second column from left); their location properties are taken from the NT4.1.1 Catalogue. After checking for data completeness on all available sources, events have been located ap- plying VELEST in single event mode; the final locations are reported in table II together with the relevant location errors, while table III shows a comparison of the obtained locations with the original positions. The new locations are compared with the macroseismic ones in fig. 4, where it is possible to see at a glance how far the epicentres have been shifted: the vectors connecting each two points indicate the change in position from macroseismic to instrumental and show very different lengths for the diverse couples. The figure also shows that the original locations were sometimes very clustered (see for example events 08/1902, 1920, 1968, 1961, 1939). The role of instrumental versus macroseismic locations for the seismicity of the last century in the NW Apennines The characteristics of the newly obtained lo- cations are reported in table II: the depth, marked by bold characters, has been computed for more than 50% of the earthquakes. At first glance, the obtained distribution of earthquakes seems com- patible with the knowledge of the recent seismic- ity of the area (Solarino et al., 2002) and with the proposed position for the main seismogenic structures of the area (e.g., the Garfagnana-north fault, Valensise and Pantosti, 2001); moreover, the less clustered aspect of the seismicity seems less «artificial» than the macroseismic one. The errors associated with the new locations (ERX, ERY, ERZ) are on average less than 5 km, and the overall RMS is sometimes less than 1 s. The usage of the standard errors in the hypo for- mat may appear inadequate or incomplete to de- scribe the uncertainties of the final locations and, as shown in Solarino (2002), could be substitut- ed by a less compact form by showing the full resolution matrix or its diagonal elements. However, at this stage the errors are only used to distinguish between locatable and non- locatable events and to give a rough idea about the final quality and reliability of the results and in such an attempt the standard errors are enough. Finally, the gap shows that for some events the station coverage is very good, while in a few cases it exceeds 200°. The analysis of the differences between loca- tions (table III, difference between latitudes and between longitudes in km) shows three different trends. There are seismic events (05/03/1902, 07/09/1920, 1931, 1939, 1951, 1959, 1972) for which the differences between the two location methods do not exceed 16 km for each coordi- 929 Table II. Index of the events relocated in this work. The list is the final result of a selection done in the area 43.6-44.6 and 9.1-10.5 after checking for multiplicity of data sources and minimum number of phase pickings (see text). The locations parameters of the second column are those of the NT4.1.1 (Camassi and Stucchi, 1997) Catalogue. The remaining columns are relative to instrumental locations and report the position, the depth and the errors associated to the locations. NC is used for depths that are not adequately constrained. Date Location as Instrumental Depth rms ERX ERY ERZ GAP in NT4.1.1 Catalogue location 05/03/1902 44.10N, 10.46E 44.16N, 10.31E NC 0.92 1.3 3.5 2.3 281 04/08/1902 44.20N, 10.20E 44.09N, 11.12E NC 1.66 0.9 3.7 2.6 246 27/10/1914 44.05N, 10.45E 44.38N, 10.82E 4.5 0.65 4.4 4.8 3.3 115 07/09/1920 44.20N, 10.20E 44.25N, 10.14E 4.3 1.70 3.6 4.6 7.1 121 29/11/1921 44.50N, 9.80E 44.76N, 10.78E NC 0.01 3.6 3.8 1.6 163 18/11/1926 44.30N, 10.00E 44.80N, 9.82E NC 2.88 3.7 4.3 1.7 113 28/10/1927 44.53N, 9.53E 44.73N, 9.84E 11.7 1.80 3.1 3.1 4.4 81 20/07/1928 44.50N, 9.61E 44.72N, 10.06E 5.9 0.89 3.1 3.2 4.1 99 03/08/1928 44.20N, 10.20E 44.50N, 11.03E 16.6 1.31 3.5 2.9 3.9 132 25/01/1931 44.25N, 10.10E 44.28N, 10.02E 5.5 1.10 3.2 3.2 0.0 161 13/06/1934 44.48N, 9.80E 44.51N, 10.52E NC 1.27 3.3 2.4 4.5 109 15/10/1939 44.16N, 10.23E 44.30N, 10.43E 42.2 1.40 4.2 4.2 3.1 143 12/08/1951 44.06N, 10.48E 44.09N, 10.56E 0.2 0.10 3.9 4.3 1.4 199 26/01/1959 44.50N, 9.50E 44.60N, 9.47E NC 0.56 2.8 3.9 3.0 118 03/08/1961 44.20N, 10.20E 43.98N, 10.25E 37.6 1.28 3.5 4.1 3.6 157 09/11/1965 44.45N, 10.30E 44.43N, 10.74E 35.0 0.19 4.4 4.3 3.7 153 07/06/1968 44.10N, 10.20E 44.58N, 10.81E 21.4 0.15 3.5 4.2 0.6 221 25/10/1972 44.41N, 9.91E 44.39N, 9.84E 39.5 0.20 4.1 4.7 4.6 167 05/06/1973 44.51N, 9.56E 44.27N, 9.30E NC 0.12 2.7 4.9 3.8 169 930 Stefano Solarino nate. The choice of this reference value is some- what arbitrary, but it represents a good compro- mise between the errors associated with the in- strumental location and the uncertainty associat- ed with the macroseismic one. It is then reason- able to say that the two locations match, if one considers that the instrumental location gives the point of rupture while the macroseismic one is more an average of the area the rupture started from. This statement is especially true for events exceeding M 5.5 (Gasperini and Ferrari, 2000), but the most widely shared definition of the macroseismic epicenter («the baricentre of the area featuring the maximum effects», Gasperini and Ferrari, 2000) involves a similar bias for all magnitude levels, though of course in proportion to them, since the maximum effects may depend upon several factors (site effects, wrong or inac- curate description, distribution of the localities). On average, the number of points used for the macroseismic locations of these events is large, say greater than 30, except for a couple of them that directly come from other sources (CP). A second group contains events that do not match: their differences not only exceed 16 km in one or both coordinates, but at least for one can be up to 80 km, which cannot be simply ex- plained by the diverse meaning of the location parameters. These are the 08/1902, 1921, 1926, 1927, 07/1928, 08/1928, 1934, 1961, 1965, 1968, 1973 events. Almost all of them have been taken from other catalogues (CP), while those based on macroseismic observations have been located with 10 (1921), 13 (1928), 13 (1965), 29 (1934) and 39 (1927) localities. This second group is probably much more difficult to locate using macroseismic data with the currently available information; on average they have a lower mag- nitude (down to 3.7). Moreover, the original cat- alogue which contained those for which macro- seismic data are not available (CP) was probably not accurate enough. It is important to emphasize that, if the loca- tions proposed in this work are reliable, some of these earthquakes actually occurred far from where they have been positioned, and this may have an impact on the microzonation and haz- ard computation. I discuss for example the event which occurred in 1934. Its original loca- tion has been obtained using 29 macroseismic observations (Monachesi and Stucchi, 1997), which is rather a good number, positioned as in fig. 5. It is evident that there is a very uneven distribution of localities and that they are espe- cially lacking on the eastern side, which can have biased the macroseismic location (grey star) towards the western area. Conversely, the instrumental location (black star in the figure) moves the epicenter to the north-east, that is in the same direction where macroseismic data are most lacking. If I accept that the macroseismic location may be biased by the poor coverage on the eastern side and I take into account the lo- cation errors of the instrumental determination, all this would render plausible a shift of the lo- Table III. Absolute differences (in km) between the latitude and the longitude of the macroseismic and instrumental locations. Bold numbers are used when the differences exceeds 16 km for any of the values. The magnitude is taken from the NT4.1.1 Catalogue. The last column of the table displays the number of points of intensity available for the macroseismic lo- cation. CP indicates the events extracted from other catalogues: for these events no quality information can be derived. Date Ms/Ix Diff. (Ia−Io), km Notes 05/03/1902 5.0/7 7−12 76 04/08/1902 5.0/7 11−−72 CP 27/10/1914 5.8/7 37−−29 588 07/09/1920 6.5/10 6−4 454 29/11/1921 4.6/5 30−−77 10 18/11/1926 4.2/5.5 55−−14 CP 28/10/1927 4.8/6 22−−24 39 20/07/1928 3.7/6 25−−35 13 03/08/1928 4.2/5.5 34−−80 CP 25/01/1931 4.0/6 4−6 CP 13/06/1934 4.9/6 8−−56 29 15/10/1939 4.9/7 16−15 56 12/08/1951 4.5/5.5 3−6 18 26/01/1959 4.2/5.5 12−2 CP 03/08/1961 4.4/6 24−−4 CP 09/11/1965 4.8/5 1−−34 13 07/06/1968 4.3/NR 53−−48 ? DB 25/10/1972 4.7/5 3−6 186 05/06/1973 4.4/4 26−−20 CP The role of instrumental versus macroseismic locations for the seismicity of the last century in the NW Apennines 931 Fig. 5. intensities for the 1934 event (DOM, Monachesi and Stucchi, 1997). The grey star shows the macro- seismic location, the black one marks the instrumental one proposed in this study. Fig. 4. Vectors connecting the original macroseismic location with the instrumental one as obtained in this work. 932 Stefano Solarino cation of the earthquake towards the east, that would mean on the other side of the watershed of the Apennines. The third group is made up of one event on- ly (1914): its location consistently differs from the macroseismic one but, the latter being ob- tained by more than 500 localities, it is reason- able to believe that instrumental data are not able to constrain the solution adequately. This is shown by both the location errors associated with this hypocenter (they are among the high- est) and by the resolution matrix (not reported), whose diagonal elements are very far from opti- mal (0.9604, 0.9584, 0.9525 and 0.8055 respec- tively for origin time, X, Y and Z coordinates). The presented analysis, although not exhaus- tive, gives a rough estimate of the potential and limitations of both data and method and thus rep- resents a good starting point for the discussion on the function of instrumental locations. 4. Discussion The obtained instrumental locations proved to be very similar, in many cases, to those pro- posed using macroseismic data. Moreover, it is always possible to define how reliable they are by looking at the associated location errors (within the limitations that these errors may in- troduce) or at the resolution matrices, and final- ly they may provide information on the depth of the events, which cannot be easily derived with macroseismic data, although several methodolo- gies have been proposed to infer depth from in- tensity observations. Provided thus that, under particular conditions, it is possible to obtain an instrumental location for older earthquakes, it is important to define the limits of their usage, that is to define what their role is. It is evident that instrumental locations can- not substitute macroseismic locations as they do not completely overlap the recent part of the parametric catalogues: instrumental data are largely available only since the sixties, while as stated above they lack for the first decades of the twentieth century, except for very large earthquakes. But instrumental data give a much better chance to distinguish different events in seismic sequences, which is not required in the macroseismic database (it is selected to avoid clusters that would bias many of the applica- tions which it is used for) but may be helpful in other studies (seismic cycles, seismotectonics and so on). For example, the event of Septem- ber 1920 was preceded the day before by an im- portant shock that is not included in the pub- lished macroseimic catalogues but was record- ed by many seismographic stations: how did this first shock affect the macroseismic obser- vations of the following large event? Could this important event be neglected when studying the way energy is released in the area? In this frame, it is very important to have at least a rough estimate of the depth of the earthquake, which can emerge, sometimes with severe lim- itations, from instrumental locations. In fact several methods have been proposed to infer depth from the attenuation of macroseismic da- ta, but seldom are they applied and in no Italian macroseismic catalogue is depth reported. At the moment, the usage of instrumental da- ta for the mentioned purposes is limited by the fact that seismic bulletins and waveforms for his- torical events are not in digital format, but this will soon change thanks to national (SISMOS, Michelini and the INGV SISMOS Group, 2004) and international (EUROSISMOS, Ferrari and Pino, 2004) projects that aim at compiling information for both instruments and seismic recordings. The correct role of the instrumental loca- tions is then to support and complement the macroseismic data and, in the few cases in which the macroseismic data are poor, to be used instead, provided that the instrumental da- ta have been carefully checked. This should not sound too odd since at the moment the most widely used parametric catalogue already con- tains a small percentage of instrumental loca- tions, and adding a few more events or some more information could only improve it. Acknowledgements An anonymous reviewer and Fabrizio Ber- nardi provided very constructive comments. Graziano Ferrari shared his precious knowledge of the older instruments and observatories. He also gave me very good hints on the older data. The role of instrumental versus macroseismic locations for the seismicity of the last century in the NW Apennines I owe special thanks to Alberto Ansaloni who provided very important information about the Observatory of Chiavari and let me thumb through the many bulletins, journals and scien- tific reports archived there. Appendix A. List of some of the reference sources used in this work. Not all the bulletins were published for the whole century. Some of them were compilations of comprehensive data, some were simply list- ing the data of a single observatory (e.g., Venezia, Quarto Castello). Finally, some other were merged in more complete compilations. Good references in this sense can be found in the TROMOS Project. Published by Title Notes Regio Ufficio Centrale Bolletino sismico Comprehensive compilation di Meteorologia e Geofisica, of data from many national Microsismi, Fascicolo I observatories Regio Ufficio Centrale Bollettino sismico Comprehensive compilation di Meteorologia e Geofisica, of data from many Macrosismi, Fascicolo II observatories Union Geodesique Bulletin du mois Comprehensive compilation et Geophysique Internationale, of data from many Bureau Central seismologique observatories de Strasbourg Raffaello Stiattesi, Spoglio delle osservazioni Contains data Osservatorio di Quarto sismiche from Observatory alone Castello (Firenze) Atti del Reale Istituto Osservazioni sismografiche Contains data Veneto di Scienze, dell’Istituto di Fisica from Observatory alone Lettere ed Arti della Regia Università di Padova Regio Ufficio Centrale Notizie sui terremoti Comments and data di Meteorologia e Geofisica osservati in Italia on the seismic activity at a national scale Int. Union of Geodesy The international Comprehensive compilation and Geophysics, the British Ass. Seismological Summary of data from many Seismological Committee, observatories the University of Oxford Caloi P., Medi E., Bollettino sismico mensile Comprehensive compilation Istituto Nazionale di Geofisica of data from many observatories Caloi P., Pannocchia G., Registrazioni sismiche Contains Rosini E. data in Roma from Observatory alone 933 This work is financed by the Istituto Nazio- nale di Geofisica e Vulcanologia and by the Di- partimento per la Protezione Civile within the DPC2004-06 research activity, under the S2 project. 934 Stefano Solarino Published by Title Notes International Seismological Bulletin Comprehensive compilation Centre of data from many foreign and national observatories International Seismological On-line Bulletin, Covers the whole century Centre, Thatcham, U.K. http://www.isc.ac.uk/Bull Osservatorio Geofisico Bollettino Mensile Bulletins were not published del Seminario Patriarcale (1920, 1927, 1934) throughout the period di Venezia (formerly of operation (1817-1938) Osservatorio Meteorologico e Geodinamico) Appendix B. Data used in this work in hypo-tape format. Readings with weight 4 and 5 have not been used due to different listing in diverse bulletins and/or data sources; for convenience, they are listed anyway. GIAC P 5 2 3 5 7 556.00 QUCA P 0 2 3 5 7 635.00 ROCC P 0 2 3 5 7 757.00 TRI P 0 2 3 5 7 708.00 FXIM P 0 2 3 5 7 636.00 PADO P 0 2 3 5 7 658.00 ROMA P 0 2 8 4223750.00 QUCA P 0 2 8 4223628.00 ROCC P 0 2 8 4223741.00 TRI P 0 2 8 4223670.00 FXIM P 0 2 8 4223627.00 SIEN P 5 2 8 4223740.00 CHIA P 0 141027 92236.00 FXIM P 0 141027 92236.00 PADO P 0 141027 92247.00 ROCC P 0 141027 92259.00 MONC P 0 141027 92265.00 DOMO P 5 141027 92257.00 MCI P 0 141027 92273.00 PADO P 5 20 9 7 55557.00 79.00 S 5 FXIM P 0 20 9 7 55556.00 ROCC P 0 20 9 7 55592.00 CHIA P 0 20 9 7 55552.00 LIVO P 5 20 9 7 55592.00 DOMO P 5 20 9 7 55580.00 MONC P 0 20 9 7 55581.00 110.00 S 0 VENE P 0 20 9 7 55578.00 SIEN P 0 20 9 7 55578.00 MCI P 0 20 9 7 55650.00 PADO P 0 21112912 445.00 62.00 S 0 POLA P 0 21112912 496.00 FXIM P 0 21112912 440.00 CHIA P 0 21112912 440.00 MONC P 0 261118225795.00 114.0 S 0 PADO P 0 261118225791.00 116.0 S 0 FXIM P 0 261118225715.00 PIAC P 0 261118225760.00 LIVO P 5 261118225864.00 74.00 S 0 MONC P 0 271028214942.00 66.00 S 0 PADO P 0 271028214945.00 70.00 S 0 FXIM P 0 271028214940.00 ROCC P 0 271028214968.00 VENE P 0 271028214957.00 84.00 S 0 TREV P 0 271028214959.00 85.00 S 0 PIAC P 0 271028214922.00 LIVO P 4 2710282149 5.00 20.00 S 4 ROMA P 0 271028214982.00 132.0 S 0 DOMO P 0 271028214990.00 The role of instrumental versus macroseismic locations for the seismicity of the last century in the NW Apennines CHIA P 0 271028214920.00 PADO P 0 28 720195353.00 75.00 S 0 PIAC P 0 28 720195330.00 ROCC P 0 28 720195383.00 142.0 S 0 TREV P 0 28 720195363.00 84.00 S 0 MONC P 0 28 720195365.00 82.00 S 0 CHIA P 5 28 720195310.00 FXIM P 0 28 720195343.00 LIVO P 4 28 720195370.00 PADO P 0 28 8 3231049.00 75.00 S 0 PIAC P 0 28 8 3231048.00 ROCC P 0 28 8 323983.00 TREV P 0 28 8 3231060.00 100.0 S 0 LIVO P 0 28 8 323970.00 FXIM P 0 28 8 323998.00 MONC P 0 28 8 3231053.00 89.00 S 0 TREV P 0 31 125104854.00 74.00 S 0 PAVI P 0 31 125104834.00 PIAC P 0 31 125104832.00 PRT P 0 31 125104830.00 FXIM P 0 31 125104831.00 LIVO P 5 31 125104870.00 60.00 S 5 ZUR P 0 31 125104870.00 108.0 S 0 NEUC P 0 31 125104868.00 109.0 S 0 TREV P 0 34 613 9 657.00 85.00 S 0 PADO P 0 34 613 9 652.00 74.00 S 0 TRI P 0 34 613 9 670.70 86.30 S 0 VENE P 0 34 613 9 659.00 79.00 S 0 PAVI P 0 34 613 9 652.00 PIAC P 0 34 613 9 637.00 PRT P 0 34 613 9 638.50 52.00 S 0 LIVO P 0 34 613 9 642.00 55.00 S 0 FXIM P 0 34 613 9 638.50 SIEN P 0 34 613 9 646.00 ROMA P 0 34 613 9 672.00 119.0 S 0 RCU P 0 39101515 573.30 107.0 S 0 MONC P 0 39101515 559.00 84.00 S 0 TRI P 0 39101515 571.00 105.0 S 0 CHUR P 0 39101515 570.00 114.0 S 0 PRT P 0 39101515 539.60 46.80 S 0 PRT P 0 51 812211948.00 57.00 S 0 BOLO P 0 51 812211955.00 67.00 S 0 PAVI P 0 51 812211969.50 91.40 S 0 SALO P 0 51 812211969.00 90.30 S 0 ROMA P 0 51 812211987.50 125.4 S 0 TRI P 0 51 812211985.00 134.0 S 0 PAVI P 0 59 126 53556.60 68.80 S 0 OROP P 0 59 126 53568.00 89.00 S 0 BOLO P 0 59 126 53569.00 93.00 S 0 RMP P 5 59 126 53742.00 PRT P 0 59 126 53568.80 88.20 S 0 FXIM P 0 59 126 53570.00 96.00 S 0 MONA P 0 59 126 53573.50 TRI P 0 59 126 53590.00 140.0 S 0 CHUR P 0 59 126 53581.40 BAS P 0 59 126 53590.40 FXIM P 0 61 8 3102645.70 63.00 S 0 PRT P 0 61 8 3102652.20 56.40 S 0 PADO P 0 61 8 3102670.00 106.0 S 0 PAVI P 0 61 8 3102680.00 100.0 S 0 RMP P 5 61 8 3102741.30 MONA P 0 61 8 3102668.00 114.0 S 0 CHUR P 0 61 8 3102680.80 111.4 S 0 RSL P 5 61 8 3102684.00 122.0 S 5 LJU P 5 61 8 3102692.00 148.3 S 5 TRI P 5 61 8 3102745.00 BOLO P 0 6511 9153512.00 24.00 S 0 PAVI P 0 6511 9153523.00 42.00 S 0 PADO P 0 6511 9153526.00 47.00 S 0 RCU P 0 6511 9153545.00 75.00 S 0 PAVI P 0 68 6 7 93468.00 83.50 S 0 PADO P 0 68 6 7 93469.00 90.00 S 0 RCU P 0 68 6 7 93496.00 AQU P 0 68 6 7 93496.00 RMP P 0 68 6 7 93497.50 GENO P 0 68 6 7 93464.00 78.00 S 0 STV P 0 68 6 7 93483.20 105.50 S 0 ROB P 0 68 6 7 93477.00 98.00 S 0 CUN P 0 68 6 7 93482.00 105.00 S 0 TRI P 0 68 6 7 93486.00 LJU P 0 68 6 7 93495.00 935 936 Stefano Solarino FUR P 0 68 6 7 93542.00 PAVI P 0 721025215631.00 45.00 S 0 BOLO P 0 721025215634.00 49.00 S 0 TNO P 0 721025215639.40 GENO P 1 721025215625.68 35.18 S 0 ALBA P 1 721025215635.98 ROB P 0 721025215635.23 51.73 S 0 SRX P 0 721025215638.53 57.53 S 0 CUN P 0 721025215639.53 59.53 S 0 VERN P 0 721025215638.93 ROA P 0 721025215640.03 ENR P 0 721025215640.43 STV P 0 721025215641.03 62.23 S 0 PNI P 0 721025215641.68 63.43 S 0 CHIA P 1 721025215620.00 25.00 S 0 LNS P 2 721025215646.10 SPF P 0 721025215649.50 LMR P 0 721025215652.50 REFERENCES BOSCHI, E., E. 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