A Harmonised Instrumental Earthquake Catalogue for Iceland and the Northern Mid-Atlantic Ridge

A comprehensive catalogue of historical earthquakes, with accurate epicentres and homogenised magnitudes is a crucial resource for seismic hazard mapping. Here we update and combine catalogues from several sources to compile a catalogue of earthquakes in and near Iceland, in the years 1900–2019. In particular the epicentres are based on local information, whereas the magnitudes are based on teleseismic observations, primarily from international on-line catalogues. The most reliable epicentre information comes from the catalogue of the Icelandic Meteorological Office, but this is complemented with 5 information from several technical reports, scientific publications, newspaper articles, and modified by some expert judgement. The catalogue contains 1272 MW ≥ 4 events and the estimated completeness magnitude is MW 5.5 in the first years, going down to MW 4.5 for recent years. The largest magnitude is MW 7.01. Such melting of local and teleseismic data has not been done before for Icelandic earthquakes, and the result is an earthquake map with no obviously mislocated events. The catalogue also lists additional 5654 earthquakes on the Mid-Atlantic Ridge, north of 43◦, with both epicentres and magnitudes determined 10 teleseismically. When moment magnitudes are not available, proxy MW values are computed with χ-regression, normally on MS , but exceptionally on mb. All the presented magnitudes have associated uncertainty estimates. The actual combined seismic moment released in the Icelandic earthquakes is found to be consistent with the moment estimated using a simple plate motion model. The catalogue is named ICEL-NMAR and it is available online at http://dx.doi.org/10.17632/7zh6xg22cv.1.

Since shortly after the IMO was established, it has been responsible for monitoring earthquakes in Iceland. From the beginning, accounts of earthquakes have been published in the IMO monthly newsletter Veðráttan (the Weather) (IMO, 1924(IMO, -2006, 60 in addition the Seismological Bulletin (IMO, 1926(IMO, -1973 was compiled and distributed to seismological centres abroad, and since 1975 computerised earthquake catalogues have been kept, and made available to scientists working elsewhere. After 1965 earthquake research took off at the University of Iceland, and has flourished ever since with a number of case studies, as well as historical summaries. The new century has seen a surge in the publication of local and global earthquake catalogues, and Iceland is not an exception. remaining ones, inaccurate teleseismically determined locations were given. To our knowledge, this is the only catalogue apart from the current one where local locations and global magnitudes have been combined. Unfortunately this catalogue was only published in a very limited distribution, and it is not available online. 70 Grünthal and Wahlström (Grünthal and Wahlström, 2003) compiled a historical catalogue of earthquakes in Central and Northern Europe until 1993, with magnitudes and locations in Iceland taken from a data file obtained from the IMO. These data were compiled at the IMO independently of the IMO catalogue discussed in Sect. 2.2.1, and are still available on the IMO website (hraun.vedur.is/ja/ymislegt/storskjalf.html). The locations are reasonably accurate, but the resulting M W magnitudes 4 https://doi.org/10.5194/nhess-2021-15 Preprint. Discussion started: 4 February 2021 c Author(s) 2021. CC BY 4.0 License. are exaggerated, often by a whole magnitude (less for the most recent earthquakes, or ∼0.2−0.3 magnitudes). The work 75 on this catalogue continued with a number of subsequent projects (Grünthal et al., 2009;Grünthal and Wahlström, 2012;Grünthal et al., 2013), under several acronyms, CENEC (CEntral, Northern and northwestern European Catalogue), EMEC (European Mediterranean earthquake catalogue), SHARE (seismic hazard harmonization), and SHEEC (SHARE European earthquake catalogue). For the Iceland region, all these projects adopt the original 2003 catalogue, adding data (locations and local magnitudes) after 1990 from IMO's catalogue. Among the products of these studies were hazard maps for Iceland where 80 the hazard was greatly overestimated in many places, among them in the Reykjavík capital area (Woessner et al., 2015).  (Storchak et al., 2013;Di Giacomo et al., 2015). The catalogue contains 40 earthquakes in the ICEL region.

Sources and data
This section discusses the primary sources used to compile the new ICEL-NMAR catalogue. These sources consist of four 90 international catalogues, used primarily to obtain and/or compute magnitudes, and several types of local Icelandic sources used as a basis for event locations. The local sources include the catalogue of the IMO, scientific publications, seismological bulletins, newsletters and technical reports, as well as newspaper articles. The section concludes with a few remarks on how individual events in different sources have been matched up.

The ISC Bulletin Event Catalog
The ISC database (2020) contains data on earthquake location and magnitude contributed by several seismological agencies from around the world. For each earthquake a single origin time (UTC) and location with multiple magnitude values are provided. The magnitudes are of several different types, but in the present work only M S , m b and M W are considered.
Magnitudes coded as m S and M s are treated as M S , and similarly for varying capitalization of m b . In addition in the period 100 1955−1970 there are a few magnitude values marked as M and these are also treated as M S cf. (Sykes, 1965). When both M and M S values are available for an earthquake the difference is small. Each magnitude is either marked ISC, to signify that the value is computed by ISC themselves, or else it is marked with the abbreviation of a submitting agency. The ISCmarked values are referred to as reviewed, and according to Storchak et al. (2017), "seismic events are reprocessed resulting in more robust and reliable mb and MS magnitudes". Di Giacomo and Storchak (2016)  relocating earthquakes and recomputing their magnitudes. They also recommend that preference be given to three agencies, CTBTO (Comprensive nuclear-Test-Ban Treaty Organization, also known as International Data Centre, IDC, Vienna), MOS (Geophysical Survey of Russian Academy of Sciences, Moscow), and USGS (United States Geological Survey).

The GCMT Earthquake Catalog
The GCMT catalogue (2020) contains data on seismic moment tensors with associated M W magnitudes of large earthquakes 110 (M W ≥ 5) around the world, starting in 1976 (Dziewonski et al., 1981;Ekström et al., 2012). This is considered to be the most authoritative catalogue providing M W (Di Giacomo and Storchak, 2016). There are 653 events in the NMAR region in this catalogue, and all but 9 of them are also in the ISC catalog, marked as originating from GCMT. In 482 cases the M W match but in 171 cases there is a mismatch of 0.1 magnitude, and the average is used here.

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Ambraseys and Sigbjörnsson (2000) published an earthquake catalogue for Iceland or more specifically for the region shown in Fig. 1. The catalogue covers exactly one century, i. e. from 1896 to 1995, and lists 422 earthquakes. The catalogue is based on teleseismic data from seismological bulletins, and information from books, journals, newspapers and reports. The authors recalculated surface magnitudes (M S ) and locations when possible. Ambraseys and Sigbjörnsson (2000) mention that the greatest outstanding problem was the epicentral accuracy, particularity for pre-1960 macroseismic and instrumental events.

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They specially remark that epicentres before 1918 reported by the British Association for Advancement of Science (BAAS) are crude, as well as epicentres estimated by the ISC before 1950, although to lesser degree (Ambraseys and Sigbjörnsson, 2000). This catalogue contains valuable information for the time period from 1900 to 1960 when fewer records are available from other catalogues.

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The USGS catalogue (2020) provides one magnitude value per earthquake (M W , M S or m b ), which is in almost all cases identical to the corresponding USGS-labeled value in the ISC database. However the locations in the USGS catalog are different from those in the ISC catalog, the difference frequently amounting to a few tens of kilometers. The Icelandic Metorological Office (IMO) in Reykjavík has been responsible for monitoring earthquakes in Iceland since shortly after its foundation in 1920 when the Mainka seismograph mentioned in the introduction was reinstalled there in 1925. A second Mainka instrument was installed in 1927, also in Reykjavík. Data processing was conducted at the IMO and the results were published in Seismological Bulletins (IMO, 1926(IMO, -1973 which were sent to several seismological agencies around the world. These results were mainly phase readings and reports of felt earthquakes along with a few locations. After 1980 the IMO reanalyzed these data and combined them with other local and global sources, e. g. the University of Iceland (UI) reports discussed in the next subsection, and Kárník (1968), to mention a few. The resulting event locations and magnitudes form the basis of IMO's catalogue for the period 1926−1952.
In 1951-1952 three Sprengnether short-period seismographs, measuring all three components of motion, were installed in Reykjavík and the old seismographs were moved to Akureyri in North Iceland and to Vík in South Iceland (Fig. 1), and in the 140 following two decades several more instruments were installed.
As detailed in the next subsection, the University of Iceland Science Institute (UISI) initiated several research projects involving seismic measurements after 1970. Many of these were in cooperation with the IMO, and at the same time IMO's network continued to expand. As before the resulting data were published in the Seismological Bulletins. The IMO catalogue 1952−1974 is based on these and a digital-only bullettin for 1974.

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From 1975 to 1986 no bulletins were published, and to fill up this gap, phase readings from the UISI and the IMO stations were merged and reanalyzed to compute locations and magnitudes. This work was carried out at the IMO after 1990, and earthquakes of magnitude M L > 3 were entered into the IMO database. The database for this period is somewhat preliminary and incomplete, as manual review is lacking. The period 1987−1990 is also in the IMO database, with results based on Mánaðaryfirlit jarðskjálfta (Monthly reports of earthquakes) (IMO, 1987(IMO, -1990, published by the IMO in cooperation with 150 the UISI. In 1991 a digital seismic system, the South Iceland Lowland (SIL) system was implemented by the IMO (Stefánsson et al., 1993;Bödvarsson et al., 1996). As the name implies, it began in South Iceland, but was gradually expanded to cover all geologically active areas in the country. In 2020 around 80 stations are in operation in the SIL-network. Even if the system did not cover the whole island to begin with, all events of magnitude M L > 4 occurring within a few tens of km offshore should 155 be present for the whole period. Locations and local magnitudes are automatically computed by the system, all automatically located events are manually reviewed, and the location recomputed. The IMO catalogue from 1991 is based on the SIL system analysis.

Data from the University of Iceland Science Institute
Research on historical seismicity at the University of Iceland relies heavily on reports by Tryggvason (1978aTryggvason ( ,b, 1979 and Technical advances and increasing interest in crustal activity following the Surtsey eruptions in 1963-1967 led to a proliferation of seismic observations in Iceland in the late 1960ies (Einarsson, 2018). Cooperation started between the UISI and 165 the Lamont-Doherty Earth Observatory (LDEO) at Columbia University in NY. A team from LDEO came to Iceland with several portable seismographs to study the background seismicity of the mid-Atlantic plate boundary (Ward, 1971). A network of six stations was operated on the Reykjanes Peninsula segment of the boundary during 1971-1976 (Björnsson et al., 2020), augmented by a dense network in the summers of 1971 and 1972 (Klein et al., 1973(Klein et al., , 1977. The work continued by building an North Iceland in 1974Iceland in , and to other parts in 1975Iceland in -1979. A telemetered network was installed in Central Iceland in 1985. These networks provided valuable data on major events such as the Krafla volcano-tectonic episode of 1975-1984(Einarsson and Brandsdóttir, 1980Brandsdóttir and Einarsson, 1979;Buck et al., 2006;Wright et al., 2012), the Hekla eruptions of 1980 and 1991 (Grönvold et al., 1983;Soosalu and Einarsson, 2002) and the Gjálp eruption in Central Iceland in 1996 (Einarsson et al.,175 1997), as well as the location of the major seismically active structures of Iceland (Einarsson, 1991). After 1991, the analog seismic stations were gradually replaced by the SIL-system discussed in the previous subsection. The last analog stations were dismantled in Central Iceland in 2010. Some of the data gathered by the seismic network discussed above, including epicentres, are documented in the Skjálftabréf (Earthquake letter) (UISI, 1975(UISI, -1988.

Combining catalogues
All the catalogues, that need to be combined for the current study, have their own version of both origin time and location of each earthquake. As proposed by Jones et al. (2000) and several later publications we consider two records that differ by less than 16 s and 100 km to refer to the same earthquake. In a few cases we have found that this window is a little too narrow and we have made an appropriate manual adjustments. Furthermore, the AMB-SIG catalogue only provides times to 190 the nearest whole minute, so for that a 90 s time window is used. For each earthquake, the ISC-time, all available locations (ISC, AMB-SIG, IMO, other local sources), and all available magnitude values of different types (M W , M S , m b ) and from different catalogues/contributors are entered into a data file. This file is then used for further processing as described below.

Earthquake locations
When accurate instrumentally determined location of an earthquake is missing, which applies to a large part of the study period, 195 several methods may be used to determine the epicentre. Sometimes the historical accounts, discussed in Sect. 2.2 provide quite accurate locations, especially in inhabited areas. For the past decades a major effort has been devoted to the mapping of surface expressions of earthquake faults in Iceland, and these often indicate the location of historical earthquakes (Einarsson, 2015).
Furthermore, the main faults tend to produce microearthquakes detected with the SIL network. By relative locations, detailed maps of the subsurface faults can be produced (Slunga et al., 1995). Combining all these methods and adding expert judgement will normally give a much more accurate locations than those provided by the international catalogues, and the same holds for many of the locations in the IMO catalogues, even before 1990.
The remainder of this section describes details of how this methodology has been applied for several subperiods of the study period.

The period until 1990 205
In the period 1900−1925 there are 22 earthquakes in the ICEL region listed in our data file. All of these are in the AMB-SIG catalogue, and 4 are also in the ISC catalogue, originally coming from Gutenberg and Richter (1949). The authors have viewed all these earthquakes on a map, checked newspapers articles for contemporary accounts of them (using the web service timarit.is mentioned in Sect. 2.2.3), as well as scientific publications, in particular the report of Ottósson (1980). The result of this scrutiny is to use the AMB-SIG location for 14 earthquakes, the aforementioned report for one event, and relocate 6 210 events using the methodology described at the beginning of this section. In the new catalogue these location sources have been specified as "Amb-Sig", "Report" and "New" respectively. Finally, for the 22 January 1910 earthquake we use the location provided by (Stefánsson et al., 2008), 20 km offshore North-Iceland. This source is marked as [1] in the catalogue, with details in an accompanying reference list.
In the period 1926−1955 there are 98 earthquakes in our data set, and their location has been scrutinised in the same way.

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Sometimes we can take into account that an origin time is within a known earthquake series. For this period additional data sources are the IMO catalogue (Sect. 2.2.1), as well as the reports of Tryggvason (1978aTryggvason ( ,b, 1979 which often provide direct epicentres. This results in using 36 AMB-SIG locations, 21 IMO locations (marked "IMetO" in the new catalogue), 34 locations from the reports, 4 computed as average of the most believable reported locations (marked "Average"), and 3 relocated (marked "New").

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In the period 1956−1990 there are 380 earthquakes in the data file. Having multiple local seismometers opens the possibility of computing locations from local measurements. Such locations have found their way into several of our sources, but the quality is variable. There are several journal articles stemming from this period providing locations for 41 earthquakes and our choice is to trust these. The relevant articles are listed in the reference list in the readme-file accompanying the catalogue, and specified as [2], [3], etc. in the catalogue itself. Some of the articles are also cited in Sect. 2.2.2 above. Available locations 225 for the remaining 338 earthquakes were viewed on a map, upto 4 locations per earthquake: From AMB-SIG, IMO, ISC, and one of the earthquake reports, newsletters or bulletins. It transpired that none of these sources could be used as an overall first choice, but instead we had to select the most believable one in each case, or sometimes take an average or relocate. The result was to use AMB-SIG for 59 cases, the IMO catalogue for 107, ISC for 36, 12 from reports, 55 locations from the Skjálftabréf (Earthquake letter) (UISI, 1975(UISI, -1988 (marked "Letter"), 14 averaged, and 56 relocated.  3.3), whereas the SIL locations are very convincing, normally accurate to a few km (1 or 2 inside the network, but somewhat more outside). The only region where the SIL-locations seem suspect is on the Reykjanes Ridge, more than 150 km offshore, or approximately south of 63 • N. This inaccuracy is not important for future work with these data e. g. in hazard analysis, and we have chosen to use the ISC locations for the relevant 40 earthquakes. In addition there are 33 ISC-earthquakes in the ISC catalogue missing from the SIL catalogue. Of these, 25 were located far offshore and 8 were in or near the Bárðarbunga 240 caldera, in the uninhabited interior of Iceland. The earthquakes near the caldera were relocated to the caldera itself, and the ISC locations for the offshore events were retained.

Accuracy of earthquake locations
To get some indication of the accuracy of event locations in the international catalogues the locations in the AMB-SIG and the ISC catalogues have been compared. For 292 events in both catalogues (period 1910−1996), the maximum mismatch in 245 location is 113 km, the median is 10.0 km, and in 90% of cases the difference is < 30 km. The accuracy does not seem to increase markedly with time or with earthquake magnitude. A similar comparison between the ISC and the USGS catalogues

Earthquake sizes
Contrary to earthquake locations, where local information is better, estimating earthquake sizes with teleseismic data is often easier and more reliable than using regional and local data. The dominant periods at teleseismic distances are longer and the structure is smoother, and therefore the waveforms fit better (Wang et al., 2009;Karimiparidari et al., 2013;Yadav et al., 2009).
Modern earthquake catalogues generally provide moment magnitudes for all earthquakes larger than about M W 4. For 255 earthquakes, whose source mechanism and magnitude have not been modeled by moment tensor inversion of seismic data, regression on surface or body-wave magnitudes is customarily used to obtain proxy M W values, and this procedure is followed here. As mentioned in the introduction a larger collection of earthquakes than is really needed in the Iceland context is used to construct the M S -M W and m b -M W regression relationships, thus killing two birds with one stone, improving the accuracy of these relationships, and getting a larger catalogue of 6926 earthquakes. The data file discussed in Sect. 2.3 above contains some 260 earthquakes that are to small to be included in the catalogue, but are used in the regression in order to improve the relationship for small magnitudes.
For each earthquake there are usually several m b -values, contributed by different agencies, and the same applies to M S , and sometimes also M W . These values must be apropriately averaged or selected before they can be used in the regression.
This subtask is dealt with in the next subsection, followed by a subsection on uncertainty in the magnitude estimates in the where the w i are normalised weights, and the sum is taken over all available M i . If the ∆ i are independent it is optimal to weigh with their inverse variance, and, even if not optimal, it is more robust to use the same weights when the ∆ i are correlated 300 (Schmelling, 1995). To be precise, uncertainty is about 3−4 times higher than when M W is found with moment tensor modeling. This reckoning is supported by both Werner (2008) and Gasperini et al. (2013).  (2) and these numbers are used directly when M g is available and M est = M g . Keeping in mind that almost all the earthquakes in the NMAR region are shallow, these uncertainties are perhaps somewhat lower than those quoted in Sect. 4.2.1. However, the accuracy of the global catalogues has probably improved since the quoted studies were carried out, and, furthermore, these studies do not explicitly specify GCMT or reviewed ISC magnitudes.

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When M g is not available, and M est is computed via Eq. (1) the error in the magnitude estimate may be partitioned into several terms: using that the w i sum to 1. Treating d and the δ i as random variables, and the ∆ i as constants this gives, The first term is given by Eq.
(2), and Var δ i and Cov(δ i , δ j ) can be approximated by σ 2 i and σ ij , the data covariance of the available pairs (δ i , δ j ). Finally, for the last term, we have where r i is the correlation between d and δ i . A reasonable constraint is that this correlation is positive: If M g overestimates M, why should M i overestimate M even more? Another constraint is that the estimated variance in M est is not smaller than 350 when the golden standard M g can be used. The second constraint corresponds to r i = σ i /(2σ d ). Selecting the middle road with the last term with i w 2 i σ 2 i , and the uncertainty estimate: The root-mean-square (RMS) average uncertainty for all cases where Eq. (1)  Appendix should be squared). Bormann et al. (2013) also recommend weighing data points in magnitude ranges with low data frequency higher (histogram equalization). We use a moderately weighted regression of this type: an earthquake with moment and surface magnitudes M W and M S gets a weight of M W + M S − 2. The effect is that the largest earthquakes weigh about twice as much as the smallest ones.

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There is freedom in the regression to fix one of the uncertainties, σ(M S ) or σ(M W ), and it is also possible to fix their ratio.
If the ratio is taken as 2.0, as in Gasperini's article, the NMAR data gives σ(M S ) = 0.176 and σ(M W ) = 0.0881.
Exactly the same method could be used to compute M W from best estimates of m b . However the NMAR dataset contains much fewer large earthquakes than the one used by Gasperini et al., so when this is attempted, the relationship turns out to be very slightly concave rather than convex (logarithmic rather than exponential). The nonlinearness is so slight that it can be 375 ignored with a linear model. For earthquakes larger than about m b = 5.75 an M S value is almost always available, and, as explained below, preferred. Thus a model valid for m b < 5.75 is constructed and used: Earthquakes in the Bárðarbunga caldera (Fig. 1)    pairs outside and 97 in Bárðarbunga. Note that a few earthquakes with m b < 3.5, and thus not included in the final catalogue, are used for the regression. A slight random jitter has been applied to the pairs to improve the visual appearance of the graphs.
As one might expect the deviation in the M S model is considerably lower than in the m b model (Fig. 3). Thus M S is used to compute a proxy M W when it is available, for 4217 events in the NMAR region, of these 933 are in the ICEL region. In the absence of an M S value the m b relation must be used, for 2954 events in NMAR, of these 379 are in ICEL. M S is available for 385 almost all large earthquakes, the ones that are important for hazard assessment. Only three m b > 5-values are used to compute proxy M W in the ICEL region and therefore the regression only uses data with m b < 5.5 (Fig. 3).
To use a somewhat round number, and to have a single M W uncertainty, the current work uses σ(M W ) = 0.09 for all the models, m b and M S , in and outside Bárðarbunga (Fig. 3, Table 1). These uncertainty values are in good agreement with the results quoted in Sect. 4.2.1, perhaps somewhat lower, which might reflect that our data is more recent and there is continuous 390 improvement in the quality of the global catalogues.
To study possible change in the M S -M W relationship or in the accuracy of the moment tensor M W values, a separate modeling was tested for a few sub-periods. A slight, somewhat erratic, improvement in the accuracy was observed, but no significant change in the relationship. Thus it was decided to use a single model for the whole period. Table 1. Parameters of exponential and linear models for MW , obtained with σ(MW ) = 0.09, c. f. Eqs. (5) and (6) where σ 2 MS is the variance estimate for the earthquake, obtained as described in Sect. 4.2.2, σ(M W ) = 0.09 as in Sect. 4.3, f is the model function given in 5, and a and b are the regression parameters (Table 1)  In all cases there is a considerable negative bias of 0.6−1.4 magnitudes, more offshore (outside the SIL network) than onshore.
One explanation for the large spread and bias of the local magnitudes is that the SIL systm's analysis is optimised towards robust magnitude estimation of smaller earthquakes than those of this comparison. Figure 4 shows the spread of the data, evidently in line with these estimates. It has no meaning to show the regression curves because of the high uncertainties. are given, and finally information on the origin time and location sources. All events smaller than M W 4 were excluded and the uncertainty was not computed for M W < 4.5. The available information on hypocentral depth is very inconsistent and it is not provided in the catalogue. The brittle part of the Icelandic crust in most areas is less than 12 km thick, and earthquakes of any significance will rupture the whole thickness (Hjaltadóttir, 2010;Pedersen et al., 2003;Stefánsson et al., 1993).

Magnitude of completeness 430
To investigate the magnitude of completeness of the new harmonised catalogue for the whole NMAR region, two methods were used. Firstly, histograms with 10−30 year bins of the earthquake count with magnitudes exceeding different thesholds were created (Fig. 6), and secondly Gutenberg-Richter models were constructed for a few selected periods and minimum magnitudes. The histograms show that the catalogue appears to be complete for M W ≥ 6 for the whole period, for M W ≥ 5.5 In the final catalogue there are a few periods with disproportionately many earthquakes connected to tectonic activity (SISZ 2000 and and volcanic activity (Krafla region 1975-1976, Hengill 1994-1999, Bárðarbunga 2014-2015.
In the wake of large earthquakes it is possible that other events are triggered by their probagating waves. These secondary 445 events can be missing from the international catalogues because their signal is lost in the coda of the primary event at teleseismic distances. An example of this are two events on the Reykjanes Peninsula triggered by the M W 6.52 South-Iceland event on 2000-06-17 15:40:41, occurring 26 and 30 seconds later, and 65 and 80 km farther west, respectively. The size of the first one was estimated to be M L 5.5 (Antonioli et al., 2006), and that of the second one M W 5.79 (Pagli et al., 2003 respectively. These are the only events not coming from one of the four international catalogues of Sect. 2.1.    (Halldórsson, 1992b;Stefánsson et al., 2008;Sólnes et al., 2013). The epicentral locations are approximate but overall the longitude is more accurate than the latitude since in most cases N-S surface faults have been mapped and linked to the largest events. A few events which differ most were investigated, and it transpired that the explanation was usually a combined effect of the regression curve difference and the underlying data difference.

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The question arises how representative the seismic activity of the catalogue period is for any period of 120 years. The answer depends on the length of the typical earthquake cycle. If the cycle is significantly longer than 120 years our sample may underestimate the seismicity greatly, e. g., if the period does not contain a characteristic maximum magnitude earthquake.
Studies of South Iceland earthquakes indicate that we may be near this critical duration of the cycle. The study of Einarsson et al. (1981) gave an average time between major earthquake sequences of about 80 years, ranging between 45 and 112 years. For comparison with our catalogue we estimate the potential seismic moment release in the two fracture zones, the SISZ and the TFZ, by a simplified geometric model of two transform faults parallel to the relative plate motion. The simplification is justified by the arguments of Sigmundsson et al. (1995), who showed that the seismic moment of many closely spaced, short transverse faults (bookshelf faults) is equivalent to that released by a single transform fault. We also assume that almost all the seismic moment is released by the transform zones and not by the divergent segments of the plate boundary or the magmatically induced seismicity. The length of the transform zones is taken as 180 km and 150 km for the South and North Iceland zones, 480 respectively, i. e. the offset of the ridge axes. The width of the fault is taken to be the thickness of the seismogenic part of the crust, about 10 km, the spreading rate is 19 mm/yr, and the shear modulus 20 · 10 9 Pa (McGarr and Barbour, 2018). The moment rate will then be: 20 · 10 9 × 19 · 10 −3 × 330 · 10 3 × 10 · 10 3 = 1.25 · 10 18 Nm/yr.
This result can be compared with the total seismic moment released in Iceland during the catalogue period, which may be 485 estimated using the catalogue data and the completeness information of Sect. 5.2. Such computation for all earthquakes ≥ M W 4 in the area shown in Fig. 6, excluding the Reykjanes Ridge and Bárðarbunga, gives a total of 1.61 · 10 20 Nm. Adding a simple correction for smaller events assuming the Gutenberg-Richter law with b = 1 raises the estimate to 1.64 · 10 20 Nm, corresponding to an annual rate of 1.37 · 10 18 Nm/yr. This agrees quite (even surprisingly) well with the result of Eq. (8).

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We have constructed a new catalogue of earthquakes in Iceland and, as a byproduct, for the Northern Mid-Atlantic Ridge. A general criteron for entry into the catalogue is that an earthquake has been instrumentally recorded by agencies outside Iceland.
Locations of events in the ICEL region ( Fig. 1) have been reassessed and proxy M W values for earthquakes without modeled moment magnitudes have been computed. The resulting moment magnitudes range from 4 to 7.08. For the ICEL region the catalogue is reasonably complete for M W ≥ 5.5 for the whole period. There are 36 earthquakes of this size onshore or less are still uncertain and controversial. Neither of them appears to have occurred on the best known structures, the Húsavík-Flatey fault or the Grímsey Oblique Rift. Stefánsson et al. (2008) suggest that the 1963 event originated on a NNE-striking fault 500 offshore Skagafjörður, based on the distribution of recent earthquakes and the focal mechanism solutions of Stefánsson (1966) and Sykes (1967). They furthermore suggest that the 1910 event originated on the eastern margin of the Grímsey Shoal.
We adopt these locations in our catalogue. Distribution of epicentres and recent bathymetric data support these suggestions (Einarsson et al., 2019).
The largest events occur in the two seismic zones, where the plate boundaries are parallell to the plate movements ( Fig. 1 and 505   5). The distance from these events to the Reykjavik capital area, where 63% of the population live, is some tens of kilometers, and the same holds for Akureyri in North Iceland, with 5% of the population. However there are several towns and villages within the zones. An important future task is to carry out a detailed analysis of the seismic hazard both in these urban areas and elsewhere in Iceland. The new catalogue should prove to be an essential resource for such seismic hazard mapping.
Data availability. The international earthquake catalogues from USGS, GCMT and ISC are freely available online. In addition we used the catalogue of Ambraseys and Sigbjörnsson (2000), as well as scattered data on individual earthquakes from various printed sources, as