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55229 NGT Innmat Nr.3-05 NORWEGIAN JOURNAL OF GEOLOGY The 31st of August 1819 Lurøy earthquake 245 The 31st of August 1819 Lurøy earthquake revisited Hilmar Bungum & Odleiv Olesen Bungum, H. & Olesen, O.: The 31st of August 1819 Lurøy earthquake revisited. Norwegian Journal of Geology, Vol. 85, pp. 245 - 252. Trondheim 2004. ISSN 029-196X. The 31st of August 1819 Lurøy earthquake has, for a long time, been considered as the largest known historical earthquake in Fennoscandia, possi- bly even on the mainland of northern Europe. Triggered by a recent claim that the magnitude (MS) of this earthquake should be 5.1 rather than 5.8, which implies a reduction in radiated energy by about a factor of 10, we have reassessed the original macroseismic observations together with the published literature. Our conclusion is that a MS magnitude of 5.8 for the 1819 Rana region earthquake still remains a reasonable, justifiable and defendable estimate. Such a magnitude is moreover consistent with present hazard models for the region, which include the possibility that magni- tude 6+ earthquakes can occur today in the most seismically active areas in Norway, such as the coastal parts of western Norway, Nordland and the Oslo rift zone. Rock avalanches and landslides, potentially triggered by earthquakes, could moreover generate tsunamis in fjords and lakes and con- stitute the greatest seismic hazard to society in Norway. Hilmar Bungum, NORSAR, POB 53, NO-2027 Kjeller, Norway. Odleiv Olesen, Geological Survey of Norway, NO-7491 Trondheim, Norway. Introduction hors (Husebye & Kebeasy 2004b). The present discus- sion is concerned with the Husebye & Kebeasy (2004a) Earthquakes differ from other environmental loading paper (hereafter called H&K). phenomena such as ocean waves in that the largest events have very long return periods, typically of the In the following we will discuss specific parts of the order of 100 years at plate margins but possibly thou- H&K paper, concentrating on the topics that are most sands of years in more stable continental regions such important for the magnitude assessment. The main as Fennoscandia (e.g. Johnston & Kanter 1990; point here is that H&K reduce the surface wave magni- Bungum et al. 2005). Given that a reasonably complete tude MS for the 1819 earthquake from 5.8 to 5.1 (and record of instrumental recordings on a global basis the local magnitude ML to 4.8), which implies a reduc- dates back only to the early 1960s, this clearly creates tion in radiated energy by about a factor of 10. First, particular problems in terms of acquiring sufficiently however, we find it appropriate to provide some com- long periods of observation for the purpose of recur- ments on the development of magnitude assessments rence estimation. This, in turn, emphasizes the impor- of earthquakes in Norway. We agree fully with H&K tance of historical observations, which for earthquakes that reported magnitudes for historical earthquakes, in is termed macroseismic or intensity data, and is essenti- Norway and elsewhere, have had a tendency to be over- ally concerned with how earthquakes have been felt by estimated. One reason for this is that historical reports people and their effects on both the built and the natu- are often unduly ‘colourful’ in their descriptions, and ral environment. that earlier scientific assessments have not filtered such effects well enough. Moreover, different assessment cri- Historical records from Norway are very scarce from teria may have been used, magnitudes may have had a earlier than the late 17th century while more systematic tendency to be ‘inherited’ indiscriminately from earlier reports on natural phenomena like earthquakes date reports, and in addition, different and poorly defined back only to the late 19th century. The few large histori- magnitude scales may have been employed. As an cal earthquakes that have been reported are therefore of example of earlier reported magnitudes we note that great importance for understanding the seismotecto- Karnik (1971) had magnitude (M) 6.0 for the 31 nics and seismic potential in this region. Consequently, August 1819 earthquake and 6.4 for the 23 October the recent papers by Kebeasy & Husebye (2003) on the 1904 Oslofjord earthquake, which is another historical 1759 Kattegat earthquake and by Husebye & Kebeasy earthquake of importance in Norway. Husebye et al. (2004a) on the 1819 Lurøy earthquake are commen- (1978) reported M 6.0 for both the 1819 and the 1904 dable. There are, however, some problems with both earthquakes, while Husebye et al. (1979) had M 6.0 and these papers that already have given rise to one critical 6.4, respectively, for the same two events. These were comment (Wahlström 2004), with a reply by the aut- commonly accepted values at that time, and it was also common that the magnitude scale was not well defined. 246 H. Bungum & O. Olesen NORWEGIAN JOURNAL OF GEOLOGY Table 1. Equation (A) From Muir (B) From Muir (C) From Muir Wood (1989) Wood & Woo (1987) Wood & Woo (1987); Muir Wood (1989) No Based on R (km) MS R (km) MS R (km) MS 1AIII log linear 630 5.7 850 5.9 800 5.9 2AIII non linear 5.8 6.2 6.2 3AIV log linear 450 5.6 530 5.7 530 5.7 4AIV non linear 5.8 6.0 6.0 Table 1. MS magnitudes for the 1819 Nordland earthquake as calculated using Eqs.(1) to (4), based on different estimates of felt areas, here expres- sed in terms of felt radii assuming a circular area. For column (A) the numbers are taken from Table 3.4 in Muir Wood & Woo (1987); for column (B) the numbers are derived by the present authors based on a felt area figure which was published both as Fig. A7.2 in Muir Wood & Woo (1987) and as Fig. 4 in Muir Wood (1989); for column (C) the numbers are taken from the text on page 234 of Muir Wood (1989). Magnitude scales and assessments ward adjustment for some of the larger historical earth- quakes, with 5.6 for the 1759 Kattegat earthquake (Muir In an effort to correct for some of the above-mentioned Wood 1989; see also Kebeasy & Husebye 2003; Wahlström inconsistencies, a major reassessment of historical 2004), 5.8 for the 1819 Lurøy earthquake and 5.4 for the earthquakes in Norway was conducted by a UK-Norwe- 1904 Oslofjord earthquake. These adjustments demon- gian team (Muir Wood & Woo 1987; Muir Wood et al. strated that earlier estimates were too high, and the ques- 1988), building on a similar effort in the UK (Woo & tion now is whether a further downward adjustment of Muir Wood 1986; see also Ambraseys (1985a,b). The these magnitudes is needed, as claimed by H&K. approach adopted was to go back to the original and primary information as a basis for a complete reassess- Eqs. (1) to (4) can all be used for deriving magnitude ment of the most important historical earthquakes. estimates, and in order to offer some insights into sen- Magnitudes were tied to the well-defined MS scale, sitivities we provide in Table 1 an overview of the based on a correlation between MS and felt area derived results for the 1819 earthquake when combining the from a set of events for which both instrumental four relations with different estimates for felt areas (or records and intensity data were available. Since this is radii, given a circular shape). The three columns use important for the present discussion of the H&K paper, values taken from (A) a table in Muir Wood & Woo we quote here the relations developed by Muir Wood & (1987), (B) a figure in the same report (reproduced also Woo (1987) between MS and felt area AIII for intensity in Muir Wood, 1989), and (C) the text from Muir Wood level III, using a log-linear and a non-linear relation, (1989). The values are seen to be in the range 5.6 to 6.2. respectively: Note that a typographical error in Muir Wood (1989) has been corrected in that an RIV of 350 km in the . MS=–0.36 + 1.0 log AIII (1) paper should be 530 km (R. Muir Wood, pers. comm. 2004), which is now used in column (C). MS=0.95 + 0.69 log AIII + 0.0006 AIII (2) Before discussing the H&K paper we would like to offer For intensity level IV, the corresponding relations from some additional evidence supporting the Muir Wood & Muir Wood & Woo (1987) read: Woo (1987) felt area based magnitude scale. This was based on a correlation between instrumental and . MS=0.90 + 0.81 log AIV (3) macroseismic data, employing 27 AIII and 25 AIV values for instrumental MS values between 4.0 and 6.1. The . MS=1.57 + 0.63 log AIV + 0.0007 AIV (4) frequency-magnitude distributions based on this scale are stable and reasonable, and moreover, consistent The scatter (standard deviation) was not provided for with instrumental ML values derived for more recent these relations, but from the plots it appears to be at earthquakes, after accounting for the difference least 0.2 magnitude units, which is common for almost between the magnitude scales (Bungum et al. 1986; any regression on magnitudes. Muir Wood et al. 1988; Bungum & Selnes 1988; Bungum et al. 1998; Lindholm & Bungum 2000). It is noteworthy that this new magnitude scale based on felt Recently, there have been two new assessments of the areas for intensity levels III and IV led to a major down- magnitude for the 1904 Oslofjord earthquake, reported NORWEGIAN JOURNAL OF GEOLOGY The 31st of August 1819 Lurøy earthquake 247 observations and models that can be used in an inde- pendent assessment.
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