Difference between revisions of "Patterns in seismology and palaeoseismology, and their application in long-term hazard assessments - the Swedish case in view of nuclear waste management"

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In the autumn of varve 10 430 vBP (∼ 10 000 cBP), a very strong seismic event occurred in the Stockholm and Mälardalen Valley (Mörner, 2003, 2008, 2011). It is recorded by multiple different methods and closely dated at one single varve, and even to the autumn sequence of this varve (Mörner, 2003, 2013a). A combined picture is given in Fig. 6, which allow us to assign an intensity of XII and magnitude of > 8 (Mörner, 2011).
 
In the autumn of varve 10 430 vBP (∼ 10 000 cBP), a very strong seismic event occurred in the Stockholm and Mälardalen Valley (Mörner, 2003, 2008, 2011). It is recorded by multiple different methods and closely dated at one single varve, and even to the autumn sequence of this varve (Mörner, 2003, 2013a). A combined picture is given in Fig. 6, which allow us to assign an intensity of XII and magnitude of > 8 (Mörner, 2011).
 +
 +
[[|center|'''Figure 6.''' The 10430 vBP event documented by nine different characteristics (Mörner, 2011).]]
  
 
Another palaeoseismic mega-event occurred in varve 9663 vBP (9150 C14 yr BP). It is documented in a similar way by multiple methods (Mörner et al., 2000; Mörner, 2003, 2008, 2011, 2013a; Mörner and Dawson, 2011). The intensity is estimated at XII and the magnitude at > 8. The characteristics are given in Fig. 7. It represents one of the most well recorded palaeoseismic events in the world.
 
Another palaeoseismic mega-event occurred in varve 9663 vBP (9150 C14 yr BP). It is documented in a similar way by multiple methods (Mörner et al., 2000; Mörner, 2003, 2008, 2011, 2013a; Mörner and Dawson, 2011). The intensity is estimated at XII and the magnitude at > 8. The characteristics are given in Fig. 7. It represents one of the most well recorded palaeoseismic events in the world.
Line 201: Line 203:
  
  
 +
==== Combination with explosive methane venting tectonics ====
 +
Methane occurs in nature in the form of gas or, in sediments and bedrock, in the form of ice (methane hydrate or clathrate). The volumetric relation between the ice and gas phases is 1 : 168, which implies a very large expansion when ice transform into gas. The transition is phase-boundary controlled by temperature and pressure (Mörner, 2011, Fig. 12).
 +
 +
During periods of permafrost, the geothermal gradient is strongly deformed towards colder conditions allowing for methane ice to be formed higher up in the bedrock. During ice ages, the vertical pressure is strongly deformed and methane ice may be formed all the way up to the surface wherever there are voids.
 +
 +
During the postglacial period after an ice age, temperature increases and pressure decreases due to land uplift. Both these processes will affect the stability of an accumulation in
 +
the bedrock of methane ice. The ice/gas transition is instantaneous. Consequently, the chances are high that this process will lead to an explosive venting of methane gas (Fig. 8).
 +
 +
This is precisely what we have found in our studies in Sweden (Mörner, 2003, 2011; Mörner and Sjöberg, 2011). The seepage of methane gas through the varved clay left spots of precipitated carbonate (isotopically linked to the deglaciation) and ending in the varve at the sea floor of the 9663 vBP palaeoseismic event (Mörner, 2003, p. 289–294), hence linking this palaeoseismic event (Fig. 7) to a simultaneous transformation of methane ice stored in the bedrock into gas seeping to the sea floor surface. This gas seepage is recorded in numerous cores and sited over a distance of 200 km from Hudiksvall to Uppsala (Mörner, 2003).
 +
 +
In association with the 9663 vBP event, there are, besides the seismotectonic effects, also records of severe deformation due to explosive methane venting. This is especially clear in the case of the Boda Cave (an old bedrock hill now fractured into a big field of fractured blocks with 2600 m cave passages in the subsurface). The surface and subsurface are fractured into big blocks, indicating an initial phase of extension followed by a period of contraction (falling back). This deformation is organized around 12 separate centra (Mörner, 2003). We believe that these centra represent centra of methane venting.
  
  

Revision as of 19:38, 29 November 2013

First publication of this report in Pattern Recognition in Physics (PRP)

Pattern Recogn. Phys., 1, 75–89, 2013
www.pattern-recogn-phys.net/1/75/2013/
doi:10.5194/prp-1-75-2013
©Author(s) 2013. CC Attribution 3.0 License.
N.-A. Mörner
Paleogeophysics & Geodynamics, Rösundavägen 17, 13336 Saltsjöbaden, Sweden
Correspondence to:
N.-A. Mörner (morner AT pog.nu[1])
Received: 12 May 2013 – Accepted: 1 July 2013 – Published: 24 July 2013


Figure 1. Means of recording seismic events over past and present time (from Mörner, 2011). For meaningful seismic hazard assessments, the seismic records must be combined with palaeoseismic analyses. Meaningful long-term predictions must be based on palaeoseismic data of high quality and completeness.
Figure 2. Dimensionless sketch (from Mörner, 1995) illustrating the remarkable long-term prediction by KBS (now: SKB) who analysed the seismicity during 25 yr, 1951–1975 (blue line), and made predictions for 100 000 to 1 million years (A: green line; KBS, 1983). Mörner (1995; B observation) showed that seismic energy release changed drastically a century ago and that Sweden was a highly seismic area during the time of deglaciation some 10 000 yr ago. Furthermore, the seismic activity increased drastically in the mid-1980s (invalidating the green line extrapolation). This indicates that palaeoseismology is the key to serious long-term seismic hazard prediction.
Figure 3. The postglacial uplift cone of Fennoscandia (yellow) with a surrounding subsidence trough (blue). Isobases give uplift and subsidence in metres over the last 15 000 yr. At the time of deglaciation the rate of uplift peaked with values 10 times higher than that of plate tectonics. The vertical uplift was linked to horizontal extension in the radial and tangential directions.
Figure 4. Cumulative distribution over time of the 59 events in the Swedish Paleoseismic-logical Catalogue (Appendix A; Mörner, 2003, 2011). There is a distinct peak in the period 11 000–9000 BP, which is the time of the peak rate of glacial isostatic uplift (left curve).
Figure 5. Distribution of palaeoseismic events in Sweden; partly in time (top left; where 50 % of the events fall in the period of maximum rates of uplift; cf. Fig. 4) and partly by area where recurrence diagrams have been constructed for five individual areas (including 44 events, or 71 % of all the events). Blue dots refer to simultaneous tsunami events.

Abstract

Seismic events are recorded by instruments, historical notes and observational criteria in geology and archaeology. Those records form a pattern of events. From these patterns, we may assess the future seismic hazard. The time window of a recorded pattern and its completeness set the frames of the assessments. Whilst instrumental records in seismology only cover decades up to a century, archaeoseismology covers thousands of years and palaeoseismology tens of thousands of years. In Sweden, covered by ice during the Last Ice Age, the palaeoseismic data cover some 13 000 yr. The nuclear industries in Sweden and Finland claim that the high-level nuclear waste can be buried in the bedrock under full safety for, at least, 100 000 yr. It seems hard, if on the whole possible, to make such assessments from the short periods of pattern recognition in seismology (< 100 yr) and palaeoseismology (∼ 13 000 yr). All assessments seem to become meaningless, maybe even misleading. In this situation, we must restrict ourselves from making too optimistic an assessment. As some sort of minimum level of the seismic hazard, one may multiply the recorded seismic hazard over the past 10 000 yr by 10, in order to cover the required minimum time of isolation of the toxic waste from the biosphere of 100 000 yr.



Introduction

In China, we have a historical documentation of seismic events for the last 4250 yr (Wang, 1987; Mörner, 1989). This is, of course, quite unique. In most other areas, we have fragments recorded over a few hundred years, or so. The recording by permanent instruments (seismographs) began at the end of the 19th century. This means that instrumental records are limited to the last half-century to the last century.

The seismic destruction of monuments (i.e. archaeoseismology) goes back a few thousands of years (e.g. Sintubin, 2011; Stiros and Jones, 1996). Sometimes, mythology may provide information of past seismic events (Piccardi and Masse, 2007).

Palaeoseismology (or earthquake geology) refers to seismic effects as recorded in geology (McCalpin, 2009; Mörner, 2003; Reicherter et al., 2009; Silva et al., 2011). This implies that the records may date to any part of the geological time. With respect to seismic hazard assessment, we must, however, focus on the continual records of the Late Quaternary, especially the Holocene (e.g. Rockwell, 2010).

When we make seismic hazard assessments, there must be some sort of relation between the time frame of observation and the time frame of extrapolation. Seismology is good for assessment of the hazard of the near future – i.e. decades up to a century. When we are forced to make assessments for a longer time period, we have to rely on palaeoseismology (Fig. 1).

Figure 1. Means of recording seismic events over past and present time (from Mörner, 2011). For meaningful seismic hazard assessments, the seismic records must be combined with palaeoseismic analyses. Meaningful long-term predictions must be based on palaeoseismic data of high quality and completeness.

In the case of nuclear waste handling, the time span needed to be covered by seismic hazard assessment increases to the immense period of 100 000 yr or more (Mörner, 2012a). The question is whether we, by this, have not extended our predictions "in absurdum" (Mörner, 2001). The present paper will address this question. IAEA (2010) has tried to establish criteria and recommendations for seismic hazard assessment in association with nuclear power installations. The focus is on the installation of nuclear power plants. It seems relevant that they firmly state that the assessment must be based on "the use to the greatest possible extent of the information collected" ("available", I would add, in this case, when the nuclear power industries take the liberty to discriminate among available observational facts), and "in accordance with the nature and complexity of the seismotectonic environment" (in this case Fennoscandia). Furthermore, they state the following:

The size of the relevant region may vary, depending on the geological and tectonic setting, and its shape may be asymmetric in order to include distant significant seismic sources of earthquakes. Its radial extent is typically 300 km. In intra-plate regions, and in the particular case of investigations into the potential for tsunamis, the investigations may need to consider seismic sources at very great distances from the site.


Results and records

In this paper we will address the situation in Sweden, because it is in Sweden and Finland (and only in those two countries) where the nuclear industry has claimed that they have a method of final deposition of the high-level waste in the bedrock that will stay intact for "at least, 100 000 years" (as claimed in Sweden) or "for 1 million years" (as claimed in Finland).

This calls for seismic hazard assessments. Because of the immense time period, we cannot apply normal means of assessment, however (Mörner, 2001, 2012c). The "methods" used are the following:

  1. Claiming what you want to claim without backing it up by observational records. Basically this is what the nuclear industry is doing.
  2. Analysing the present seismic records (in Sweden, the nuclear industry selected the time window 1951–1976) and making straight-line extrapolations into the distant future (KBS, 1983; La Pointe et al., 2000).
  3. Analysing the palaeoseismic records over the last 13 000 yr and multiplying the records for the time period of 100 000 yr (Mörner, 2003, 2011, 2012a).
  4. Substituting observations for computer modelling of the changes in stress and strain in the bedrock (Lambeck, 2005; Lund et al., 2009).
  5. Claiming that we cannot undertake serious and meaningful assessments over such time periods (Mörner, 2001).


Claiming what you want

In the absence of adequate background data for a serious seismic hazard assessment, it may be tempting to give up, and simply claim what you want to claim. Having started nuclear power without first having solved the handling of the high-level nuclear waste puts an "a posteriori" pressure of "solving" the waste management. One way out of the dilemma was simply to claim that "all works well" and "the waste can be stored under full safety for 100 000 yr" (KBS, 1983; SKB, 2011; Posiva, 2012). This is to substitute science for hope.


Using seismological records

Sweden has an excellent seismological record from the beginning of the 19th century (Båth, 1978). Both Sweden and Finland have low to moderately low seismic activity today, whilst it at the time of deglaciation some 10 000 yr ago was high to very high (Mörner, 1985, 2003, 2011). A seismologic pattern of events achieved from a time window as short as decades can never provide meaningful seismic hazard assessments for longer-term time periods (as illustrated in Fig. 1). Still, this has been done both in Sweden and Finland.

KBS (1983) analysed a 25 yr period (1951–1976) and made predictions for the next 105 –106 yr. This is, of course, to violate the rules of serious hazard assessments. Still, this is what was done. Worse was that it came to form the illusive idea that seismic activity will not pose any problem for a safe deposition in the bedrock for, at least, 100 000 yr (SKB, 2011). In Finland, similar views were expressed (e.g. Posiva, 2012), and they even claimed that the safety would last for 1 million years (STUK, 2011).

Assessing their methodology, the present author extrapolated their seismic activity line backwards (Mörner, 1995) and noted that there was a major change in the level of energy release some 50 yr back in time and the occurrence of high-magnitude palaeoseismic events some 10 000 yr ago (Fig. 2). Furthermore, in the mid-1980s, a high number of events occurred which violated the extrapolation applied by KBS (1983). This should, of course, have invalidated the methodology applied by the nuclear power industries in Sweden and Finland.

Figure 2. Dimensionless sketch (from Mörner, 1995) illustrating the remarkable long-term prediction by KBS (now: SKB) who analysed the seismicity during 25 yr, 1951–1975 (blue line), and made predictions for 100 000 to 1 million years (A: green line; KBS, 1983). Mörner (1995; B observation) showed that seismic energy release changed drastically a century ago and that Sweden was a highly seismic area during the time of deglaciation some 10 000 yr ago. Furthermore, the seismic activity increased drastically in the mid-1980s (invalidating the green line extrapolation). This indicates that palaeoseismology is the key to serious long-term seismic hazard prediction.

The Swedish nuclear industry (SKB, 2011) still relies on the extrapolation of present-day seismic activity (La Pointe et al., 2000), which, they claim, can as a maximum only generate 1 magnitude 6 (M6) event in 100 000 yr (or 1 M7 event in 1 million years) in the region of the proposed nuclear waste repository at Forsmark. This is strongly contradicted by our observational facts covering the last 13 000 yr (Mörner, 2003, 2011).

Figure 3. The postglacial uplift cone of Fennoscandia (yellow) with a surrounding subsidence trough (blue). Isobases give uplift and subsidence in metres over the last 15 000 yr. At the time of deglaciation the rate of uplift peaked with values 10 times higher than that of plate tectonics. The vertical uplift was linked to horizontal extension in the radial and tangential directions.

Using the palaeoseismic records

With the introduction of palaeoseismological studies in Sweden, it became evident that the seismic activity had changed considerably back in time (e.g. Mörner, 1985, 1991, 2003, 2011; Mörner et al., 2000), primarily as a function of the very high rates of glacial isostatic uplift at the time of deglaciation some 10 000 yr ago. The uplift of Fennoscandia has the form of a somewhat skewed cone (Fig. 3) with a maximum uplift of 800 m in 13 000 yr. This means very high vertical rates of uplift, which exceed the maximum horizontal plate motions by a factor of ten. At the same time, the horizontal extensional forces in the radial and tangential directions are high, giving strain rates up to two ten potencies higher than today. Therefore, it is not surprising that Fennoscandia, at the time of deglaciation, was a highly seismic area (Fig. 4), despite the fact that it represents an intra-plate shield area.

Figure 4. Cumulative distribution over time of the 59 events in the Swedish Paleoseismic-logical Catalogue (Appendix A; Mörner, 2003, 2011). There is a distinct peak in the period 11 000–9000 BP, which is the time of the peak rate of glacial isostatic uplift (left curve).

The "seismic landscape" of Fennoscandia at the time of deglaciation is characterized by frequent high-magnitude palaeoseismic events (Figs. 4–5, Table 2, Appendix A). Mörner (2011) recorded 7 high-magnitude events during 102 yr from varve year 10 490 to 10 388 BP. Several events must have reached a magnitude of 8 to > 8 (Mörner, 2011). Active faults occur all from northernmost to southernmost Sweden (Mörner, 2004). In northernmost Fennoscandia, there are a number of very large and long fault scarps denoting high-magnitude events (or repeated movements) – i.e. in Norway (Olesen, 1988; Dehls et al., 2000), Sweden (Lundqvist and Lagerbäck, 1976; Bäckblom and Stanford, 1989; Lagerbäck, 1990) and Finland (Kujansuu, 1964) as shown on the map of “active tectonics and postglacial palaeoseismics” (Mörner, 2004, Fig. 5)

Table 1. Maximum earthquake magnitudes by time and methodology.
Methodology Time unit Maximum magnitude
Seismology last 100 yr < 4.8
Historical data last 600 yr < 5.5
Late Holocene PS last 5000 yr >> 6 to ∼ 7
Deglacial phase PS 9–11 000 yr BP >> 8

In palaeoseismology, we study both primary structures (i.e. faults and fractures in direct association to the epicentre) and secondary effects from the ground shaking (i.e. rock and sediment slides, sediment deformations, liquefaction, tsunami events, turbidites, magnetic grain rotation, etc.). By means of different established criteria (Wells and Coppersmith, 1994; Reicherter et al., 2009; Silva et al., 2011), we may assess seismic intensity and magnitude. A key factor is dating. The varve chronology in Sweden offers an exceptional means of dating seismic events to one single varve year, in a few cases even to the season of a year (Mörner, 2003, 2011, 2013a).

Figure 5. Distribution of palaeoseismic events in Sweden; partly in time (top left; where 50 % of the events fall in the period of maximum rates of uplift; cf. Fig. 4) and partly by area where recurrence diagrams have been constructed for five individual areas (including 44 events, or 71 % of all the events). Blue dots refer to simultaneous tsunami events.

With increasing time units, the maximum earthquake magnitude increases dramatically from below 4.8 to well above 8 (Table 1). This implies that we can only achieve a meaningful long-term hazard assessment if the palaeoseismic records (PS) of past earthquakes are included.

Table 2. Time/magnitude distribution of palaeoseismic events in Sweden.
Time unit M5–6 M6–7 M7–8 M>8 Total
∼ 30 000 2 1 - - 3
12 000–13 000 - 1 - 1 2
11 000–12 000 - - 2 - 2
10 000–11 000 - 11 2 2 15
9000–10 000 2 6 4 3 15
8000–9000 1 2 1 - 4
7000–8000 1 4 - - 5
6000–7000 - - 1 1 2
5000–6000 - - 1 - 1
4000–5000 - 4 - - 4
3000–4000 - 3 - - 3
2000–3000 1 3 - - 4
1000–2000 1 - - - 1
< 1000 - 1 - - 1
Events in total: 8 35 12 7 62
13 000–9000 3 20 9 6 38
8000–0 3 15 2 1 21


Pattern recorded in Swedish palaeoseismic events

The main catalogue of palaeoseismic events in Sweden (Mörner, 2003, p. 301–308) used to include 52 separate events. A few additional events have been added (e.g. Mörner, 2008) and today the catalogue includes a total of 62 events (Appendix A). Their temporal distribution is given in Fig. 5. There is a clear maximum at the period of maximum glacial isostatic uplift (Fig. 4).

In five areas, it was possible to establish recurrence diagrams (Fig. 5; cf. Mörner, 2003, p. 310). In four of these areas (the Mälardalen–Stockholm region, the Forsmark region, the Hudiksvall region and the Umeå region), there is a clear effect of the isostatic uplift, generating high-frequency events during the early deglacial phase. In the Stockholm region seven events were recorded and dated by varves during a period of 102 yr (Mörner, 2011). This implies very high seismic activity. Also the magnitudes were exceptionally high with events even exceeding M8 (Table 1; Mörner, 2003, 2012). Adams (2005) showed that the distribution of events followed the international relations established by Wells and Coppersmith (1994).


Mode of recording

Because of the land uplift, former sea beds and sea bed sediments are now exposed on land where much more detailed investigations can be applied than in a shelf position (Mörner, 2009a). Thanks to this, it was possible to combine multiple criteria in the identification of individual palaeoseismic events (Mörner, 2003, 2011). Furthermore, the glacial clay and silt is annually varved allowing for dating with an annual time resolution (Mörner, 2013a).

In the autumn of varve 10 430 vBP (∼ 10 000 cBP), a very strong seismic event occurred in the Stockholm and Mälardalen Valley (Mörner, 2003, 2008, 2011). It is recorded by multiple different methods and closely dated at one single varve, and even to the autumn sequence of this varve (Mörner, 2003, 2013a). A combined picture is given in Fig. 6, which allow us to assign an intensity of XII and magnitude of > 8 (Mörner, 2011).

[[|center|Figure 6. The 10430 vBP event documented by nine different characteristics (Mörner, 2011).]]

Another palaeoseismic mega-event occurred in varve 9663 vBP (9150 C14 yr BP). It is documented in a similar way by multiple methods (Mörner et al., 2000; Mörner, 2003, 2008, 2011, 2013a; Mörner and Dawson, 2011). The intensity is estimated at XII and the magnitude at > 8. The characteristics are given in Fig. 7. It represents one of the most well recorded palaeoseismic events in the world.

Even in the Late Holocene, when the main glacial isostatic uplift was virtually over, there are events recorded by multiple methods indicating an intensity of X and a magnitude of 7 (Mörner, 2009b, 2011). This is important as it increases the seismic hazard from the present-day maximum of M4.8 to M7 (Table 1; cf. Fig. 2).

The total number of palaeoseismic events recorded today is 62. Their temporal distribution is as follows: 3 events at around 30 000 BP, 38 events during the deglacial period 13 000–8000 BP (i.e. 61 %), and 21 events in the last 8000 yr with 13 in the last 5000 yr. Their magnitude distribution is given in Table 2.

In a temporal context, this implies that the maximum magnitudes recorded grow from 4.8 by recent instrumental cover in the following manner back in time: after 109 yr a M5.4 event is observed and recorded, after about 900 yr the first M ∼ 7 event is recorded by palaeoseismology, after 6100 yr the first M8 event occurs, and in the period 9000–11 000 BP several high-magnitude events are recorded indicating that Sweden by then was a highly seismic region due to the exceptionally high rates of uplift (Mörner, 1991, 2003, 2011).

This is the database (Table 2, Fig. 5, Appendix A) upon which we have to base our estimates on future seismic hazards. The seismological database is far to short and non-representative to be used for long-term predictions (as illustrated in Fig. 2).


Combination with explosive methane venting tectonics

Methane occurs in nature in the form of gas or, in sediments and bedrock, in the form of ice (methane hydrate or clathrate). The volumetric relation between the ice and gas phases is 1 : 168, which implies a very large expansion when ice transform into gas. The transition is phase-boundary controlled by temperature and pressure (Mörner, 2011, Fig. 12).

During periods of permafrost, the geothermal gradient is strongly deformed towards colder conditions allowing for methane ice to be formed higher up in the bedrock. During ice ages, the vertical pressure is strongly deformed and methane ice may be formed all the way up to the surface wherever there are voids.

During the postglacial period after an ice age, temperature increases and pressure decreases due to land uplift. Both these processes will affect the stability of an accumulation in the bedrock of methane ice. The ice/gas transition is instantaneous. Consequently, the chances are high that this process will lead to an explosive venting of methane gas (Fig. 8).

This is precisely what we have found in our studies in Sweden (Mörner, 2003, 2011; Mörner and Sjöberg, 2011). The seepage of methane gas through the varved clay left spots of precipitated carbonate (isotopically linked to the deglaciation) and ending in the varve at the sea floor of the 9663 vBP palaeoseismic event (Mörner, 2003, p. 289–294), hence linking this palaeoseismic event (Fig. 7) to a simultaneous transformation of methane ice stored in the bedrock into gas seeping to the sea floor surface. This gas seepage is recorded in numerous cores and sited over a distance of 200 km from Hudiksvall to Uppsala (Mörner, 2003).

In association with the 9663 vBP event, there are, besides the seismotectonic effects, also records of severe deformation due to explosive methane venting. This is especially clear in the case of the Boda Cave (an old bedrock hill now fractured into a big field of fractured blocks with 2600 m cave passages in the subsurface). The surface and subsurface are fractured into big blocks, indicating an initial phase of extension followed by a period of contraction (falling back). This deformation is organized around 12 separate centra (Mörner, 2003). We believe that these centra represent centra of methane venting.



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