Mega Bc.zip
A megathrust earthquake is a very large earthquake that occurs in a subduction zone, a region where one of the earth's tectonic plates is thrust under another. The Cascadia subduction zone is located off the west coast of North America. From mid Vancouver Island to northern California the Juan de Fuca Plate is subducting beneath the North American Plate. The two plates are continually moving towards one another, yet become "stuck" where they are in contact. Eventually the build-up of strain exceeds the friction between the two plates and a huge megathrust earthquake occurs.
mega bc.zip
The recurrence time varies from subduction zone to subduction zone. In the Cascadia subduction zone 13 megathrust events have been identified in the last 6000 years, an average one every 500 to 600 years. However, they have not happened regularly. Some have been as close together as 200 years and some have been as far apart as 800 years. The last one was 300 years ago.
Megathrust earthquake are the world's largest earthquakes. The last Cascadia earthquake is estimated at magnitude 9. A megathrust earthquake in Chile in 1960 was magnitude 9.5, and one in Alaska in 1964 was magnitude 9.2.
The Cascadia fault, on which megathrust earthquakes occur, is located mostly offshore, west of Vancouver Island, Washington, and Oregon, although it does extend some distance beneath the Olympic Peninsula of Washington State. The large distance between the Cascadia fault and the urban centres limits the level of shaking that the urban areas are exposed to.
The sudden submergence of the outer coast when a megathrust earthquake occurs kills vegetation which can be dated. Megathrust earthquakes also cause underwater landslides off the continental shelf into the deep ocean. The landslide deposits can be recognized in core samples taken from the ocean floor.
No. Earthquake shaking, in the frequencies that damage buildings, increases to a maximum between a magnitude 7 and 8 earthquake, then the shaking simply involves a bigger area. However, the duration of shaking for a megathrust earthquake is much longer. It can be several minutes. This long duration can result in damage to some types of buildings that might not be damaged at the same strength of shaking produced by a smaller earthquake.
The Kobe earthquake was right beneath the city and the megathrust earthquake will be about 150 kilometres from Vancouver. The damage pattern would be very different. We can get a good example of the kinds of damage Vancouver can expect to experience if we look at what happened to Anchorage, Alaska, during the 1964 magnitude 9.2 megathrust earthquake. Anchorage is about the same distance from the Alaska subduction fault. Small buildings generally had little or no damage, unless they were affected by landsliding. Almost all the damage involved large buildings or large structures such as bridges.
No. Vancouver Island is part of the North American plate. The fact that there is water between Vancouver Island and the mainland is function of the current position of sea level. However, the west coast of Vancouver Island will drop as much as a metre or two when the next megathrust earthquake occurs.
New ideas and observational facts are natural parts of scientific development; and growing knowledge. When we are facing necessary revisions of older material, the quality of previous documentation plays a vital role. The archaeological material by Fagerlund and Hamilton [15] and Ullén [16], which we are revising below, both meet this high-quality criterion. The geological map descriptions are less useful, but includes reports of strange records of clay above gyttja clay [17] [18] [19] [20], which can now be understood in terms of off-shore re-working by the mega-tsunami here described (below).
The C14-dates obtained from Sites Q and R (Figure 14) give far too old ages, which implies that they both represent reworked and redeposited organic matter (the 13C values indicate brackish environment). The C14-date from Site S was 2396 29 C14-years BP or 455 50 cal. yrs BC, an age which corresponds to the climatic changes at the Sub-Boreal to Sub-Atlantic boundary; viz. a short period of intensive drought [31] followed by a general change to cooler and wetter climate conditions [32]. The site was isolated from the Baltic at about 1050 AD. A 2-point sedimentation rate graph is presented in Figure 15. It proposes that the tsunami ended at 1175 BC at the fine clay/gyttja clay boundary, an age which corresponds well with other dates of the mega-tsunami event (Appendix).
The tsunami at Site T (Figure 19) starts with erosion of the seabed surface followed by the deposition of 2.5 cm silt (bedload), 19.5 cm of gyttja and clay (a mixture of re-deposition of organic matter and setting of suspended particles) and 127.5 cm of fine clay (representing the settling of suspended particles). Gyttja 2-5 cm above the silt layer was C14-dated at 5113 31 BP or 3882 82 cal. yrs BC (indicating the re-deposition of older reworked material). This means that the upper 149.5 cm, in fact, represents one huge tsunamite divided into 3 parts of bedload transport and settling of suspended particles in a mega-graded-bedding unit (black arrow in Figure 19).
Theoretically, the tsunami might have reached all the way in to Örebro (Figure 4). With the new tool of a mega-tsunami event at about 3000 BP, it seems urgent also to revisit geological and archaeological sites in the entire Lake Mälaren region (Figure 4). Lake Hjälmaren has a present level of +21.8 m (drained by about 1.3 m in 1878-1887), so even this region might have been affected by the tsunami. We have investigated the map description and relevant literature of the area without finding any firm records of the Lake Mälaren 3000 BP mega-tsunami here described.
The recording of the mega-tsunami tsunami event seems now firmly fixed in Figure 29. It represents a tsunami wave height of at least 14.5 m, which implies that Lake Hjälmaren (+24 m) must also have been invaded by the tsunami. A record of this seems to occur in Ullavisjön (Magnusson, 1970), but we were not able to document this by coring at the site.
Sediments cores in the lake record a tsunami bed over a distance of at least 6 km. It is dated at 3055 70 BP or 1303 91 cal. yrs BC. Previously, it was interpreted as a local lake tsunami generated by a slide [23]. Now, it seems obvious that it was contemporaneous with the mega-tsunami event recorded in the whole of the Lake Mälaren valley at about 3000 BP.
As a part of an archaeological investigation, we also undertook a geo-tectonic analysis of the Tolan site and Sandaskogen area ( [24] and supplementary material to [11] ). Obviously, the area had suffered severe ground-shaking. In lake Sandakärret at +35.5 m, a tsunami bed was recorded (in 3 cores) consisting of unsorted and angular grains. It was dated at 3073 33 C14-yrs BP or 1315 85 cal. yrs BP (in full agreement with other dates of the mega-tsunami). In order to reach into the lake basin, the tsunami-wave need to have had a run-up of 20.5 m (which is the highest value recorded).
After a careful investigation of available data, it now seems possible to identify a significant tsunami event and provide an absolute date in terms of the Swedish varve chronology [42]. The effects of a mega-tsunami are revealed in the following three facts (as illustrated in Figure 32):
In conclusion, we think we can now provide an absolute annual age of the mega-tsunami at Ångermanälven of 1171 yrs BC or 3121 yrs BP. The C14-date in the varve above of 1306 87 cal. yrs BP represents redeposited organic matter set in suspension at the tsunami run-up (Appendix). Besides, we determine the run-up at about 14 m and the submarine bedload effects at about 10 m.
The sea level position at the time of the mega-tsunami is estimated from the known rate of uplift, giving an elevation of +15.5 m for HY2000, +15.2 m BP for 1950 (C14) and +15.0 m for HY00. Because the eustatic component is likely to have been slightly below present sea level, +15.0 m was chosen as likely sea level position at the time of the mega event. Back in time, the rate of isostatic uplift has increased, but for the last 3000 years it seems to have remained stable [47]. We may therefore, conclude that sea level 3121 years before HY200 is likely to have been at +15 m to +16 m. In this paper we used +15 m.
At about 3000 BP or 1200 cal. yrs BP something both unique and revolutionary happened: the Kaali impact occurred in Estonia (e.g. [49] ) and initiated violent ground shaking, methane venting tectonics and a mega-tsunami affecting the whole of the Baltic [2] [10] [11] [22]. In this paper, we have explored the tsunami effects in general and specifically in the Enköping area. We have combined geological and archaeological data, and, indeed, where ever we work, we find evidence of a major tsunami event at about 3000 BP when sea level was at about +15 m in the Stockholm area.
Figure 35 graph is quite unique in recording both the on-shore run-up values, and the submarine erosional/depositional effects. The tsunami deformations are symmetrically ordered in 13.5 m around the +15 m sea level at the time of the mega-tsunami event. This implies quite symmetrical waves (Figure 36) before the tsunami hits the coast and runs-up over the land surface causing erosion, deformation and re-deposition (as observed by coring, e.g. Figure 13, and reinterpretation of archaeological data; e.g. Figure 25, Figure 27).
This paper presents geological evidence suggesting the occurrence of a mega-tsunami in eastern Sweden around 3000 BP, more exactly 1171 BC. A primary question is whether this catastrophe is also traceable in the archaeological material of the Lake Mälaren region. For archaeologists, there are several key and methodological problems, not least: what is there to look for, where is such evidence found and what is important to document and analyze?
At a late stage (April-June, 2020), the Ångermanälven analysis (Section 5) was performed. Evidence of the occurrence of a mega-tsunami was established, and it was found that the tsunami must have occurred in varve-year 1171 BC or 3121 BP, an age which fits perfectly well with previous C14-dates of the tsunami event in Sweden (Appendix, [10] [11] and the age of the Kaali impact [10] [11] [49]. 041b061a72