done. If an 11 m height is taken as wave height, we would be dealing with a ground-shaking magnitude of M 7.9 by applying Figure 1 relations. The situation is quite similar to that of the 2900 BP event (Figure 5), in basic structure as well as in tsunami height.

4. Discussion

Figure 1 relations between tsunami wave height and earthquake magnitude [3] [4] implies an improved and simplified tool of converting observed tsunami wave height into corresponding earthquake magnitude as compared to preceding graphic relations [23] .

Sweden has a database of 17 postglacial tsunami events [10] [12] [14] . The wave heights documented is consistent with a very high seismicity, not only in Late Glacial time, but also in Mid-Holocene and Late Holocene time (Figure 1).

Two paleoseismic events were selected for a test of the significance of Figure 1 relations established; viz. the 10,430 vBP and the 9663 vBP events, because both of these event were established by means of a multiple criteria [10] [11] , besides they both represent very strong events with well-established tsunami heights. This implies the comparison between multiple independent parameters. The data referring to the 10,430 vBP event are consistent with a mega-event of a magnitude of about M 8.5 - 9.0 (Figure 3) and the data referring to the 9663 vBP event to an event of magnitude of M 8.4 - 8.5 (Figure 4).

In the Late Holocene, there were two major tsunami events (Figure 1). Both of those events were generated by methane venting tectonics, however [10] [20] . Hence, there is no straightforward application of Figure 1 relations. In order to obtain some sort of quantification of the forces involved and magnitude of the ground shaking, Figure 5 was drawn. The tsunami height of the 2900 BP event is consistent to a magnitude (rather magnitude equivalent) ground shaking of M 8.0, which seems to harmonize with the violence of the structures observed (Figure 5). The 3000 - 4000 BP event from the Stockholm area give a similar picture.

The process of methane venting tectonics [20] has by this (Figure 5) obtained a first serious quantification as to corresponding ground shaking, and both events were found to be consistent with a magnitude M 8.0 equivalent. This makes it a very dangerous factor for long-term stability and must be considered seriously in hazard assessment [5] [11] .

5. Conclusions

A new tool for the conversion of observed tsunami heights to corresponding causation earthquake magnitude is presented (Figure 1).

It is quite successfully tested on multiple independent magnitude estimates from two paleoseismic events in Sweden (Figure 3, Figure 4). The general agreement is very good.

Methane venting tectonics is a novel process of severe bedrock deformation [10] [20] . Big tsunami events were generated at an event occurring 2900 BP (the Skålboberget event in central Sweden) and at 3000 - 4000 BP (the Kvarnberget event in the Stockholm area). The tsunami heights documented correspond to an earthquake equivalent magnitude of about M 8.0 (Figure 5), indicating that methane venting tectonics implies violent deformation and ground shaking equivalent to high-magnitude seismic events.

Cite this paper

Mörner, N.-A. (2017) Converting Tsunami Wave Heights to Earthquake Magnitudes. Open Journal of Earthquake Research, 6, 89-97. https://doi.org/10.4236/ojer.2017.62005

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