PALEOSIS Region II: The Upper Rhine Graben System

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Region II:
The Upper Rhine Graben System
(Alsace and Basel area)

1. Introduction

2. Methodological approach

3. Identfying trench sites

4.Regional effects of large paleo- earthquakes

4.1 Caves

4.2. Paleoliquefactions

5. Upcoming investigations

Authors references

Regional references

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4.1. Earthquakes records in endo-karst

In natural caves, strong ground motions may brake speleothems (stalagmites, stalactites) (Postpischl et al., 1991). A Recent workshop in Han/Lesse (Belgium) showed the influence of tectonics (vertical movements, eartquakes shaking) on the endo-karst system.This point of view needs to be supported by good observations and studies.

The study of growth anomalies of speleothems in karstic environment provides potential evidence for paleoearthquakes. These data are useful to study major earthquakes recurrence times in areas where evidence for historic seismicity is lacking. A preliminary study has been carried out in the epicentral area of the 1356 Basel earthquake (Io = VII-VIII, macroseismic magnitude = 6.2). The Bättlerloch and Dieboldslöchli caves, situated in the area of greatest damage, show growth anomalies of speleothems probably related to a seismic event. The first U/Th disequilibrum measurements by alpha spectrometry show recent ages (less than several tens of thousands of years and probably historic). ¹⁴C dating by AMS of carbonate laminations taken on both sides of the anomalies confirm the evidence of a seismic event around 1300 AD. More accurate dating by U/Th TIMS are presently carried out in order to compare the information brought by the two different dating methods.


Structural framework of western end of Blauen-Landskorn anticline and location of the Bättlerloch et Dieboldslöchli caves (structural features after Bitterli-Brunner et Fischer, 1988)	Geological cross-section of Blauen anticline and Frontal Thrusting of Jura (after Bitterli-Brunner and Fischer, 1988). Location of the Dieboldslöchli cave.

Transversal cross sections of the sample ECH3 and ECH4 taken on the fault plane in the room of the Dieboldslöchli cave. The stalagmitic growths, few centimetres long, seal the break attesting to a sudden break of the stalagmitic stump. The growth axis of the re-growths is suddenly displaced of some centimetres upward on the fault plane. Sampling locations for dating and dating results are indicated on sample ECH3.	Sketch illustrating the location of the samples in the Dieboldslöchli cave. The shear of the stalagmites could be caused by a seismic shake and the offset of the growth axis could indicate a reverse movement of some centimetres on the fault plane.