Advanced School on Non-linear Dynamics and Earthquake Prediction

Observations of intraclast breccia layers in the Dead Sea basin, formerly termed “mixed layers,” provide an exceptionally long and detailed record of past earthquakes and defi ne a frontier of paleoseismic research. Multiple studies of these seismites have advanced our understanding of the earthquake history of the Dead Sea and of the processes that form the intraclast breccias. In this paper, we describe a systematic study of intraclast breccia layers in laminated sequences. The relationship of intraclast breccia layers to intraformational fault scarps has motivated the investigation of these seismites. Geophysical evidence shows that the faults extend into the subsurface, supporting their potential association with strong earthquakes. We defi ne fi eld criteria for the recognition of intraclast breccias, focusing on features diagnostic of a seismic origin. The fi eld criteria stem from our understanding of the mechanisms of breccia formation, which include ground acceleration, shearing, liquefaction, water escape, fl uidization, and resuspension of the originally laminated mud. Comparison between a dated record of breccia layer and the record of historical earthquakes provides an independent test for a seismic origin. The historical dating is signifi cantly more precise and accurate than the radiocarbon dating of breccia layers. Yet, assuming that the lamination of the sediments shows an annual cycle, the precision of counting laminae may approach the precision of the historical record. A similar accuracy is then expected for the intervals between earthquakes. We review our work based on counting laminae representing the historical period, mutually corroborating the seismic origin and the annual lamination. The correlation of documented historical earthquakes with individual breccia layers provides quantitative estimates for the threshold of ground motion for breccia formation in terms of earthquake magnitude and epicentral distance. The investigation of breccia layers and the associated historical earthquakes has underscored cases in which a breccia layer represents a pair of earthquakes. We Agnon, A., Migowski, C., and Marco, S., 2006, Intraclast breccias in laminated sequences reviewed: Recorders of paleo-earthquakes, in Enzel, Y., Agnon, A., and Stein, M., eds., New frontiers in Dead Sea paleoenvironmental research: Geological Society of America Special Paper 401, p. 195–214, doi: 10.1130/2006.2401(13). For permission to copy, contact editing@geosociety.org. ©2006 Geological Society of America. All rights reserved. 196 A. Agnon, C. Migowski, and S. Marco spe 401-13 page 196

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