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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS006 Oral Presentation 1930
Magnetic fabric of earthquake-triggered structures
Dr. Shmuel Marco
Geophysics and Planetary Sciences Tel Aviv University IASPEI
The microscopic fabric of clastic sediments reflect their transport, deposition, and deformation history.
We characterize the fabric of clastic dikes and seismites layers by measuring the anisotropy of magnetic
susceptibility (AMS). We assume that different sources of dike fill have different microfabrics, which
manifest in different magnetic fabrics. We studied over 250 Holocene clastic dikes, exposed by deep
fluvial incision into the lacustrine 70-15 ka Lisan Formation in the Dead Sea basin. Typically, they are
vertical, up to 30-40 m high, and up to 0.4 m thick. In map view they appear in a radial arrangement
spanning a sector of 60 degrees with projected strikes that converge at a structural dome above a rising
salt diapir. Field relations and AMS analyses show that 'passive' dikes filled from above have vertical
Kmin directions, compatible with sedimentary features. Conversely, horizontal to sub-horizontal Kmin
directions occur in dikes that show segmentation typical of horizontal propagation of the fractures and
lateral material transport. Vertical zoning of the clay and silt symmetric about the center of the dikes is
regarded as evidence of multiple injection events. We conclude that the upward indentation of a salt
diapir induced a local stress perturbation, which triggered the formation of a radial set of fractures.
Subsequently the fractures were filled by horizontal injection of fine clastics pressurized by earthquake
vibrations. Sedimentation from above occurred where the tops of the dikes reached the surface and
remained open.
Keywords: earthquakes, magnetic susceptibility, clastic dikes