Focal mechanism of prehistoric earthquakes deduced from pseudotachylyte fabric

Fault pseudotachylytes form by frictional melting during seismic slip and therefore are widely interpreted as "earthquake fossils." Rapid movement along a rupture surface typically forms a pseudotachylyte generation vein, the thickness of which increases with earthquake magnitude. The dire...

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Bibliographic Details
Published in:Geology (Boulder) 2015-06, Vol.43 (6), p.531-534
Main Authors: Ferré, Eric C, Geissman, John W, Chauvet, Alain, Vauchez, Alain, Zechmeister, Matthew S
Format: Article
Language:English
Subjects:
active faults
Alps
Cottian Alps
displacements
Dora Maira Massif
Earth Sciences
Earthquakes
Europe
fabric
Fabrics
Fault lines
fault planes
faults
focal mechanism
Geological faults
Italy
Kinematics
magnitude
metamorphic rocks
mineral composition
mylonites
Orientation
paleoseismicity
Piedmont Alps
Piemonte Italy
Prehistoric
pseudotachylite
Rocks
Sciences of the Universe
Seismology
Slip
slip rates
Southern Europe
stress fields
structural analysis
Structural geology
Tectonics
textures
Veins
Western Alps
Western Europe
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Summary:Fault pseudotachylytes form by frictional melting during seismic slip and therefore are widely interpreted as "earthquake fossils." Rapid movement along a rupture surface typically forms a pseudotachylyte generation vein, the thickness of which increases with earthquake magnitude. The direction and sense of seismic slip cannot always be determined due to the generally complex geometry of pseudotachylyte veins. Here we show, for the first time, that the orientation of the magnetic fabric of fault pseudotachylytes indicates both direction and sense of seismic slip. The magnetic fabric, acquired in a manner similar to that of other magmas, arises in this case from the asymmetric preferred orientation of paramagnetic grains during viscous shear of the friction melt. This kinematic information, coupled with fault plane orientation and generation vein thickness, provides new and critical insight for the earthquake focal mechanism. The magnetic fabric of pseudotachylytes therefore not only constitutes a valuable kinematic criterion for these fault rocks, but also could expand our knowledge of prehistoric seismic events.
ISSN:0091-7613
1943-2682
DOI:10.1130/G36587.1