Seismic slip direction deduced from the magnetic fabric of frictional melts

Abstract:

The direction of seismic slip constitutes an important focal mechanism parameter particularly for ancient-prehistoric earthquakes. Here I show that in most cases, the magnetic fabric of fault pseudotachylytes indicates the direction of coseismic slip as a result of the dominant simple shear nature of frictional melt flow. The magnetic fabric arises from the shape or crystallographic preferred orientation of magnetically anisotropic minerals (paramagnetic silicates or ferromagnetic oxides) in a viscous medium. This fabric, acquired in a manner similar to that of other magmas, becomes rapidly frozen as the frictional melt is quenched. 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 on prehistoric seismic events.