A Comparison Between Modeling and Field Observations of Off-Fault Strain around Pseudotachylyte Fault Veins, Norumbega Shear Zone, southern Maine

Static stress changes caused by fault motion may be of significant magnitude around fault bends, ends, and intersections, and have been shown to partially explain aftershock distributions aftershocks (Poliakov et al., 2002). In the brittle-ductile transition zone, these stress concentrations may be relaxed after earthquakes by ductile flow. Small-scale deformation features adjacent to pseudotachylyte-filled fault veins may record deformation in response to static stress changes in the wallrock caused by slip on non-planar or intersecting fault surfaces. Where pseudotachylyte fault veins bend, the general deformation pattern between two pseudotachylyte veins was characterized by a simple pattern: when one fault bends away from the other, the deformation in between the two is characterized by mm- to cm-scale pseudotachylyte injection veins; where the faults are parallel to each other, the deformation style is characterized by ductile features. The pseudotachylyte photo-mosaic was then used in Coulomb3 software in order to build an idealized fault model. The rake and slip of the earthquakes which formed the pseudotachylytes are not known, so we assumed that the shear zone is primarily dextral, and we use the average scaling of normal earthquakes to make estimates of the slip. The predicted stress change orientation and magnitude distribution were produced by Coulomb3. We compare the stress change distributions from the models to the distribution of small near-fault structures mapped in the field. The distribution of stresses is similar to the distribution of shortening and extensional strain measured in the wallrock, therefore we conclude that static stress changes were accommodated plastically in the compressional region where the two faults propagated parallel to each other and in a brittle manner in the extensional region behind the rupture tip in the post-seismic time interval.