Source Mechanisms of Laboratory Earthquakes During Fault Nucleation and Formation

Identifying deformation and pre-failure mechanisms preceding faulting is key for fault mechanics interpreting precursors to rupture. This study presents the results of a new robust derivation first motion polarity focal mechanism solutions (FMS) applied acoustic emission (AE). FMS are solved using least squares minimization fit between projected measurements deviatoric stress field induced by dilatational (T-type), shearing (S-type), compressional (C-type) sources. 4 × 10 cm cylindrical samples Alzo Granite (AG, porosity <1%) Darley Dale Sandstone (DDS, ?14%) underwent conventional triaxial tests in order investigate relationships increasing confining pressure (5, 10, 20, 40 MPa), failure mode, role microstructural features. Results highlight that S-type events occur very low numbers with poor spatial correlation structure. Instead, driven complex interplay compactant dilatant (T-type) regions deformation. C-type earliest precursor related crack nucleation T-type mark cracks opening, onset fracture growth characterized periodic cycles coalescence. For AG single sequence able lead dynamic failure, while DDS several needed coalescence take place due competition deforming multiple sites. The occurrence C- also consistent quasi-static premonitory phase, or foreshock, before critical length allows development planar localization.