Publication Date:
2022-02-28
Description:
Identifying deformation and pre-failure mechanisms preceding faulting is key for fault
mechanics and for interpreting precursors to fault rupture. This study presents the results of a new
and robust derivation of first motion polarity focal mechanism solutions (FMS) applied to acoustic
emission (AE). FMS are solved using a least squares minimization of the fit between projected polarity
measurements and the deviatoric stress field induced by dilatational (T-type), shearing (S-type), and
compressional (C-type) sources. 4 × 10 cm cylindrical samples of Alzo Granite (AG, porosity 〈1%) and
Darley Dale Sandstone (DDS, porosity ≈14%) underwent conventional triaxial tests in order to investigate
the relationships between increasing confining pressure (5, 10, 20, and 40 MPa), deformation and failure
mode, and role of microstructural features. Results highlight that S-type events occur in very low numbers
with poor spatial correlation to fault structure. Instead, deformation is driven by a complex interplay
between compactant (C-type) and dilatant (T-type) regions of deformation. C-type events are the earliest
precursor related to crack nucleation and T-type events mark new cracks opening, with the onset of
fracture growth characterized by periodic cycles of coalescence. For AG a single sequence is able to
lead to dynamic failure, while for DDS several cycles are needed for coalescence to take place due to the
competition between dilatant and compactant deforming regions induced by multiple fracture nucleation
sites. The occurrence of C- and S-type events is also consistent with a quasi-static premonitory phase, or
foreshock, before a critical nucleation length allows the development of a planar localization.Identifying deformation and pre-failure mechanisms preceding faulting is key for fault
mechanics and for interpreting precursors to fault rupture. This study presents the results of a new
and robust derivation of first motion polarity focal mechanism solutions (FMS) applied to acoustic
emission (AE). FMS are solved using a least squares minimization of the fit between projected polarity
measurements and the deviatoric stress field induced by dilatational (T-type), shearing (S-type), and
compressional (C-type) sources. 4 × 10 cm cylindrical samples of Alzo Granite (AG, porosity 〈1%) and
Darley Dale Sandstone (DDS, porosity ≈14%) underwent conventional triaxial tests in order to investigate
the relationships between increasing confining pressure (5, 10, 20, and 40 MPa), deformation and failure
mode, and role of microstructural features. Results highlight that S-type events occur in very low numbers
with poor spatial correlation to fault structure. Instead, deformation is driven by a complex interplay
between compactant (C-type) and dilatant (T-type) regions of deformation. C-type events are the earliest
precursor related to crack nucleation and T-type events mark new cracks opening, with the onset of
fracture growth characterized by periodic cycles of coalescence. For AG a single sequence is able to
lead to dynamic failure, while for DDS several cycles are needed for coalescence to take place due to the
competition between dilatant and compactant deforming regions induced by multiple fracture nucleation
sites. The occurrence of C- and S-type events is also consistent with a quasi-static premonitory phase, or
foreshock, before a critical nucleation length allows the development of a planar localization.
Description:
Published
Description:
e2020JB021059
Description:
1T. Struttura della Terra
Description:
JCR Journal
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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