# Crustal stress & geodynamics

As part of my work at the Stanford Stress & Crustal Mechanics Group and the Stanford Center for Induced & Triggered Seismicity, I am building a high spatial resolution stress map of the central and eastern USA. We have compiled >600 new orientations of the maximum horizontal compression (*S*Hmax) as well as a map of the faulting regime (relative principal stress magnitudes). Our new data show a surprisingly variable stress field across parts of the region—and broadly consistent patterns in others—which provides an opportunity to understand the contribution of tectonic and local, geodynamic factors to the stress field.

In 2016, we published a paper in *Geophysical Research Letters *presenting our first 200 stress orientations. The results show a surprisingly variable but coherent stress field across the area. In 2018, we added some 100 more stress orientations in the prolific Permian Basin of west Texas and southeast New Mexico. Our stress map continues to grow.

The table below indicates the latest quality criteria for stress measurements, as published in the supplementary materials of Lund Snee and Zoback (2018). These are similar to those used by the World Stress Map (WSM), with some modifications, but these criteria newly include metrics for *S*Hmax orientations from aligned microseismic events defining hydraulic fractures (HFM) and for relative stress magnitudes (*A*ϕ) from formal focal mechanism inversions (FMF). I strongly discourage the use of individual focal mechanisms (FMS) as direct indicators of *S*Hmax orientation.

Stress Indicator* | A | B | C | |
---|---|---|---|---|

Drilling-Induced Tensile Fractures (DIF) | Ten or more distinct tensile fractures in a single well with standard deviation (sd) ≤ 12˚ and with highest and lowest observations at least 300 m apart | At least six distinct tensile fractures in a single well with sd ≤ 20˚ and with highest and lowest observations at least 100 m apart | At least four distinct tensile fractures in a single well with sd ≤ 25˚ and with highest and lowest observations at least 30 m apart | |

Focal Mechanism Inversions (FMF) | (Directions) | Formal inversion of ≥ 35 reasonably well-constrained focal mechanisms resulting in stress directions with sd ≤ 12º | Formal inversion of ≥ 25 reasonably well-constrained focal mechanisms resulting in stress directions with sd ≤ 20º | Formal inversion of ≥ 20 reasonably well-constrained focal mechanisms resulting in stress directions with sd ≤ 25º |

(Relative Magnitude, ϕ) | Formal inversion of ≥ 35 reasonably well-constrained focal mechanisms resulting in ϕ with sd ≤ 0.05 | Formal inversion of ≥ 25 reasonably well-constrained focal mechanisms resulting in ϕ with sd ≤ 0.1 | Formal inversion of ≥ 20 reasonably well-constrained focal mechanisms resulting in ϕ with sd ≤ 0.2 | |

Wellbore Breakouts (BO) | Ten or more distinct breakout zones in a single well (or breakouts in two or more wells in close proximity) with sd ≤ 12˚ and with highest and lowest observations at least 300 m apart | At least six distinct breakout zones in a single well with sd ≤ 20˚ and with highest and lowest observations at least 100 m apart | At least four distinct breakout zones in a single well with sd ≤ 25˚ and with highest and lowest observations at least 30 m apart | |

Microseismic Alignments Along Hydraulic Fractures (HFM) | Twelve or more distinct linear zones associated with HF stages, with sd ≤ 12˚ | Eight or more distinct linear zones associated with HF stages, with sd ≤ 20˚ | Six or more distinct linear zones associated with HF stages, with sd ≤ 25˚ | |

Shear Velocity Anisotropy from Crossed-Dipole Logs (SWA)† | Anisotropy ≥ 2% present at a consistent azimuth, with highest and lowest observations at least 300 m apart, and with sd of fast azimuth ≤ 12˚ | Anisotropy ≥ 2% present at a consistent azimuth, with highest and lowest observations at least 100 m apart, and with sd of fast azimuth ≤ 20˚ | Anisotropy ≥ 2% present at a consistent azimuth, with highest and lowest observations at least 30 m apart, and with sd of fast azimuth ≤ 25˚ |

*The shallowest measurement must be at least 100 m deep and also sufficiently deep that measurements are not affected by topography.

†In addition to anisotropy ≥ 2%, measurements should ideally have an energy difference between fast and slow shear waves ≥ 50% and a minimum energy ≥ 15%.