Stress Regime Assignment


The stress magnitudes are defined using the standard geologic/geophysical notation with compressive stress positive and S1>S2>S3, so that S1 is the maximum and S3 the minimum principal compressive stress.

The following categories are used to describe the tectonic stress regime:

NF: Normal faulting

NS: Predominately normal faulting with strike-slip component

SS: Strike-slip faulting
(includes minor normal or thrust component)

TS: Predominately thrust faulting with strike-slip component

TF: Thrust faulting

U: Unknown (largely breakout data with no stress magnitude information available; also see below).

Besides the standard NF, TF, and SS categories, combinations of NF with SS (NS) and TF with SS (TS) exist (Zoback, 1992). NS represents data where the maximum stress or P-axis is the steeper plunging of the P- and B-axes. TS represents data where the minimum stress or T-axis is the steeper plunging of the B- and T-axes. The plunges (pl) of P-, B-, and T-axes (or S1, S2, and S3 axes) used to assign the stress data to the appropriate stress regime are given in the table below (according to Zoback, 1992).

blind

P/S1-axis

B/S2-axis

T/S3-axis

Regime

SH-azimuth

pl > 52

pl < 35

NF

azim. of B-axis

40 < pl < 52

pl < 20

NS

azim. of T-axis +90

pl < 40

pl > 45

pl < 20

SS

azim. of T-axis +90

pl < 20

pl > 45

pl < 40

SS

azim. of P-axis

pl < 20

40 < pl < 52

TS

azim. of P-axis

pl < 35

pl > 52

TF

azim. of P-axis



For some overcoring (OC) and hydraulic testing of pre-existing fractures (HFG) measurements, the magnitudes of the full stress tensor are determined and the SH azimuth can be calculated directly from the eigenvectors of the tensor. However, the stress regime characterization is still based on the plunges of the principal axes.

The exact cutoff values defining the stress regime categories are subjective. In this attempt Zoback (1992) used the broadest possible categorization consistent with actual P-, B-, and T-axes values. The choice of axes used to infer the maximum horizontal stress (SH) orientation is displayed in the table above, e.g. the SH orientation is taken as the azimuth of the B-axis in case of a pure normal faulting regime (NF) and as 90° + T-axis azimuth in the NS case when the B-axis generally plunges more steeply than the T-axis.

If data fall outside of the ranges the tectonic regime can not be assigned. When the focal mechanism comes from the routine analysis of the Global CMT catalogue the data record will not be entered into the database. If the focal mechanism comes from a regional study it is given an E-quality and unkown tectonic regime (U). E.g. this holds on in particular for focal mechanism all three axes have moderate plunges (between 25° and 45°) and when both P- and T-axes have nearly identical plunges in the range of 40° to 50°.