Stress Indicators in the WSM Database

The WSM compiles information of the contemporary crustal stress from a wide range of stress indicators. Basis of the compilation is the WSM quality ranking scheme to guarantee comparability of the various stress indicators that reflect the stress state of very different rock volumes. The stress indicators are grouped into four categories:

  • Earthquake focal mechanisms
  • Well bore breakouts and drilling-induced fractures
  • In-situ stress measurements (overcoring, hydraulic fracturing, borehole slotter)
  • Young geologic data (from fault-slip analysis and volcanic vent alignments)  

A detailed description of the different stress indicators in the context of the WSM project can be found in Zoback & Zoback (1991), Zoback et al. (1989), Zoback & Zoback (1980), and Sperner et al. (2003). The quality assignment of the individual stress data record is guaranteed through: (1) The WSM quality ranking scheme for each stress indicator according to international standards and (2) the WSM standardized regime assignment. For more technical details please read the WSM Technical Report 16-01. If you want to create your own stress map please use our online software tool CASMO.

WSM Quality Ranking Scheme

The success of the WSM is based on the WSM quality ranking scheme for each stress indicator. This quality scheme is mainly based on the number, the accuracy, and the depth of the observation. The WSM quality ranking scheme was introduced by Zoback & Zoback (1989), and Zoback & Zoback (1991), and then refined and extended by Sperner et al. (2003) as well as Heidbach et al. (2010). It is internationally accepted and guarantees reliability and global comparability of the stress data.  

Each stress data record is assigned a quality between A and E, with A being the highest quality and E the lowest. A quality means that the orientation of the maximum horizontal compressional stress SHmax is accurate to within ±15°, B quality to within ±20°, C quality to within ±25°, and D quality to within ±40°. For the most methods these quality classes are defined through the standard deviation of SHmax. E-quality data records do not provide sufficient information or have standard deviations greater than 40°. These data records are mainly for well bores, contain no stress information, but are only kept for book keeping purposes that these data have been processed. In general, data records with A-, B- and C-quality are considered reliable for the interpretation of the crustal stress state. For more technical details we refer to the WSM Scientific Technical Report 16-01.

Tectonic Stress Regime Assignment

From the stress indicators that provide absolute or relative stress magnitudes the tectonic regime is derived according to the WSM stress regime categorization table. The stress magnitudes are defined using the standard geologic/geophysical notation with compressive stress positive and S1>S2>S3, with S1 as the maximum, S2 as the intermediate and S3 as the minimum principal stress.

Besides the standard regime of normal faulting (NF), thrust faulting (TF), and strike-slip (SS), combinations of NF with SS (transtension, NS) and TF with SS (transpression; TS) are defined (Zoback, 1992). NS represents a data record where S1 or the P-axis is the steeper plunging of the P- and B-axes. TS represents data where the S3 or the 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).

Tab.1 Stress regime assignment for data records from earthquake focal mechanism solutions.
P/S1-axisB/S2-axisT/S3-axisStress regimeSHmax orientation
pl≥52°pl≤35°NFazimuth of B-axis
40°≤pl<52°pl≤20°NSazimuth of T-axis +90°
pl<40°pl≥45°pl≤20°SSazimuth of T-axis +90°
pl≤20°pl≥45°pl<40°SSazimuth of P-axis
pl≤20°40°≤pl<52°TSazimuth of P-axis
pl≤35°pl≥52°TFazimuth 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 SHmax orientation 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 (SHmax) orientation is displayed in the table above, e.g. the SHmax 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 and the label is set to U=unknown stress regime. 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°.

Possible Plate Boundary Events (PBE)

Plate boundaries are characterized by faults with preferred orientations and presumably include major faults with a low coefficient of friction which can be easily reactivated. Thus, the derivation of stress orientations from a single focal mechanism is not always an unambiguous matter, because of its dependence on the mechanical behaviour of the involved fault zone. In the case of weak faults the angle between the principal stress axes S1, S2, and S3 and the principal strain axis P, B, and T from the moment tensor might be as large as 90° (McKenzie, 1969). The scientific debate about the strength of plate boundary faults - whether they are weak or strong - is still going on (e.g. Provost and Houston, 2003). Users should be aware that stress orientations derived from single focal mechanism solutions (FMS) along weak plate boundaries might have a higher degree of uncertainty.

Although it is beyond the objectives of the WSM project to take part in such debates, it is the task of the WSM to provide its users with stress data that have been reliably quality controlled. To this end, FMS data records located near plate boundaries have been flagged as Possible Plate Boundary Events (PBE) if they fulfill the following criteria as explained in more detail by Heidbach et al. (2010):

  1. The event is located within a critical horizontal distance relative to the closest plate boundary segment. The critical distances depend on the types of plate boundaries. We estimated them by means of statistical analysis as being 45 km for continental transform faults, 80 km for oceanic transform faults, 70 km for oceanic spreading ridges, and 200 km for subduction zones.
  2. The angle between the strike of the nodal plane and the strike of the plate boundary is smaller than 30°.
  3. The tectonic regime of the FMS reflects the plate boundary kinematics, i.e. thrust faulting (TF, TS) near subduction zones, strike-slip faulting (SS, NS, TS) near oceanic and continental transforms, and normal faulting (NF, NS) near oceanic spreading ridges.

Stress data sets flagged as PBEs are not down-ranked in quality in the WSM database, but by default they are not plotted on stress maps created with CASMO or in the pre-defined stress maps offered in the download area. However, to allow the user to define their own selection criteria for FMS data records we substantially extended in the stress map interface CASMO the filter options for this data type.

WSM Scientific Technical Report 16-01

The WSM Scientific Technical Report STR 16-01 presents and explains details of the WSM Database Release 2016. It explains the WSM quality ranking scheme, the stress regime assignment and the procedure of the assignment of the possible plate boundary label.

The report also presents the WSM analysis guidelines of the most common stress indicator and gives the detailed explanation of each data record field in the WSM database.