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Andrew J. Michael coauthored a comparison of stress-orientation measurements with competing strength models for the San Andreas Fault. The study combines earlier observations with new inversions of focal mechanisms, finding that similar measured orientations can support different interpretations.^[ref]

Fault geometry and the distribution of available slipping surfaces affect how orientation is translated into strength. Measuring a stress direction therefore does not by itself determine an absolute friction coefficient.

Dip-slip faulting is relative motion on a geological fault predominantly along the steepest direction within the fault plane. That direction is perpendicular to the fault strike and follows the fault dip. It differs from strike-slip faulting, whose motion is mainly along strike.

For an inclined plane, normal faulting lowers the hanging wall relative to the footwall; reverse faulting raises it. The surface step is only a projection of the full fault slip, so vertical offset alone is not the total displacement unless the geometry is known. Mixed motion belongs to oblique-slip faulting, and an observed focal mechanism must be interpreted with independent evidence for the actual fault plane.

The United States Geological Survey explains lower-hemisphere focal mechanisms, compression shading, and the ambiguity between the two candidate fault planes.

Jeanne L. Hardebeck coauthored a focal mechanism method that propagates uncertainty in first-motion polarity, source position and seismic structure. Synthetic tests and closely spaced earthquake groups provide checks on the inferred solutions.^[ref]

The method matters for stress studies because a sharply fitted mechanism can still change substantially when an assumed ray path changes.

Extension accommodated by a hanging wall moving down relative to the footwall. Its focal mechanism has an extensional dip-slip pattern, with the centre unshaded under the convention that shaded quadrants denote compression.

Fault slip with both dip-slip and strike-slip components. The focal mechanism can have an off-centre nodal intersection, with the central polarity indicating its dip-slip sense under a specified shading convention. A reverse component with horizontal shear is compatible with transpression.

For a double-couple seismic source, measure P-wave first-motion polarity signs and use source-to-station takeoff angles from the location and velocity model to plot them on a focal sphere. Fit the alternating radiation-sign quadrants, optionally adding amplitude ratios and waveform information. The two nodal planes determine a focal mechanism, conventionally expressed by fault strike, fault dip and fault rake.

A more general waveform inversion fits a seismic moment tensor to several components and stations. Its double-couple part supplies candidate slip and normal vectors, but a general moment tensor should not be forced to be double-couple without justification. Polarity calibration, near-nodal observations, path effects and takeoff-angle errors limit the result.

Point-source radiation alone interchanges the fault plane and auxiliary plane without changing the double couple. Resolve this fault-plane ambiguity using mapped rupture, aftershock geometry, geodesy or finite-rupture directivity. A dense set of polarity picks does not by itself remove that fundamental ambiguity.

The decisive change was from moving continents within a supposedly permanent ocean floor to moving coherent tectonic plates that include both continental and oceanic lithosphere. Several independent observations made that change testable.

Continental connections supplied the initial problem. In the early twentieth century, Alfred Wegener assembled matching continental margins, correlatable rock belts and mountain chains, shared fossil assemblages, and paleoclimate indicators into the case for continental drift. Reassembling Gondwana, for example, makes geographically separated glacial deposits and striation directions compatible with one ice-sheet system. Fits at the continental shelf edge are physically more meaningful than exact fits to modern coastlines, which have been modified by erosion and sea-level change. The evidence suggested relative movement, but did not justify the proposed forces: continents cannot readily plough independently through strong oceanic rock. A convincing mechanism and independent motion measurements were missing.

Oceanography changed the boundary conditions. Echo-sounding surveys revealed a connected spreading ridge system, axial valleys, subduction trenches and long fracture zones. Seismic refraction surveys established systematic differences between continental and oceanic crust. Thin oceanic sediment cover, high ridge heat flux and the lack of very old exposed oceanic basement made permanent, ancient ocean basins increasingly implausible. Harry Hammond Hess proposed creation of oceanic lithosphere at ridges and recycling at trenches: seafloor spreading without requiring an expanding Earth. Slow mantle convection supplied a physically plausible connection between surface transport and internal heat loss. The mechanism need not have been completely quantified before its kinematic predictions could be tested.^[ref]

Magnetic records supplied a clock and a speedometer. Dated continental lavas established a sequence of geomagnetic reversals. Marine magnetometers then found elongated, approximately paired marine magnetic stripes on either side of ridges. The 1963 interpretation by F. J. Vine and D. H. Matthews combined repeated reversals with seafloor spreading: cooling basalt locks in remanent magnetization, and continuous motion converts a time sequence into a spatial sequence. If a polarity interval lasts $\Delta t$ and occupies width $w$ on one ridge flank, its half-spreading rate is $w/\Delta t$. Matching several reversals tests the predicted sequence, rather than merely fitting a single stripe.^[ref] Different apparent polar wander paths for different continents also became mutually consistent after continental reconstruction, favoring relative continental motion over independent magnetic-pole histories.

Earthquakes identified creation, translation and consumption. Improved global seismology located shallow earthquakes along ridges and transforms, and dipping Wadati-Benioff zones beneath trenches. Their progressive focal depths identified cold, coherent slabs entering the mantle. Focal mechanisms showed extension at spreading boundaries, tangential slip at transforms and convergence at trenches. In 1965, John Tuzo Wilson's transform fault interpretation made a particularly discriminating prediction: active slip occurs between offset ridge segments, with a sense that can be opposite to that inferred by treating the entire lineament as an ordinary offset fault. The outer fracture zones are inherited traces, not continuing active plate boundaries. Agreement of focal mechanisms with the transform prediction linked magnetic and seismic evidence.

The late-1960s synthesis made these observations one quantitative theory. Relative motion of rigid tectonic plates on a sphere can be described by an Euler angular velocity vector, with $\mathbf{v}=\mathbf{\omega }\times \mathbf{r}$. Spreading rates, transform azimuths and slip directions must fit the same rotation, and relative angular velocities must close around plate circuits. The resulting framework accounted for global boundary geometry and earthquake mechanisms without moving continents separately from the seafloor. Deep-ocean drilling beginning in 1968 subsequently confirmed that basement and overlying sediments generally become older away from ridges; this was a powerful confirmation of an already emerging synthesis, not a prerequisite discovered decades earlier.^[ref]

Thus plate tectonics succeeded through agreement between geological reconstruction, magnetic chronology, ocean-floor structure, seismology and rigid-plate kinematics. Modern satellite geodesy verifies motions directly, but belongs to the later confirmation of the theory, not its original 1960s evidence.

Support for the left curve comes from high laboratory friction and substantial in situ differential stress; support for the right curve comes from low inferred fault traction, weak fault materials or high pore pressure. No single observation directly measures the whole plotted envelope.

For the left, stronger shallow-crust model:

For the right, weak shallow-layer model:

To discriminate, measure the strength-controlling quantities through depth and across both faults and their surroundings. Obtain borehole stress, pore pressure and temperature profiles where feasible; use borehole breakouts and hydraulic tests to constrain effective stress. Recover fault and host rocks and test their friction and material creep at relevant fluid conditions. The right curve specifically predicts an end to the shallow weakening near its sharp rise: investigate changes in permeability of a porous medium, alteration and fault gouge with depth, rather than testing only a surface sample.

Combine those measurements with heat flux transects, magnetotelluric sounding for conductive fluids, accurate earthquake depths and focal mechanisms, and GNSS or InSAR observations of deformation. The latter can help constrain the depth distribution of locking and postseismic relaxation, but neither locking nor seismicity alone gives absolute strength. Earthquake stress drop is a change in stress, not the pre-event stress. Low heat flux can also reflect heat transport or transient weakening; high electrical conductivity can have causes other than free water. A joint explanation of all these data is needed to choose between the models locally.

Assuming pure dip-slip faulting, use an east–west striking, $65^{\circ }$ north-dipping normal fault, with the northern hanging wall moving down dip. In a right-hand-rule strike convention this is strike $270^{\circ }$, dip $65^{\circ }$, rake $-90^{\circ }$. The auxiliary nodal plane strikes east–west and dips $25^{\circ }$ south. The focal mechanism has a predominantly vertical P axis and a predominantly north–south horizontal T axis; the steep central direction is dilatational.

The sketch is a double-couple seismic moment tensor calculation, not a claim that first motions uniquely identify which nodal plane slipped. The mapped north-dipping surface supplies that additional geological choice.

A reverse or thrust fault earthquake is the expected end member: a hanging wall moves upwards relative to its footwall, accommodating north–south tectonic shortening. A plausible double-couple seismic source focal mechanism has an approximately east–west fault strike, a moderately north- or south-dipping fault plane, and up-dip fault slip. For example, right-hand-rule strike $090^{\circ }$, southward fault dip about $32^{\circ }$, and fault rake $+90^{\circ }$ specify a pure thrust mechanism.

In a lower-hemisphere equal-area plot, with shaded regions denoting compressional P first motions, this focal mechanism has a shaded central region and two unshaded outer lobes. The P axis is approximately horizontal north–south and the T axis approximately vertical for a symmetric idealization; for a single $32^{\circ }$ plane their axes are slightly tilted. The P and T axes of a double-couple seismic source need not exactly equal the regional principal stress axes. The nodal curves represent both the fault plane and its auxiliary plane; additional evidence is needed to choose between them.^[ref]

Peter M. Shearer coauthored a method for inferring focal mechanisms from P wave first-motion polarities. It varies source location and the velocity model, collecting acceptable mechanisms rather than treating every ray direction as exact.^[ref]

The spread of acceptable solutions measures stability of the inference and helps distinguish real fault-orientation variation from measurement uncertainty.

Shortening accommodated by a hanging wall moving up relative to the footwall. The centre of its dip-slip focal mechanism is compressional under the usual lower-hemisphere convention. Low-angle reverse faulting is commonly called thrust faulting.

Predominantly horizontal relative motion along a fault. A pure strike-slip double couple has two vertical nodal planes; a focal mechanism gives both possible planes, and independent evidence selects the actual one.