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The asthenosphere is a relatively weak region of the upper mantle below the mechanically strong lithosphere. A seismic low-velocity zone often occurs in this region, particularly beneath oceanic lithosphere, but mechanical weakness and seismic speed are distinct properties.

A low-velocity zone has a wave-speed decrease with depth over some interval. Surface rays do not turn within its negative-gradient interior; rays penetrating it can return only after encountering sufficient speed recovery below. This can produce shadow ranges, multiple travel-time branches and poor turning-depth coverage. A wave-specific low-velocity zone is not identical to a mechanical layer boundary.

In a zone with $dv/dz<0$, a descending ray bends towards the vertical instead of turning back towards the surface. A ray cannot have its ordinary smooth diving-ray turning point within that decreasing-speed interval. There are rays turning above the zone and rays that penetrate it and turn deeper, after speed has increased sufficiently. The latter spend extra time traversing slower material, so their branch is delayed relative to a simple extrapolation of the original uniform-gradient curve.

Turning depths within the low-velocity zone are missing. In a piecewise-gradient example this can leave a range interval without a returning diving ray: a geometrical shadow zone between the shallow and deeper branches. The lower row of the original plot shows that situation, with the interrupted shallow branch and the delayed deeper branch. The boundaries are profile-dependent; reflected, refracted or diffracted energy can still arrive, and smoothing an idealized velocity maximum can change the branch limits. Thus the robust signature is the interruption/reorganization of the ordinary turning-ray family, not a universal claim of zero recorded signal. The supplied description alone does not specify unique numerical travel times or shadow-zone widths.

Elastic rebound. Tectonic motion loads a fault whose shallow portion is locked. Rock on either side deforms and stores elastic strain energy while slip is inhibited. When failure permits rapid slip, part of the stored energy becomes seismic radiation and part is dissipated in friction and damage. The adjacent rock partly relaxes, while a permanent displacement remains across the fault. Geodetic strain accumulation and coseismic offsets support this loading–release picture.^[ref]

The model is useful for brittle crustal faulting but is not a prediction of precise recurrence, rupture size or frictional evolution. Aseismic creep can release deformation without a large sudden event. Viscoelastic flow, afterslip, fluid-pressure changes and interacting fault segments complicate a simple locked-fault cycle. Deep-focus earthquakes and non-shear source components require additional physics; not every earthquake source is adequately described by a shallow brittle double couple. Rebound also need not release all the pre-existing stress.

A thin negative-gradient velocity zone. In horizontal layering, use the seismic ray parameter $p=\sin i/v$, the conserved horizontal slowness. A ray turns when $pv=1$. Downward passage through decreasing speed makes $pv$ smaller, so a surface ray cannot turn in the negative-gradient interior. Rays either turn above the zone or penetrate it and turn below where speed has recovered sufficiently. This produces missing turning depths and a seismic shadow range from a low-velocity zone in the ideal ray model.

Retain all branches usingThe travel-time slope is $dT/dx=p$. Just below the critical value that permits passage beyond the upper speed maximum, the ray makes a finite extra excursion through the low-velocity region. Range can jump, and the deep branch can fold back before resuming its normal trend, giving multiple arrivals outside the shadow. A first-arrival curve alone hides some of these branches. Define the seismic intercept time byThe $x$–$p$ representation displays the gap and fold; the $\tau$–$p$ representation has separate shallow and deep branches and a branch jump at the critical ray parameter for the piecewise-linear cusp model. Do not join them by invented turning rays. Diffraction and finite-frequency energy soften a purely geometrical shadow.

The figures use an explicit illustrative profile, not measured Earth parameters: speed rises from 1 to 2 over depths 0 to 1, falls to 1.8 by depth 1.2, then rises with its original gradient. The asthenosphere beneath oceanic lithosphere is an important setting for a low-velocity zone. A major P-wave speed reduction also occurs at the boundary between the mantle and the outer core of the Earth; S waves do not propagate through the liquid outer core of the Earth.^[ref]

A ray's intercept time subtracts its tangent travel-time contribution at the receiver range. For a smooth branch, $dT/dx=p$ implies $d\tau /dp=-x$. Thus an intercept-time curve encodes range as its negative slope. Separate turning branches around a low-velocity zone must be retained rather than joined by fictitious rays.