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Catherine A. Rychert and Peter M. Shearer used fifteen years of global seismic wave observations to image a sharp interface near the lithosphere-asthenosphere boundary. Conversions from P waves to S waves constrain the interface's depth and sharpness: a transition that preserves high-frequency converted phases is too sharp to be explained by a broad thermal gradient alone. Their interpretation therefore considers composition, melting and seismic anisotropy, and distinguishes the interface beneath oceans from a possible internal boundary inside thicker cratons.^[ref]

A seismology researcher studying lithosphere and asthenosphere structure. Institutional profile: mit.whoi.edu/faculty/5644/

Hugo Benioff investigated whether sequences of earthquakes could reveal large faults extending far below the Earth's surface. His 1949 study used accumulated estimates of elastic-rebound strain increments to compare events as possible movements on one structure. He interpreted the Tonga–Kermadec and South American sequences as activity on great inclined faults reaching roughly $650\mathrm{km}$ depth. The observations helped establish the deep geometry later associated with a Wadati-Benioff zone; the original fault interpretation belongs to the historical development preceding modern subduction theory.^[ref]

A seismology researcher associated with deep earthquake distributions, the Wadati-Benioff zone and seismograph development.

Kiyoo Wadati established that some earthquakes originate far deeper than the shallow sources then commonly assumed. His 1927 study analyzed recorded waveforms and seismic travel times, including the 27 July 1926 event interpreted as occurring about $350\mathrm{km}$ beneath central Japan. The argument used instrumental observations to constrain source depth, rather than inferring it solely from surface damage. He also investigated the geographical distribution of deep events, helping make deep earthquake sources an observable part of the Earth's interior.^[ref]

A seismology researcher associated with the recognition of deep earthquakes and the Wadati-Benioff zone.

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.

R. D. van der Hilst uses seismology to investigate Earth's interior. His teaching develops the connection between measured seismic arrivals and wave propagation, while his research includes global seismic tomography used to constrain mantle circulation.^[ref][ref]

The inferential step is important: a wave-speed anomaly is an observation-dependent model, and interpreting it as temperature, composition or flow requires additional physical assumptions.

Renee C. Weber studies planetary interiors through seismology. Her lunar-core research reanalysed deep-moonquake seismograms from the Apollo stations, combining weak arrivals to identify waves that interacted with the lunar core.^[ref]

The method uses repeatable sources and known station geometry to improve signals hidden in scattered wave trains. Interpreting the enhanced arrivals still requires a seismic velocity model and reliable phase identification; the core is inferred from wave propagation rather than directly photographed.

Roberta Rudnick is a geologist who investigates the composition and origin of continental crust. Much of the lower continental crust is inaccessible to direct sampling, so its composition must be inferred by combining geochemistry, petrology and seismology. Her 1995 synthesis with David M. Fountain linked laboratory measurements of elastic wave speeds in rocks to seismic refraction surveys, using granulites to constrain the composition of the deep continental crust.^[ref]

Their reconstruction distinguished a compositionally heterogeneous, on average mafic lower continental crust from an intermediate-composition middle continental crust. The distinction matters for Earth's heat budget: the abundance of potassium, thorium and uranium, and hence heat production from radioactive decay, varies between rock types and with depth. Seismic wave speeds alone are not a unique chemical analysis; different rocks can have overlapping elastic properties, making independent constraints from geochemistry essential.^[ref]

Research author of Nature and composition of the continental crust: A lower crustal perspective by Roberta L. Rudnick and David M. Fountain.

Seismology studies elastic waves generated by earthquakes, impacts and controlled sources, and uses them to investigate planetary interiors. A seismometer converts ground motion into a recorded seismogram. Arrival times, waveforms and amplitudes then constrain both the source and the material traversed by the waves.

P waves respond to compression and S waves to shear. Their different speeds and their reflections or conversions at interfaces help distinguish layers. Locating an earthquake requires several stations because its origin time and location must be estimated along with the propagation model. A single late arrival can reflect a deeper source, a slower path or a mistaken seismic phase assignment.