Overview
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History of Seismology
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-   1800s: Cauchy, Possion, Stokes, Rayleigh and others developed the theory of
    elastic wave propagation, and described the existence of body wave
    (compressional and shear wave) and surface wave in solid materials.
-   1857: Robert Mallet described the idea that earthquakes radiate seismic
    waves away from a hypocenter and that they can be located by projecting
    these waves backward to the source. His work represented the first
    significant attempt at observational seismology.
-   1889: First teleseismic observation, made in Potsdam for a Japan event.
-   1898: E. Wiechert introduced the first seismometer with viscous damping,
    capable of producing useful records for the entire duration of an
    earthquake.
-   1900s: B. B. Galitzen developed the first electromagnetic seismographs.
-   1900: Richard Oldham reported the identification of P, S, and surface
    waves on seismograms
-   1906: Richard Oldham detected Earth's core from the absence of direct P
    and S arrivals at source–receiver distances beyond about 100°.
-   1909: Andrija Mohorovicic found Moho.
-   1907: the first widely used traveltime tables were produced by Zo ̈ppritz
-   1914: Beno Gutenberg published traveltime tableswith core phases
-   1936: Inge Lehmann discovered the solid inner core
-   1940: Harold Jeffreys and K. E. Bullen published the final version of
    their travel time tables for a large number of seismic phases, which is
    called JB table.
-   1946: an underwater nuclear explosion near Bikini Atoll led to the first
    detailed seismic recordings of a nuclear bomb.
-   1949: a Soviet nuclear bomb.
-   1961: the Worldwide Standardized Seismograph Network (WWSSN) was established


Earth's structure
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- Crust: 6 km under the oceans, 30–50 km beneath continents.
- Mantle: 84% of Earth's volume and 68% of the mass
- Outer Core
- Inner Core