P waves

P waves are seismic body waves in which particles oscillate parallel to the direction of travel. They propagate through solids, liquids, and gases.

Longitudinal wave: A wave in which the medium's oscillations are parallel to the direction the wave travels. P waves are the seismic example of this broader wave pattern.
Compressibility: A measure of how much a material's volume changes under applied pressure. Resistance to compression helps set P-wave speed.
S wave: A seismic body wave in which particles move perpendicular to the direction of propagation. Unlike P waves, S waves cannot travel through liquids.
Earthquake hypocenter: The point within Earth where an earthquake rupture begins. P-wave arrival times help locate the earthquake's origin.
Richard Oldham: A British geologist who used seismic observations to identify Earth's core in 1906. His analysis of seismic arrivals helped reveal a distinct central layer.
Elastic wave: A mechanical disturbance that propagates through a material by deforming and restoring it. P waves move through Earth as elastic disturbances.
Shear modulus: A measure of a material's resistance to deformation by shear stress. Together with compressibility and density, it governs P-wave speed in solids.
Surface wave: A seismic wave that travels along Earth's surface rather than through its interior. Surface waves generally travel more slowly than P waves and cause strong surface motion.
P-wave first-motion focal mechanism: A representation of earthquake faulting inferred from the initial directions of P-wave motion at stations. The first push or pull recorded by P waves constrains fault movement.
Inge Lehmann: A Danish seismologist who proposed Earth's solid inner core in 1936. She explained unexpected P-wave arrivals by proposing a solid inner core.

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