Slide 1
Earthquake waves probe inaccessible layers
Earthquakes send seismic waves through the planet. Measuring their arrival times and paths helps scientists infer structures they cannot directly visit.
More to Read
A single seismometer records motion at one location. Networks of stations provide crossing paths, making it possible to separate source location and timing from the properties of the rocks along the route.
Slide 2
Compression and shear test different material properties
Compressional waves squeeze material along their travel direction. Shear waves move it sideways and require resistance to shear deformation.
More to Read
Ordinary liquids transmit compressional waves but do not carry bulk shear waves as solids do. The absence of direct shear-wave passage through the outer core is therefore a major clue to its liquid state.
Slide 3
Boundaries can bend, reflect or change wave types
Seismic waves change speed when material properties change. At boundaries, waves can reflect, refract and convert between wave types.
More to Read
Travel-time curves show arrivals that differ from a simple uniform-Earth prediction. Abrupt changes and missing arrival regions help identify boundaries, while gradual changes reveal variation within layers.
Slide 4
Missing direct waves constrain the hidden layers
Some regions receive no direct waves of a particular type from an earthquake. These shadow zones constrain the size and properties of deep layers.
More to Read
A missing direct arrival does not mean the station records no motion at all. Reflected, converted and surface waves can still arrive, so seismologists identify phases rather than treating every signal as equivalent.
Slide 5
Wave speeds need other evidence to identify composition
Wave speeds constrain density and elastic properties, but do not uniquely name every mineral. Laboratory experiments and other measurements refine the interpretation.
More to Read
High-pressure experiments test how candidate materials behave under deep-Earth conditions. Combining those results with gravity, chemistry and seismology produces a stronger model than assigning composition from one velocity measurement alone.
Sources
- The Interior of the Earth
US Geological Survey
- Body Waves Inside the Earth
US Geological Survey