Seismologists use waves generated by earthquakes to interrogate the layers and constituents of the Earth. Earthquakes generate a variety of surface and body waves, which are detected using seismometers at locations all around the world. Seismologists have developed a wide variety of techniques to process these waves and make inferences about the Earth’s interior.

  

In the Physical Acoustics Lab (PAL) we use laser ultrasonics to excite and detect elastic waves in a Braeburn apple. We then borrow techniques from seismology to measure the physical properties of the apple, whose structure bares some resemblance to the Earth. Beyond the use of the word “core”, both objects have a thin skin/crust that covers a mantle/exocarp.

Below is a comparison between our waveforms in an apple and those in the Earth due to the 2011 Tohoku Earthquake. The scales are vastly different, but the waveforms look alike; the zig-zagging pattern are caused by surface waves traveling around the two spherical objects.

Unlike the Earth, apples age quite rapidly. Below you can see how the wavefield in the apple changes, as a function of time: