What can earthquakes and marsquakes tell us about the planets?
P waves and s waves form a group called body waves, which cut through the rock. The fastest are p-waves (primary or compression waves), which compress rocks as they pass through them. The movement of these waves is akin to someone at the back of a line jostling the person in front of them. As P waves pass through different materials, they speed up and slow down, changing their path. So, by mapping how these P waves change direction, scientists can attempt to piece together a planet’s internal stratification and composition.
The other type of body wave is called a s wave (secondary or shear wave), and it is the second fastest type of seismic wave. S waves move rock perpendicular to the direction the wave is moving, like shaking a beach towel and throwing it on the sand. To propagate, S waves require a material with some internal strength and stiffness. This means that s waves cannot pass through liquids, including the liquid metallic outer core of our planet.
The last two seismic waves form a group called surface waves. On Earth, it is the waves that cause the most damage. The first kind, waves of love, oscillate the surface from side to side (horizontally). Meanwhile, the other guy, Rayleigh waves, move across the surface like an ocean wave (vertically). When we feel the ground shake during an earthquake, Love waves and Rayleigh waves are what we usually feel.
Inside looking out
Inside the Earth, p and s waves get faster as you penetrate deeper into the planet’s crust and mantle. There are some variations as the waves progress from fragile crust to dense core malleable mantle, but in general they move faster at greater depth. This is due to the changing pressure (and therefore density) of the Earth’s interior, as well as changes in the minerals that make up the mantle.
When seismic waves hit the Earth’s mantle-core boundary, great things happen. First, s waves disappear because the Earth’s outer core is liquid. Remember, s waves shear rocks, and with nothing to shear, they disappear. However, s waves return to the solid inner core of the planet. This is because s waves, or secondary waves, are primarily a byproduct of the movement of p waves, or primary waves.
Meanwhile, p-wave velocities drop dramatically at the mantle-core boundary. This is because they change from silica-rich rocks to an iron-nickel core. In fact, inside the Earth, the speed of P waves drops by almost 50%. However, P waves speed up again as they penetrate deeper into the Earth’s outer and inner core.
After the s and p waves pass through the Earth’s core, their velocities change again – but this time in reverse, as they now venture from the core into the crust.
In the dark
Planets and moons are spheres. So whether it is an earthquake, quake, or moonquake, an earthquake will send seismic waves that travel in an arc shape through the interior of the world (see image below). When these waves hit different materials, their speed increases or decreases, causing their trajectory to bend. This phenomenon is known as refraction, and it’s the same thing that happens when light (which can be a wave) passes through a lens.