Seismic waves are vibrations generated primarily by earthquakes that travel through Earth’s interior, providing scientists with the only direct means to investigate our planet’s deep structure. These waves come in two main forms: the faster Primary waves (P-waves) and the slower Secondary waves (S-waves), also known as shear waves. S-waves are characterized by a unique motion that makes their speed highly dependent on the material they pass through. Understanding the velocity of S-waves is fundamental to seismology.
The Nature and Typical Velocity of S-Waves
S-waves are defined by their distinct side-to-side motion, where the ground is displaced perpendicular to the direction the wave is traveling. This transverse, or shearing, motion means S-waves can only propagate through solid materials that possess the ability to resist a change in shape.
The velocity of S-waves varies significantly depending on their location, but they are consistently slower than P-waves in any given material, typically traveling at about 60% of the P-wave speed. Near Earth’s surface, S-waves generally travel at speeds ranging from approximately 2 to 5 kilometers per second. This relatively slower pace is why they always arrive at a seismograph station after the P-waves, which is how they earned their designation as “secondary” waves. As S-waves travel deeper into the Earth, their speed generally increases, reaching around 7.2 kilometers per second just before they encounter the boundary of the core.
Material Properties Governing S-Wave Speed
The speed of an S-wave is determined by two fundamental properties of the transmitting medium: its rigidity and its density. The relationship dictates that S-wave velocity is proportional to the square root of a material’s shear modulus, or rigidity, divided by its density. Rigidity measures a material’s resistance to being sheared or twisted, which is the exact deformation an S-wave imposes on the rock.
A higher shear modulus directly translates to a faster S-wave velocity, making rigidity the positive factor in the speed calculation. Conversely, density acts as the negative factor; all else being equal, a denser material slows the wave down. However, within the Earth’s interior, rigidity and density do not change independently. Under the immense pressure of deeper layers, materials become both denser and significantly more rigid.
This interplay results in a phenomenon known as the velocity-density paradox. As depth increases, the speed of S-waves generally rises because the increase in a material’s rigidity under pressure is much greater than the simultaneous increase in its density.
S-Waves and Mapping Earth’s Internal Structure
The way S-waves interact with different materials allows seismologists to map out the distinct layers of the planet’s interior. The velocity changes drastically at boundaries between layers, such as the transition from the crust to the mantle. These abrupt speed shifts are used to precisely delineate the depth and composition of internal structures.
S-waves provide evidence for the state of the Earth’s core. Because S-waves require shear rigidity to propagate, they cannot travel through liquids, which have a shear modulus of zero. When S-waves encounter the core-mantle boundary, they are completely blocked, proving that the outer core is in a molten, liquid state.
This complete cessation of S-wave travel creates a massive area on the opposite side of the Earth from an earthquake, called the S-wave shadow zone. This zone, which begins at an angular distance of about 103 degrees from the earthquake epicenter, records no direct S-waves. The existence and size of this large shadow zone confirmed the presence of a liquid outer core.