Seismic waves can be roughly classified into body waves, which travel inside the earth, and surface waves, which travel only along the earth's surface. Surface waves are classified into Rayleigh wave associated with volumetric change and Love waves associated with shear deformation. Here, we will discuss the Love wave.
First, let's take a look at how Love waves propagate (Figure 1). Think of ○ as a marker in the ground to know how it oscillates. In the figure, you can see how it is deformed horizontally. Move the cursor to the figure and press s on the keyboard; the Love wave starts to propagate in the right direction. The circle moving in the front is shown in red, and the circle moving in the back is shown in light blue. When the wave reaches the right end, it reappears at the left end and repeatedly propagates (periodic boundary condition).
Love waves are generated when a soft layer is overlying a hard layer. For example, a soft crust overlying a hard mantle. In this figure, circles are placed every 10 km along the vertical axis (depth) and every 6.25 km along the horizontal axis (horizontal). The fourth circle in the depth direction is at the boundary between the crust and the mantle (Moho plane). You can see how the waves are efficiently propagating horizontally through the crust.
Let's take a closer look at the waves and their propagation: press r to reset the screen, and pause (press s again) when the Love wave is near the right edge.
You can see that the shape of the wave distorts more than it was at the beginning. The red area extends both horizontally and vertically. The green area sticks near the surface. You can see that the green part arrives later. The propagation speed changes depending on the wavelength of the wave, and the shape of the wave is distorted. This phenomenon is called dispersion. In this case, waves with longer wavelengths travel faster and waves with shorter wavelengths travel slower. This is because long waves are strongly affected by the hard layer (deep layer: mantle).
- Pause and restart: Press "s".
- Reset: Press "r".