D1 Seismology

Topic

Seismology is the primary diagnostic method for studying the interior of the solid Earth; it uses recordings of elastic waves generated by earthquakes—or even artificial sources such as controlled explosions or vibrator trucks—to infer both the planet's internal structure and the parameters of the source that generated each earthquake. International and regional seismographic networks continuously record ground motion using broadband seismometers, enabling the detection of earthquakes ranging from slightly negative magnitudes to magnitudes of 9.5 or higher anywhere on the planet.

Seismic waves generated by an earthquake fall into two broad categories: body waves, which travel through the planet's interior, and surface waves, which travel only along the surface. Among body waves, compressional waves travel the fastest—at speeds between 6 and 8 kilometers per second in the Earth's crust and up to 13.7 kilometers per second in the lower mantle—whereas shear waves travel more slowly, at speeds between 3.5 and 4.5 kilometers per second. Regarding surface waves, one type moves via purely horizontal shearing, while the other moves in a retrograde elliptical motion as it propagates.

The time difference between the arrival of shear waves and compressional waves at a single station forms the historical basis seismology has used to pinpoint the exact location of an earthquake. The ratio between compressional wave speed and shear wave speed depends on an elastic property of the rock itself, and this ratio changes significantly when the rock contains fluids: a fluid-saturated rock exhibits a notably higher velocity ratio than the same rock in a dry state.

The seismic moment tensor provides the most complete description of an earthquake source: it is a symmetric, 3x3 matrix that characterizes the set of equivalent forces that produced the earthquake. When an earthquake source involves pure shear—without any associated volume change—the sum of the diagonal elements of the corresponding matrix is ​​exactly zero; seismologists refer to this configuration as a "double couple." By mathematically decomposing this seismic moment tensor, it is possible to determine the nodal planes of the ruptured fault, the axis along which the rock was compressed during the earthquake, and the axis along which it was stretched; together, these three elements define the earthquake's focal mechanism.

Ideally, an earthquake's focal mechanism reflects the tectonic kinematics of the region where it occurred: subduction earthquakes produce reverse-faulting mechanisms, earthquakes in extensional zones produce normal-faulting mechanisms, and earthquakes along transform plate boundaries produce strike-slip mechanisms. This establishes a link between the elastic wave record—with which this description began—and the internal structure of the earthquake itself, which that record allows us to reconstruct.

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