Waves and Seismic Activity

Vocabulary flashcards reviewing seismic wave types, recording instruments, measurement scales, and energy relationships.

Seismometer

An instrument for measuring the seasonal winds and movements of the Earth, the first of which was created by Zhang Heng in China in $132\,\text{BC}$.

Seismograph

An instrument used to record earthquake waves by capturing Earth's movement in three components: north-south, east-west, and vertical.

Body Waves

Seismic waves that travel through the interior of the Earth, consisting of P-waves and S-waves.

P-waves

Primary, compressional, longitudinal body waves that alternate compressions ("pushes") and dilations ("pulls") along the ray path in the direction of wave propagation, traveling at $5-8\,\text{km/s}$ in typical continental crust.

S-waves

Secondary, shear, transverse body waves that create alternating transverse motions perpendicular to the direction of propagation; they travel at $3-4\,\text{km/s}$ in typical continental crust and cannot travel through fluids such as molten material, air, or water.

Surface Waves

Seismic waves that travel along the surface of the Earth, including Rayleigh waves and Love waves.

Rayleigh Waves

Surface waves ($V_R$ or R waves) that advance in a backward rotating elliptical motion at speeds of $2-4\,\text{km/s}$ on the Earth's surface.

Love Waves

Surface waves ($V_L$ or L waves) that propagate in a transverse horizontal motion perpendicular to propagation and parallel to the Earth's surface; they travel at $2-4\,\text{km/s}$ and do not travel through air or water.

S-wave Time Minus P-wave Time

The difference between S-wave arrival time and P-wave arrival time, which shows how far away an earthquake was from the recording station without indicating direction.

Richter Magnitude

An open-ended scale based on the size of the largest amplitude wave measured $100\,\text{km}$ away, where a change of $1$ unit equals a $10\times$ change in seismic wave amplitude.

Seismic Moment Magnitude

A measurement scale for earthquakes that measures the total energy released and provides greater accuracy for large earthquakes.

Mercalli Intensity

A scale ranging from $\text{I}$ to $\text{XII}$ that measures earthquake size based on the structural and surface damage caused.

Charles Richter

The seismologist who showed that earthquakes with larger intrinsic energy produce larger ground motion amplitudes at a given distance, calibrating his scale using maximum shear amplitudes.

Moment Magnitude Scale ($M_w$)

An earthquake magnitude scale introduced by Thomas Hanks and Hiroo Kanamori as a successor to the Richter scale, which depends on the seismic moment ($M_0$) related to the area of rupture movement.

Earthquake Energy Scaling

The principle that an increase of $1$ unit in magnitude represents $32$ times more energy released (e.g., an $M_w\,8$ releases $1,000,000$ times more energy than an $M_w\,4$, and an $M_w\,5.3$ releases about as much energy as the Hiroshima atomic bomb).