Seismic waves

Seismic waves

Authors: Cardito Marta, De Pofi Laura, Fiorino Anna, Naderi Melika, Solinto Gaia,

Earthquakes

Earthquakes are violent "ground movements" caused by movements in the Earth's crust occurring on a large scale.

Seismograph and seismogram

Seismograph

A seismograph is an instrument that records seismic movements, reproducing the ground motion in a graphical image called a seismogram.

Seismogram

A seismogram is a graph resulting from recordings by a seismograph, representing ground displacement, velocity, or acceleration over time.

Faults

A fault is a fracture in the Earth's crust along which the two fractured rock blocks move reciprocally.

Types of faults:

Normal: The hanging wall moves downward compared to the footwall due to tensile stress. Reverse: Conversely, the hanging wall moves upward due to compressive stress. Strike-slip: No vertical movement; only lateral sliding along the fault direction.

In normal or reverse faults, the offset is vertical; in strike-slip faults, it is horizontal. Mixed cases exist where the displacement is oblique. Movement along the fault generates an earthquake.

Earthquake origins

The theory of elastic rebound explains earthquake origins. Blocks on either side of the fault are stationary and undistorted initially (intact wall). Forces acting on the blocks cause ground and stone wall deformation, resulting in cracks. Stress exceeds the breaking point, leading to sudden movement that produces the earthquake. The stone wall breaks into two parts. The movement of the fault blocks is indicated by the two misaligned parts of the wall (horizontal offset).

Epicenter and hypocenter

Epicenter

The point where earthquake propagation begins on the surface is called the epicenter ("epi" means above).

Hypocenter

The earthquake's point of origin deep within the crust is called the hypocenter ("hypo" means below) or focus.

Types of earthquakes

Earthquakes can be classified into various types depending on their origin:

Tectonic earthquakes

Collapse earthquakes

Tectonic earthquakes originate from movements along faults. They are the most frequent and powerful earthquakes, as they release a large amount of energy.

Collapse earthquakes originate from the collapse of mountains, caves or landslides. They are infrequent and have a very limited and localized power.

Volcanic earthquakes

Explosion earthquake

Volcanic earthquakes originate from the volcanic activity of magma underground or in the volcanic vent. They are less frequent than tectonic earthquakes and have a lower power. They have a superficial origin and a very limited radius from the epicenter.

An explosion earthquake is the result of the detonation of a nuclear and/or chemical device. The power depends on the object causing the explosion.

Scales of measurement

From the point of view of the energy released, the earthquake can be classified using two scales of measurement:

Mercalli scale

The Mercalli scale is based on the effects of the earthquake on the environment, on buildings and on people, associating each earthquake with a degree on a scale from zero to twelve (maximum catastrophe).

Richter scale

The Richter scale is based on the amount of energy released by the earthquake, i.e. on the magnitude of the earthquake, it measures the intensity of the earthquake regardless of the presence of man in the place (e.g. desert). Currently the maximum magnitude recorded in an earthquake is nine.

Magnitude

The magnitude (M) is the ratio between the amplitude of the maximum ground movement recorded for a given earthquake and that of a standard earthquake recorded by a standard seismograph placed at a distance of 100 km from the epicenter; this value is used as a unit of measurement.

Seismic waves

Body waves

Earthquake energy travels from the focus in the form of seismic waves.

P-waves (Primary waves): P-waves are compressional waves that travel through solids, liquids, and gasses. They are the fastest seismic waves and have a push-pull or compressional motion. P-waves move in the same direction as the wave is propagating, causing particles in the material to vibrate parallel to the direction of the wave. P-waves can travel through the Earth's interior, and they are the first to be recorded on seismographs during an earthquake.

S-waves (Secondary waves): S-waves are shear waves that only travel through solids. They are slower than P-waves and have a side-to-side or perpendicular motion. S-waves cause particles to move perpendicular to the direction of the wave, resulting in a shaking or shearing motion. S-waves cannot travel through liquids, and their absence in certain areas during seismic studies has helped scientists determine the existence of liquid layers within the Earth.

Seismic waves

Surface waves

L-waves (Love waves): Love waves are surface waves that travel along the Earth's surface and are responsible for horizontal ground motion. Love waves move with a side-to-side motion, similar to S-waves, but they are confined to the Earth's outer layers. Love waves are typically the slowest among seismic waves and are responsible for the damage caused during earthquakes due to their side-to-side motion.

R-waves (Rayleigh waves):Rayleigh waves are also surface waves that travel along the Earth's surface. They cause both vertical and horizontal ground motion. Rayleigh waves have a rolling motion, with particles moving in elliptical paths in the direction of wave propagation. Rayleigh waves are slower than P-waves but faster than Love waves. They contribute significantly to the shaking experienced during earthquakes and can travel through both solid and liquid layers.

Class activity

During the class activity, we used three seismographs to analyze a significant seismic event. We focused on the amplitude of the seismic waves recorded, comparing the results obtained by the Labriola seismograph with those of two other schools.

The amplitude of the waves from the Labriola seismograph was greater due to the proximity of the hypocenter compared to the other two sites.

Class activity

To analyze seismic waves, we used a spring. In the first experiment, the spring was stretched horizontally, with two participants grasping its ends. By applying an impulse parallel to the spring, a longitudinal wave was generated, identified as a P wave. The impulse propagated back and forth, and both boys felt a tug in the same direction as the impulse. Pulling the spring to the right or left generated a transverse wave, the S wave. In this case, the hand moved laterally from right to left.

In the second experiment, one participant raised the spring as high as possible, while the other lowered it as far as it would go, simulating the epicenter and hypocenter, respectively. Using the P wave the first pulse for the participant simulating the epicenter was perceived as a vertical movement of the hand, up and down. However, when the S wave arrived, the movement of the hand was horizontal.

Thanksfor your attention

Students: Cardito Marta, De Pofi Laura, Fiorino Anna, Naderi Melika, Solinto Gaia.Teacher: Lippo Silvia Maria School: Liceo Scientifico Arturo Labriola 2023/2024

Thanks to professor Luigi Cerri of Città della Scienza