Earthquake Magnitude, Seismic Waves, and Zones

Understanding Earthquake Magnitude and Intensity

The moment magnitude is determined from an estimate of the area that breaks along a fault plane during the earthquake, the amount of movement or slippage at failure, and the stiffness of the rocks near the focus of the quake.

The intensity of an earthquake is often indicated by the Modified Mercalli Scale, which is a qualitative measure. The intensity of an earthquake is usually displayed on a map.

Seismic Waves Explained

When a fault is generated, the rocks break suddenly and violently, releasing energy in the form of seismic waves.

P Waves

The P waves, also called compressional or primary waves, are the most rapid of the two main types. They can travel through a solid, liquid, or gas. P waves travel faster through solids than liquids.

S Waves

The S waves, also called secondary or shear waves, can travel only through solid materials. They travel slower than P waves and have an average speed through the crust of three kilometers per second.

When P and S waves reach the surface, they form complex surface waves that move across the Earth's surface.

How Earth Materials Affect Seismic Waves

The different types of Earth materials behave differently in a quake. This difference is related to their degree of consolidation. Seismic waves move faster through rock substrate than through unconsolidated sediment or soil.

Key Seismological Terms

Seismogenic Source

It is a crustal volume considered to have uniform seismicity.

Tectonic Structure Capable (Active Fault)

It is a tectonic structure that can generate earthquakes or deformation on the surface, i.e., an active fault. The concept of an active fault is defined by the age of its last movement. From a neotectonic point of view, it is defined by deformations presented during the current tectonic regime.

Seismotectonic Province

Can be defined as a region with geological, geophysical, and seismic similarities. It is therefore assumed to have uniform potential seismic activity. Earthquakes can occur anywhere in the province, even when seismic data show concentrations or preferred locations.

Understanding the Control Earthquake

The control earthquake is the maximum earthquake expected in each seismogenic source. Within each seismogenic source, the control earthquake is situated at the closest distance to the site.

If the site is located within a seismotectonic province, the control earthquake will be located at a distance of 15 km from the site.

Deterministic Seismic Hazard Assessment

This method considers the hazard as defined by the peak ground motion generated by different control earthquakes.

The results of the deterministic method may therefore lead to extremely conservative conclusions.

Factors Modifying Local Seismic Response

• High amplifications are greatest in soft soil areas.
• The duration of the earthquake increases in elevated areas.
• On slopes, differential displacements can occur.
• The horizontal component of motion is amplified more than the vertical component on parts of cliffs or edges of slopes.