Earthquake Measurement Scales and Seismic Risk in Spain

Richter and Mercalli: Measuring Earthquake Intensity

A magnitude of 4.2 on the Richter scale represents the amount of energy released by an earthquake. The Richter scale, proposed in 1935, is measured on an exponential scale of 9 degrees, ranging from 1 to 9. In this scale, each grade represents a significant increase in energy; specifically, a difference of two units corresponds to 1,000 times more energy released.

Another seismic scale is the Mercalli scale, which was defined in 1902. It measures the number and severity of the effects of an earthquake on buildings and the landscape. It is measured on a scale of 12 degrees, ranging from I to XII.

Key Differences Between Seismic Scales

• Richter Scale: Measures the magnitude and energy released at the hypocenter or focus. This is the area underground where the fracture or rupture of rocks begins and seismic waves propagate.
• Mercalli Scale: Represents the violence felt at different sites. It measures the intensity, damage to buildings, objects, and the impact on people.

Epicenter and Hypocenter Definitions

The epicenter of an earthquake is the point on the Earth's surface vertically above the hypocenter. The hypocenter is the point inside the Earth where the earthquake originates.

Tectonic Features and Seismic Activity

Seismic activity is often associated with the following geological features:

• Ridges
• Subduction fronts
• Transform faults

Seismic Incidence in the Iberian Peninsula

The South and Southeast areas of the Iberian Peninsula (including Granada, Malaga, Almeria, Murcia, and Alicante), along with the Pyrenees, are where the highest incidences of seismic activity occur. These regions have historically experienced the most destructive earthquakes, characterized by a high frequency of intermediate-depth tremors.

Seismic Risk in the Region of Murcia

In connection with the rest of Spain, the Region of Murcia is located in a medium-high seismic zone. It is considered to have moderate seismicity according to national standards. Notably, the annual rate of earthquakes in Murcia is double the average of the Iberian Peninsula.

Seismotectonic Zones of Murcia

1. Guadalentín Valley: This is an important area delimited to the North by a strike-slip fault parallel to the Librilla-Totana-Alhama-Lorca road. This is known as the Alhama de Murcia or Guadalentín fault.
2. Vega Media and Baja del Segura: Located further East of the Guadalentín, this area possesses very similar geological characteristics.
3. Fortuna-Mula: In the lineup of these towns, there is a match with an important seismotectonic belt and a fault of the same name.

Global Seismic Activity and the Ring of Fire

In 1976 and 1983, frequent seismic activity was recorded in areas belonging to the Pacific Ring of Fire. This is an area on the edge of tectonic plates where seismic and volcanic activity is extremely intense. For example, the intensity of earthquakes in Guatemala is notably high compared to other regions.