Earthquakes and Oceanic Expansion: A Detailed Look

Earthquakes: When the Earth Shakes

The Dynamics of Plate Tectonics: The Earth's lithosphere is responsible for most earthquakes. Some tectonic plates pull apart, others collide, and some slide along one another. These interactions generate tensions that accumulate slowly, causing elastic deformation in the rocks. When the rocks fracture, they release the accumulated energy, giving rise to earthquakes.

Earthquakes are generated on ridges, in subduction zones, and on transform faults. They are due to the brutal ground shaking caused by the fracturing of rocks at depth, which suddenly releases large amounts of energy accumulated over years. Seismic waves are generated in an area called the hypocenter (a point several kilometers deep) and are picked up by seismographs, which record seismograms. The epicenter is the area on the Earth's surface located directly above the hypocenter.

Seismic Waves

Seismic waves require physical media to spread. When an earthquake is generated, three main types of waves are produced:

• Primary (P-waves): Compression waves that cause a succession of compressions and expansions back and forth in the same direction as the wave moves. They are the first to arrive at a particular point and are recorded first in all seismograms. They spread through solid media.
• Secondary (S-waves): Transverse waves that cause the rocks to move up and down. They are recorded second in seismograms and do not propagate through fluid media.
• Surface Waves: The last waves to arrive. They spread over the surface and are of two types:
  • Rayleigh Waves: Produce an elliptical motion.
  • Love Waves: Cause horizontal movement from one side to another.

Magnitude and Intensity of an Earthquake

Richter Scale

The Richter scale measures the magnitude of an earthquake and is a measure of the energy released by the seismic event. The Richter magnitude scale has no theoretical upper limit, although no earthquake with a magnitude greater than 9.6 has ever been recorded.

MSK Scale (Medvedev-Sponheuer-Karnik)

The MSK scale consists of twelve degrees (denoted by Roman numerals from I to XII) and measures the intensity of an earthquake. The intensity is different at each location and varies with distance from the epicenter. Therefore, an earthquake will have a unique magnitude and different intensities in each location. Points that have the same intensity are joined by curves called isoseismal lines.

Oceanic Expansion

Ocean Floor Expansion: The activity of oceanic ridges is responsible for fragmenting continents and forming new oceans between the fragmented continental blocks. Oceanic lithosphere is continuously generated on either side of the ridge axis, leading to the expansion of the deep ocean. These are also called constructive plate boundaries. The process of opening an ocean can be distinguished in four successive stages:

1. Start of the Ridge (Upwelling Stage): The upwelling of a magma plume collides with the continental lithosphere, which bulges and forms a thermal dome.
2. Young Ridge (Rifting Process): The thermal dome in the lithosphere stretches, breaks, and leads to a triple junction. This system consists of three major radial faults that can evolve into valleys called rifts. The rift is a depression or graben formed by the collapse of the central blocks, generated by a staggered fault system on both sides of the rift valley. In some zones, magma escapes through fissures and can accumulate in the valley bottom, forming basaltic lava plains or volcanic cones.
3. Middle-Aged Ridge (Red Sea Stage): An intermediate form is reached in the rifting process when the triple junctions and associated rifts join, completing the fracturing of the continental lithosphere. Eventually, the continued collapse of the rift valley causes inundation by seawater, leading to the formation of a narrow sea, like the Red Sea. The breaking of the lithosphere decreases the pressure in the upper mantle, which favors the formation of magma that ascends and extends over the flooded valley floor on either side of the ridge.
4. Mature Ridge (Atlantic Stage): If adequate ridge activity continues, new oceanic lithosphere is generated, the ocean floor expands, and the narrow sea evolves into a great ocean.