Understanding Earthquakes: Types, Risks, and Earth’s Structure
Types of Earthquakes
Earthquakes and Plate Tectonics
Converging Areas:
- Zones of seismicity and largest earthquakes with higher magnitudes.
- Convergence between two continental plates can form mountain ranges.
- Convergence between oceanic and continental plates leads to the formation of an ocean trench where subduction occurs. The oceanic plate (denser) subducts under the continental plate.
Divergent Zones:
- Earthquakes associated with mid-ocean ridges.
- Located in rift parallel faults.
- Earthquakes of lower magnitude compared to those in convergent zones.
Conservative Limits:
- Earthquakes at transform faults.
Horizontal movement of plates in opposite directions.
In essence, these three regions are seismic because they coincide with the boundaries between tectonic plates. These earthquakes are considered interplate and represent about 95% of all earthquakes. Earthquakes that occur within tectonic plates represent 5% and are called intraplate earthquakes.
Minimizing Seismic Risk
- Prevention:
Geological studies of the land: Construction of buildings or infrastructure should not be performed on active faults.
Earthquake-resistant buildings: Buildings must comply with anti-seismic regulations.
Staff training: Evacuation plans must be in place.
Evacuation plans: These should be common knowledge, and simulations should be conducted.
Education: People should be aware of emergency plans.
- Behavior During an Earthquake:
Before:
Identify safer locations inside and outside the house.
Explain procedures to children and ensure adults in the home are responsible for them.
Secure cabinets, gas cylinders, and any materials that could be released.
Have a flashlight, extra batteries, radio, fire extinguisher, first aid kit, canned food, and bottled water readily available.
During:
Avoid panic.
Do not rush to exits.
Use stairs instead of elevators.
Move away from sharp objects (e.g., windows).
Take cover under a doorway, in the corner of a room, or under a sturdy table or bed.
If you are on the road, move away from tall buildings or isolated structures.
After:
Remain calm and avoid panic.
Do not rush to exits or stairs.
Do not light matches or lighters. Use a flashlight.
Turn off electricity or gas.
Shut off water, gas, and electricity as soon as possible.
Check for fires and call the fire department if necessary.
Check for injuries and call emergency services if needed.
Release domestic animals.
Move away from beaches and rivers due to the possibility of tsunamis.
Calm children and the elderly.
- Do not walk through the streets to observe the damage.
Measuring Earthquakes
Modified Mercalli Scale:
Measures the intensity of an earthquake based on people’s descriptions. These reports are used to create isoseismal maps.
Seismic Wave Propagation:
Direct wave: The initial wave originating from the earthquake’s focus that does not interact with the source or any surface of discontinuity, thus not experiencing reflections or refractions.
Reflected wave: A new wave that propagates from a surface of discontinuity in the opposite direction within the same medium as the initial wave.
Refracted wave: The wave transmitted through a surface of discontinuity into a second medium.
Model of Internal Structure and Dynamics of the Geosphere
Structure and Dynamics of the Geosphere
Internal Discontinuities in the Geosphere:
The study of seismic wave propagation revealed the following:
- If the Earth were homogeneous, seismic waves recorded at stations distant from the epicenter would have arrived sooner than observed.
- The greater the epicentral distance, the larger the difference between the actual arrival time of seismic waves and the expected arrival time.
- The greater the epicentral distance, the deeper the seismic waves travel, and the greater their speed. Therefore, the speed of seismic waves increases with depth.
- S waves do not propagate in media with zero stiffness (liquid media), while P waves decrease their speed in such media.
- The speed of waves decreases with increasing density.
- The speed of waves increases with depth, indicating that stiffness increases more than density with depth.
- Seismic waves can be deflected or absorbed during their journey, revealing the existence of media with different compositions. Thus, the Earth is heterogeneous.
- There are surfaces of discontinuity revealed by changes in wave behavior.
Existing Discontinuities:
- Mohorovicic discontinuity (Moho): Separates the oceanic crust and upper mantle, located about 5 km deep under oceans and 30 to 70 km deep under continents. There is an increase in the speed of seismic waves at this discontinuity.
- The 400 km discontinuity: A realignment of minerals occurs, including the transformation of olivine to spinel.
- The 670 km discontinuity: Similar to the 400 km discontinuity, it involves a remineralization of spinel, which transforms into perovskite.
- Gutenberg discontinuity: Located at 2900 km depth, it separates the lower mantle and outer core. S waves disappear, and P waves slow down. This discontinuity explains the existence of a shadow zone.
- Lehmann discontinuity: At 5150 km depth, it separates the outer core (liquid) from the inner core (solid). There is an increase in shear wave velocities, indicating an increase in stiffness. The inner core is solid due to immense pressure, which outweighs the temperature factor.
Shadow Zones:
- Areas on the Earth’s surface between 11,500 and 15,900 km from the epicenter of an earthquake where no P or S waves are received by seismograph stations.
Terrestrial Models:
- Chemical Model (rock composition):
- Crust:
- Continental (granite)
- Oceanic (basalt)
- Mantle:
- Upper (peridotite)
- Lower (peridotite)
- Core:
- Outer (Fe-Ni liquid)
- Inner (Fe-Ni solid)
- Crust:
- Physical Model (stiffness/density):
- Lithosphere: Rigid and brittle behavior.
- Asthenosphere: Low rigidity and plastic behavior, deformable and malleable.
- Mesosphere: Rigid.
- Endosphere:
