Understanding Earthquakes and Earth’s Structure

Posted on May 15, 2024 in Geology

The Soil Structure and Seismic Methods

Earthquakes

Definition: A sudden jolt of the Earth’s interior.

Why does it occur? It is due to failures in the Earth’s crust.

Definition of failure: A fracture of the field accompanied by a shift.

Elements of a Failure

Fault Plane or Fault Surface

The area where the fracture occurs and the displacement occurs.

Types of Failure

1. Normal Fault or Direct: The fault plane dips toward the collapsed lip. The sunken lip rests on the up. Normal faults are caused by divergent forces that pull the opposite directions. The earthquake fracture increases the volume.
2. Reverse Fault or Indirect: The fault plane dips toward the raised lip. The raised lip rests on the sunken one. Convergent forces push towards each other. The force produced by the thin volume of the fault is less than the initial volume of the field.
3. Horizontal Fault: There is no vertical displacement of the blocks, there is only horizontal scrolling. No raised lip or sunk.

Grabens

Staggered set of faults that define a depression. The forces responsible for these structures are divergent forces. Normal faults are involved.

Tectonic Horts or Pillars

Staggered set of faults that delimit an elevation. The forces responsible for these structures are divergent. The forces involved are normal faults.

Earthquakes

Hypocenter

The point located inside the Earth where there is a shift. It is located on the fault plane.

Epicenter

Point located on the surface, vertically above the hypocenter. It is the point where the earthquake has more strength.

Seismic Waves

There are 3 types:

1. P Waves: They are the fastest. Transmitted through solids and liquids. The particles vibrate in the direction of the earthquake by expansion and compression. They are the first to arrive at the seismograph.
2. S Waves: They are slower than the P waves. They involve only solids; they stop at a liquid medium, i.e., its speed. Particles vibrate in the perpendicular to the earthquake. They are the second to reach the seismograph.
3. L Waves: They are the slowest of the three. Transmitted through solids and liquids. These waves are produced by the vibration of the Earth’s surface and are responsible for catastrophes (buildings, bridges). From the geological point of view, they do not provide information from inside the Earth.

Discontinuity

The speed of seismic waves depends on the nature of the rocks where they travel. If materials are very rigid, the speed is higher. If materials are more plastic, the speed is lower. When speed is constant in a constant field, it indicates that it is the same type of rock.

Graphs of the Deep Seismic Discontinuities

1. Mohorovicic Discontinuity: Separates the crust from the mantle. Shows that the two layers are solid. Variable depths: mountains 70km, plains 30km, oceans 5-10 km.
2. Gutenberg Discontinuity: Located at 2900 km depth. Separates the mantle from the core. At 2900 km, the core begins to be liquid, as shown by the S waves that stop.
3. Repetti Discontinuity: Located at 700 km depth. Separates the mantle at the top and bottom. Both are solid.
4. Wiechert-Lehmann Discontinuity: At 5,100 km depth. This discontinuity shows that the nucleus is divided into: a) outer core (2900-5100 km) liquid, b) inner core (5100-6370km) solid because P wave speed increases a lot.

Crust

There are two types of crust: oceanic and continental.

Differences Between Continental Crust and Oceanic Crust

Horizontal Structure of Ocean Depths

1. Continental Shelf: Soft, gently sloping zone. Has a maximum depth of 200 m. It is a photic zone and an area with numerous fishing grounds. It is really the continent under the sea.
2. Continental Slope: A steep area that goes from 200m to 4000m. It marks the boundary between continental crust and oceanic crust.
3. Abyssal Plain: Broad plain at 4000m depth formed by basalt.
4. Oceanic Dorsal: 2000m mountain range in the middle of the ocean. It has a crack called the oceanic rift, where lava erupts from the mantle.
5. Guyots: Cone-shaped structure with a flat top. Its origin is volcanic.
6. Ocean Trenches: Not present in all oceans. Depressions that can reach 11,000 m deep. Example: Mariana Trench in the Pacific Ocean.

Horizontal Structure of Continents

1. Continental shelf may be underwater.
2. Orogenic or mountain ranges: recent ranges.
3. Cratons or shields: ancient ridges destroyed by erosion, forming plains.

Mantle

The mantle extends from the Moho to the Gutenberg discontinuity (2900km). It consists of rocks denser than the crust (density = 5 g/cm³).

The rock believed to form the mantle is peridotite. It has much olivine or peridot. This mineral contains much iron and magnesium.

Core

The nucleus is divided into two zones:

1. The outer core is liquid.
2. The inner core is solid.

It has an estimated density of 13.2 g/cm³. It is made up mainly of iron and sulfur; there is also carbon and nickel.

Geodynamic Model of the Earth

• Lithosphere: Rigid layer. Includes the crust and a bit of the upper mantle. It is approximately 100km thick.
• Asthenosphere: Located under the lithosphere, it includes a portion of the upper mantle. It is plastic and responsible for many movements of the lithosphere. It appears to exist in some areas.
• Mesosphere: Includes some of the upper mantle and the entire lower mantle.
• Endosphere: The entire core.
• Layer D: The innermost part of the mantle, almost melted due to the heat from the core.

Epeirogenic Movements

They are vertical movements of continents. There are two types:

• Epeirogenic+ Movements: This indicates that the continent is rising. Example: Norway and Finland (2 cm per year).
• Epeirogenic- Movements: The continents are sinking. Example: Galician coast.

What Causes Epeirogenic Movements?

Isostasy. It assumes that the crustal blocks float on a denser material, the mantle. These blocks float in balance, and breaking this balance originates vertical movements to readjust.

How Can We Break the Balance?

1. Adding weight to the crustal blocks causes the continents to sink. This happens with intense glaciations or sedimentation.
2. Removing weight from the continents causes them to rise. Examples: a) intense erosion, b) melting (in the case of Norway and Finland).

Eustatic Movements

Refer to sea level. There are two types:

• Eustatic+ Movements: Increases the water level because there are more gallons of sea water. Happening now because we are in an interglacial period or thaw.
• Eustatic- Movements: Lower sea level because there is less water. This happens in ice ages.