Notes on Earthquakes
Geologic Hazards and Resources: Chapter 3 - Earthquakes
Overview of Earthquakes
Date: September 19, 2017
Location: Central Mexico
Depth: 50 km
Magnitude: 7.1
Tectonic Plates Involved: North American Plate and Cocos Plate
Geography Illustration: Page includes data such as location coordinates (60°W), seismic activity details, and fault line diagrams.
What is an Earthquake?
Definition: An earthquake is defined as the vibration of the Earth produced by the rapid release of energy in the form of mechanical waves.
Cause of Earthquakes: Movements that result in earthquakes are typically associated with faults, which are breaks in the Earth's brittle lithosphere.
Types of Faults
Normal Faults: Occur when the lithosphere is under tensional forces.
Reverse Faults: Occur due to compressional forces acting on the lithosphere.
Strike-slip Faults: Occur where shear forces are prevalent in the lithosphere.
Epicenter and Focus
Epicenter: The geographic location where an earthquake is reported, which does not represent the actual slip point.
Focus: The point beneath the ground where the slip occurs along the fault plane.
Elastic Rebound Theory
Explanation: The events leading up to, during, and immediately after an earthquake can be explained by the elastic rebound theory, which includes:
Rocks on either side of a fault move and bend.
Elastic energy is stored in the deformed rocks.
The frictional force maintaining the rocks together is exceeded.
A rapid slippage occurs at the focus, resulting in vibrations (earthquakes) as the deformed rocks return to their original shape (elastic rebound).
Aftershocks: They are explained through the same process where rocks resettle back to equilibrium positions.
Mechanics of Earthquakes
Types of Seismic Waves
Body Waves:
a) P-waves (Primary waves):Nature: Compressional waves.
Movement: Material compresses then dilates in the direction of wave movement.
Speed: Fastest seismic waves.
b) S-waves (Secondary waves):
Nature: Shear (transverse) waves.
Movement: Material moves perpendicular to wave direction.
Note: S-waves cannot travel through liquids.
Surface Waves:
a) Rayleigh Waves: Cause the ground to move up and down.
b) Love Waves: Cause the ground to move side to side.Wave Velocities:
P-waves: 4.8 km/s in granites (~11,000 mph), 1.4 km/s in water (~3,100 mph).
S-waves: 3 km/s in granites (~6,700 mph).
Seismology
Definition: The study of earthquake waves.
Historical Context: Seismology dates back almost 2000 years to ancient China.
Seismograph: Instruments recording seismic waves; modern versions use stationary mass on rotating drums or magnetic tapes.
Global Seismographic Network: A network of seismographs around the world used for data sharing and understanding of earthquakes.
Finding an Earthquake Epicenter
Requirements: Recordings from at least three different seismic stations are necessary.
Distance Measurement: Each station calculates the time interval between the arrival of the first P-wave and the first S-wave to ascertain distance to the epicenter.
Circle Method: Circles with a radius equal to the distance to the epicenter are drawn around each station, with the intersection point indicating the epicenter.
Measuring Earthquakes
Intensity Measurement: The degree of earthquake shaking at a location based on damage experienced.
Magnitude Measurement: The estimation of energy released at the earthquake source.
Scales Used
Modified Mercalli Intensity Scale: Developed by Giuseppe Mercalli based on California buildings. Provides qualitative measurements that vary per individual experience.
Richter Magnitude Scale (ML): Created by Charles Richter in 1935, measuring maximum ground shaking caused by S-waves.
Moment Magnitude Scale (Mw): Developed in 1979, offers a standard measurement based on the energy released by an earthquake. Richter and Moment Magnitude scales are logarithmic.
Recent Earthquake Examples
Indonesia:
Date: August 17, 2018, and October 1, 2018
Magnitude: 7.5
Casualties: >1300 dead.
Type: Strike-slip fault with a shallow focus of 10 km.
Richter Scale Limitations: The scale only goes up to 10, but it doesn’t imply that larger earthquakes can't happen as historical records are limited to human history.
Earthquake Damage Factors
Ground Shaking: The primary cause of destruction, with variability based on ground type and structural construction.
Seismic Wave Material: The material that seismic waves pass through affects the extent of shaking.
Liquefaction Process: Water-rich sediments can liquefy during seismic waves, leading to significant infrastructure damage. Features of liquefaction include sand blows, which typically occur in earthquakes of magnitude 5.5 and above.
Substrate Influence on Damage
Illustrations show how different foundation materials result in varying damage levels for identical earthquakes.
Infrastructure Risks: Ground shaking may disrupt utility lines, which can lead to fires.
Tsunami Generation
Earthquakes occurring in oceans may lead to tsunamis, which are distinct from tidal waves.
Building Reinforcement Methods
Steel Truss Design: These structures allow flexibility to accommodate ground motion, as shown in the retrofitting of the University Hall at UC Berkeley.
Different construction types in similar circumstances display varied resilience to similar earthquake magnitudes.
Earthquake Risk Assessment
Seismic Activity Mapping: Overview of cities based on tectonic boundaries:
City 1: Transform boundary - Moderate risk (10%)
City 2: Subduction zone - High risk (85%)
Cities 3 & 4: Hot spot and Rift - Low risk (5%)
Historical Earthquake Insights
Top Strongest Earthquakes
1960 Chile: 9.5
1964 Alaska: 9.2
2004 Sumatra: 9.2
2011 Japan: 9.1
Top Deadliest Earthquakes
1556 Shaanxi, China: 830,000+ deaths.
1976 Tangshan, China: 255,000 deaths.
2010 Haiti: 316,000 deaths.
Case Studies of Significant Earthquakes
Chile 2010 Earthquake
Date: February 27, 2010
Magnitude: 8.8
Impact: Blackout affecting 93% of the population, significant structure collapse, and loss of life (around 150 casualties).
New Zealand 2010 Earthquake
Date: September 4, 2010
Magnitude: 7.1
Casualties: M6.3 aftershock on February 22, 2011 caused 185 deaths.
Alaska 1964 Earthquake
Magnitude: 9.2
Aftershocks: Six significant aftershocks causing landslides and destruction (e.g., 45-foot tsunami).
Earthquake Prediction Challenges
Current Limitations: Short-term earthquake prediction methods are unreliable; scientists focus on long-term predictions.
Seismic Gap Theory: Method assessing regions on active fault lines that haven’t moved recently to predict possible future quakes.
Case of Italy's L'Aquila: Implicated scientists after failing to predict a significant earthquake in 2009, highlighting challenges in public communication about risks and predictions.
Conclusion
Earthquakes are complex geological events influenced by a variety of factors, including geological setting, material properties, and human infrastructure. Understanding their mechanics, measurement methodologies, damage potential, and the challenges of prediction are crucial for mitigating risks associated with such natural disasters.