GEOL102-Lecture-24 Earthquake Processes and Seismic Waves

Introduction

Based on the lecture, the focus is on understanding how earthquakes occur, the mechanics behind them, and the resulting seismic waves. Thes aspects are vital for understanding the impacts of earthquakes on human activity and landscapes.

Overview of Earthquake Mechanics

Transition to Seismic Waves

  • The discussion shifts from structural consequences of earthquakes to fundamental processes involved.

  • Emphasizes the need to understand the description of faulting processes, the nature of seismic waves, and their measurement.

Key Questions

  • What is the actual process of an earthquake?

  • How do we describe different types of seismic waves?

  • What properties are significant for characterizing seismic waves?

  • How is data collected during earthquakes to better understand these processes?

Understanding the Earthquake Process

Elastic Strain Concept

  • Elastic Strain: The buildup of elastic strain in a fault due to stress until it overcomes the fault's strength and results in an earthquake.

  • The process illustrates a cyclic pattern: build-up of strain → fault slip → release of strain → repeat.

  • Ties back to Elastic Rebound Theory derived from observations of the 1906 San Francisco earthquake, which had a magnitude of 7.9 and ruptured approximately 500 kilometers of the San Andreas Fault.

San Francisco Earthquake Example

  • Demonstrated significant damage to the city, exacerbated by subsequent fires.

  • Utilizes a notable example of a fence offset approximately 2.6 meters by the earthquake.

Fault Behavior During Earthquakes

Fault and Rock Interaction

  • Fault: A weak point in rock layers typically remaining stationary.

  • In the interseismic period, while the fault is locked, the rocks on either side of the fault continue to move due to tectonic forces.

  • Ongoing debate on whether stresses come from the brittle upper crust or the more viscous underlying layers.

Interseismic and Coseismic Periods

  • Interseismic: Time during which elastic strain builds up while faults remain stuck.

  • Coseismic: The time frame during which the earthquake occurs, characterized by the fault slipping after a significant build-up of strain.

  • Post-seismic: Observable gradual return of strain that might not result in complete recovery, characterized by viscous behaviors below the fault line.

Rupture Dynamics

Hypocenter vs. Epicenter

  • Hypocenter: The 3D location where the earthquake starts, including depth (z-axis).

  • Epicenter: The projection of the hypocenter on the surface, defined by latitude and longitude only.

  • The relationship between the depth of the fault and the epicenter can significantly impact identification on maps.

Rupture Propagation

  • The process through which the earthquake rupture spreads is not instantaneous across the fault but occurs progressively (typically at speeds between 2-3 km/s).

  • Slip refers to the amount of movement that occurs at a specific point along the fault.

Slip Variations

  • Most slip occurs near the hypocenter, but variations may occur based on local geological conditions.

  • The fault slip varies spatially along the fault, which affects the overall earthquake magnitude.

Wave Types and Properties

Different Seismic Wave Types

  • Body Waves: Two types, P-waves (compressional) and S-waves (shear).

  • Surface Waves: Two types, Love waves (side-to-side movement) and Rayleigh waves (rotational motion akin to ocean waves).

Properties of Seismic Waves

  • Amplitude: The distance from the resting position to the crest/trough of the wave, indicating the wave's magnitude.

  • Wavelength (λ): The distance between two successive crests or troughs of a wave, crucial for understanding wave behavior.

  • Frequency (f): The number of crest passages per unit time; inversely related to period (T), which is the time between successive crests.

  • Speed of waves varies:

    • P-waves: 6-8 km/s (fastest, primary wave)

    • S-waves: 3-5 km/s

    • Love Waves: ~2 km/s

    • Rayleigh Waves: 1-3 km/s (slowest, but most destructive).

Seismographic Techniques

Basics of Earthquake Measurement

  • Primary historical method: Using a pendulum system with paper to record seismic waves.

  • Modern techniques employ digital seismometers that measure ground oscillations more accurately.

    • Strong motion seismometers: Designed for high amplitude records, capable of capturing displacements, velocities, and accelerations.

Summary and Relevance

  • Understanding the mechanics of earthquakes, wave types, and measurement techniques lays the groundwork for anticipating impacts on the built environment.

  • This elucidation is valuable for construction practices and emergency preparedness regarding seismic activity.

  • Further exploration into the effects of seismic wave propagation and fault slip will continue to contribute to both scientific knowledge and public safety.

Conclusion

  • The discussion highlights the recurring patterns in fault behavior, the types of seismic waves produced, and how they can be measured to better understand and prepare for earthquakes.