Seismic Waves and Earthquakes Study Notes

Seismic Waves

  • Introduction to seismic waves

    • Seismic waves are the energy waves produced by earthquakes. They carry energy away from the source of the earthquake into the Earth and beyond.

  • Installing MyShake

    • Students are instructed to install the MyShake app on their smartphones for class interaction and to contribute data regarding seismic events.

Waves

  • General Definition

    • Waves are periodic disturbances that transfer energy from one point to another. They can be mechanical (requiring a medium) or electromagnetic (can travel through vacuum).

Seismic Waves Overview

  • Types of Seismic Waves

    • Seismic waves are categorized based on their paths through the Earth:

      • Body Waves

      • Travel through the interior of the Earth. Consist of:

      • P-waves (Primary Waves)

        • Characterized by first arrival at a seismograph station.

        • Particle motion is described as push-pull motion.

        • Fastest seismic waves, traveling through solid, liquid, and gas.

      • S-waves (Secondary Waves)

        • Arrive second at a seismograph station.

        • Particle motion is side-to-side (shear) or vertical motion.

        • Can only travel through solids.

      • Surface Waves

      • Travel along the Earth's surface. They are generally slower than body waves and include:

      • Love Waves

        • Move the ground side-to-side horizontally.

      • Rayleigh Waves

        • Cause an elliptical motion, rolling along the surface similar to ocean waves.

Making a Seismogram

  • Using MyShake Application

    • Install MyShake on your smartphone.

    • To record seismic activity:

      1. Place the smartphone flat on a desk.

      2. Open the sensor page on the app.

      3. Understand the axis of movement:

      • Vertical vibration is measured along the z-axis (P-wave).

      • Horizontal vibrations are along the x-axis (side-to-side) and y-axis (up and down).

Seismic Waves Detection Example

  • 1906 San Francisco Earthquake

    • Recorded seismic waves at Cheltenham, Maryland, showing the arrival times of P-waves, S-waves, and surface waves over a time range of 0 to 55 minutes.

USGS ShakeMap

  • Recent Earthquake of Interest

    • Ridgecrest Earthquake (July 5th, 2019)

      • Magnitude: M7.1

      • Distance from Los Angeles: 186 km

      • Notable Arrival Times:

      • P-wave: 20:22 (UTC)

      • S-wave: 20:43 (UTC)

      • Origin Time: 03:19:53 (UTC)

      • Significant distances to other cities:

      • Columbia, CA: 353 km

      • Boston, MA: 3991 km

  • Arrivals and Behavior of Seismic Waves

    • P and S wave arrivals are discussed with their respective times providing insight into earthquake wave propagation.

    • Highlight the difference in travel time which indicates the distance from the earthquake epicenter.

Locating Earthquakes

  • Determining Earthquake Location

    • Using seismic waveforms at different stations provides data on earthquake information.

    • Requires three observations of seismic arrival times from three different stations to determine:

      • Latitude

      • Longitude

      • Origin time

    • Needs to calculate distance based on the S-P time (the difference in arrival of P and S waves).

Source Mechanism

  • Understanding Fault Types

    • Analysis of seismic waves helps understand the types of fault movements and characteristics of earthquakes.

    • Types of Faults:

      • Normal Faults

      • Associated with divergent boundaries; characterized by tension stress.

      • Reverse Faults

      • Associated with convergent boundaries; characterized by compression stress.

      • Strike-slip Faults

      • Associated with transform boundaries; characterized by horizontal shear stress.

    • Moment Tensor Analysis

      • Graphical representation (beach-ball diagrams) is used to illustrate the focal mechanism of earthquakes indicating the type of faulting that occurred.

Real-World Applications of MyShake

  • Overview of MyShake Functionality

    • Global seismic network with over 4.2 million downloads and active 2.5 million users.

    • Enables earthquake detection and provides estimates regarding locations based on P and S wave arrival data.

    • The application can detect seismic activity when the phone is stationary and charging.

Conclusion and Key Learnings

  • Seismometers and accelerometers record ground motion which can be explored using the MyShake app.

  • Seismic waves are essential in understanding the interior structure of the Earth and informing about earthquake characteristics and locations.

  • Utilizing P- and S-wave arrival times at a network of seismic sensors allows for more accurate determination of earthquake parameters.

  • The different types of seismic waves and their properties play a crucial role in earthquake studies and hazard assessment.