Lab11

.pdf

School

University of British Columbia *

*We aren’t endorsed by this school

Course

210

Subject

Geology

Date

Apr 3, 2024

Type

pdf

Pages

10

Uploaded by ChefEnergy13358 on coursehero.com

UBC Geological Engineering - EOSC 210 Lab 11: Earthquake Hazards 1 Lab 11 EARTHQUAKE HAZARDS Learning Objectives: 1) Interpret seismograms to map an earthquake epicenter. 2) Calculate the energy released from an earthquake. This lab is meant to introduce you to the basic principles of earthquake analysis. Earthquakes often result in great destruction and loss of human property and life, therefore, it is important to understand their causes and effects. Seismic Waves Earthquakes are shock waves caused when crustal plates move relative to one another. Stress builds up in the crustal rock until it exceeds the strength of the rock. The rock fails and slip along fractures that form. The failure of the rock releases a great amount of energy which travels through the earth in the form of waves. The point in the earth’s interior where the earthquake occurs is called the focus . The projection of this point to the earth’s surface is called the epicentre . This point is usually where the earthquake is most strongly felt. There are three kinds of waves which emanate from the earthquake source, traveling in all directions: 1) P-waves - The P-wave, or primary wave, is a compressional wave which travels in a push/pull motion. These are the fastest waves and are the first to arrive at the recording station. 2) S-waves - The S-wave, or secondary wave, travels in a transverse sinusoidal motion. They are slower than the P-waves and arrive at the recording station later. 3) L-waves - The L-wave, or surface wave, travels along the periphery of the earth in an elliptical pattern. L-waves are the slowest of the waves but have a large amplitude causing the greatest damage of the three waves.
UBC Geological Engineering - EOSC 210 Lab 10: Earthquake Hazards 2 P-wave motion : dilatations compressions wave front wave direction S-wave motion : wave direction Measuring Earthquakes Scientists have set up a web of instrumentation around the world to detect even the smallest and most remote earthquakes. Basic systems include geophones which sense the seismic motions in the earth, amplifiers which magnify the signal and seismographs which physically record the measured vibrations. The vibrations are recorded on paper wrapped around a drum that moves with time. This record is called a seismogram . Interpreting Seismograms The interpretation of seismograms makes possible the determination of the position of an earthquake’s epicentre. The first step in interpreting a seismogram is to d etermine the starting position (i.e. arrival times) of the P- and S-wave deflections. The first vibrations recorded mark the arrival of the primary (P) wave with the arrival of the secondary (S) wave coming a short time later and having a higher amplitude trace. The seismogram is broken up into regular intervals representing time (each line on the seismogram represents 15 minutes) allowing for the easy determination of the arrival time. Once the arrival times are determined, it is possible to calculate the distance the waves have traveled by assuming an average wave velocity. The calculated distance is equal to the distance from the epicentre to the geophone where the earthquake is detected. Graphs such as the one on the following page can be used to simplify this process. These types of graphs are constructed based on world-wide seismic data. Since P- and S- waves travel at slightly different velocities, the lag in arrival times is proportional to the distance between the geophone and the epicentre. To determine the distance from the epicentre to the geophone determine the difference in arrival times between the P- and S-waves. Plot this point on the Y-axis of the chart. Project a line from this point to the S-P line and then project another line from this intersection point down to the X-axis (make sure all lines are perpendicular to the axis). This will give the distance that the geophone is from the epicentre of the earthquake. Since the waves from the earthquake travel radially, calculated distances from two other geophone sites are required to triangulate the earthquake’s epicentre.
UBC Geological Engineering - EOSC 210 Lab 10: Earthquake Hazards 3
UBC Geological Engineering - EOSC 210 Lab 10: Earthquake Hazards 4 Assignment 1 of 3: Earthquakes Part of a seismogram recorded at the Frobisher Bay seismograph station, North West Territories, on June 30 th , 1964 is given on the next page. Distinguish between “background noise” and the record of vibrations associated with an earthquake. Keeping in mind that only a part of the seismogram is reproduced (note the time markings), answer the following questions. 1). What was the arrival time of the P-wave (to the nearest second)? 2). What was the arrival time of the S-wave? Since P- and S-waves are propagated from the focus with slightly different velocities, the lag in arrival times is proportional to the distance between the seismograph and the epicentre. Use the Time-Distance chart (given on the preceding page) to relate the difference in arrival times to the distance from the epicentre. 3). What is the observed difference (in seconds) in the arrival times? 4). How far was Frobisher Bay away from the epicentre? 5). What was the travel time of the P-wave? 6). What was the travel time of the S-wave?
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
  • Access to all documents
  • Unlimited textbook solutions
  • 24/7 expert homework help