BACKGROUND:
Damage caused by earthquakes varies depending on many factors. In order
to predict what the damage may occur, many factors must be considered. These
include:
- Intensity of the earthquake
- Distance from the focus
- Type of soil/rock buildings are built on
- Type of material the building is composed of
- Probability of a landslides if in a mountainous area
- Probability of tsunami (large ocean waves) if near a coastline
If a city is close to the epicenter of a large earthquake (6.0 or above
on the Richter scale) it may have significant damage. The damage may be
substantial if the buildings are not reinforced. Brick buildings are more
prone to "fall" apart. In addition, if the city is situated on
unconsolidated soil or landfill, damage will likely be greater, because
these materials settle and/or liquify during earthquakes.
Earthquakes are caused by the sudden movement and fracturing of rock
masses along preexisting faults. A fault is a broken surface within the
Earth’s crust. The point on the fault at which the displacement begins is
called the focus of the earthquake. The point on the surface of the earth
directly above the focus is the epicenter. The energy released by
earthquakes travels through the Earth as seismic waves.
Scientists have developed many scales to measure the intensity of
earthquakes. Two common ones are the Richter Scale and Modified Mercalli
Scale. The Richter Scale measures the size of the waves produced by the
earthquake, hence the energy it releases. The Richter Scale is absolute, so
the same Richter magnitude wherever they measure the earthquake. The
Modified Mercalli Scale describes what a person feels during an earthquake.
This scale is relative because it changes depending on how far you are away
from the epicenter, and how much damage occurs around you.
P-waves and S-waves are easily distinguishable on a seismogram. P-waves
are faster than S-waves, so the first shaking recorded by the seismogram is
always due to P-waves. When S-waves arrive, their energy combines with the
P-waves to amplify the shaking. This makes the waves bigger, and the shaking
stronger. The first arrival of S-waves is thus marked by a distinct increase
in the size of the waves on the seismogram.
PROCEDURE:
- Review and discuss the concept that volcanoes and earthquakes provide
data for understanding the movement of the lithosphere, as explained by
the theory of plate tectonics.
- Show the class images of earthquake damage. You can use slides or
transparencies, or the presentation below. We recommend showing the
students the pictures before they complete the lab, so they can
understanding the damage earthquakes can cause. Explain that damage in
urban areas depends on many factors.
If you wish to customize or create your own presentation, here is
a website that have good earthquake damage information:
http://www.sfmuseum.org/1906/06.html
Damage from the San Francisco earthquake in 1906. A link to photographs
is towards the bottom of the each page.
- Review the Richter Scale and the Modified Mercalli Scale with the
class.
- Introduce the assignment to the students. Explain that interpreting
the seismograms and describing potential earthquake damage both require
imagination. The point is not to get the "right" answers, but
to speculate on what might happen. This is a prediction, after all!
As a class, you may want to have the students find the geographic
locations of the seismograms on the worksheet. Use the United States
placemats. Try to get the students to locate cities near the epicenters
that might be effected by the earthquakes.
- Select a few students to read one or more of their answers to the rest
of the class. The class should conclude that according to the
seismogram, the San Fernando Valley earthquake appears to be the most
intense and would probably cause the most damage.
Here is an example of a student response:
Seismogram 1. The earthquake was probably a sharp quake. Maybe a 5 or 6
on the Richter scale. However, conditions in the ocean created a large
tsunami (or tidal wave) and it caused major destruction along the coast.
Downtown Anchorage was not affected, except for small items thrown from
shelves.
