Earthquakes
Earth Sciences 2
Name __________________
Problem 1. Earthquake Activity. Determining Mercalli Intensity scale exercise.
For this exercise we are going to use. the Morgan Hill Earthquake of
1984. The Morgan Hill earthquake, also called the Halls Valley earthquake,
occurred on 24 April 1984 along the Calaveras fault 18 km east of San Jose and
22 km north of Morgan Hill. It had a magnitude of ML=6.2 and a focal depth of
8 km. The earthquake was felt in California and Nevada over an area of 120,000
km2 and caused damage estimated to be worth $7.5 million. After the
earthquake, many cracks were found in the ground, but no unequivocal
evidence of surface faulting was found. The aftershocks define an area that
extends approximately 30 km southeast of the epicenter along the Calaveras
fault. This is the section of the fault that has generally regarded as having
ruptured at depth during the earthquake.
On the next page you have the lists reports of the effects of the Morgan Hill
earthquake in different places. Complete the table by assigning Mercalli
intensities for all of the locations. Intensity is usually given in Roman
numerals, You can give intermediate values (e.g., 4.5) if you feel it is
appropriate.
You can you used the
map to help you
estimating the
Intensity
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Earthquakes
Earth Sciences 2
Name __________________
Location
Distance to
Epicenter
Distance to
Fault
Description of Shaking
Antioch
72 km
72 km
Berkeley
70 km
70 km
Coyote
32 km
8 km
Gilroy
36 km
12 km
Hollister
58 km
32 km
Jackson
Oaks
18 km
2 km
Los Angeles
530 km
500 km
Felt by several; frightened a few;
hanging objects swung slightly.
Felt by many and frightened some; small
objects moved but did not overturn;
hanging pictures swung out of place.
Felt by all; some windows broken out;
light and heavy furniture was
overturned; many objects thrown from
store shelves; chimneys fell.
Many items thrown from store shelves;
many cracks in walls; one wardrobe
toppled; several chimneys fell.
Plate glass window jumped from the
frame and shattered; several windows
broken out; many dishes broken; light
furniture overturned; few items thrown
from shelves; hairline cracks in walls.
At least four houses knocked off
foundations; walls shifted; many
chimneys down; pictures fell off walls;
refrigerator toppled and furniture broke.
Not felt.
Morgan Hill
22 km
6 km
Sacramento
145 km
145 km
Salinas
75 km
49 km
Hanging pictures fell; windows cracked;
light and heavy furniture overturned.
San Jose
18 km
18 km
San Martin
27 km
6 km
Cracks in some walls and plaster fell; no
glass broken; dishes broken; many items
thrown from store shelves; pictures fell;
some chimneys cracked.
Felt by all; many cans of paint fell from
shelves; many dishes broken; heavy
furniture overturned; many items
thrown from store shelves; some
chimneys fell; cracks in walls.
Mercalli
Felt by all; light damage to 47 homes; a
few chimneys fell; mobile homes fell off
supports; many items thrown from store
shelves.
Felt by a few; slight swaying of curtains
on the 16th floor of downtown building.
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Earthquakes
Earth Sciences 2
Name __________________
Based on you answers answer the following questions
Q1 Which location felt the strongest intensity. ?
Q2 Is there a relation between distance from epicenter or distance to fault and the Intensity? Why?
Explain ?
Q3 This earthquake was in California where there is a Transform plate boundary Q. What type of
faults you find in this kind plate motion? Also draw and example.
Q4 Can Tsunamis be generated with this type of Fault? Please explain Why or Why not?
Q5 When there is and earthquake in the ocean sometimes a Tsunami is generated which wave travels
the fastest the Seismic Waves or the Water wave of the Tsunami. ?
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Earthquakes
Earth Sciences 2
Name __________________
Modify Mercalli Intensity Scale (Source USGS)
I.
II.
Not felt except by a very few under especially favorable conditions.
Felt only by a few persons at rest, especially on upper floors of buildings.
III.
Felt quite noticeably by persons indoors, especially on upper floors of buildings.
Many people do not recognize it as an earthquake. Standing motor cars may rock
slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV.
Felt indoors by many, outdoors by few during the day. At night, some awakened.
Dishes, windows, doors disturbed; walls make cracking sound. Sensation like
heavy truck striking building. Standing motor cars rocked noticeably.
V.
VI.
Felt by nearly everyone; many awakened. Some dishes, windows broken.
Unstable objects overturned. Pendulum clocks may stop.
Felt by all, many frightened. Some heavy furniture moved; a few instances of
fallen plaster. Damage slight.
VII.
Damage negligible in buildings of good design and construction; slight to
moderate in well-built ordinary structures; considerable damage in poorly built or
badly designed structures; some chimneys broken.
VIII.
Damage slight in specially designed structures; considerable damage in ordinary
substantial buildings with partial collapse. Damage great in poorly built
structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy
furniture overturned.
IX.
X.
Damage considerable in specially designed structures; well-designed frame
structures thrown out of plumb. Damage great in substantial buildings, with
partial collapse. Buildings shifted off foundations.
Some well-built wooden structures destroyed; most masonry and frame structures
destroyed with foundations. Rails bent.
XI.
Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent
greatly.
XII.
Damage total. Waves seen on ground surfaces. Lines of sight and level distorted.
Objects thrown upward into the air.
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