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Locating the source of earthquakes
Focus - the place within Earth where
earthquake waves originate
Epicenter on an earthquake– location on
the surface directly above the focus
Epicenter is located using the difference in
velocities of P and S waves
Earthquake focus and epicenter
Locating the epicenter of an earthquake
Three seismographs needed to locate an epicenter
Each station determines the time interval between the
arrival of the first P wave and the first S wave at their
location
A travel-time graph then determines each station’s
distance to the epicenter
On a map, draw a circle around the seismic station
Radius of circle= distance to epicenter
Repeat for other seismic stations (Triangulation)
The three circles will meet at a point; that point is the
epicenter
Travel-time graph used to find distance to
epicenter
The difference in
arrival time of the
First P wave and S
Wave in the
Example is 5 minutes
Typical seismic record. Note the time interval (about 5 minutes)
between the arrival of the first P waves and the arrival of the first S
waves
The records obtained from seismographs, called seismograms, provide a great deal
Of information about the behavior of seismic waves
Earthquake epicenter is located using the distance
obtained from three seismic stations
Earthquake belts
95% of energy released by earthquakes
originates in narrow zones that wind
around the Earth
These zones mark of edges of tectonic
plates
Locations of Earthquakes
Most occur along tectonic plate boundaries:
– Around the Pacific Ocean (Circum-Pacific belt is major
site of earthquakes) (‫)الحزام المطوق للمحيط الهادي‬
– Mediterranean Sea area through Iran and on the
Himalayas
– Asia (Indonesia, Himalayan region)
– Mid ocean ridges (‫)مرتفعات وسط المحيطات‬
Some occur far from plate edges
Locations of earthquakes with magnitude equal to
or greater than 5 for the period 1980 to 1990
Circum Pacific Belt
Oceanic ridge system
Depths of Earthquakes
Earthquakes originate at depths ranging
from 5 to nearly 700 kilometers
Definite patterns exist
– Shallow focus occur along oceanic ridges
(less than 70 km)
- Intermediate Earthquakes: 60-300 km
– Deep earthquakes occur in western Pacific
east of oceanic trenches (greater
than 300 km
-90 % of earthquakes are found in a depth
less than 100 km
Earthquake in subduction zones
Measuring the size of
earthquakes
Two measurements describe the size of an
earthquake
Intensity – a measure of earthquake shaking at a
given location based on amount of damage
Intensity of an earthquake depends on:
– Distance to epicenter
– Building materials and design
– Type of ground material-soil, rock, etc.
Magnitude – estimates the amount of energy
released by the earthquake
Intensity scales
Modified Mercalli Intensity Scale was
developed using California buildings as its
standard
Ranges from I (felt by only a few) to XII
(total destruction)
Magnitude scales
Richter magnitude - concept introduced by Charles Richter
in 1935
Richter scale
– Measures earthquake magnitude
– Determined by measuring the amplitude of the largest
waves on the the seismogram
– A logarithmic scale
– Each number on the Richter Scale is tens times greater
in wave amplitude. The wave amplitude of an
earthquake of “magnitude 6”is 10 times more than the
amplitude of a “ magnitude 5 earthquake”
– Each number on the Richter Scale involves an energy
release about 32 times as great. Thus, an earthquake
with a magnitude of 6.5 releases 32 times more energy
than one with a magnitude of 5.5
Magnitudes scales
Moment magnitude was developed
because Richter magnitude does not
closely estimate the size of very large
earthquakes
– Derived from the amount of
displacement that occurs along a fault
and the area of the fault that slips
Earthquake destruction
Amount of structural damage depends on
Intensity and duration of vibrations
Nature of the material upon which the
structure rests (hard rock good, soft bad)
Design of the structure
Results of Earthquakes
Vibration of the ground-Causes damage to structures
Thixotropic sediment causes ground to flow. Thixotropic is a
term used to refer to a solid that flows when vibrated or jolted
Tsunami-seismic sea waves
Fires
Landslides
Well water levels fluctuate; 1964 Alaska caused wells in SE
US to fluctuate by 3.5 meter
Tsunamis, or seismic sea waves
Earthquakes under the ocean
Destructive waves called “tidal waves”
Result from “push” of fault block or
under sea landslide on water
In open ocean height is > 1 meter
In shallow coast water wave can be > 30
meters
Very destructive
Formation of a tsunami by displacement of
the ocean floor
The speed of a wave moving across the ocean correlates
with the ocean depth.
Tsunami 1960, Hawaii
Can earthquakes be predicted
Short-range predictions
Goal is to provide a warning of the location
and magnitude of a large earthquake within
a narrow time frame
Research has concentrated on monitoring
possible precursors – phenomena that
precede a forthcoming earthquake such as
measuring uplift, subsidence, and strain in
the rocks
Earthquakes cannot be predicted
Short-range predictions
Currently, no method exists for making
short-range earthquake predictions
Long-range forecasts
Calculates probability of a certain
magnitude earthquake occurring over a
given time period
Probability of a major earthquake from 1988 to 2018 on the San Andreas Fault
Major historical earthquakes
1811 and 1812-Three large earthquakes, the largest
in U.S. history occurred near New Madrid,
Missouri (7.9)
1886 Charleston, South Carolina (7.9)
1906 San Francisco, CA (7.8)
1964 Alaska (9.2). Strongest earthquake to hit US
this century
1989 San Francisco, California (Loma Prieta) (6.1)
Importance of Earthquakes
Earthquake waves provide information about
the internal structure and composition of earth
Earthquakes are indictors of present-day
tectonic activity or earth movements