Each station determines the time interval between the

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 is used to determine
each station’s distance to the epicenter

Locating the source
of earthquakes

Locating the epicenter of an earthquake
•
A circle with a radius equal to the distance
to the epicenter is drawn around each
station
•
The point where all three circles intersect is
the earthquake epicenter

Finding an
earthquake
epicenter

Measuring the size
of earthquakes

Two measurements that describe the size of
an earthquake are
•
Intensity – a measure of the degree of
earthquake shaking at a given locale based
on the amount of damage
•
Magnitude – estimates the amount of energy
released at the source of the earthquake

Measuring the size
of earthquakes

Intensity scales
•
Modified Mercalli Intensity
Scale was developed using
California buildings as its
standard
•
The drawback of intensity
scales is that destruction may
not be a true measure of the
earthquake’s actual severity

Measuring the size
of earthquakes

Magnitude scales
•
Richter magnitude - concept introduced by
Charles Richter in 1935
•
Richter scale
•
Based on the amplitude of the largest seismic
wave recorded
•
Accounts for the decrease in wave amplitude
with increased distance

Measuring the size
of earthquakes

Magnitude scales
•
Richter scale
•
Largest magnitude recorded on a Wood-
Anderson seismograph was 8.9
•
Magnitudes less than 2.0 are not felt by humans
•
Each unit of Richter magnitude increase
corresponds to a tenfold increase in wave
amplitude and a 32-fold energy increase

Measuring the size
of earthquakes

Magnitude scales
•
Other magnitude scales
•
Several “Richter-like” magnitude scales have
been developed
•
Moment magnitude was developed because none
of the “Richter-like” magnitude scales
adequately estimates very large earthquakes
•
Derived from the amount of displacement that
occurs along a fault

Measuring the size
of earthquakes

Earthquake destruction

Amount of structural damage attributable
to earthquake vibrations depends on
•
Intensity and duration of the vibrations
•
Nature of the material upon which the
structure rests
•
Design of the structure

Earthquake destruction

Destruction from seismic vibrations
•
Ground shaking
•
Regions within 20 to 50 kilometers of the
epicenter will experience about the same
intensity of ground shaking
•
However, destruction varies considerably
mainly due to the nature of the ground on
which the structures are built

Important historical
earthquakes
•
Shaanxi, China, 1556 - ~830,000 deaths M 8.0
•
Tangshan, China, 1976 - ~650,000 deaths M 8.2
•
Aleppo, Syria, 1138 - ~230,000 deaths M 8.5
•
Sumatra, Indonesia, 2004 – 227,898 deaths M 9.1
•
Haiti, 2010 – 222,570 deaths M 7.0
•
Gansu, China, 1920 - ~220,000 deaths M 7.8-8.5
•
Damghan, Iran, 856 - ~200,000 deaths M 8.0
•
Xining, China, 1927 - ~200,000 deaths M 7.9
•
Kanto, Japan, 1923 – 142,800 deaths M 7.9
•
Ashgabat, Turkmenistan, 1948 – 110,000 deaths M 7.3
•
Messina, Italy, 1908 – 100,00-200,000 deaths M 7.1

Largest earthquakes
since 1900
•
Valdivia, Chile, 1960 – M 9.5
•
Anchorage, Alaska, 1964 – M 9.2
•
Sumatra, Indonesia, 2004 – M 9.1