Earthquakes and California Tectonics

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Earthquakes and California Tectonics

• Chapman University ESCU 234
• Online Course

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Lecture 1 A Brief History of Earthquakes and
Seismology

• In the last 500 years, more than 7 million people
  have died from earthquakes and many more have
  seen their food sources and local economies
  destroyed
• In the 20th century alone, earthquakes have
  caused over 50 of deaths caused by natural
  disasters
• The hazard that earthquakes present to a growing
  population is often the primary concern of
  scientists and engineers

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Yet earthquakes have also proven to be a great
source of geologic knowledge - structure of
earths interior - composition of earths
interior
Seismology, the scientific study of earthquakes,
is an extraordinarily young science only about
100 years. Although people have speculated about
the causes of earthquakes for 1,000s of years.
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The Earliest Records

• The oldest written records of earthquakes have
  been traced back to 1831 B.C. in the Shandong
  Province of China. The record states merely of
  the shaking of Taishan Mountain.
• But the record is fairly complete only from 780
  B.C.
• Some historical reports are so detailed that from
  them, modern studies have been able to establish
  the distribution of damage and the size of the
  earthquakes

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Example

• On September 2, 1679, the greatest known
  earthquake near Beijing, China, was mentioned in
  the records of 121 cities. When modern
  researchers compared the descriptions of building
  damage, ground cracks and other features near the
  source together with reports of shaking, with
  recent earthquakes, they concluded it was similar
  to the 1906 San Francisco earthquake (magnitude
  7.9).
• Despite their careful documentation, Chinese
  scholars were unable to achieve any insight into
  the causes of the catastrophic ground shaking

• It was thought that earthquakes and other
  natural disasters were caused by supernatural
  forces such as the Namazu.

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Early Greek Ideas

• Thales 580 B.C. believed that movements of water
  produced earthquakes
• Anaximenes 526 B.C. believed that Earths
  interior was hollow and rocks falling in the
  interior would strike other rocks causing
  vibrations
• Anaxagoras 426 B.C. viewed fire as the cause of
  at least some earthquakes
• Seneca A.D. 63 believed that earthquakes were
  caused by air finding its way into underground
  passages

Seneca
Thales
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Aristotle 384-322 B.C.

• Like many of his contemporaries was convinced of
  a central fire inside the earth. The fire
  would rise and if obstructed would burst
  violently causing vibrations and noise
• A later modification suggested that these
  subterranean fires would burn away supports of
  the outer parts of the Earth. The ensuing
  collapse would create earthquakes.
• Although incorrect, his ideas were widely
  accepted until the 18th century.
• Other important observations of Aristotle
• Different types of quakes based on shaking was
  mainly vertical or diagonal
• Whether quake was associated with vapors
• Places where subsoil is poor are shaken more
  because of the large amount of wind they absorb

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Modern Enlightenment
A new era dawned in the 18th century under the
influence of Sir Isaac Newtons writings of waves
and mechanics. His laws of motion provided the
physical theory needed to explain earthquake
waves.
Sir Isaac Newton

• 1750 is often referred to as the year of the
  earthquakes because London, England was jolted
  by earthquakes several times. Still and many
  writers showed, earthquakes were usually believed
  to caused by either supernatural causes or causes
  hypothesized by the Greek scholars of long ago.
  In fact, a writer in a scientific journal
  apologized to those who are apt to be offended
  by any attempts to give a natural account of
  earthquakes.

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The Lisbon Earthquake of 1755

• The scientific study of earthquakes received an
  important boost in 1755 when a disastrous
  earthquake struck in the Atlantic Ocean, several
  hundred kilometers south-southwest of Portugal on
  November 1st.
• 60,000 residents of Lisbon were killed
• ocean waves 30-40 above high tide level swamped
  the city
• fires burned for 3 days

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• The Lisbon earthquake of 1755 was a major source
  of inspiration for one of the first modern
  Fathers of Seismology, the British engineer
  John Michell (1724-1793). In his description of
  the earthquake written in 1760, he classified two
  types of earthquake waves
• A tremulous vibration followed by,
• Wavelike undulation of the Earths surface

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Michell also concluded that the speed of
earthquake waves could actually be measured from
their arrival times at two different points.
From eyewitness accounts he calculated that the
waves in the Lisbon earthquake traveled about 500
meters per second (m/s). Although incorrect, it
was the first attempt at such a calculation.
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Robert Mallet and the Great Italian Earthquake of
1857
The earthquake of December 16, 1857 near Naples
in southern Italy provided Robert Mallet the
opportunity to study seismic effects extensively
and lay a firm foundation for modern seismology.
Mallet did much to further the interaction
between engineering, geology, and mechanics. His
goal was to take earthquake studies from a stage
of mystery by applying physical and engineering
principles to the search for the real nature of
earthquakes.
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• In Mallets landmark work The First Principles
  of Observational Seismology he coined much of
  the basic vocabulary describing earthquakes that
  we still use.
• Mallet is also notable for being the first to
  work with artificial earthquakes. He exploded
  charges of gunpowder underground, then recorded
  the waves by watching the surface of a container
  of mercury placed at a distance from the charge.
  A stopwatch gave him the elapsed time between the
  explosion and ripples on the mercury surface.
• From these observations he deduced that
  earthquake waves travel at different speeds
  through different materials. For the first time
  it was clearly understood that seismic waves are
  affected by the physical properties of the
  different rocks through which they pass.
• - sandy soil 280 m/s
• - granite 600 m/s (Too small!)

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• Believing that earthquakes such as that which
  struck Naples were produced by volcanic sources,
  he drew attention to the nearness of volcanoes
  such as Mt. Vesuvius to the area. From his
  notion of an explosive source (which is wrong!),
  Mallet inferred correctly that the seismic waves
  would start at a point, the focus or hypocenter.
• Further, he suggested that seismic waves
  resembled sound waves traveling in the air. From
  this he concluded that the first motion of the
  ground would show a regular direction away from
  the initiation point.
• Used
• objects thrown from heights
• overturned objects
• directions of cracks in buildings
• To calculate the position of a hypocenter.
• Based on his studies, Mallet calculated the
  depth of the Naples earthquake to 6 ½ miles
  beneath the surface.

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Knowing what we do today about earthquakes and
seismology, Mallets methods were impractical.
Nevertheless, this was the first attempt to
estimate the origin of an earthquakes
motion. Not until 50 years later with the
development of the modern seismograph could
earthquake hypocentral depths be calculated (and
still not all that well today!).
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Four Key Earthquakes
In the mid to late 1800s two organizations were
created that contributed much to the science of
earthquakes - 1857 Geological Survey of India -
1879 United States Geological Survey An early
work of the USGS was done by M. L. Fuller in 1912
who published evidence on three extraordinarily
large shocks which occurred along the Mississippi
River - December 16, 1811 - January 23, 1812 -
February 7, 1812 (reportedly the largest)
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• All together, over 1870 earthquakes shook the
  area between December 16 and March 16. Eight of
  these were severe, felt in Louisville, Kentucky,
  200 miles away.
• Damage occurred over 30,000 to 50,000 square
  miles bordering the Mississippi River southward
  from New Madrid, Missouri.
• The largest of these shocks awakened President
  Madison in the White House rang church bells in
  Boston, Massachusetts, and toppled chimneys in
  Cincinnati, Ohio.

Significance At the time, it was a geological
puzzle why such immense earthquake energy was
released in a continental interior. Earthquakes
were always considered to occur on the edges of
continents.
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On June 12, 1897 another very important event
struck the province of Assam in northeast India.
As reported by Richard D. Oldham, head of the
Indian Geological Survey

• The earthquake was felt over an area of 1 ¾
  million square miles
• Completely devastated 9,000 square miles
• Fewer than 1,000 people died because of low
  population density and few large structures
• Shaking (strong) lasted about one minute
• Persons were thrown to the ground
• Powerful accelerations of the ground threw
  boulders straight up
• Sandy soils behaved as a liquid and reported
  homes sinking until only the roof was visible
• Oldham reported that in heavily shaken areas,
  people saw visible waves ( 1 ft. high) moving
  across the ground
• Vertical throw of 35 feet along a deep thrust
  fault

R. D. Oldham
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The Contribution of the 1906 San Francisco
Earthquake
The turning point in our understanding of the
causes of earthquakes came from the studies of
the earthquake that shook central and northern
California on April 18, 1906.

• No active volcanoes nearby, therefore geologists
  were not tempted to turn to old Greek notions
• Source of the 1906 earthquake lay beneath a
  readily traversed area that surveyors had already
  covered with survey markers showing distances and
  relative heights between points (note that these
  were not available in either the 1811-1812 and
  1897 earthquakes)
• This allowed mapping of the ground deformation

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To study the quake the State Earthquake
Investigation Commission was set up under
University of California, Berkeley professor
Andrew Lawson. Scientists assembled under Lawson
compared geodetic measurements prior to and after
the quake. Their report contained the fundamental
theory that has dominated seismology to this day.
The report concluded that the severe ground
shaking had been generated by sudden slip on what
Lawson called the San Andreas Fault. The fault
had slipped over a section extending for more
than 400 km from San Juan Bautista south of San
Francisco Bay to a point 250 km north of San
Francisco.
The San Andreas fault south of San Francisco
following the 1906 earthquake.
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The Japanese Earthquake of 1923
By the 1920s scientists were looking for
patterns in seismic events that might point to
the location of future earthquakes. Dr.
Fusukichi Omori, Director of the Seismological
Institute of Japan had been studying the apparent
distribution of large earthquakes in the vicinity
of Honshu. In 1922 Omori wrote that the vicinity
near Tokyo was seismically quiet but that regions
60 km away it were active (this is called a
seismic gap). Thus he felt the area was due for
a particularly devastating earthquake.
Approximately one year after his prediction, on
September 1, 1923 at about noon when the streets
of Tokyo were filled with people, a particularly
devastating quake hit, the Kwanto earthquake.
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Devastation along Ningyocko St., Tokyo
Seismogram from Kwanto Earthquake
Refugees in front of the Imperial Palace
following the 1923 earthquake
Damage at Miyamachi Tagaata-gun Shiuoka-ken