Download The California earthquake of April 18, 1906 ranks as one of the most

The San Francisco Earthquake 1906
The California earthquake of April 18, 1906 ranks as one of the most significant earthquakes of
all time. Today, its importance comes more from the wealth of scientific knowledge derived
from it than from its sheer size. Rupturing the northernmost 296 miles (477 kilometers) of the
San Andreas fault from northwest of San Juan Bautista to the triple junction at Cape Mendocino,
the earthquake confounded contemporary geologists with its large, horizontal displacements and
great rupture length. Indeed, the significance of the fault and recognition of its large cumulative
offset would not be fully appreciated until the advent of plate tectonics more than half a century
later. Analysis of the 1906 displacements and strain in the surrounding crust led Reid (1910) to
formulate his elastic-rebound theory of the earthquake source, which remains today the principal
model of the earthquake cycle.
At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt
widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25
seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong
shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to
south of Los Angeles and inland as far as central Nevada. The highest Modified Mercalli
Intensities (MMI's) of VII to IX paralleled the length of the rupture, extending as far as 80
kilometers inland from the fault trace. One important characteristic of the shaking intensity noted
in Lawson's (1908) report was the clear correlation of intensity with underlying geologic
conditions. Areas situated in sediment-filled valleys sustained stronger shaking than nearby
bedrock sites, and the strongest shaking occurred in areas where ground reclaimed from San
Francisco Bay failed in the earthquake. Modern seismic-zonation practice accounts for the
differences in seismic hazard posed by varying geologic conditions.
As a basic reference about the earthquake and the damage it caused, geologic observations of
the fault rupture and shaking effects, and other consequences of the earthquake, the Lawson
(1908) report remains the authoritative work, as well as arguably the most important study of a
single earthquake. In the public's mind, this earthquake is perhaps remembered most for the fire
it spawned in San Francisco, giving it the somewhat misleading appellation of the "San
Francisco earthquake". Shaking damage, however, was equally severe in many other places
along the fault rupture. The frequently quoted value of 700 deaths caused by the earthquake and
fire is now believed to underestimate the total loss of life by a factor of 3 or 4. Most of the
fatalities occurred in San Francisco, and 189 were reported elsewhere.
The following figure shows the extent of the 1906 rupture seen at the surface. (Slip on offshore
segments of the San Andreas fault north and south of Shelter Cove is inferred from comparisons
of geodetic observations made before and after 1906).
The total length is 296 miles (477 kilometers). For comparison, the 1989 Loma Prieta
earthquake had a rupture length of about 25 miles (40 km).
"At almost precisely 5:12 a.m. local time, a foreshock occurred with sufficient force to be felt
widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25
seconds later, with an epicenter near San Francisco... Violent shocks punctuated the strong
shaking, which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to
south of Los Angeles and inland as far as central Nevada." - from Ellsworth (page 159).
http://quake.wr.usgs.gov/info/1906/index.html
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The San Francisco Earthquake 1906
Based on what we learned from the 1989 Loma Prieta earthquake, the best guess is that in places
where the offset approached 20 ft, it probably took 4 to 5 seconds to move this distance.
This would imply a speed of 4 to 5 ft/sec or approximately 3 miles/hr.
Note, however, that the rupture itself probably propagated at a speed of about 2.7 km/sec or
5,800 mi/hr. (See the paper by Wald and others.)
Intensity today is measured on the Modified Mercalli Intensity scale. (Thanks to ABAG Association of Bay Area Governments - for this table.)
Many intensity values of 8 (Moderate Damage) and 9 (Heavy Damage) occurred in a broad band
along the rupture and extending as much as 60 miles to the east of the San Andreas fault.
The magnitude of 1906 has been redetermined in two recent studies that used rather different
approaches:
M = 7.7
David Wald, Hiroo Kanamori, Donald Helmberger, and Thomas Heaton (Cal Tech and USGS)
studied recordings of the 1906 seismic waves made at 96 observatories around the world. Data
from 12 of these observatories were of good enough quality to be digitized and modeled, and
suggest a surface wave magnitude (Ms) of 7.7. They also found that most of the shaking energy
came from two separate areas on the fault -- one between Point Reyes and Fort Ross to the north
and the second on the San Francisco Peninsula to the south.
M = 7.9
Wayne Thatcher, Grant Marshall, and Michael Lisowski (U.S.G.S.) have re-evaluated the
geodetic data from before and after the 1906 earthquake. The offset on the fault plane produces
ground deformation over a broad region around the fault. This deformation manifests itself as
changes in the angles and distances between benchmarks in geodetic networks. Using the
observed changes, these scientists constructed models of the slip distribution on the fault plane in
1906. Based on the amount of slip, these models suggest a moment-magnitude (Mw) of 7.9.
M = 8.3?
The traditional magnitude of 8 1/4 or 8.3 comes from Richter (1958). However, the Richter
magnitude scale was developed for local earthquakes recorded on high-frequency seismometers.
The preferred descriptor for large earthquakes rich in low frequencies is the moment magnitude
as used by Thatcher et al (above). This magnitude is directly proportional to energy release and
can be obtained from analysis of broad-spectrum seismograms or from the product of the rupture
area and average fault slip. Each unit step in moment magnitude is equivalent to roughly a factor
of 32 in energy release, thus it would require roughly 30 1989 Mw=6.9 Loma Prieta earthquakes
occurring simultaneously to equal the energy release of 1906.
http://quake.wr.usgs.gov/info/1906/index.html
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The San Francisco Earthquake 1906
Why can't scientists agree on magnitudes for earthquakes?
For older earthquakes, the data are often of poor quality or the few stations existing at the time
may not have been in the best locations to record the information needed. There are also a
number of different ways to measure the "size" of an earthquake, which because of details of the
rupture process for an individual earthquake, may not always agree, even for recent earthquake.
Dead - More than 3,000
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A report of U.S. Army relief operations (Greely, 1906) recorded:
o 498 deaths in San Francisco
o 64 deaths in Santa Rosa
o 102 deaths in and near San Jose
A 1972 NOAA report suggested that 700-800 was a reasonable figure.
Gladys Hansen and Emmet Condon, after extensive research, estimated that over 3000
deaths were caused directly or indirectly by the catastrophe. The population of San
Francisco at the time was about 400,000.
Homeless - 225,000

225,000 from a population of about 400,000. (photo) (photo) (photo)
Buildings Destroyed - 28,000
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"The 3-day conflagration following the earthquake caused substantially more damage
than did the earthquake. The area of the burned district covered 4.7 square mile..."
(NOAA report). By one count:
o Wood buildings lost = 24,671 (photo)
o Brick buildings lost = 3,168 (photo) (photo)
o Total buildings lost = 28,188 (photo) (photo) (photo)
Monetary Loss - More than $400 million

Estimated property damage (NOAA report): $400,000,000 in 1906 dollars from
earthquake and fire, $80,000,000 from the earthquake alone. (photo) (photo) (photo)
http://quake.wr.usgs.gov/info/1906/index.html
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The San Francisco Earthquake 1906
What are earthquakes?
Most people living in California have at one time or another experienced an earthquake.
Without warning we wake up in the middle of the night because of the sometimes
violent, sometimes slow rolling motion of the earth; other times we are driving on the
freeway, walking in the mall, or even just watching television when an earthquake
occurs. So what are earthquakes? What causes them? Why do earthquakes feel so
different from one another?
Earthquakes are the Earth's natural means of releasing stress. When the Earth's plates
move against each other, stress is put on the lithosphere. When this stress is great
enough, the lithosphere breaks or shifts. Imagine holding a pencil horizontally. If you
were to apply a force to both ends of the pencil by pushing down on them, you would
see the pencil bend. After enough force was applied, the pencil would break in the
middle, releasing the stress you have put on it. The Earth's crust acts in the same way.
As the plates move they put forces on themselves and each other. When the force is
large enough, the crust is forced to break. When the break occurs, the stress is
released as energy which moves through the Earth in the form of waves, which we feel
and call an earthquake.
http://quake.wr.usgs.gov/info/1906/index.html
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