LECTURE 1 NOTES - HISTORICAL INTRODUCTION (updated 07/15/96)
Instructor: Professor Barbara Romanowicz
Director of Seismological Laboratory
Office Hours: Thursday 2-4 pm , upon appointment only
475 Mc Cone Hall
HISTORICAL INTRODUCTION
Likelihood of being in an earthquake quite high:
worldwide: 1 person in 8000 will die in an earthquake
10 times as many will be injured
last 500 yrs over 7 M people died in earthquakes, many more
had their local economies destroyed (cf Kobe, January '95)
Never a pleasant experience because we take it for granted that the
ground under our feet is stable and firm and we can count on it more
than anything else in the world. Roof above our heads protects us from
all natural elements: this is not so for earthquakes. Contrary perhaps to
intuition the greatest hazard does not come from the earthquake itself
but from man-made construction being damaged during the earthquake. In an open
field, the risk is very small that you will be seriously injured
(except avalanches and floods in mountaineous areas).
Losses/damage are very different in different parts of the world.
Depends how big, how far, how long the shaking lasts construction,
underlying soil, foundations time of day, place in the world with respect
to "belts" of seismicity
Contrast
Armenian Earthquake December 7, 1988 : 25,000 dead M= 6.8
500,000 homeless
Loma Prieta, Oct 17, 1989: 63 died M=7.1
3700 injured
Earthquakes are sources of destruction (can't have everybody living in
tents everywhere).
------------------------> motivation for scientists and engineers
Negative and positive aspect:
- destruction
+ source of geological knowledge
We want to understand:
o Where,
o When,
o How big,
o WHY and HOW?
o How can we protect ourselves?:
- anticipate earthquake occurrence
- build to withstand the shaking
- minimize casualties by:
-rapid warning
-knowing how to react (education)
-organizing emergency relief
- predict earthquakes??????
earthquake source ----- propagation of waves ----- effects of earthquakes
where their properties damage
when what do we learn from protection against
how big them (Earth structure) effects:
what physical process earthquake eng.
(macro/microscopic scales) building codes
warning, emergency relief,
prediction
Seismology is a very young subject compared to physics, chemistry,
biology, astronomy:
first telescope 1600
first seismograph : second half of XIXth century
Remarkable progress since the 1880's on
- causes of earthquakes
- nature of seismic waves
- patterns of seismic activity
- internal structure of the earth
(Two aspects of earthquakes:
- the phenomenon itself: why, where, when, how?
- illumination of the earth's deep interior)
Some milestones:
1882: Von Rebeur Paschwitz, Potsdam, Germany, picks up and recognizes
Japanese earthquake on his seismograph - revolutionized seismology (eg
Copernicus and revolution of the earth?) started the field of "global
seismology".
1906 earthquake (San Francisco): theory of elastic rebound
~1960Ős: acceptance of theory of plate tectonics
We now know earth's outer layers ---> plates, move with respect to
each other and as they rub against each other, they cause earthquakes
to occur.
Earthquakes have been around forever, certainly since mankind appeared
on Earth and there have been speculations about causes of earthquakes
for thousands of years.
I will entertain you today, as an introduction, on the evolution of
thinking from early superstitions to modern understanding of the
sources of shaking.
Earliest Records
China - written - 1831 b.c., fairly complete since 780 b.c.
Detailed historical records permit to establish the distribution of
damage and, by] comparison with modern events, the size (~) of
historical earthquakes.
Prevailing thinking in antiquity: link to other natural catastrophes
floods, droughts, cholera outbursts
---> supernatural intervention
---> religious interpretation
Invoke supernatural animals:
China: Earth rides on the back of a giant ox. When the animal moves->
the Earth shakes and trembles.
Mongolia: Frog
Japan: catfish "namazu", prone to mischief, had to be controlled by a god.
awhen god looked avaw, Namazu wriggled around-> earthquake.
India: several beliefs. e. g. Earth held up by 34 elephants standing on the
back of giant turtle. Turtle standing on a Cobra snake. When any of them
moves, the Earth trembles.
West Africa: giant carries the Earth on his head. All plants are his hair...
When he turns his head, the Earth shakes.
Powerful animals often combined with activities of gods.
Scandinavia: the god Loki was tied to a rock in an underground cave as a
punishement for killing his brother. A serpent dropped poison down on him,
which was caught in a bowl by his sister. When his sister emptied the bowl
on him, Loki had to twist and turn to avoid the poison, causing earthquakes.
Greece: Power of gods invoked in natural catastrophes. The temper tantrums
of the god Poseidon, god of see and earthquakes caused the Earth to shake.
The trident he carries causes the Earth to shake when he strucks it with it.
Reccuring themes: power and wrath (punishement).
Earthquakes often associated with significant theological events.
Many accounts in the Bible (falling of Walls of Jericho ~1100 b.c.,
parting of the Red Sea) can be related to earthquakes (e.g. video by
A. Nur)
Same in arabic documents (Taher)
St Matthew at the time of Christ's death: "... and the Earth did quake,
and the rocks rent..."
The release of Paul and Silas from jail in Macedonia:
"...and suddenly there was a great earthquake so that the foundations of
the prison were shaken: and immediately all the doors were opened and
everyone's bands were loosened" (acts 16:26).
First steps towards physical understanding: Greece
Many intersting ideas
~ 580 bc Thales believed globe floated on oceans and mvt of water
produced shaking
~ 526 bc Anaximenes rocks of Earth cause shaking, falling in the
interior, striking other rocks
~ 428 bc Anaxagoras Fire inside the earth
384-322bc Aristotle First rational account, pragmatic approach
analogies with atmospheric events: thunder, lightning
subterranean events: volcanic activity "central fire" in the
earth:
firestorms rise rapidly and if obstructed burst violently
causing vibrations and noise
Seneca -4 +65: (Compaigne, Italy ,earthquake of AD 63)
Shaking is the result of air finding its way into
underground caves
and corridors -> compressed --> violent storms
Pliny ~ 100 ad: "Tremors of the Earth never occur except when the sea is calm
and the sky so still that birds are unable to soar because all the
breath that carries them has been withdrawn"
Earthquakes often interpreted as punishements bestowed by angry gods:
Greek Mythology: Poseidon (ruler of the sea) causes earthquakes when angry
Roman mythology: Neptune (earthq., tidal waves)
Lisbon , 1755: All Saints Day when people were in church: seen as
punishement for sins (60,000 killed).
Japan: "catfish" movements cause earthquakes
These views largely subsisted to the 18th century
For example, until at least 1700: the idea of vapors as cause of earthquakes
wave were very persistent: often with humanistic concept of the Earth:
i. e. the Earth possessed circulatory system where blood and bodily fluids
were associated with underground rivers.
When the system was upset/perturbed->the Earth became sick --> earthquakes
and other natural catastrophes.
e.g. Shakespeare:
"Diseased Nature oftentimes breaks forth
In strange eruptions: off the teeming earth
Is with a kind of colic pinch'd and vex'd
By the imprisoning of unruly wind
Within her womb which for enlargement striving
Shakes the old beldam earth and topples down
Steeples and moss grown towers"
Henry IV, part 1 (Act III, scene 1)
in 1758 Isnard suggested relieving pressure and preventing earthquakes by
digging deep shafts into the Earth.
18th Century: some milestones
By that time the following changes in the thinking allowed the progress of
science.
1.) Concept that God does not interfere in the affairs of Nature
2.) Evaluation of hypotheses through experimentation
o Newton: 1) laws of motion -> physical formalism for explanation
of earthquake waves
2) laws of gravitation -> understand geological forces that
shape the earth
o Galileo: work on gravitational attraction
o Interest in geological effects: landslides, sea level changes,
damages to structures
o still : causes due to gases and fires: description page 6
o Lisbon, 1755 source of inspiration for John Mitchell (british
engineer)
memoir: earthquake waves are set up by "shifting masses of rocks miles
below the surface" ---- > towards understanding the earthquake source
description of motion:
2 types : "tremulous vibration" followed by "wavelike undulation"
1st attempts at measuring speed of waves from arrival times at two
different points. (500 m/s wrong but first estimate: actually several
km/s).
mitchell proposed an analogy with a carpet raised at one edge and
brought down again -> the air underneath is propelled, passed to the other side
"raising the cloth in a vawe all the way as it goes"
----- > towards understanding wave propagation
but still thought that vapors caused earthquakes.
Important step in the development of earthquake science is still missing:
recognition that faulting is the principal cause of earthquakes. That
was not easy since so few ruptures go all the way to the surface. Also
obscured by perceived relation with volcanic eruptions, with wind causing shaking...
o New world settlements
realization that earthquakes are a worldwide phenomenon not necessarily
linked to volcanoes
1663 Three rivers , Quebec, felt sharplyin New England
1755 Cape ann, Mass. 3mn duration, felt from Chesapeake Bay
to Nova Scotia, chimneys fell in Boston.
19th century
o Dec 16, 1857, Naples: Robert Mallet
3 months visit, wrote :
"the first principles of observational seismology"
- earthquake waves travel at different speeds through different
materials
- seismic waves resemble soundwaves travelling through the air
- first modern catalogs of earthquakes
- introduced concept of focus, hypocenter: origin of seismic waves
- first reliable maps of zones of predicted earthquake effects:
intensity maps
tried to determine the position of hypocenter from direction of fall
of objects.
got 6 1/2 miles depth for Naples earthquake. Not unrealistic allthough
impractical.
50 yrs later seismologists could measure focal depth accurately.
o Foundation of USGS (1879)
o Geological Survey of India (1857) --->documentation on great
earthquakes
Into the 20th century.... some key earthquakes
o 1891 Mino Owari earthquake in Japan. Observers noted that a fault scarp
had formed when the shaking occured: 100 km long, up to 5 m hight, cutting
and offsetting roads.
In his publilcation reporting on this earthquake, Professor Koto (Imperial
University) clearly established cause and effect.
(the shaking is a consequence of the faulting, not the other way around).
o Fuller's work (1912) on New Madrid earthquakes 1811-1812 (S.
Missouri)
Series of big earthquakes, 8 severe, 3 main ones (dec-Feb), unusual
sequence (not like mainshock/aftershock...).
Damage 30,000-50,000 square miles south of N. Madrid. Awakened
President Madison in the White House, rang church bells in Boston,
chimneys fell in Cincinnati.
"sunken country": 240 by 60 km wide sunk by 1 to 3 m --> water rushed
in, new lakes, swamps: Reelfoot lake in Tennessee formed (8 by 2
miles) descriptions of sand ejected from cracks in the ground, many trees
broken geological puzzle: why in such mid-continent
now we know some more: repetition ~ 1000 yrs but many bogus forecasts
- one in 1989.
o Assam earthquake of 1897 (Oldham, geological survey of India,
provided detailed documentation , help to understand geological
relation)
felt over 1.75 M square miles, devastated 9000 square miles.
Not too many deaths <1000 because not densely populated area. Strong
shaking ~1mn, boulders thrown straight up, sandy soils behaved like
liquids
visible waves moving across the ground ~1ft high
hypothesis by Oldham: thrusting toward the Himalaya range along
structural fault under the Assam Hills. difficult terrain to document
o 1882 Von Rebeur Paschwitz
o 1906 San Francisco earthquake M 8.25 (april 18, 1906)
Turning point in our understanding of causes of earthquakes.
no active volcanoes --> no temptation to link underground
explosions/volcanic events
readily accessible land already surveyed to determine distances and
relative heights between points. These geodetic measurements allowed
well-trained geologists to map the ground deformation (compare before
and after, map ground displacements)
theory = basis of present day seismology "elastic rebound
theory"(Reid):
"sudden slip on the San Andreas fault" (Lawson), later mapped as
stretching from Mexican Border to Mendocino.
blocks of rocks west slod several feet northward
over more than 400 km: San Juan Bautista to Point Arena
passing just west of Golden Gate straight
felt area 180,000 sq km
few deaths recent estimates ~up to 3000
San Francisco then counted 400,000 people. Structures then erected
without systematic regard to eq. hazard: wooden, unreinforced brick
buildings, partially reinforced.
Obs: well built wooden structure and masonry relatively unaffected
when on firm ground and firm foundations.
Wharf area at foot of Market Street: reclaimed marshland by dumping
refuse on mudflats of SF bay ---> severe damage
high rise steel frame bldgs not structurally damaged
19 story Spreckles bldg
16 story Chronicle bldg
fire hazard was known (report in 1905- read page 15 Bolt #2), large
part of the city burned in the next few days.
Creation of a commission led by H.F. Reid to investigate the causes and
effects of this earthquake. It was Reid's idea to look at the surveys
conducted in 1851-1867, 1874-1892 and just after the earthquakes
findings related to the surveys:
o movement along the fault at the time of 1906 earthquake
o surprise: ground near fault had been moving for 50 years prior to
the earthquake, but not right on the fault --> elastic rebound theory:
rocks are elastic until they reach their elastic limit. If a force is
continuously applied, they will store elastic energy,
and then suddenly release it when they break.
other findings of commission:
o damage not uniformly spread out from source along SAF:
concentrations in cities such as Santa Rosa, San Jose, built in
alluvial valleys or basins: soft soil amplified the shaking.
Berkeley: brick chimneys broken, upper walls of brick buildings
thrown down, U.C. no serious damage
Stanford: much worse, closer to SAF bul also # construction-
unreinforced masonry
o much damage from fire
o engineers learnt something about which types of bldg design would
provide safest construction
o -careful mapping of intensity of shaking ---> basic guide to shaking
hazards in SF Bay Area.
o creation of the Seismological Society of America
o Japan Earthquake of 1923
Japan first country where estimates of seismic potential were made.
Prof. Omori (1868-1923) head of Seismological Institute of Japan
studied the distribution of earthquakes in Honshu, and in 1922 made a
forecast:
p19 Bolt 2...
Sagami Bay, Sept 1, 1923 "Kwanto earthquake" about noon (streets full
of people) shaking amplified by soft soil of filled areas in downtown.
Upper town on firmer soil experienced less ground motion. 50% of brick
buildings destroyed, 16 high rise steel frame buildings not damaged,
fires broke out, half of 2.5 M inhabitants were homeless, 68,000 died.
lead to very important reports written by subcommittees of
specialists:" seismo,geol., geotetic, engineering, civil, architecture
(unfortunately in Japanese : took time to have impact elsewhere).
changes in depth of Sagami Bay: 690 ft depression, in other areas
uplifted by 820 feet. Many faults, had 6 ft vertical displacement over
many miles.
Now expect another great earthquake on east coast of Honshu to affect
Tokyo-Osaka.
o Alaska 1964 "Good Friday earthquake"
second largest known this century
occurred at a depth of 30 km under northern Price William Sound,
horizontal extent 800 km roughly parallel to Aleutian trench.
along shoreline: beds of sea life raised 10 m above sea level
geodetic level lines: 200,000 km2 deformed
most slip under the oceans, only some land fault scarps (Montague
Island)
Generated large water waves: "tidal waves" "Tsunami"
Kenai peninsula 19 mn
Kodiak Isl 34 km -->devastated developments along ALaska coast
Anchorage 100 km from fault slip
temporary change of soil and sand to liquid state: liquefaction
o Loma Prieta 1989 M 7.1
felt strongly, seen on television (World series earthquake)
63 deaths, 8000 homeless
40 km segment of SAF in Santa Cruz mountains
15 sec of shaking at source, great variability of shaking:soft soil,
filled grounds 3 times to 5 times stronger and longer duration
damage in Watsonville, Santa Cruz but epicentral area sparsely
populated.
peninsula: much less than 1906
highest intensity: isolated areas in SF and Oakland
- Marina
-Oakland: City Hall
Bay Bridge (built 1936)
West Bay section twin suspension bridge with piers founded on rock,
not damaged
East Bay: 4 simple spans and cantilevers designed for moderate eq.
intensities, failure of upper span between 2 piers (connecing bolts
were severed)
Cypress viaduct (construction 1954-57)
collapsed in section with thick soil deposits
41 deaths (only one other freeway viaduct built this way in SF, also
damaged)
4000 bridges affected, cost to transportation system: $1.8 billion.
INTENSITY OF SHAKING (R. Mallet, 1857, Italy)
Drawing lines on a map between places of equal damage = lines of equal
intensity: "isoseismal", to determine the center of shaking
The rate at which effects diminish with distance determine the
relative size of earthquakes.
Intensity scale:
classifies the degree of damage to structures of human construction
and the amount of disturbance to surface of ground
1902 Mercalli (version fit for California)
Descriptive scale, inaccurate:
depends on density of population, regional nature of soils and quality
of construction.
More accurate scale that does not depend on type of construction,
density of population, soil conditions ----- > magnitude scale, 1930's
Intensity scale still important to determine approximate size of large
earthquakes:
access to historical records
many regions still no seismographs for strong ground motion
Theory has much evolved since the 1920;s
- now have a theory involving the deformation of the whole Earth
explaining why there are earthquakes in Japan and California but not
in the Paris Basin.
Also explains deep trenches on ocean floors, volcanoes, mountain
ranges (established 1960's) ----> principles of geology, plate
tectonics (2 lectures)
- seismology provides key tools for probing structure and dynamics of
the Earth's interior (tomography): earthquake waves carry with them
evidence of geological sources and structural variations deep in the
Earth.