Transcript Lecture 15 Earthquake Hazards

Lecture 17 Earthquake
Hazards
 Big
earthquakes
 Earthquake damages: aftershocks,
amplification, liquefaction, landslides,
fire
 Earthquake hazard mitigation
 Tsunami
Big earthquakes
appr. annual frequency of earthquakes
description
great
major
strong
moderate
mag
>8
>7
>6
>5
on the order of
1
10
100
1000

Distribution of earthquakes M>=5 for 1980-1990.
 earthquake
magnitude and energy
equivalence
mag
2
6
10

energy release appr. equivalence
600x1012 ergs 1000 pound of explosives,
rumbling of trains, smallest human can feel
600x1018 ergs 1946 Bikini atomic bomb test
600x1024 ergs annual US energy consumption
1811-1812 New Madrid, Missouri, three major
earthquakes, M~8, largest in contiguous US
 1964 Alaska earthquake, M~9, 131 death, one of
the largest ever recorded.
 examples
of most destructive
earthquakes

1556 Shanxi Huaxian, China earthquake, 830
thousands deaths, possibly the greatest natural
disaster ever.

1976 Tangshang, China M7.8, 240 thousand deaths.

1994 North ridge, CA, M6.7, 61 deaths, damage
exceeding $15 billion.

1995 Kobe, Japan, M6.9, 5472 death, damage
exceeding $100 billion.

Areas away from plate boundaries are not
necessarily immune from earthquakes. This is
damage to Charleston, S. Carolina caused by the
Aug 31, 1886 earthquake there. This was the
greatest earthquake in the eastern US. Strong
vibrations were felt even in Chicago.
Destruction from seismic
vibrations
The amount of structural damage due to
vibrations depends on:
 strength of earthquake
 duration (and after shocks)
 distance from epicenter
 the site materials
 building design, building natural periods
Earthquake intensity

Modified Mercalli intensity scale (P.408)
measures the damage from an earthquake at
a specific location.

The intensity ranges from I (not felt) to XII
(total destruction).

Appr. relationship between MM and
magnitude and ground acceleration (P.409)

Damage caused to a five-story JC Penney building,
Anchorage, Alaska by 1964 Alaskan earthquake.
Very little structural damage was incurred by the
adjacent building. (NOAA)
Resonance with building natural periods
 At
close distances, the most powerful
vibrations are in 0.5-5 Hz, produced by S
and short-period surface waves (Lg).
 Typical
building of 10 storeys has T=1s;
each storey adds 0.1s.
aftershocks:
After a main earthquake, there are aftershocks in the following
minutes, hours, days, months, or years -- The number of
aftershocks decreases with time. The chance of one or more
aftershocks with equal or larger magnitude within 7 days can be
over 50% in California.

Aftershocks may cause previously damaged, yet still standing,
structures to collapse. Thus, for engineers, damaged public
buildings should be examined immediately and closed down if
necessary to minimize the risks of aftershocks.

Example: 1952 Ken county, CA earthquake (M7.7) had a M5.8
aftershock, which caused more damage to Bakersfield than the
main one.
amplification by soft sediments
Massive bedrock provides best
foundation because it passes wave
motions on, resulting less vibration to
building structure.
 Soft sediments generally amplify the
vibrations more than solid bedrock.


Seismograms from an aftershock of 1989 Loma Prieta
earthquake show that shaking is greatly amplified in
soft mud as compared to firmer materials. The portion
of the Cypress Freeway structure in Oakland, CA that
stood on soft mud (dashed red line) collapsed during
the main shock.
liquefaction

Saturated fine sands and silts are subject to
liquefaction during earthquake vibrations, in
which water rises and a stable soil turns into
a mobile fluid that has weak shear strength.

Underground objects such as sewer lines
may literally float toward the surface and
buildings settle and collapse.

Effects of liquefaction. The tilted building rests on
unconsolidated sediment that behaved like quicksand
during the 1985 Mexican earthquake. (J.L. Beck)

“Mud volcanoes”
produced by 1989
Loma Prieta
earthquake. They
formed when geysers
of sand and water shot
from the ground, an
indication of
liquefaction. (R.Hilton)
Landslides

A photo of Turnagain Heights landslides
caused by the 1964 Alaskan earthquake.

Landslides of Turnagain Heights, Alaska caused by
the 1964 Alaskan earthquake. In less than 5 min, as
much as 200 m of the bluff area was destroyed.
(USGS)
Fire caused by earthquakes

San Francisco in flames after the 1906 earthquake.
The greatest destruction was caused by the fires,
which started when gas and electrical lines were
severed.
Fire hazards after earthquakes

Ignited by broken gas and electrical lines, toppled stoves

Added fuel from chemicals, rubbers, gas stations

Fire services not alerted because of lack of information

Roads blocked by earthquake damages

Water in emergency tanks underground run out

Shaking hazard map based on past earthquake
activities and how far shaking extends from quake
sources. Colors show the levels of horizontal shaking
that have a 10% chance of being exceeded in a 50-

Aerial view of the collapsed freeway interchange between I-5
and the Antelope Valley Freeway (State 14) after Northridge
Earthquake, Jan. 17, 1994 (Mw 6.7). (photo: Kerry Sieh)
Tsunami

Tsunami means "great wave in harbor" in Japanese.
The name is fitting as these giant waves have
frequently brought death and destruction to harbors
and coastal villages.

In physics, tsunami is just like ordinary wind driven
ocean waves. They are gravity waves: Gravity is the
primary restoring force of the motions. But tsunamis
are distinguished by particularly long periods (2002000s) and wavelengths of tens of kilometers.

A tsunami hit Hilo, Hawaii on April 1, 1946, 4h 55m
after it originated from a large earthquake in Aleutian
trench. Tsunami runup reached 16m. Total 159 killed
in the five main islands, including 96 deaths in Hilo.
No warning was issued. (UC Berkeley)
Tsunami (continued)

These long period ocean waves can travel thousands
of kilometers across the ocean with the speed (5009500 km/hr) equivalent to that of jetliners. The wave
speed decreases with the decrease of ocean depth.

The height of a tsunami in the open ocean is usually
less than 1 meter (so it can pass undetected), but the
waves can sometimes exceed 30 m as they slow
down in shallow water and pile up.

Illustration of a tsunami generated by displacement of ocean
floor. The wave speed decreases with the decrease of ocean
depth. The height of a tsunami in the open ocean is usually less
than 1 meter (so it can pass undetected), but the waves can
sometimes exceed 30 m as they slow down in shallow water
and pile up. (Tarbuck and Lutgents)
Tsunami are excited by large-scale
submarine displacements of water due
to submarine landslides, submarine
volcanic eruptions, sea bottom faulting
(earthquakes), etc.
Tsunami at Hawaii

Being in the middle of the Pacific Ocean, surrounded
by a ``ring of earthquakes'', Hawaii is exposed to real
damages and threats of tsunamis. On 1946, April
Fools Day, a large earthquake occurred in Aleutian
trench, 4h 55m later, large tsunami hit Hawaii.
Tsunami sunup reached 16m. Hilo was the most
affected. Total 159 killed in the five main islands,
including 96 deaths in Hilo. No warning was issued.

Two years later, the now Tsunami Warning System in
Hawaii was set up. The system took full advantage of
the fundamental relationship between tsunamis and
earthquakes: (1) large earthquakes can generate
tsunamis; (2) seismic waves travel at 30 to 60 times
the speed of a tsunami (i.e. minutes vs hours).

Tsunami travel times to Honolulu, Hawaii from various
locations of Pacific rim. (Tarbuck and Lutgens)
Examples of tsunami warning

1957, March, 9. A magnitude 8 quake occurred in the
Aleutian Trench. A Tsunami Watch was issued.
4h55m later, 10 ft waves hit Hilo. However Kauai had
larger damages. The runup reached 32 ft. No loss of
life.

1960, May 22. A magnitude 8.5 quake shook Chile. A
large tsunami was immediately excited. It traveled
6600 miles in 15h and hit the Hawaii islands. Hilo
again had the most extensive damage with 61 deaths.
The tsunami warning was very accurate but the
public response was a failure.