You can see the result of waves
moving through the rigid surface and
thru human constructions on the
earth’s surface, which often fail as
they are compressed or sheared by P
and S and L waves.
Rayleigh and or Love Waves!
Compression (and slip)
Compression
(maybe p waves)
Some form of rolling surface wave (L) probably accelerated the ground
up into these supports and they failed.
Faulting itself can express at the surface and also
cause damage: Surface rupture w/ lateral slip
along fault
1855! Wairarapa Earthquake (here’s a vertical rupture, a classic,
old fault scarp, worn down (eroded) over time to less slope)
Kobe, 1995
Kobe, 1995
Ground accelerations due to seismic waves cause
various types of structural failure
Izmet, Turkey, 1999
Loma Prieta, 1989
Izmet, Turkey, 1999
7.4 Anatolian Fault
14K fatalities
And Ground Failure p 88-89
Loma Pieta, 1989
Alaska, 1964
Nisqually eg (WA) Feb., 2001; 6.8: ground failures like this are most common in
unconsolidated material—looks like that here- and especially when wet!
EQ Hazard Assessment
• Richter is insufficient for hazard assessment– too much
attention to the geophysical phenomenon: energy release;
making EQ comparable but not linked to likely damage on the
surface.
• Mercalli is a measure of what impacts and damage actually
result in a given eq.
• In between these two the newest innovaiton is projecitons and
measures of:
– Ground motions caused by waves moving through different
materials, & geographical settings.
Which can then be linked to:
– Likely damage
• And like all hazard assessments, we’re interested in:
frequency/probability; duration; locational details, and local
impacts like ground failure; liquefaction; landslide; etc.)
Ground Motions (Box 6.2)
• Seismologist and hazards mangers have
settled on a key measure of “ground
motions”.
• Motion measured as “ peak or instantaneous
acceleration” in g’s (acceleration due to
gravity, 9.8 m/s-2 ).
• 1g may do in some bldgs (un-reinforced
masonry); .2 causes more damage, .5 is Kobelike damage.
• >.5 g is major motion and devastation!
What the heck is “Acceleration” and
“acceleration due to G” anyhow?
• Acceleration: change in speed over time (often
instantaneous, or short period; measured compared to a
falling object at sea level which speeds up at 9.8 m/s/s
(aka: 9.8ms-2) aka: 32.1 ft/s-2
• Why use it?
– Common factor in engineering force evaluation so also
a measure of “force” applied, or “load factor”.
– Easy to measure (drop something at sea level and
record it! Also easily available accelerometers that can
record accelerations);
– Experienced by people in planes and cars and
spaceships (up to 3gs in short episodes in some rides;
only 3 g’s on Space Shuttle for equipment’s sake).
car
sports car
race car
truck
starting
0.3- 0.5 > 0.9
1.7
< 0.2
braking
0.8 - 1.0 > 1.3
2
~ 0.6
cornering 0.6 - 1.0 > 2.5
3
Here’s example ground motion risk map---as we go over steps in EQ risk
assessment we’ll use this new USGS approach.
10% chance in 50 years
• Still, like all measure ground acceleration can still be
an awkward measure for earthquake shaking, but you
can imagine it, think in terms of distance moved:
• 32.1 ft/s-2 not too uncommon in amusement parks.
• So a .1g ground motion would move you 3 feet in a
second!
• And a .5g ground motion moves you 16 feet in a
second! Now that is major destruction! (notice that
.5g earthquake shaking is catastrophic, and the text
says that biggest fault slips are about 15 feet).
Problem expressing magnitude vs.
probability in useful way
• USGS has settled on a mixed p and r relative to a peak or
exceedance magnitude of ground acceleration as % of g.
• 10% chance (p) in 50 years is typical map (see previous)
– Aka as: 1% chance in 5 years
– Aka as: 20% chance in 100 years
– Aka as: Return period = about 500 years
To capture risk of shaking at any given point (like your
address) they must account for possible shaking from
all possible sources of shaking---that is all possible
faults, each has its own p and r at different intensities.
That’s a lot in some areas, especially CA where this was
designed, so a lot of calculation goes into thes emaps!
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What is the likelihood of a large earthquake at location X?
Our analyses do not answer your question as stated, "probability of large earthquake at a location," but in
a more suitable manner for safety. Danger comes, not only from large earthquakes AT a location, but also
large earthquakes further away, and close, smaller earthquakes. Inasmuch as smaller earthquakes are
more likely to occur, some attention has to be paid to the contribution to hazard from these events, too.
By focusing on GROUND SHAKING caused by earthquakes of all magnitudes and distances from the site,
rather than the size of the earthquake "at" a site, we get a truer picture of the hazard.
The maps give a good impression of overall relative earthquake ground motion shaking hazard for various
locations in the vicinity of the locations you are interested. You can see which locations look more or less
hazardous than others.
More particularly, our maps give the probability of various levels of ground motion being exceeded in 50
years.
For instance, using the data available at our web site for Chico, California, and for San Diego, California,
one can get an idea of the approximate probability of strong, damaging ground motion at each location.
Inspecting peak ground acceleration maps for ground motions having 10 percent probability of being
exceeded in 50 years and ground motions having 2 percent probability of being exceeded in 50 years, we
would find approximately that around San Diego, the ground motion that has 10 percent probability of
being exceeded in 50 years is about equal to the ground motion around Chico that has 2 percent
probability of being exceeded in 50 years. That ground motion is 30 percent g. (See the data from the zipcode look-up, below.)
From one of the answers at the Frequently-Asked Questions page at our site,
<<2.What is "% g"? What is the relation to building damage? >> we find that an approximate threshold for
shaking that causes damage is 10 percent g. So, 30 percent g is very strong shaking indeed and has about
1 chance in 10 of occurring in San Diego in 50 years. In 20 years this would be about 1 chance in 25. In
Chico, this level of shaking has about 1 chance in 50 of occurring in 50 years or about 1 chance in 125 in 20
years. This may be a reasonable number to use for the likelihood of the occurrence of a big, nearby
earthquake.
The USGS Fine Print: Here’s some of how USGS explains their ground
motion risk maps
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These maps depict earthquake hazard by showing, by contour values, the earthquake ground
motions that have a common given probability of being exceeded in 50 years.
The ground motions being considered at a given location are those from all future possible
earthquake magnitudes at all possible distances from that location. The ground motion
coming from a particular magnitude and distance is assigned an annual probability equal to
the annual probability of occurrence of the causative magnitude and distance.
The method assumes a reasonable future catalog of earthquakes, based upon historical
earthquake locations and geological information on the recurrence rate of fault ruptures.
When all the possible earthquakes and magnitudes have been considered, one can find a
ground motion value such that the annual rate of its being exceeded has a certain value.
Hence, on a given map, for a given probability of exceedance, PE, locations shaken more
frequently, will have larger ground motions.
For a LARGE exceedance probability, the map will show the relatively likely ground motions,
which are LOW ground motions, because small magnitude earthquakes are much more likely
to occur than are large magnitude earthquakes. For a SMALL exceedance probability, the
map will emphasize the effect of less likely events: larger-magnitude and/or closer-distance
events, producing overall LARGE ground motions on the map.
The maps have this format, because they are designed to be useful in building codes, in
which we assume that, for the most part, all buildings would be built to the same level of
safety. For other applications, maps of another format might be more useful.
For instance, many buildings across the US are built more or less the same, regardless of
earthquake hazard. If we knew that a particular type of building was likely to fail at a
particular ground motion level, we could make a map showing contours of the likelihood of
that ground motion value being exceeded, due to earthquakes.
•http://earthquake.usgs.gov/research/hazmaps/design/index.php
What is "% g"?
When acceleration acts on a physical body, the body experiences the acceleration as a force. The force we
are most experienced with is the force of gravity, which caused us to have weight. The units of acceleration
of the map are measured in terms of g, the acceleration due to gravity. An acceleration of 11 feet per
second per second is 11*12*2.54 = 335 cm/sec/sec. The acceleration due to gravity is 980 cm/sec/sec, so an
acceleration of 11 feet/sec/sec is about 335/980 = 0.34 g. Expressed as a percent, 0.34 g is 34 % g.
What is the relation to building damage?
Pre-1940 dwellings are likely to perform poorly in earthquake shaking
Pre-1975 dwellings are likely to have some vulnerabilities to earthquake shaking
Some post-1985 dwellings, built to California earthquake standards, have experienced
severe shaking (60 % g) with only chimney damage and damage to contents.
You can see EQ hazard maps at:
• http://earthquake.usgs.gov/hazards/products/
and create your own here:
• http://earthquake.usgs.gov/research/hazmaps/ind
ex.php
Let’s go through the steps that yield hazard risk maps like these.
10% chance in 50 years
Hazard Assessment
• Monitoring, data collection: to determine magnitude /
frequency relationships (thus probability)
– Often extrapolate to higher magnitudes
– Some modeling and simulation may help, especially for rare
events, few observations
– May also include historical and pre-historic reconstructions
for more time depth (old fault scarps of events centuries
before)
• Translate into hazard elements, e.g.:
– Eq mag (ML) = ground shaking (%g accel) at some
probability
(Not unlike effort to create a better historical
hurricane data set)
Hazard Assessment (cont)
• Calculate damage / loss curves---or refer to current
engineering practice to choose “standard” event or
risk threshold (g with 10% chance in 50 years)
• This standard risk allows you to create a map: very
important in EQ where local conditions affect ground
motions, but difficult-requires lots of field work, geodata.
• Map populations and property at risk
• Apply loss curves
• Go to Risk/hazard Management