Transcript EARTHQUAKE EFFECTS, PATTERNS, AND RISK

EARTHQUAKE EFFECTS,
PATTERNS, AND RISK
Earthquake Effects - Surface Faulting
Landers, CA 1992
Thrust Fault Example
Normal Fault Example
Dixie Valley-Fairview Peaks, Nevada earthquake
December 16, 1954
Rupture on a Fault
Total Slip in the M7.3 Landers Earthquake
Earthquake Effects Ground Shaking
Northridge, CA 1994
Earthquake Effects - Ground Shaking
Loma Prieta, CA 1989
KGO-TV News ABC-7
NATURAL PERIOD OF
BUILDINGS
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Natural Period, T = 1/f
Typical values:
Stories:
Value:
2
0.2 sec
10
1
20
2
30
3
Earthquake Effects - Ground Shaking
Kobe, Japan 1995
Earthquake Effects - Ground Shaking
Kobe, Japan 1995
TSUNAMIS
• Earthquakes beneath the ocean floor sometimes
generate immense sea waves, or tsunamis.
• These waves travel across the ocean at speeds as great
as 750-960 kilometers per hour and may be 15 meters
high or higher by the time they reach the shore.
• During the 1964 Alaskan earthquake, tsunamis engulfing
coastal areas caused most of the destruction at Kodiak,
Cordova, and Seward and caused severe damage along
the west coast of North America, particularly at Crescent
City, California. Some waves raced across the ocean to
the coasts of Japan.
Earthquake Patterns in Time and
Space
• Foreshock-main event-aftershock pattern: increase in number of
microearthquakes in area of future rupture preceding a main event,
and the decrease in earthquake activity after a main event
• Quiescence: an anomalous loss of normal background
microearthquakes for hours or days preceding a main event
• Seismic Gaps: an absence of expected earthquakes on a segment
of an active fault
• Slip-rate diagrams: cumulative slip vs. time to estimate repeat times
and average amounts of slip
EARTHQUAKE RISK ESTIMATES
• To estimate the danger of destructive
earthquakes in a region, a number of factors
need to be assessed:
– 1) earthquake history--instrumental-- provides
location, size, and timing of events in last 50 years
– 2) non-instrumental history-- provides location,
estimated size (or at least damage), and timing
– 3) geologic history of fault movement at archeological
sites and prior to human history.
PALEOSEISMOLOGY
• The earthquake study will identify active
faults, and the pattern of events. Patterns
to examine include repeat times, amount
of movement at each event, whether
preceded by microearthquakes or other
changes in terrain, water level, animal
behavior, etc.
Prediction vs. Forecast
Prediction vs. Forecasting
• PARAMETER
PREDICTION
FORECAST
• LOCATION:
+/- <5 km
+/- 50km
• TIME:
+/- hours or days
+/-decades to
centuries
• SIZE:
+/- 1 Magnitude
+/- 2 M
LOCATION
• Seismologists use Instrumental Records of the
arrival times of the P and S waves. Location
quality has improved with more stations carefully
calibrated, but can be as bad as +/- 50 km in
some remote regions!
• Paleo-seismologists use:
– Historical Records
– Geological Field Data: scarps, gouge, offsets,
landslides, disturbed archaeological sites
– Evidence of ancient tsunamis
Time
• Seismologists use Instrumental Records.
Can narrow the time down to a fraction of
a second.
• Paleoseismologists: dated historical
records, isotopic dating of offset features.
Some rock types are undatable for this
purpose. Most radiometric dates are +/- 50
to 150 years.
Size
• Seismologists: instruments. Depends on
careful calibration. Can be accurate to
0.01 Magnitude units.
• Paleoseismologists: estimates based on
size of rupture zone (fractured rock),
amount of slip, damage reports, how far
inland and how high above sealevel
tsunami deposits are found