Download Physical Geology - Introduction

Our Hazardous Environment
GEOG 1110
Dr. Thieme
Lecture 4:
Earthquakes
Earthquake - a vibration produced by the rapid release of stored energy.
The focus is the source of the quake, typically on a fault or plane of
slippage in the Earth's crust
The epicenter is the point on the Earth's surface immediately above
the focus, determined from the seismic waves generated by the quake.
Shallow-focus (<70 km) earthquakes tend to have the
largest magnitude, as large as 8.6 on the Richter scale.
Strongest intermediate-focus (70-300 km) earthquakes
have magnitudes less than 7.5, and deep-focus (>300 km)
do not exceed 6.9 in Richter magnitude.
Earthquake Magnitude
• Richter scale (quantitative and based
on amplitude of seismic waves)
• Moment - determined from the area
ruptured along the fault plane, more
or less equal to the old Richter values
• Mercalli scale (Modified Mercalli
Intensity Scale) - semi-quantitative
and based on damage caused by the
quake.
Earthquake Intensity - Mercalli Scale
J.C. Penney store in Anchorage, AK destroyed by
the "Good Friday" earthquake in 1964.
Twisting of steel frames
and disintegration of
concrete blocks
Local uplift of shorelines (up to 1 m)
Strike-slip (Transform)
fault
Reverse (Thrust)
fault
Normal
fault
Buried or "Blind" Faults
Epicenter Location
• Measure S-P Interval
in mm
• Convert to Distance in
km using nomogram
• Triangulate to a point
of intersection
• Epicenter is a point on
the Earth's surface
directly above the
earthquake focus
Earth Surface Material Differences
Effects of 1989 Loma Prieta earthquake
on the Cypress Freeway in Oakland, CA
Seismic shaking is much greater in unconsolidated sediment,
in more saturated materials, and in materials with laminar bedding.
Large earthquakes in central Mexico (1985, 2003) occur due to
transform motion at a "Triple Junction" situation like that in California.
Mexico City was built on a drained lake basin. Even though the city
is over 400 km east of the epicenter on the west coast, liquefaction
of the lake deposits resulted in subsidence and severe shaking during
the 1985 event.
Undamaged buildings with
vertical steel frames next to
completely demolished structures.
Shattered windows caused
by twisting motion in a
Mexico City building, 1985
Buildings vibrated against one another because the seismic wave
periods coincided with the spacing of stories.
"Seismic Retrofit" - University
Hall on the University of
California, Berkeley, campus 1997
Chemistry Building
Architecture Building
Engineering Building
"Seismic Retrofit" - Tohoku
University, Sendai, Japan - 2003
The earthquake of January, 1995 in Kobe, Japan occurred near
another "Triple Junction" where transform plate motion
occurs inboard of the plate boundary.
Landslide where faulting
destabilized the hillslope.
Paved roads cracked and
offset several meters.
Offset of agricultural terraces
Suspension Bridge pylon offset a meter.
Liquefaction of landfill and
estuarine muds
Cretaceous granites are overlain by the
Osaka formation, a Plio-Pleistocene
sedimentary unit consisting of alluvium
interbedded with marine clays.
Fill for the harbor was quarried from
granite residuum in the interior.
Intraplate Seismicity (New Madrid Zone)
Prediction based on Past Frequency?
Richter Magnitude Recurrence Interval
4.0
14 months
5.0
10-12 years
6.0
70-90 years
7.0
254-500 years
8.0
550-1200 years
Prediction based on "Gaps" with stored energy
Paleoseismology
Figure 2.21