Lecture 4: Earthquakes

Our Hazardous Environment

GEOG 1110Dr. Thieme

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.

Local uplift of shorelines (up to 1 m)

Twisting of steel framesand disintegration of

concrete blocks

Strike-slip (Transform)fault

Reverse (Thrust)fault

Normalfault

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

Seismic shaking is much greater in unconsolidated sediment,in more saturated materials, and in materials with laminar bedding.

Effects of 1989 Loma Prieta earthquakeon the Cypress Freeway in Oakland, CA

Large earthquakes in central Mexico (1985, 2003) occur due totransform motion at a "Triple Junction" situation like that in California.

Mexico City was built on a drained lake basin. Even though the cityis over 400 km east of the epicenter on the west coast, liquefaction

of the lake deposits resulted in subsidence and severe shaking duringthe 1985 event.

Shattered windows causedby twisting motion in a

Mexico City building, 1985

Undamaged buildings withvertical steel frames next to

completely demolished structures.

Buildings vibrated against one another because the seismic waveperiods coincided with the spacing of stories.

"Seismic Retrofit" - University Hall on the University of California, Berkeley, campus - 1997

"Seismic Retrofit" - Tohoku University, Sendai, Japan - 2003

Chemistry Building

Architecture Building

Engineering Building

The earthquake of January, 1995 in Kobe, Japan occurred nearanother "Triple Junction" where transform plate motion

occurs inboard of the plate boundary.

Landslide where faultingdestabilized the hillslope.

Paved roads cracked andoffset several meters.

Offset of agricultural terraces

Suspension Bridge pylon offset a meter.Liquefaction of landfill and

estuarine muds

Cretaceous granites are overlain by theOsaka formation, a Plio-Pleistocene

sedimentary unit consisting of alluviuminterbedded 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?

550-1200 years8.0

254-500 years7.0

70-90 years6.0

10-12 years5.0

14 months4.0

Recurrence Interval

Richter Magnitude

Prediction based on "Gaps" with stored energy

Paleoseismology

Figure 2.21