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 (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