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Geophysics, Lecture Notes- Physics - 15, Study notes of Physics

University of London (Lond)Physics

Overview of head budget and earth, Heat flow and depth of oceans, convection in mantle, thermal structure of core

Typology: Study notes

2010/2011

Uploaded on 09/10/2011

aisha-92
aisha-92 🇬🇧

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PX266 Geophysics (2010/11)
Lecture 11 Handout – Earthquakes
Dr. Gavin Bell
Above: four basic types of faults
Below: intensity distribution for two large earthquakes in the USA
The New Madrid quakes have a much longer “range” due to the weak attenuation of
seismic waves passing through the simple geological structures of the Mid West
(recall Bouguer anomaly across the USA – pretty featureless here). The west coast
lies on a tectonic plate boundary and more attenuation occurs in the complex
geological structures found here.
pf2

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PX266 Geophysics (2010/11)

Lecture 11 Handout – Earthquakes

Dr. Gavin Bell

Above : four basic types of faults

Below : intensity distribution for two large earthquakes in the USA

The New Madrid quakes have a much longer “range” due to the weak attenuation of

seismic waves passing through the simple geological structures of the Mid West

(recall Bouguer anomaly across the USA – pretty featureless here). The west coast

lies on a tectonic plate boundary and more attenuation occurs in the complex

geological structures found here.

Modified Mercalli Intensity Scale

I. People do not feel any Earth movement. II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. (N.B. Cartoons from Hong Kong earthquake awareness web site) V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers. VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

Further study

Four problems to tackle on Seismology and earthquakes, questions 14-17 (2 are A

questions and 2 are B questions).

You should be aware of the different fault types but don’t need to memorise them.

Make sure you understand why the logarithmic earthquake magnitude scales have the

forms they do and what the difference is between magnitude and intensity.

Note – we don’t consider earthquake fault plane solutions (re. directionality of

seismic wave displacements) in this course.