EES215

Lecture 6

Results of whole earth seismology: gradual increase in density; rigidity and compressibility with depth with major discontinuities at boundaries - whole earth model; inverse theory; lab studies on minerals give velocities as fx of p,T

Example: Earthquake near Sumatra, recorded at our seismometer. The record shows that first major response occurred about 20 min after the earthquake à based on Bullard diagram the earthquake occurred in the shadow zone, i.e. the first response is a PP wave. Minor earlier signals are from PKIKP. Large amplitude waves are surface waves, which arrived more than 1 hour after the earthquake.

Crustal seismology used for determination of composition and layering of crust (exploration; presence of faults; stability of crust etc.)
Approach (land): set off source (explosion; thumper); array of receivers
(sea) sonar signals from ship (single, multiple channel; sidescan sonar) -- near surface features

crustal seismology - example of two layers; reflected; refracted waves; head wave; time-travel diagram; application for the investigation of near surface layering

Magnitude of earthquake: Richter scale - logarithmic scale based on amplitude and distance of earthquake; definition of energy; power; energy content; magnitude distribution of earthquakes; types of earthquakes associated with plate boundaries; mechanism solutions Magnitude of earthquakes - depth; type of earthquake

Environmental considerations: earthquakes influence shape of surface on geologic time scale; specific events have consequences for human life.
Seismology main method for study of solid earth (composition; structure; layering); other methods: gravity; magnetism; conductivity.

Seismic hazard maps are based mostly on historical records. While most of the high-risk zones in the USA are associated with the plate boundary along the Pacific Coast, there are also interplate risk zones in mid-continent and along the East Coast. Fig. 1

Earthquake prediction: Information needed:

Location à ~ 10 km

Time à ~ 1 day

Magnitude à one order of magnitude (i.e. will be Magnitude 6,7 or 8)

Prediction in most cases cannot come close to the accuracy needed à more emphasis in building codes and other preventative measures.

Plate tectonics, earthquakes: what is the energy source?

Potential sources of energy: Heat from radioactive decay à important in mantle and crust

Gravitational settling à release of potential energy: important in the core, growth of inner core at expense of outer core

Rotation (minor)