Earthquakes

Geology 101, Fall 2012

Stress and strain

• Stress is the stored energy built up due to plate motion; strain is the volume change on a rock due to stress

Faults

• At fracture point, the rock breaks; the resulting offset between the two parts of the rock is a fault. The energy released is an earthquake

Types of faults

• The different types of plate boundaries lead to different types of faults

Basic earthquake terminology

Earthquake energy

• Radiates away as body waves

Earthquake energy

• When body waves reach the surface, they transfer into surface waves

Seismograph

Seismogram is a record of all waves

Measures of earthquakes

• Richter scale (Charles Richter and Beno Gutenberg, Caltech, 1935)

• Based on movement of the seismograph stylus

• Logarithmic scale, meaning that each 1 point increase in magnitude means a 10-fold increase in movement of the stylus

• Not accurate for big earthquakes, or distant ones

Measures of earthquakes• Moment magnitude scale is

commonly used today to report earthquake “magnitudes”.

• It is still logarithmic, but is now based on a formula that looks at other parts of the seismogram than just the largest movement.

• The formula calculates the energy released by the quake, which is more relevant to engineers and planners than the Richter scale magnitude.

Basic math of logarithmic scale

The point is, for each increase of 1 point on the moment magnitude scale, the energy released by the earthquake goes up by a factor of 30 or so.

Measures of earthquakes

• Modified Mercalli Intensity (MMI), developed by Guiseppe Mercalli, 1884 and 1906

• Measures effects on humans and their structures

• Often a better indicator of potential earthquake hazard than the moment magnitude scale

Nisqually (2001) quake intensity

Aftershocks occur as stress transfers to unrelieved parts of fault or rock

Earthquake prediction – which one is true?

Interlude about resource extraction

When organic material dies...

• If oxygen can be excluded, the decay process produces a material called kerogen from various fats and other biological compounds.

• They are rich in carbon (C), hydrogen (H) and oxygen (O), which will burn well, but also contain nitrogen (N), sulfur (S) and phosphorus (P) which produce noxious gases (like SO2 and NOx) when burned.

Petroleum needs three rocks

• Source rock, which contains the organic material in the first place (and allows the organics to “cook”)

• Reservoir rock to which the petroleum migrates and is stored.

• Cap rock to prevent the petroleum from rising further.

Oil and natural gas can be trapped

Since oil is less dense than water, it will “float” on ground-water. This allows trapping by an imperme-able layer.

Using seismic studies profitably

Using vibrator trucks to shake the ground, and listening with an array of geophones, petroleum engineers can determine oil-rich layers.

Interlude about wave behavior

• When a wave passes through a boundary, it can either reflect off the boundary or pass through it in a slightly different direction.

• First shown for light, the same physics works for seismic waves.

Probing the Earth’s interior

Changes in wave speed indicate deep Earth structures

Specifically, the Mohorovicic (Moho) discontinuity, where seismic speeds suddenly increase as they get deeper, shows the change from crust materials to mantle within the lithosphere.

Note that the mantle does get more solid toward the core (speeds increase), but that the outer core is liquid.

S-wave properties give us the size of the outer core

P-wave properties give us the size of the inner core