1. Earthquake Basics

• Earthquakes are an energy release in the form of seismic waves caused by the sudden rupture of rocks along a fault. Focus, Epicenter.

• ELASTIC REBOUND THEORY OF EQ's:

Rocks on either side of a fault undergo elastic strain as they are stressed by tectonic forces. When they finally rupture, the built-up stress is released and the rocks "rebound" to their original undeformed shape.

2.  Plate Tectonics and Earthquake Distribution

Global distribution of earthquakes primarily coincides with plate boundaries.
Shallow-focus EQ's can occur at all boundaries.
Deep-focus EQ's only occur at subduction zones; the largest EQ's also occur here.

3. Types of Seismic Waves

• P-waves (compressional): alternating compression and dilation in the direction of wave propagation (~5.5 km/s). Can travel though solids, liquids, and gases.
• S-waves (shear or secondary): up-down motion that is perpendicular to the direction of wave propagation (~3 km/s). Can travel though solids only.
• Surface waves: slowests waves; travel on or near the Earth's surface; very destructive; have a complex transverse or elliptical motion.

4. Earthquake Magnitude

• Mercalli Intensity Scale: ( I to XII)

Subjective based on damage and human perception.
• Richter Magnitude Scale: (from -2 to infinity; values of >9 are essentially impossible).

Based on the largest amplitude seismic wave measured on a seismograph.  Base-10 logarithmic scale; thus M=7 EQ has amplitude on a seismograph 10x larger than M=6 EQ.
Energy increase is ~32x.
• Seismic Moment:

More quantitative and accurate than Richter Scale. Based on energy released; determined by three factors: 1) amount of slip on fault; 2) rupture surface area; 3) rock strength. Much better than Richter for estimate of energy release for large EQ's (ie. M>7). Richter Scale underestimates the magnitude of large EQ's.
 
• Largest EQ ever recorded: South-Central Chile, 1960 (moment = 9.5; Rich = 8.5)

5. Destructive Effects

• Ground Motion: - affects of ground motion depend on several factors such as: 1) EQ magnitude; 2) distance to focus, 3) type of seismic waves; 4) local geology
• Fires: - broken gas and water lines.
• Liquefaction - the loss of shear strength of water-saturated, unconsolidated materials.
• Tsunami ("tidal waves") - caused by sudden displacement of sea floor or submarine landslide triggered by EQ's. Travel at speeds of ~600-800 km/hr in the open ocean, grow to ~20 m when they approach coast.

6. Earthquake Mitigation

• Seismic hazard maps: based on EQ history; probability of max horizontal ground motion.
• Seismic Gaps: recurrence interval of large EQ's; faults that lack recent activity.
• Precursor phenomena:
• increase of frequency and magnitude of foreshocks.
• rapid ground deformation.
• changes in physical properties of rocks (electrical and P-wave velocity).
• fluctuations in groundwater well levels.
• anomalous animal behavior