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Definition of an earthquake: It is the vibration of Earth produced by the rapid release of energy.
The focus: The point within the Earth where rock displacement generates the release of energy.
Epidenter: The point on the Earth’s surface directly above the focus.
Seismic energy travels in waves in all directions from the focus.
Thousands of earthquakes occur globally each day, yet only about 15 are significantly large, greater than a 7.0 magnitude.
Over 1 million earthquakes occur annually, most of which are not felt by humans.

Magnitude: 7.0
Location: Haiti, a nation recognized as the poorest in the Western Hemisphere.
Casualties and destruction:
- 316,000 people died.
- 300,000 were injured.
- 280,000 homes destroyed or damaged.
Epicenter located 15 miles from Port-au-Prince (the capital), 6 miles deep, along a fault similar to the San Andreas Fault.
Extreme damage attributed to deep magnitude, shallow depth, local geology, and absence of building codes.
Over 52 aftershocks greater than 4.5 magnitude were felt post-event.
The area is yet to recover fully from this disaster.

Types of seismic waves:
- Surface waves: Travel along the Earth’s outer layer and cause significant ground movement.
- Body waves: Travel through Earth’s interior, subdivided into Primary (P) waves and Secondary (S) waves.
Focus, Epicenter: Focus is the origin deep within the Earth; the epicenter is directly above it.
Magnitude and Frequency of Earthquakes:
- Licensed instruments can detect earthquakes globally.
- Most earthquakes occur along pre-existing faults due to rock displacement, with movement being vertical or horizontal.
- The theory of plate tectonics explains the fault movement and most earthquakes occur in conjunction with plate boundaries.

Proposed by H.F. Reid post the 1906 San Francisco earthquake.
Key Mechanism: Tectonic stresses deform rocks until slippage occurs at the weakest point, releasing energy (
elastic rebound).
Example: Pacific plate moved 9.7 meters northward past North American plate during the quake.
The vibration felt is the result of rocks snapping back to their original shape.

San Andreas Fault: Most studied; exhibits transform boundary tensions between North American and Pacific plates.
- Displacement and variations in fault behavior, including slow creeping movement, small, frequent quakes, and major seismic events.
Faults are classified as:
- Normal faults: Hanging wall moves down.
- Reverse faults: Hanging wall moves up.
- Thrust faults: A type of reverse fault with a dip less than 45°.
- Strike-slip faults: Horizontal movement.

Seismic Wave Types:
- P waves (Primary waves):
- Compress and expand materials; travel through solids, liquids, and gases.
- S waves (Secondary waves):
- Move at right angles; only travel through solids; cause shape change and do not travel through gases or liquids.

Epicenter determination: Using the velocity difference between P and S waves to locate the earthquake source.
Triangulation Method: Three seismic stations record waves; draw circles to find the overlapping area for precise epicenter location.
Circum-Pacific Belt accounts for about 95% of seismic energy release.

Intensity: Refers to shaking severity and damage level, assessed based on personal estimates.
Magnitude: Energy released calculated from seismic records.
Intensity Scales: Modified Mercalli Intensity Scale which assesses damage but does not accurately represent energy.
Magnitude Scales:
- Richter Scale: Measures amplitude of surface waves; logarithmic scale.
- Moment Magnitude (MW): Based on fault displacement and rock properties; accurately portrays large earthquakes.

Destruction factors include:
- Quake intensity and duration.
- Structural integrity and local geology.
- Liquefaction occurs when water-saturated sediments cause structures to shift.
Notable earthquakes:
- 1964 Good Friday Alaskan Earthquake: 9.2 magnitude, significant damage due to aftershocks, and 131 fatalities.
Tsunami information:
- Generated by underwater seismic activity; travel unnoticed at low heights in the open ocean but can rise dramatically in shallow waters.

Tsunami Formation: Vertical oceanic floor displacement or landslides. E.g., the 2004 Indian Ocean tsunami from a 9.1 earthquake killed over 230,000 people.
Travel Speed: 300-600 mph; height increases in shallow water.
Historical examples of significant tsunamis:
- 2011 Japan earthquake and tsunami: 9.0 MW; 15,861 deaths and extensive damage.
Tsunami warning systems starting after major events to monitor global seismic activity and predict tsunami risk.

Major zones of earthquakes (95% occurrence):
- Circum-Pacific belt: most active.
- Alpine-Himalayan belt due to continental collisions.
- Oceanic Ridge System shows frequent seismic activity.
- Transform faults that exist within continents like the San Andreas Fault.

Rock deformation leads to folding and faulting, significantly along plate boundaries.
Types of deformation:
- Elastic Deformation: Temporary shape change.
- Ductile Deformation: Permanent flow without breaking at high pressures.
- Brittle Deformation: Fracturing occurs at lower temperatures and pressures.

Types of folds caused by compression:
- Anticlines: Upfolding of rocks.
- Synclines: Downfolding of rocks.
Orientation variations: symmetrical, asymmetrical, and overturned folds.

Fault classifications:
- Dip-slip faults: Vertical movement along the fault plane (normal and reverse faults).
- Strike-slip faults: Horizontal motion.
The San Andreas Fault serves as a prominent example.

Processes resulting in mountain systems involving folding, faulting, and more.
Young mountains: e.g., the Appalachians and the Himalayas.
Two types of mountains:
- Island Arc-type: Subduction-related volcanic chains.
- Andean-type: Involves continental margin collision and sediment accumulation.

Earthquakes and resultant geologic processes like folding, faulting, and mountain building are critical in understanding Earth's dynamic nature. The study of seismology not only informs us of potential hazards but also unveils the geological history shaping our planet.