Tectonic Plate Movement: By far the most common cause (approx. 90%). Strain energy builds up at locked plate boundaries (strike-slip, convergent, divergent) over decades or centuries until the shear stress exceeds the rock's strength, causing a sudden rupture.

Volcanic Activity: Earthquakes frequently precede or accompany volcanic eruptions, associated with the underground movement of magma and the sudden expansion of trapped volcanic gases.

Human Activity: Termed "induced seismicity." This includes massive reservoir impoundment (dam construction altering pore pressures), deep mining blasts, and deep-well fluid injection (e.g., wastewater disposal from fracking).

Reverse / Thrust Faults (Subduction Zones): Produce the most powerful "megathrust" earthquakes globally (e.g., 1960 Valdivia, 2011 Tohoku). Massive energy release.

Strike-Slip Faults (Transform Boundaries): Produce highly destructive, shallow earthquakes (e.g., 1906 San Francisco).

Normal Faults (Divergent Boundaries): Produce generally smaller magnitude, shallow earthquakes.

P-Waves (Primary/Compressional): The fastest seismic waves ($6-13 \text{ km/s}$). They propagate via longitudinal compression and dilation (like sound waves), pushing and pulling the rock parallel to the direction of travel. They can pass through solids, liquids, and gases, but generally cause minor, high-frequency rattling damage.

S-Waves (Secondary/Shear): Slower than P-waves ($3.5-7.5 \text{ km/s}$). They propagate by shearing the rock side-to-side or up-and-down, perpendicular to the direction of travel. Crucially, because liquids cannot sustain shear stress, S-waves cannot travel through liquids (e.g., they are entirely blocked by the Earth's liquid outer core). S-waves cause significant, damaging horizontal shaking to rigid structures.

Love Waves: Move the ground from side to side in a horizontal plane perpendicular to the direction of propagation. This intense horizontal shearing is devastating to building foundations.

Rayleigh Waves: Move the ground in a complex, rolling, elliptical motion (similar to ocean waves), moving the surface both vertically and horizontally.

Ground Shaking: Direct dynamic loading on structures. Mitigated by ductile design, base isolation, and tuned mass dampers.

Liquefaction: A devastating geotechnical phenomenon where saturated, loose, granular soils (like clean sands or non-plastic silts) completely lose their shear strength and temporarily behave like a dense, viscous liquid during intense, cyclic shaking.
Mechanism: The rapid shaking tries to densify the loose sand, but the water in the pores cannot escape fast enough. This causes a massive, instantaneous spike in pore water pressure, which completely negates the effective normal stress holding the sand grains together. Buildings built on shallow foundations over liquefied soil will simply sink, tilt wildly, or completely overturn intact (e.g., the infamous 1964 Niigata, Japan earthquake). Light, buried structures like empty sewer pipes or fuel tanks will forcefully float to the surface. Mitigated by deep foundations (piles), soil compaction, or vibro-replacement.

Surface Rupture: Permanent offset of the ground surface along the fault trace. Structures cannot withstand this; mitigation requires avoiding construction directly on active fault lines (setbacks).

Landslides and Rockfalls: Earthquakes frequently trigger mass wasting on steep slopes or coastal bluffs. Mitigated by slope reinforcement, retaining walls, or avoiding high-risk zones.

Tsunamis: Massive sea waves generated by sudden vertical displacement of the seafloor during an offshore subduction zone earthquake. Coastal facilities must consider tsunami inundation zones.

$d$ = Distance to the epicenter (km)

$T_s$ = Arrival time of the S-wave (seconds)

$T_p$ = Arrival time of the P-wave (seconds)

$k$ = An empirical velocity constant for the local crust (often simplified to roughly $8 \text{ km/s}$ for introductory problems).