Seismic Waves & Locating Earthquake Epicenters

Learning Competency & Module Goals

Describe and relate the global distribution of active volcanoes, earthquake epicenters, and major mountain belts to Plate-Tectonic Theory (Code: S10ES-Ia-j-36.1).
By the end of the lesson you should be able to:
- Compare & contrast all types of seismic waves.
- Explain the physical process of earthquake generation.
- Locate earthquake epicenters through the triangulation method.
- Justify the importance of disaster preparedness before, during, and after earthquakes.

Key Terms & Concepts in Seismology

Seismology: from the Greek “seismos” (earthquake); the scientific study of earthquakes and the propagation of seismic waves through, within, and around Earth.
Seismograph: the primary instrument that detects, records, and measures seismic waves.
Wave (general): a rhythmic disturbance that transports energy without transporting matter from place to place.
Seismic Wave: energy released by sudden rock rupture (or an explosion) that travels through the Earth; recorded by a seismograph.

Classification of Seismic Waves

Two overarching families:
- Body Waves: travel through Earth’s interior.
- Surface Waves: confined to Earth’s exterior layers.

Body Waves

Characteristics
- Higher frequency than surface waves → useful for probing interior structure.
Types
- P-waves (Primary / Compressional)
- Fastest seismic wave; arrives first.
- Particle motion: alternating push–pull (longitudinal), analogous to sound waves rattling windows during loud thunder.
- Propagation medium: both solids and fluids (e.g., mantle + outer-core liquid, water).
- S-waves (Secondary / Shear / Transverse)
- Arrive after P-waves (slower velocity).
- Particle motion: up–down or side-to-side perpendicular to propagation direction.
- Medium restriction: only solids → outer liquid core blocks them, creating an S-wave shadow zone.

Surface Waves

Characteristics
- Generated when body waves reach the surface; arrive after P and S.
- Lower frequency; often largest amplitude → responsible for most felt shaking & damage.
Types
- Love Waves (L-waves)
- Named after mathematician A. E. H. Love (1911).
- Horizontal, snake-like, side-to-side shearing of the ground.
- Fastest surface wave.
- Rayleigh Waves
- Predicted by Lord Rayleigh (1885).
- Rolling motion (retrograde ellipse) similar to ocean waves; moves ground both up–down and side-to-side in direction of travel.
- Usually produces the strongest, longest-lasting shaking felt by people.

Internal Earth Structure Revealed by Seismic Discontinuities

Mohorovičić Discontinuity (Moho)
- Discovered 1909 by Andrija Mohorovičić.
- ~ $50\ \text{km}$ depth average; marks abrupt velocity increase → boundary between crust & mantle due to density contrast.
Gutenberg Discontinuity
- Identified by Beno Gutenberg via P-wave shadow zone (between $103^{\circ}$ and $142^{\circ}$ epicentral distance).
- Separates solid mantle from fluid outer core; bends P-waves causing their temporary disappearance.
Lehmann Discontinuity
- Proposed 1936 by Inge Lehmann.
- Reveals solid inner core inside the liquid outer core; inferred from subtle P-wave reflections/refractions and complete S-wave attenuation in liquid layer.

Earthquakes: Definition & Mechanics

Earthquake: ground shaking caused by sudden release of accumulated elastic strain energy when stress exceeds frictional resistance along a fault (rock fracture/discontinuity).
Hypocenter (Focus): subsurface point where rupture initiates.
Epicenter: surface point directly above the hypocenter.
Seismic Sequence
- Foreshocks: smaller quakes that precede the main event in same area.
- Mainshock: principal, largest-magnitude quake.
- Aftershocks: smaller subsequent quakes on/near the same fault; can last weeks–months proportional to mainshock size.

Global Seismicity & Plate Tectonics

Pacific Ring of Fire (Pacific Rim)
- Horseshoe-shaped belt encircling the Pacific Ocean.
- Hosts most of Earth’s active subduction zones → frequent volcanism & earthquakes.
- Typical normal earthquake magnitude: up to $6.9$ ; average recurrence ~every $2.5$ days.
Fault Line: surface trace of a fault; visible expression of subsurface fracture.

Triangulation Method for Locating an Epicenter

Determine time lag ( $t_d$ ) between arrival of P-wave and S-wave at each seismic station (from seismogram).
Compute distance from station to epicenter using $d = \frac{t_d}{8\ \text{s}} \times 100\ \text{km}$ where:
- $d$ = distance to epicenter.
- $t_d$ = P–S arrival time difference.
- Constant $8\ \text{s}$ reflects that at $100\ \text{km}$ distance the S–P gap is $8\ \text{s}$ .
Convert distance to map scale (e.g.
$1\ \text{cm} : 100\ \text{km}$ ) and set compass radius.
Draw circle around each station; radius = computed distance.
The intersection of three (or more) circles pinpoints the epicenter.

Worked Example Snippets (from transcript tables)

Cavite station: $t_d = 28\ \text{s} \Rightarrow d = \frac{28}{8}\times100 = 350\ \text{km}$ .
Virac station: $t_d = 40\ \text{s} \Rightarrow d = 500\ \text{km}$ .
Vigan station: $t_d = 12\ \text{s} \Rightarrow d = 150\ \text{km}$ .
Draw corresponding radii of $3.5\ \text{cm}, 5\ \text{cm}, 1.5\ \text{cm}$ on a map with scale $1\ \text{cm}:100\ \text{km}$ and find the common intersection.

Warning Signs & Precursors (Not Always Reliable)

Unusual animal behaviour.
Changes in atmospheric conditions (humidity, cloud patterns, odd stillness).
Electromagnetic disturbances (radio/TV static, unusual light phenomena).
Audible booming or rumbling sounds.

Earthquake-Related Hazards

Ground shaking (intensity depends on magnitude, distance, local geology).
Surface rupture (visible ground cracking/displacement along fault trace).
Liquefaction (water-saturated soils lose strength, behave like fluid ⇒ building collapse).
Tsunamis & seiches (long-period water waves in oceans, lakes, reservoirs).
Sinkholes, subsidence, lateral spreading.
Landslides & rockfalls (especially on steep, unstable slopes).
Fire (gas-line rupture, electrical faults).

Earthquake Preparedness & Readiness

Develop and routinely review a family preparedness plan.
Identify & secure hazards (fasten shelves, water heaters, heavy objects).
Learn your locality’s seismic risk zoning and building codes.
Practice the “Drop, Cover, and Hold On” drill:
- Drop to hands and knees.
- Cover head & neck under sturdy furniture or with arms.
- Hold on until shaking stops.
Protect property: retrofit buildings, brace foundations, install automatic gas shut-off valves.
Safeguard important documents (backup copies, fire-/water-proof containers).

Ethical, Social & Practical Implications

Public education and drills save lives by reducing panic and ensuring coordinated response.
Building to seismic-resistant standards is an ethical responsibility of engineers, architects, and governments, especially in high-risk zones (e.g., Pacific Rim).
Early-warning systems (P-wave sensors linked to automatic alerts) can provide crucial few-second lead time for critical infrastructure shutdowns (trains, surgeries, power plants).
Equitable access to preparedness resources mitigates disproportionate impact on vulnerable populations.

Quick Reference Formulae & Figures

P–S time-distance conversion: $d = \frac{t_d}{8}\times100$ (km).
P-wave shadow-zone range: $103^{\circ} - 142^{\circ}$ .
Average Moho depth: $\approx 50\ \text{km}$ (varies $5 – 70\ \text{km}$ depending on oceanic vs continental crust).
Typical earthquake magnitude in Ring of Fire: $\le 6.9$ .

Revision Strategy

Memorise wave definitions & particle motions with sketches.
Practise at least 3 triangulation problems using real seismograms.
Rehearse “Drop, Cover, Hold-On” steps aloud.
Use flashcards for discontinuity names, discoverers & significance (Moho, Gutenberg, Lehmann).
Relate hazards to local examples to internalise risk awareness.