Plate Tectonics, Seismic Waves & Earthquake Hazards – Comprehensive Bullet-Point Notes

OBJECTIVES

• Describe and map the distribution of:

  • Active volcanoes

  • Earthquake epicenters

  • Major mountain belts

• Illustrate the above specifically within the CALABARZON region.

QUICK REVIEW / CONTEXT

• Topic sits inside Plate-Tectonic Theory and regional hazard awareness.

• Spatial patterns of volcanoes, quakes, and mountains are primary evidence for moving plates.

KEY DEFINITIONS

• Earthquake – shaking of Earth’s surface due to sudden release of energy in the lithosphere → generates seismic waves.

• Seismic Wave – packets of energy that propagate through or along Earth and are recorded by seismographs.

TYPES OF SEISMIC WAVES

• Body Waves (travel through the interior)

  • P-Waves (Primary / Compressional)

  • Fastest; first to arrive.

  • Motion: push–pull parallel to wave propagation ⇒ longitudinal compression & dilation.

  • Pass through solids, liquids, gases.

  • Analogy: sound waves in air.

  • S-Waves (Secondary / Shear)

  • Arrive after P’s.

  • Motion: particles move perpendicular to propagation, giving a “wiggle.”

  • Travel only through solids (liquids lack shear strength).

• Surface Waves (restricted to Earth’s exterior; largest amplitudes; most destructive)

  • Love Waves (L-Waves)

  • Horizontally polarized shear; side-to-side movement.

  • Do not propagate through water; instead shove water laterally along basin edges.

  • Rayleigh Waves

  • Retrograde elliptical motion (up–down + back–forth); combines longitudinal & transverse components.

  • Amplitude decays exponentially with depth below surface.

VARIETIES OF EARTHQUAKE VIBRATION (WAVE PARAMETERS)

• Period (T) – time for one full cycle → distance between successive peaks in the time domain.
  $T = \frac{1}{f}$

• Wavelength (\lambda) – physical distance between two consecutive crests in space.

• Amplitude (A) – maximum positive or negative displacement from equilibrium.

• Frequency (f) – number of cycles per second, measured in Hertz (Hz). $f = \frac{1}{T}$

EARTHQUAKE SIZE & CHARACTERISTICS

1. INTENSITY (WHAT PEOPLE/STRUCTURES FEEL)

• Qualitative/ semi-quantitative; varies with location.

• Modified Mercalli Intensity (MMI) Scale (I – XII, but transcript lists I–X):

  • I Not felt

  • II–III Weak (noticed by few; upper floors)

  • IV Light (windows rattle; like passing truck)

  • V Moderate (many awakened; dishes break)

  • VI Strong (slight damage; heavy furniture moves)

  • VII Very Strong (moderate damage; chimneys fall)

  • VIII Severe (partial collapses)

  • IX Violent (substantial buildings shift)

  • X Extreme (most masonry destroyed; rails bent)

• ShakeMaps – computer-generated contour maps showing peak ground acceleration (PGA) or velocity; guide emergency response. Example: 13 Nov 2008 synthetic M $7.8$ event (ShakeOut scenario) with color gradations of intensity & damage potential.

2. MAGNITUDE (ENERGY RELEASE, SINGLE VALUE PER EVENT)

• Derived from maximum seismograph motion; logarithmic.

a. Richter Magnitude $M_L$

• Empirical; local scale; each integer step ⇒ $10$× amplitude & $\sim31.6$× energy.

• TNT equivalents:
  • $M=3$ ≈ $20{,}000\,\text{kg TNT}$
  • $M=6$ ≈ $60{,}000{,}000\,\text{kg TNT}$
  • $M=9$ ≈ $20\text{ trillion kg TNT}$.

• Frequency relationship: big quakes far less common than small ones.

b. Moment Magnitude $M_w$

• Based on seismic moment $M<em>0 = \mu A D$ (rigidity × fault area × average slip). $M</em>w = \frac{2}{3}\log<em>{10}(M</em>0) - 10.7$

• Captures total energy; preferred for M>7.

c. Gutenberg–Richter Frequency–Magnitude Law

• Statistical relationship: $\log_{10} N = a - b M$

  • $N$ = number of events ≥ magnitude $M$;

  • b-value ≈ $1.0$ globally (steeper lines ⇒ fewer big events).

• Plot in transcript shows variable b-values (0.49–1.21) for Tonga & South America/Philippines data bins.

3. PEAK GROUND ACCELERATION (PGA)

• Max instantaneous acceleration during shaking at a site.

• Sentinel instrumentation trigger: ± $5\,\text{mg}$ ($1\,\text{g}=9.81\,\text{m/s}^2$).

• Example Christchurch series:
  • 2016 Kaikoura: $50\,\text{mg}$
  • 2010 Darfield: $300\,\text{mg}$
  • 2011 Christchurch CBD: >1000\,\text{mg}.

SECONDARY GROUND EFFECTS – LIQUEFACTION

• Definition: saturated, unconsolidated soils lose strength/rigidity under cyclic shaking and behave as a fluid.

• Process: grains temporarily lose contact; water pressure equalizes; post-shake reconsolidation can leave differential settlement.

• Hazards: tilting buildings, buried tank floatation, lateral spreading of ground toward rivers/bays.

CONNECTIONS TO PLATE TECTONICS & MOUNTAIN BELTS

• Volcanoes, epicenters, and orogenic belts align mainly along plate boundaries:
  • Convergent (subduction) → volcanic arcs, deep quakes, fold–thrust mountains.
  • Divergent (ridges) → shallow quakes, volcanic ridges.
  • Transform (strike-slip) → linear quakes, minimal volcanism (e.g., San Andreas).

• CALABARZON (Philippines): near Philippine–Sea Plate subduction ⇒ clustered volcanism (Taal, Banahaw), frequent quakes, Sierra Madre & related uplifts.

STUDY ASSIGNMENT / NEXT STEPS

• Advance reading:
  • Different plate-boundary types (divergent, convergent, transform).
  • Characteristic geologic/tectonic signatures.
  • Community risk awareness relative to boundary proximity.

ETHICAL & PRACTICAL IMPLICATIONS

• Urban planning must incorporate hazard maps (ShakeMaps, liquefaction zones).

• Building codes tied to expected PGA & MMI levels.

• Public education on intensity scales improves emergency response.

SUMMARY OF KEY TAKEAWAYS

• Seismic energy travels as body (P, S) and surface (Love, Rayleigh) waves, each with diagnostic motions and damage potential.

• Earthquake vibration is characterized by period, wavelength, amplitude, and frequency.

• Size is recorded as intensity (location-specific) and magnitude (event-wide); logarithmic nature underlines why small differences in $M_w$ imply huge energy jumps.

• PGA and secondary processes (liquefaction) dictate ground failure risks.

• Frequency-magnitude statistics (Gutenberg-Richter) guide long-term probability forecasts.

• Spatial distribution of geological hazards reflects plate-tectonic dynamics and informs regional preparedness (e.g., CALABARZON).