Comprehensive Study Notes on Earthquakes and Earthquake Hazards

Historical Seismic Activity and Recent Large Earthquakes in the Philippines

• November 30, 1619 (M8.0 Earthquake):

  • Lal-lo and Tocolano, Cagayan: Churches collapsed.

  • Dingras, Ilocos Norte: Churches partially collapsed.

  • Sinait and Vigan, Ilocos Norte: Churches were damaged.

• February 14, 1934 (M7.6 Earthquake & Tsunami):

  • Originated as an offshore earthquake along the Manila Trench.

  • Maximum tsunami height recorded was $2\,m$.

  • Vigan, Ilocos Sur, and San Esteban were affected, with an estimated inundation extent of approximately $2\,m$.

• July 16, 1990 (M7.8 Luzon Earthquake):

  • Affected Area: Central and Northern Luzon.

  • Casualties: $1,283$ dead; $2,786$ injured; $321$ missing.

  • Property Damage: $25,305$ houses totally damaged; $77,249$ houses partially damaged.

  • Intensities Recorded:

    • PEIS VIII: Very Destructive.

    • PEIS VII: Destructive.

    • PEIS VI: Very Strong.

  • Notable Structural Impacts: Collapsed or damaged infrastructures included the Carmen Bridge (Rosales), Magsaysay Bridge (Dagupan), and Saint Mary’s Academy (Agoo).

• Other Significant Historical Earthquakes:

  • 1968: $M7.3$ Casiguran.

  • 1973: $M7.0$ Ragay Gulf.

  • 1976: $M7.6$ Moro Gulf (recorded as $M8.0$ in some records, causing $3,739$ deaths).

  • 1983: $M6.5$ Laoag.

  • 1990: $M7.1$ Panay.

  • 1994: $M7.1$ Mindoro (recorded as $M6.8$ in some records, causing $78$ deaths).

  • 2012: $M6.9$ Negros (Negros Oriental).

  • 2013: $M7.1$ Bohol.

  • 2017: $M6.7$ Surigao.

  • July 6, 2017: $M6.5$ Leyte (caused building collapses in Kananga and surface ruptures in Tongonan, Ormoc).

Fundamental Concepts and Types of Earthquakes

• Verbatim Definition: An earthquake is a weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth’s surface.

• Artificial Earthquakes:

  • Result from man-made activities such as nuclear or chemical explosions and dynamite explosions.

• Natural Earthquakes:

  • Tectonic Earthquakes: Produced by the sudden movement along faults and plate boundaries.

  • Volcanic Earthquakes: Produced by the movement of magma beneath volcanoes.

Theory of Plate Tectonics

• Internal Structure of the Earth:

  • Lithosphere: The outer layer composed of the crust and uppermost mantle, measuring approximately $60\,km$ to $100\,km$ thick. It is broken into pieces called tectonic plates.

  • Mantle: The layer beneath the lithosphere.

  • Core: The innermost layer of the Earth.

• The Driving Mechanism: Convection currents within the mantle serve as the driving force for plate movements.

• Tectonic Plate Boundaries:

  • Convergent Boundaries: Plates move toward each other. Includes subduction zones (forming trenches and volcanic arcs like the Himalayas or various island arcs) and collision zones.

  • Divergent Boundaries: Plates move apart. Examples include oceanic spreading ridges like the Mid-Atlantic Ridge and the East African Rift.

  • Transform / Strike-Slip Boundaries: Plates slide past each other laterally. A primary example is the San Andreas Fault.

• Plate Motion Rates (Specific examples):

  • Relative plate movement in the Philippines region ranges around $10.2\,cm/year$ to $10.7\,cm/year$.

Faults and Philippine Seismicity

• Fault Definition: A fracture, fissure, or zone of weakness where movement or displacement has occurred or may occur again.

• Active Fault Criteria:

  • Demonstrates historical and contemporary seismicity.

  • Fault slip is based on displaced rock or soil units of known age.

  • Includes displaced landforms.

  • Definition by age: Active faults are those that moved within the last $10,000$ years.

• Types of Faults:

  • Normal Fault: Associated with divergent motion where one block moves down relative to another.

  • Thrust or Reverse Fault: Associated with convergent motion where one block moves up over another.

  • Strike-Slip Fault: Associated with lateral movement (e.g., the Digdig Segment of the Philippine Fault Zone).

• Philippine Seismic Statistics:

  • The Philippines records an average of $20$ earthquakes per day.

  • Approximately $100$ to $150$ earthquakes are felt per year.

  • There have been roughly $90$ destructive earthquakes in the past $400$ years (about 1 every 5 years).

Measuring Earthquake Strength: Magnitude and Intensity

• Magnitude:

  • Refers to the energy released at the origin (focus/hypocenter).

  • Measured using a seismograph and reported in Arabic numerals (e.g., $6.3$, $7.5$).

  • Each step increase in magnitude represents a $32$-fold increase in energy released.

• Intensity:

  • Refers to the perceived strength or effects of an earthquake felt at the surface.

  • Based on relative effects on people, structures, and the environment.

  • Reported using Roman Numerals (e.g., IV, IX).

  • In the Philippines, the PHIVOLCS Earthquake Intensity Scale (PEIS) is used.

The PHIVOLCS Earthquake Intensity Scale (PEIS)

• I. Scarcely Perceptible: Perceptible only under favorable circumstances. Delicately balanced objects disturbed slightly.

• II. Slightly Felt: Felt by few individuals at rest indoors. Hanging objects swing slightly.

• III. Weak: Felt by many people indoors, especially on upper floors. Like the passing of a light truck. Dizziness and nausea may be experienced.

• IV. Moderately Strong: Felt generally by people indoors and some outdoors. Heavy sleepers awakened. Like a heavy truck passing. Windows, doors, and plates rattle.

• V. Strong: Generally felt by most people. Hanging objects swing violently. Small, unstable objects may fall. Liquids spill.

• VI. Very Strong: People are frightened and run outdoors. Some lose balance. Heavy objects move. Wall plaster may crack. Very old houses slightly damaged.

• VII. Destructive: Difficult to stand on upper floors. Heavy furniture overturns. Old/poorly built structures suffer considerable damage. Limited liquefaction and landslides.

• VIII. Very Destructive: People are panicky. Many well-built buildings considerably damaged. Bridge foundations destroyed. Railway tracks bent/broken. Boulders thrown out.

• IX. Devastating: People forcibly thrown to the ground. Most buildings totally damaged. Utility posts and monuments tilted or broken. Widespread liquefaction and landslides.

• X. Completely Devastating: Practically all man-made structures destroyed. Massive landslides and liquefaction. Changes in river courses. Ground distorted into undulations.

Factors Determining Earthquake Perception

1. Magnitude: Shaking is more intense for larger earthquakes.

2. Epicentral Distance: Shaking intensity decreases as distance from the fault/epicenter increases.

3. Local Soil Conditions: Soft, loose soil shakes more intensely than hard rock at the same distance.

Earthquake Hazards

• 1. Ground Rupture:

  • Creation of new or renewed movement of old fractures where blocks move in opposite directions.

  • Mitigation: Avoid construction on top of active faults. Structures should be at least $5\,m$ away from the fault trace.

• 2. Ground Shaking:

  • The disruptive up-and-down and sideways motion experienced during an earthquake.

• 3. Liquefaction:

  • Verbatim Definition: A process where particles of loosely consolidated and water-saturated deposits are rearranged into a more compact state, squeezing water and sediments toward the surface in the form of sand fountains, creating a condition resembling “quick sand.”

  • Prone Areas: Low-lying, water-saturated areas with loose, sandy, or silty deposits (river banks, coastlines, reclaimed areas).

• 4. Earthquake-Induced Landslide:

  • Slope failure in steep or hilly areas triggered by shaking.

• 5. Tsunami:

  • Japanese for "harbor waves."

  • Giant sea waves generated by large-scale displacement of the sea floor, usually by offshore earthquakes ($M6.8$ or greater), submarine landslides, or volcanic eruptions.

  • Physical Properties:

    • Faster in deep water ($1000\,kph$ at depth, slowing to $30$-$50\,kph$ at shoreline).

    • Height increases as water depth decreases (from $<1\,m$ at sea to over $30\,m$ at shore).

Earthquake and Tsunami Monitoring and Preparedness

• Philippine Seismic Network:

  • Consists of $36$ manned stations and $51$ unmanned stations.

• Tsunami Warning System (TeWS):

  • Uses tide gauges and wet sensors (e.g., stations at Bolinao, Subic, Zamboanga).

  • Local Tsunami: Generated by nearby trenches/faults; lead time of $2$ to $30$ minutes. Rely on natural signs (SHAKE, DROP, ROAR).

  • Distant Tsunami: Generated outside the Philippines; lead time of $1$ to $24$ hours.

• Emergency Kit Essentials:

  • First aid kit, water purification tablets, AM/FM radio, high-energy food/water, tools, ropes, candles, flashlights, whistle, blanket, and personal hygiene items.

• Structural Safety (Self-Check for CHB Houses):

  • Standard load-bearing exterior walls should be at least $6\,inches$ thick.

  • Box-type/Regular shapes behave better during shaking.

  • Check if built by a licensed professional and according to standards (post-1992).