PHIVOLCS Earthquake Intensity Scale (PEIS) 2025: Complete Guide to Understanding Earthquake Strength in the Pilipinas

Introduction: Understanding How Earthquake Strength is Measured

When an earthquake strikes the Pilipinas, PHIVOLCS (Pilipino Institute of Volcanology and Seismology) reports two critical measurements: magnitude and intensity. While magnitude measures the energy released at the earthquake's source, intensity describes what people actually feel and the damage that occurs at specific locations. The PHIVOLCS Earthquake Intensity Scale (PEIS) is the standard system used nationwide to communicate earthquake effects to the public. This comprehensive guide explains every intensity level, how it differs from magnitude, and what each level means for your safety.

1. Magnitude vs. Intensity: Critical Differences

1.1 What is Earthquake Magnitude?

Magnitude is a single numerical value that represents the total energy released by an earthquake at its source (hypocenter/focus).

Key Characteristics:

• Single Value: Each earthquake has only one magnitude, regardless of location
• Measurement Method: Calculated from seismograph recordings of ground motion amplitude
• Scale Type: Logarithmic scale (each whole number increase = 10x amplitude increase, ~32x energy increase)
• Common Scales:
  • Richter Scale (older, local earthquakes)
  • Moment Magnitude Scale (Mw) - modern standard used by PHIVOLCS
• Example: The 1990 Luzon Earthquake had a magnitude of M7.8 everywhere

1.2 What is Earthquake Intensity?

Intensity measures the effects of an earthquake at a specific location, describing shaking strength, human perception, and damage levels.

Key Characteristics:

• Location-Specific: Varies from place to place for the same earthquake
• Measurement Method: Based on observations, reports, and damage assessments
• Scale Type: Descriptive scale with defined levels (I to X in PEIS)
• Factors Affecting Intensity:
  • Distance from epicenter (closer = higher intensity)
  • Depth of earthquake (shallow = higher intensity)
  • Soil/geology (soft soil amplifies shaking)
  • Building construction quality
  • Local topography
• Example: The 1990 Luzon Earthquake (M7.8) produced Intensity X in Baguio City, Intensity VII in Manila, and Intensity III in Mindanao

1.3 Visual Comparison Table

2. The PHIVOLCS Earthquake Intensity Scale (PEIS)

2.1 History and Development

The PHIVOLCS Earthquake Intensity Scale was developed to provide a standardized, culturally relevant way to describe earthquake effects in the Pilipinas.

Evolution:

• Pre-1996: Pilipinas used the Modified Mercalli Intensity (MMI) Scale
• 1996: PHIVOLCS developed PEIS, adapted to Pilipino building types and local conditions
• Key Adaptations:
  • References to common Pilipino building materials (bamboo, light materials, concrete)
  • Consideration of local construction practices
  • Cultural references familiar to Filipinos
  • Simplified language for public understanding

2.2 PEIS Structure and Levels

PEIS uses Roman numerals I through X to describe increasing levels of earthquake intensity. Each level has specific criteria based on:

• Perception: What people feel and how they react
• Objects: Behavior of hanging objects, furniture, liquids
• Structures: Effects on different building types
• Nature: Mga epekto sa lupa, landslides, ground cracks

3. Complete PEIS Intensity Levels (I - X)

Intensity I - Scarcely Perceptible

Description:

The weakest intensity level, felt only by very few people under favorable conditions.

Effects:

• Perception: Felt by very few individuals at rest indoors, especially on upper palapag of buildings
• Objects: No movement of objects
• Structures: No damage to any type of structure
• Nature: No observable effects in nature

Equivalent Magnitude Range:

Typically M2.0-3.0 at close distances or M4.0-5.0 at greater distances

Safety Actions:

• None required - no hazard present
• Most people will not notice this level

Intensity II - Slightly Felt

Description:

Felt by few individuals, particularly those at rest or in upper palapag.

Effects:

• Perception: Felt by few individuals at rest indoors, like a passing light vehicle
• Objects: Hanging objects may swing slightly
• Structures: No damage
• Nature: No observable effects

Equivalent Magnitude Range:

Typically M3.0-4.0 nearby or M5.0-5.5 at moderate distances

Safety Actions:

• None required
• Good opportunity to practice "Drop, Cover, Hold" awareness

Intensity III - Weak

Description:

Felt by many people indoors, especially in upper palapag. Some people outdoors may notice it.

Effects:

• Perception: Felt by many people indoors, vibration like a passing heavy vehicle
• Objects: Hanging objects swing noticeably, standing vehicles may rock slightly
• Structures: No damage to structures
• Nature: No observable effects

Equivalent Magnitude Range:

Typically M4.0-4.5 nearby or M5.5-6.0 at moderate distances

Safety Actions:

• Be aware of earthquake occurrence
• Review family earthquake plan
• No immediate danger

Intensity IV - Moderately Strong

Description:

Generally felt by people indoors and by some people outdoors. Light sleepers may be awakened.

Effects:

• Perception: Felt by most people indoors, some awakened from sleep
• Objects: Hanging objects swing considerably, dishes and glassware rattle, windows and doors rattle
• Structures: No damage to structures, but items may fall from shelves
• Nature: Possible slight sloshing of liquids in containers

Equivalent Magnitude Range:

Typically M4.5-5.0 nearby or M6.0-6.5 at moderate distances

Safety Actions:

• Stay calm, assess situation
• If indoors: stay indoors, move away from windows
• If outdoors: stay outdoors, away from buildings
• Check for any fallen objects after shaking stops

Intensity V - Strong

Description:

Generally felt by most people, many frightened and run outdoors. Significant indoor effects.

Effects:

• Perception: Generally felt by most people, many frightened and run outdoors
• Objects: Hanging objects swing violently, dining utensils clatter and fall, small unstable objects overturn or fall
• Structures:
  • Strong shaking noticed by people driving vehicles
  • Light wall materials (bamboo, light wood) may crack
  • No damage to well-built structures
• Nature: Liquids spill from filled open containers, small church bells may ring

Equivalent Magnitude Range:

Typically M5.0-5.5 nearby or M6.5-7.0 at moderate distances

Safety Actions:

• During shaking: Drop, Cover, Hold (under sturdy table/desk)
• After shaking: Check for injuries, inspect building for damage
• Be prepared: Aftershocks are likely
• Check mga suplay: Ensure emergency kit is accessible

Intensity VI - Very Strong

Description:

Many people frightened and run outdoors. Notable damage to poorly built structures. Well-built structures generally safe.

Effects:

• Perception: Many frightened, people have difficulty standing or walking
• Objects: Heavy objects or furniture move or may overturn, small church bells ring
• Structures:
  • Slight damage to old, poorly built structures
  • Cracks in walls of some masonry buildings
  • Damage to bamboo and light material houses
  • Well-designed and built structures experience no damage
• Nature: Trees noticeably shaken, some branches may break

Equivalent Magnitude Range:

Typically M5.5-6.0 nearby or M7.0-7.5 at moderate distances

Safety Actions:

• During shaking: Drop, Cover, Hold - DO NOT run outside during shaking
• After shaking:
  • Evacuate if building shows structural damage
  • Check for gas leaks, water line breaks, electrical damage
  • Expect aftershocks
  • Listen to official emergency broadcasts
• Community response: Check on neighbors, especially elderly and vulnerable

Intensity VII - Destructive

Description:

Most people frightened and try to run outdoors. Significant structural damage, especially to poorly built structures.

Effects:

• Perception: Most people frightened, difficult to stand, noticed by drivers of moving vehicles
• Objects: Big church bells ring, hanging objects quiver, furniture breaks, liquids disturbed
• Structures:
  • Considerable damage to poorly built or badly designed structures
  • Some collapse of older masonry buildings
  • Cracks appear in well-built structures
  • Walls of bamboo and light materials collapse
  • Old or weak chimneys may break or fall
• Nature:
  • Trees shake strongly, some branches break off
  • Possible landslides in unstable slopes
  • Changes in flow of springs and wells

Equivalent Magnitude Range:

Typically M6.0-6.5 nearby or M7.5+ at moderate distances

Safety Actions:

• Immediate: Drop, Cover, Hold until shaking stops
• Evacuation:
  • Exit damaged buildings carefully after shaking stops
  • Watch for falling debris
  • Stay away from damaged buildings
• Emergency response:
  • Activate emergency plan
  • Use emergency mga suplay (7-day kit)
  • Provide first aid as needed
  • Report casualties to authorities
• Communication: Contact family members via text (voice calls may overload network)

Intensity VIII - Very Destructive

Description:

Widespread damage, many buildings severely damaged or destroyed. People panic and struggle to stand.

Effects:

• Perception: People panicky, difficult or impossible to stand
• Objects: Heavy furniture overturned, objects thrown into the air
• Structures:
  • Many poorly built structures severely damaged or destroyed
  • Partial collapse of some well-built structures
  • Serious damage to reinforced concrete structures
  • Fall of walls, chimneys, columns, monuments, towers
  • Underground pipe breaks
• Nature:
  • Numerous landslides and rockfalls
  • Ground cracks may appear, especially in wet ground
  • Changes in well water levels
  • Trees broken or uprooted

Equivalent Magnitude Range:

Typically M6.5-7.0 nearby or M8.0+ at moderate distances

Safety Actions:

• During shaking: Drop, Cover, Hold - protect head and neck
• After shaking:
  • Expect widespread damage and disruption
  • Evacuate damaged buildings immediately but carefully
  • Avoid areas with fallen power lines
  • Stay away from damaged buildings, bridges, overpasses
• Emergency survival:
  • Use emergency mga suplay (expect services disrupted for days)
  • Provide first aid, rescue if safe to do so
  • Set up temporary shelter if home is uninhabitable
  • Conserve water and food mga suplay
• Community response:
  • Organize with neighbors for mutual aid
  • Follow instructions from emergency authorities
  • Report trapped persons to rescue teams

Intensity IX - Devastating

Description:

Massive destruction, most buildings severely damaged or collapsed. Mga epekto sa lupa widespread.

Effects:

• Perception: People thrown about, impossible to remain standing
• Objects: Objects thrown violently into the air
• Structures:
  • Most buildings severely damaged or destroyed
  • Many well-built structures destroyed
  • Even earthquake-resistant structures suffer serious damage
  • Large-scale collapse of bridges, overpasses
  • Dams, dikes may be seriously damaged
  • Monuments and statues displaced, rotated, or toppled
• Nature:
  • Widespread landslides, rockfalls, avalanches
  • Ground cracks several centimeters wide
  • Ground permanently deformed (uplift, subsidence)
  • Sand and mud ejection (liquefaction)
  • Large waves on bodies of water

Equivalent Magnitude Range:

Typically M7.0+ nearby or M8.5+ at moderate distances

Safety Actions:

• During shaking: Drop, Cover, Hold - protect yourself from falling debris
• Immediate aftermath:
  • Expect catastrophic damage and casualties
  • Evacuate collapsed/severely damaged structures if safe
  • Watch for secondary hazards (fire, gas leaks, flooding)
  • Stay away from coastlines (tsunami risk)
• Survival priorities:
  • Provide emergency first aid and rescue if trained and safe
  • Use emergency mga suplay (expect no services for weeks)
  • Seek shelter, prioritize injured and vulnerable
  • Conserve all resources (water, food, medical mga suplay)
• Long-term:
  • Follow evacuation orders from authorities
  • Organize community mutual aid
  • Report missing persons and casualties
  • Prepare for extended displacement

Intensity X - Completely Devastating

Description:

Total or near-total destruction. The most severe intensity level, rarely experienced.

Effects:

• Perception: People thrown violently, cannot control movement
• Objects: Nearly all objects thrown and tossed into the air
• Structures:
  • Nearly all structures destroyed, including well-built earthquake-resistant buildings
  • Total destruction of infrastructure (roads, bridges, utilities)
  • Dams and dikes destroyed
  • Railroad tracks bent or destroyed
• Nature:
  • Massive landslides and ground failures
  • Large ground cracks (meters wide)
  • Severe ground deformation (uplift, subsidence >1 meter)
  • Widespread liquefaction
  • Rivers dammed, courses changed
  • Waterfalls created or destroyed

Historical Occurrence:

Intensity X has been rarely observed in the Pilipinas. The 1990 Luzon Earthquake (M7.8) produced Intensity X in limited areas of Baguio City.

Equivalent Magnitude Range:

Typically M7.5+ very close to epicenter or M9.0+ at moderate distances

Safety Actions:

• Survival focus:
  • Immediate survival is the only priority
  • Seek any available shelter and protection
  • Rescue trapped persons if safe to attempt
• Post-disaster:
  • Expect complete loss of infrastructure and services
  • Self-sufficiency required for extended period (weeks to months)
  • Community organization critical for survival
  • Follow all evacuation and emergency orders

4. Practical Application: How PHIVOLCS Determines Intensity

4.1 Intensity Data Collection Methods

Citizen Reports

• Online Reporting: PHIVOLCS website and mobile apps accept intensity reports
• Social Media Monitoring: Twitter, Facebook posts analyzed for intensity indicators
• Mga Ulat sa Hotline: Phone calls to PHIVOLCS Mga Emergency Hotlines

Field Surveys

• Mabilisang Pagsusuri Teams: PHIVOLCS staff deployed to affected areas within hours
• Structural Engineers: Assess building damage and categorize by intensity criteria
• Documentation: Photographs, videos, interviews with residents

Instrumental Data

• Seismographs: Ground motion recordings correlate with intensity levels
• Strong Motion Instruments: Measure peak ground acceleration (PGA) in affected areas
• Correlation Models: Mathematical relationships between PGA and intensity

4.2 Creating Intensity Maps (Isoseismal Maps)

After significant earthquakes, PHIVOLCS produces intensity maps showing how intensity varied across affected regions.

Map Features:

• Isoseismal Lines: Contour lines connecting areas of equal intensity
• Color Coding: Different colors represent different intensity levels
• Epicenter: Marked location of earthquake origin
• Fault Lines: Active faults shown for context

Interpretation:

• Higher intensities generally occur closer to epicenter
• Elongated patterns may indicate fault rupture direction
• Localized high intensities may show soil amplification effects

5. Case Studies: Historical Pilipino Earthquakes and Intensity Distributions

5.1 1990 Luzon Earthquake (M7.8, July 16)

Intensity Distribution:

Key Lessons:

• Same M7.8 earthquake produced intensities ranging from X to III depending on location
• Local soil conditions (Dagupan's soft soils) amplified shaking
• Distance dramatically affects intensity (X at 15km, III at 800km)

5.2 2013 Bohol Earthquake (M7.2, October 15)

Intensity Distribution:

Key Lessons:

• Shallow earthquake (depth ~10km) produced high intensities despite M7.2 magnitude
• Historical structures (Spanish-era churches) particularly vulnerable at Intensity VII+
• 6 km visible ground rupture demonstrated surface faulting at Intensity IX areas

5.3 2019 Cotabato Earthquakes (M6.3, M6.6, M6.9, October)

M6.9 Event Intensity Distribution:

Key Lessons:

• Earthquake sequence (three M6+ events in weeks) caused cumulative damage
• Buildings weakened by first earthquake collapsed in subsequent events
• Importance of post-earthquake building inspections before re-occupancy

6. Intensity vs. Magnitude: Practical Scenarios

6.1 Scenario 1: Shallow vs. Deep Earthquakes

Shallow Earthquake (Depth: 5-10 km)

• Example: M6.5 at 8 km depth
• Intensity at Epicenter: VIII-IX (Very Destructive to Devastating)
• Affected Radius: High intensities (VII+) within 30-50 km
• Characteristics: Very strong shaking near epicenter, rapid intensity decrease with distance

Deep Earthquake (Depth: 100-150 km)

• Example: M6.5 at 120 km depth
• Intensity at Epicenter: IV-V (Moderately Strong to Strong)
• Affected Radius: Moderate intensities (IV-V) over very wide area (200+ km)
• Characteristics: Moderate shaking over large area, less structural damage

6.2 Scenario 2: Soil Effects

Same Earthquake, Different Soil Types

Real-World Example:

During the 1990 Luzon Earthquake, Dagupan City (on soft sediments) experienced Intensity VIII and widespread liquefaction, while areas of similar distance on bedrock experienced Intensity VI-VII.

7. Understanding Intensity for Personal Safety

7.1 "Drop, Cover, Hold" by Intensity Level

Intensity I-III: Awareness

• Action: Note the earthquake, no protective action required
• Purpose: Practice awareness, good time to review emergency plans

Intensity IV-V: Precautionary

• Action: Drop, Cover, Hold if shaking intensifies
• Purpose: Protect from falling objects, practice safety response

Intensity VI-VII: Protective

• Action: Immediately Drop, Cover, Hold
• Purpose: Critical protection from falling objects and structural damage
• After shaking: Inspect building, evacuate if damaged

Intensity VIII-X: Survival

• Action: Drop, Cover, Hold - protect head and vital organs
• Purpose: Survive catastrophic shaking and structural collapse
• After shaking: Immediate evacuation from damaged structures, render aid if safe

7.2 Building Safety by Intensity

When to Evacuate Buildings

8. Intensity Reporting and Citizen Science

8.1 How to Report Earthquake Intensity to PHIVOLCS

Online Reporting Platforms

• PHIVOLCS Website: www.phivolcs.dost.gov.ph - "Report an Earthquake" form
• Mobile Apps: PHIVOLCS official earthquake app (iOS and Android)
• Social Media: Tag @phivolcs_dost on Twitter with #PHEarthquake

Information to Provide

• Location: Exact address or coordinates
• Time felt: When you felt the earthquake
• What you felt:
  • Light, moderate, or strong shaking
  • Duration of shaking
  • Indoor or outdoor perception
• Observed effects:
  • Objects moved, fell, or broken
  • Building damage (cracks, collapse)
  • Mga epekto sa lupa (cracks, landslides)
• Photos/Videos: Visual na ebidensya of effects (if safe to document)

8.2 Importance of Citizen Reports

Why Your Report Matters

• Mabilisang Pagsusuri: Citizen reports provide immediate intensity data before field teams arrive
• Saklaw: Reports from remote areas where PHIVOLCS has no instruments
• Pagmamapa ng Pinsala: Helps prioritize emergency response and resource deployment
• Halaga sa Agham: Contributes to understanding of earthquake effects and improving building codes
• Kaligtasan ng Publiko: Accurate intensity maps help other communities prepare for aftershocks

9. Frequently Asked Questions

Q1: Why do PHIVOLCS reports use both magnitude and intensity?

Answer: Magnitude tells us the earthquake's size (energy released), which is important for scientific analysis and comparison. Intensity tells us the actual effects people experience and damage that occurs, which is critical for emergency response and public safety. Both measurements provide complementary information.

Q2: Can a small magnitude earthquake cause high intensity?

Answer: Yes, if the earthquake is very shallow and you are near the epicenter. For example, a M4.5 earthquake at 2 km depth could produce Intensity VI-VII locally, even though it's a relatively small magnitude. Conversely, a M7.0 earthquake at 150 km depth might only produce Intensity IV-V.

Q3: Why does intensity vary so much for the same earthquake?

Answer: Intensity depends on:

• Distance: Farther from epicenter = lower intensity
• Depth: Deeper earthquakes produce lower intensities at surface
• Soil type: Soft soils amplify shaking (higher intensity)
• Building quality: Poor construction shows more damage at same shaking level
• Topography: Ridge tops may experience amplified shaking

Q4: How quickly does PHIVOLCS determine intensity after an earthquake?

Answer:

• Preliminary intensity: 5-15 minutes (based on instrumental data and initial reports)
• Revised intensity: 1-2 hours (incorporating citizen reports and field observations)
• Final intensity map: Days to weeks (after comprehensive field surveys)

Q5: Is PEIS the same as the Richter Scale?

Answer: No. The Richter Scale measures magnitude (energy released), while PEIS measures intensity (effects observed). They are completely different measurement systems. PEIS is more similar to the Modified Mercalli Intensity Scale, adapted for Pilipino conditions.

Q6: Can intensity be predicted before an earthquake?

Answer: No, intensity cannot be predicted because it depends on the earthquake occurrence itself. However, intensity can be estimated for scenario earthquakes (e.g., "If the West Valley Fault produces a M7.2 earthquake, we expect Intensity VIII in these areas"). These estimates guide preparedness planning.

Q7: Why do some areas experience higher intensity than closer areas?

Answer: This happens due to:

• Soil amplification: Soft soil areas amplify shaking more than nearby bedrock areas
• Basin effects: Valleys filled with sediment can trap and amplify seismic waves
• Directivity: Fault rupture propagation can focus energy in certain directions
• Building resonance: Certain building heights resonate with earthquake frequencies, increasing damage perception

10. Key Takeaways and Mga Aksyon na Dapat Gawin

10.1 Essential Understanding

1. Magnitude ≠ Intensity: One earthquake has one magnitude but many intensities depending on location
2. Intensity is personal: What you feel and observe at your location determines intensity
3. Distance matters: Intensity generally decreases with distance from epicenter
4. Soil amplifies shaking: Soft soils increase intensity; bedrock reduces it
5. Building quality is critical: Same intensity causes different damage depending on construction
6. Depth is crucial: Shallow earthquakes produce higher intensities than deep ones

10.2 Practical Response Guide

10.3 Immediate Action Checklist

• ☐ Learn the 10 intensity levels and what each means
• ☐ Understand that magnitude and intensity are different
• ☐ Know your area's soil type (affects intensity)
• ☐ Practice "Drop, Cover, Hold" for Intensity V+
• ☐ Install PHIVOLCS earthquake app for reporting
• ☐ Learn how to report earthquake intensity
• ☐ Review when to evacuate buildings after earthquakes
• ☐ Understand that intensity determines damage, not magnitude alone
• ☐ Prepare emergency mga suplay for Intensity VII+ scenarios
• ☐ Share this knowledge with family and community

11. Resources and Further Information

11.1 PHIVOLCS Contact Information

• Main Office:
  • Address: PHIVOLCS Building, C.P. Garcia Avenue, UP Campus, Diliman, Quezon City
  • Hotline: (02) 8426-1468 to 79
  • Email: info@phivolcs.dost.gov.ph
  • Website: www.phivolcs.dost.gov.ph
• Earthquake Information:
  • Real-time earthquake bulletin: www.phivolcs.dost.gov.ph/index.php/earthquake/earthquake-information3
  • Intensity reporting: Via website or mobile app

11.2 Mga Mga Emergency Hotlines

• National Emergency: 911
• PHIVOLCS: (02) 8426-1468 to 79
• NDRRMC: (02) 8911-5061 to 65
• Pilipino Red Cross: 143

References

• PHIVOLCS (1996). "PHIVOLCS Earthquake Intensity Scale (PEIS)"
• PHIVOLCS (2023). "Understanding Earthquake Intensity and Magnitude"
• PHIVOLCS Earthquake Bulletins and Intensity Reports (various dates)
• PHIVOLCS (2015). "Guidelines for Earthquake Intensity Determination"
• Intensity maps from 1990 Luzon, 2013 Bohol, and 2019 Cotabato earthquakes