MEASUREMENT OF EARTHQUAKES: MAGNITUDE VS. INTENSITY

GROUP 3

Seismograph

a machine that records seismic waves or ground motion.

Magnitude

is proportional to the energy released by an earthquake at the focus.

Magnitude

Arabic Numbers (e.g. 4.8, 9.0)

Intensity

is the strength of an earthquake as perceived and felt by people in a certain locality.

Intensity

t is represented by Roman Numerals (e.g. II, IV, IX)

Sultan Kudarat Swarn (Jan 2026)

Over 2,640 earthquakes were recorded off the coast of Kalamansig between January 19 and 30, 2026, with the largest being a magnitude 5.9 ang 5.0.

Davao Oriental Earthquake (Oct 2025)

A massive 7.4 magnitude earthquake struck off the coast of Davao Oriental on October 10, 2025, followed by a 6.8 magnitude aftershock later that day.

Cebu Earthquake (Sept/Oct 2025)

A 6.9 magnitude earthquake struck Cebu Province on September 30, 2025, which was identified as the strongest ever recorded in that area.

Magnitude Scales

are logarithmic tools used to measure the "size" or total energy of a phenomenon, most commonly used in seismology to quantify earthquakes.

Richter Magnitude scale

is a logarithmic scale developed by seismologists Charles F. Richter and Beno Gutenberg to quantify the magnitude of seismic energy released by earthquakes, first introduced in the early 1930s.

Moment Magnitude Scale (Mw)

The current standard for global reporting.

Richter Local Magnitude (ML)

It measures the maximum amplitude of seismic waves recorded on a specific type of seismograph.

Moment Magnitude Scale (Mw)

It measures total energy release by considering physical factors

Body-Wave Magnitude (Mb)

Based on the amplitude of "P" (primary) waves. It is primarily used to measure deep earthquakes or those at great distances (teleseisms).

Surface-Wave Magnitude (Ms)

Calculated from the amplitude of surface waves that travel along the Earth's crust. It is useful for distant, shallow earthquakes but also saturates around magnitude 8.

Duration Magnitude (Md)

Estimates magnitude based on the length (duration) of the seismic wave recording.

Distance from Epicenter

intensity generally decreases as distance increases.

Hard rock

tends to transmit seismic waves efficiently without amplifying them, resulting in lower intensity.

Soft soils

can trap and amplify seismic energy, increasing shaking intensity significantly

Liquefaction-Prone Areas

can cause buildings to sink or tilt, which increases the observed intensity effects.

Shallow Earthquakes

(less than 70 km deep)

Shallow Earthquakes

seismic waves reach the surface with more energy, often causing higher intensity levels over a smaller area.

Deep Earthquakes

(more than 300 km deep)

Deep Earthquakes

waves spread out over a larger area but with less intensity at any single point.

Building and Infrastructure Quality

The way structures are designed and built directly affects how much damage they sustain.

Duration of Shaking

Longer periods of shaking can cause more damage to structures and feel more severe to people.

Mercalli Intensity Scale

developed by the Italian volcanologist Giuseppe Mercalli in 1884 and expanded to include 12 degrees of intensity in 1902 by Adolfo Cancani.

Mercalli Intensity Scale

A.K.A Modified Mercalli Intensity Scale.

Mercalli-Cancani-Sieberg (MCS)

A 12-grade system originally used in Southern Europe to judge quakes based on human perception and visible damage.

Modified Mercalli Intensity (MMI)

Developed in 1931 as a 12-level update to the MCS specifically for California, now widely used across North America.

Medvedev–Sponheuer–Karnik (MSK)

Medvedev–Sponheuer–Karnik (MSK)

European Macroseismic Scale (EMS-98)

Used in Europe since the late 90s, this 12-level scale refines the MSK and MMI to better categorize damage to contemporary construction styles.

Japanese Meteorological Agency (JMA) Intensity Scale

A 7-level system (with subdivisions) unique to Japan that blends physical observations with automated sensor data like horizontal acceleration.

I. Scarcely Perceptible

Felt by very few people; barely noticeable.

II. Slightly Felt

Felt by a few people, especially on upper floors.

III. Weak

Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake.

IV. Moderately Strong

Felt by many indoors, few outdoors. May feel like heavy truck passing by.

V. Strong

Felt by almost everyone, some people awakened. Small objects moved. Trees and poles may shake.

VI. Very Strong

Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls. Chimneys may be slightly damaged

VII. Destructive

Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly built structures. Some wall may fall.

VIII. Very Destructive

Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse.

IX. Devastating

Considerable damage to specially built structures, buildings shifted off foundations. Ground cracked noticeably. Wholes destruction. Landslides.

X. Completely Devastating

Most masonry and frame structures and their foundations destroyed, Ground badly cracked. Landslides. Wholesale destruction.

XII

Total damage. Waves seen on ground. Objects thrown up into air.

XI

Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground.