Seismic Intensity Calculator
Convert between earthquake magnitude scales and estimate ground motion intensity.
Earthquake magnitude and intensity are related but distinct measurements. Magnitude measures the energy released at the earthquake source, while intensity describes the effects felt at a specific location.
The Richter Scale and Moment Magnitude: The original Richter scale (ML) has largely been replaced by the Moment Magnitude scale (Mw) for scientific purposes, though both are logarithmic and give similar values for moderate earthquakes. Each whole number increase represents a 10-fold increase in measured amplitude and approximately 31.6 times more energy released.
Energy relationship:
Energy (joules) = 10^(1.5 × Magnitude + 4.8)
This means:
- A magnitude 5.0 earthquake releases about 2 × 10^12 joules
- A magnitude 6.0 releases about 6.3 × 10^13 joules (31.6 times more)
- A magnitude 7.0 releases about 2 × 10^15 joules (1,000 times more than 5.0)
Comparing earthquake magnitudes:
The energy ratio between two earthquakes is:
Energy Ratio = 10^(1.5 × (M2 - M1))
So a magnitude 8.0 earthquake releases approximately 31,623 times more energy than a magnitude 5.0 earthquake.
Modified Mercalli Intensity Scale (MMI): While magnitude is a single number for an earthquake, intensity varies by location. The MMI scale ranges from I (not felt) to XII (total destruction):
- I-II: Not felt to barely felt
- III-IV: Felt indoors, rattling of dishes
- V-VI: Felt by everyone, some damage to poorly built structures
- VII-VIII: Significant damage, difficult to stand
- IX-X: Major damage, buildings shifted off foundations
- XI-XII: Near-total destruction
Approximate relationship between magnitude and maximum MMI:
- Magnitude 3.0-3.9: MMI II-III (barely felt)
- Magnitude 4.0-4.9: MMI IV-V (light damage)
- Magnitude 5.0-5.9: MMI VI-VII (moderate damage)
- Magnitude 6.0-6.9: MMI VII-IX (strong to major damage)
- Magnitude 7.0-7.9: MMI IX-X (major to extreme damage)
- Magnitude 8.0+: MMI X-XII (extreme to catastrophic)
Peak Ground Acceleration (PGA): PGA estimates the actual ground shaking in terms of gravitational acceleration (g). A PGA of 0.1g means the ground acceleration is 10% of gravity. Buildings are typically designed to withstand PGA values common in their seismic zone.
Depth matters: Shallow earthquakes (less than 20 km deep) cause more surface damage than deeper earthquakes of the same magnitude because the energy has less distance to dissipate before reaching the surface.