California Earthquake Intensity Calculator
Estimate Peak Ground Acceleration (PGA) and Modified Mercalli Intensity (MMI) shaking by magnitude and fault distance based on USGS ShakeMap standards.
Typical shallow California crustal earthquakes occur between 4 and 10 miles deep.
Felt by everyone. Difficult to stand; loose furniture moves; glassware falls.
Slight damage to unreinforced structures
USGS ShakeMap Standards & Seismic Scales
Seismological parameters established by the USGS Earthquake Hazards Program and CGS
Instrumental ground-motion attenuation equations and Modified Mercalli Intensity (MMI) ShakeMap standards.
Modified Mercalli Intensity (MMI) & Peak Ground Acceleration Scale
| MMI Scale | Peak Accel. (%g) | Perceived Shaking | Potential Structural Damage |
|---|---|---|---|
| I – II | < 0.17% g | Barely Felt / Imperceptible | None |
| III – IV | 0.17% – 3.9% g | Light to Moderate | None to negligible; loose objects rattle |
| V – VI | 3.9% – 18.0% g | Moderate to Strong | Slight; dishes break, furniture moves |
| VII – VIII | 18.0% – 65.0% g | Very Strong to Severe | Moderate to heavy in older structures |
| IX – X+ | > 65.0% g | Violent to Extreme | Catastrophic structural collapse |
How Earthquake Ground Motion Is Calculated
Mathematical formulas and step-by-step arithmetic breakdown
Worked Example Calculation: Mw 7.0 Earthquake at 15 Miles Distance
Consider a major Mw 7.0 earthquake on the Hayward Fault with a focal depth of 6 miles, modeled for a home 15 miles away:
- Moment Magnitude: Mw 7.0
- Surface Distance to Fault: 15.0 miles
- Focal Hypocenter Depth: 6.0 miles
- Slant Hypocentral Distance (R): 16.2 miles (26.0 km)
- Peak Ground Acceleration (PGA): 26.1% g (0.2608 g)
- Modified Mercalli Intensity: MMI VII-VIII (Very Strong to Severe)
- Expected Shaking: Alarm and panic; vehicle driving disturbed. Masonry walls crack and fail.
Travel Planning: Estimate how far and how long it takes to drive between major California cities with our California city-to-city driving distance calculator.
California-Specific Nuance: Shallow Faults & Basin Amplification
Why California earthquakes generate disproportionately violent shaking
The Strike-Slip Shallow Crust Hazard
Unlike subduction zone earthquakes in the Pacific Northwest or Japan where fault rupture occurs 20 to 50 miles below the Earth's surface, the San Andreas fault system consists of strike-slip transform boundaries where plates grind horizontally directly through populated urban valleys. Ruptures regularly breach the surface, meaning the focal depth is exceptionally shallow (often 4 to 8 miles).
Sedimentary Basin Resonances
The Los Angeles Basin and San Francisco Bay mud flats act like bowls of gelatin during major seismic events. Seismic waves slow down as they enter soft alluvium, increasing their amplitude and duration. A home built on unreinforced bay fill can experience up to twice the peak ground acceleration of an identical home built on solid granite in the foothills.
Frequently Asked Questions About California Earthquakes
Geological safety guidelines from the USGS and Cal OES
Magnitude (typically measured on the Moment Magnitude scale, Mw) measures the total seismic energy released at the earthquake source, which is a single number for each event. Intensity (measured on the Modified Mercalli Intensity scale, MMI) measures the actual ground shaking and damage at a specific geographic location, which decreases with distance from the fault.
Peak Ground Acceleration (PGA) is the maximum rate of change in ground velocity during an earthquake, expressed as a fraction or percentage of standard Earth gravity (g, ~9.8 m/s²). PGA is the primary engineering metric used by structural engineers to design earthquake-resistant buildings under the California Building Code.
Most California earthquakes along the San Andreas fault system occur at shallow crustal depths between 4 and 10 miles (6 to 16 km). Because they are shallow, ground shaking near the epicenter is much more intense than for deep subduction earthquakes of the same magnitude.
Local soil conditions play a critical role. Bedrock sites (such as the hills of San Francisco or the Hollywood Hills) shake significantly less than soft sedimentary basins (such as the Marina District, Silicon Valley, or the Los Angeles Basin), which can trap and amplify seismic waves and trigger soil liquefaction.
Geological studies indicate the southern segment of the San Andreas Fault ruptures with a magnitude 7.5+ “Big One” on average every 150 to 200 years. The last major rupture on the southernmost segment occurred over 300 years ago in 1690.
Follow the California Office of Emergency Services (Cal OES) protocol: Drop, Cover, and Hold On. Drop onto your hands and knees, take cover under a sturdy desk or table, and hold on until the shaking stops.