No one can say the exact day an earthquake will strike. But scientists can say where hazards happen and how often, and for volcanoes they can often spot warning signs weeks or months ahead. That is the difference between a prediction (a specific time and place) and a forecast (a chance over a span of time).
- Earthquakes: records of past quakes and fault slip give the chance of a large quake in a region over decades. The U.S. Geological Survey estimated a 72% chance of at least one magnitude 6.7 or larger quake in the San Francisco Bay Area between 2014 and 2043.
- Volcanoes: rising magma causes swarms of small earthquakes, swelling of the ground (measured with tilt meters and GPS) and more volcanic gas. These signs often come before eruptions.
- Early warning: systems such as ShakeAlert detect the fast P waves of a quake that has already started and send alerts seconds before the stronger shaking arrives.
Key idea
Forecasts give the chance of a hazard over a time span, based on records and warning signs. If an event happens on average once every $T$ years, the chance in any one year is about $\tfrac{1}{T}$.
Why it works
Faults load up with stress over decades to centuries and release it at irregular times, so the exact date can't be known. But the long-term average can. If a fault has had 5 large quakes in 1,000 years, the average gap (recurrence interval) is $1{,}000 \div 5 = 200$ years, and the chance in any one year is about $\tfrac{1}{200}$, or 0.5%. That chance helps communities decide how strongly to build.
Worked example: Recurrence
A town's records show 4 big floods in 120 years. What is the average time between floods, and the chance of one in any year?
- Average interval: $120 \div 4 = 30$ years.
- Yearly chance: $\tfrac{1}{30}$, about 3.3%.
Answer: About 30 years; about a 1 in 30 chance each year
Worked example: Volcano signs
Around a quiet volcano, small earthquakes become frequent, the ground starts to bulge and gas output rises. What do these signs suggest?
- All three signs come from magma moving up into the volcano.
- An eruption may be coming; scientists raise the alert level, though it may still be weeks away or may not happen.
Answer: Magma is rising; an eruption may be coming
Common mistake
Believing scientists can predict the exact day of an earthquake. No one can yet; forecasts give the chance of a quake over years or decades. A related trap: a "100-year flood" is not one that comes exactly every 100 years. It has a 1% chance every year, so two can happen in back-to-back years.