On July 6th, sports fans like me tuned in to the NBA summer league to watch the debut of Zion Williamson and R.J. Barrett, two highly anticipated rookies. I never expected the ESPN commentators to do play-by-play for an earthquake, but that is exactly what happened. According the U.S. Geological Survey (USGS), a magnitude 7.1 (M7.1) earthquake occurred that evening. It was centered 17 km to the north - northeast of Ridgecrest, California. NBA summer league games were in Las Vegas, but the earthquake was felt there and in many places across the region. Earlier in the week, an M6.4 earthquake impacted the region. In the immediate aftermath of the earthquake, preliminary estimates of magnitude ranged from 6.9 to 7.1. While it may seem like splitting hairs, the scale used to estimate earthquake magnitude is a logarithmic scale, and small increments matter more than you think. This distinction is important when communicating hazards associated with earthquakes and hurricanes, which also uses a well-known logarithmic scale.
For those curious, a M7.1 earthquake is 5 x bigger than a M6.4, but is 11.2x stronger in terms of energy release. The Richter scale is logarithmic; meaning, a M7.1 would be 10x bigger than a M6.1 and 100x bigger than a M5.1 (for example). #EarthquakeLA
While many people are familiar with the Richter scale, the magnitude of most earthquake is now estimated with the moment magnitude scale. According to the USGS website,
The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes....Moment Magnitude (MW) is based on physical properties of the earthquake derived from an analysis of all the waveforms recorded from the shaking.
Bowen's tweet also mentioned energy release. This may be a more meaningful measure because it is tied to potential damage with engineered structures. Earthquakes release energy across several frequencies. The graphic below illustrates the relationships associated with a whole number increase in magnitude (M) and energy released. According the the USGS website, "While each whole number increase in magnitude represents a tenfold increase in the measured amplitude, it represents an 32 times more energy release."
Though not a geologist or seismologist, I have noticed that the messaging challenge is very similar to what my community, meteorology, faces with hurricane threats. People often perceive a category 3 hurricane to be just "a little" bit worse than a category 2 hurricane. The Saffir-Simpson wind scale (below), used to categorize hurricanes, is also a logarithmic scale. This means that changes in wind speed associated with hurricane categories can yield dramatically different damage outcomes. The National Weather Service Jet Stream website offers a very clear example:
When the cost from hurricane related damages are normalized (normalization takes into account inflation, changes in population, and changes in wealth to arrive at a common level for comparison) the result shows an eighth-power increase1 in damages from category to category. What this means is the potential damage from a hurricane is 28 power. For example, a doubling of the wind speed from 75 mph (121 km/h) to 150 mph (241 km/h) is not a doubling or quadrupling of potential damage but a 256 times increase.
If you think about how relatively rare category 3, 4, or 5 storms are, it is not surprising that they cause well over 85% of the damage associated with hurricanes, according to NOAA.
Communicating risks associated with hurricanes and earthquakes is already a challenge. The "linear" or incremental mental model that people use to assess damage potential as a function of small increments can be deceiving and dangerous. I wrote this article to shatter such perceptions as California braces for more earthquakes and coastal communities brace for hurricane season.
