Courtesy of: Matthew Fontanesi, Structural Engineer, AEI Consultants
Last night’s seismic event in the Eastern Bay Area portion of California was a small-moderate earthquake. Perhaps it felt big—bumps in the night are unsettling and the surge of adrenaline heightens your experience and the memory of the event in a way that trucks rumbling past the office won’t capture. Most people will conclude that an earthquake was big or small based on the Magnitude. People can recite that a M7.0 is 1000x times bigger than a M6.0 event. When it comes to people and property, ignore Magnitude. It doesn’t predict what we care about, damage, or measure the metadata that causes damage: movement or acceleration. As technical people, we need to avoid fuzzy descriptors like “bigger” or “more intense”.
Magnitude is an estimate of energy released by an earthquake. It was first put to a scale by Charles Richter in the 1930s (hence, Richter Scale), but his model went out of fashion in the 1970s. It has been replaced by a dozen newer scales that vary greatly depending on earthquake type (mechanism), soil characteristics, bedrock age, depth, and the agency applying it. The value that we typically see reported is “Moment Magnitude” or Mw. It’s reported with great fanfare in bold red letters. And it is nearly irrelevant. Magnitude is barely mentioned in the Building Code. And it can be negative!
What we care about most when considering the effects of an earthquake on a building is based on high school physics:
The force an earthquake applies to a building is based on the mass of the building and the acceleration of the structure.
When a building is designed, the building code tells us what acceleration value to use based on USGS calculations that take into account soil type, proximity to faults, how active those faults are, and the estimated lifespan of the structure, and if it has any special post-disaster requirements. The weight of the building is estimated which provides the forces that the designs are required to resist. The basic idea is that a 1,000,000lb building, designed for 1g of acceleration will need to resist 1,000,000lbs of lateral force.
The second critical idea here is that earthquake shaking dissipates with distance. The reports from last night’s event highlight that shaking was felt as far away at Lake Tahoe (approximately 200 miles.) But we intuitively know that any shaking near Tahoe is really, really faint. Predictably, acceleration dissipates with distance too. This the other reason that reporting an earthquake’s Magnitude is irrelevant. Everywhere on the planet experienced last night’s earthquake, so everywhere survived a M4.5. It would have been imperceptible beyond a few hundred miles, but big earthquakes can be measured across the world. In 2016 there was a large earthquake in Kaikoura, New Zealand that was picked up by the Large Hadron Collider in Switzerland, 11,800 miles away.
Using a top-line number doesn’t provide any nuance that could indicate local intensity. It would be broadly equivalent to describing a hurricane as a Category 5. That may be indicative of something, but the real information that people experiencing it care about is rain fall rates and volume, sustained wind gust speeds, fetch of the winds, tides, etc. That nuance is critical to understanding what we’re really dealing with and what our response should look like.
So here’s what the Peak Ground Acceleration measurements looked like last night:
This Peak Ground Acceleration value, and Peak Spectral Acceleration are our best data points for predicting damage. Is 0.2g of acceleration a lot? That’s a question of relativity. For example, new building in Walnut Creek will be designed for approximately 1.0g of PGA. Older buildings are probably designed for half that value and brick homes from the late 1930s would be designed for 0.02g (the first seismic parameter was a result of the 1933 Long Beach Earthquake). So why haven’t all of the old buildings fallen down from earthquakes? Because they were also designed for wind and wind forces will control the lateral design if the seismic parameters are very low.
- Magnitude is an easy number to report because it’s a single number, but it doesn’t translate to damage.
- Acceleration is the data point most relevant to structural engineers.
We recommend browsing the USGS Earthquake Event page to see all of the interesting information they provide, for free, nearly in real time.
Feel free to reach out to AEI Consultants if you have any questions for one of our Structural Engineers.