San Benito Geology: Earthquake 101

This is the fourth in a series of six monthly articles about the geology of San Benito County by retired Earth Science teacher Jim Ostdick. The topics are the nature of science, how do earthquakes occur and how are they measured.
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This article was contributed by Jim Ostdick.

People often ask scientists these questions about various topics: How do you know that is true? How can you be sure that is right? People were not around to see that happen, so how can you say it did? Skepticism is a healthy part of the scientific process and these are reasonable queries.

However, it is important to understand that science is an ongoing, rigorous search for truthful, accurate, tested explanations. Each succeeding generation adds to and improves upon our understanding of how the physical world works. Every new interpretation is subject to intense scrutiny. It is likely that your question has been asked many times for hundreds of years.

Anyone who has been humbled by the stresses of peer review by a panel of experts will testify to the intensity of scientific investigation. To be accepted for publication as part of the scientific record, your methods and data analysis are subject to strict standards and are statistically measured to a 95 percent confidence interval. Your conclusions must reflect tight logic and they must allow for further study. Sometimes, as often as not, results of your research pose more questions than they answer.

By the time enough work is done on a particular problem, like how do earthquakes occur or how can they be measured, there is a strong probability that the science is sound and that it can be trusted. Do we know it all? Not by a long shot. Is it amazing how far we have come? Absolutely!

If you have been following this series, you know quite a bit about faults in our local region. From years of study, we know that earthquakes occur along these and other faults, which are breaks in Earth’s crust that happen in response to stresses by shifting tectonic plates. When will earthquakes happen next? That’s the tricky part. That’s the new frontier in seismology.

There have been some mighty big quakes in California over the past 30 years. There is no reason to believe that they will stop happening. How can we expand our understanding in order to warn citizens of the next big one?

The biggest quakes in recent California history:

  1. 7.3 M, Landers, CA, June 28, 1992
  2. 7.2 M, Cape Mendocino, CA, April 25, 1992
  3. 7.2 M,  Off coast of Northern CA, June 15, 2005
  4. 7.1 M, Hector Mine, CA, Oct. 16, 1999
  5. 7.0 M, Honeydew, CA, Aug. 17, 1991
  6. 7.0 M, Cape Mendocino, CA, Sept. 1, 1994
  7. 7.0 M, Northridge, CA, Jan. 17, 1994
  8. 6.6 M, San Simeon, CA, Dec. 22, 2003
  9. 6.6 M, Off coast of Northern CA, June 17, 2005
  10. 6.2 M, Joshua Tree, CA, April 23, 1992
  11. 6.0 M, Central CA, Sept. 28, 2004
  12. 5.6 M, Sierra Madre, CA, June 28, 1991
  13. 5.4 M, Chino Hills, CA, July 29, 2008
  14. 5.4 M, San Francisco Bay Area, CA, Oct. 31, 2007

The “M” next to the number on the list refers to the Richter Magnitude of a tremor as measured by the amplitude of the largest seismic wave recorded on a seismogram. Charles Richter, a Cal Tech seismologist in the 1930s, created a logarithmic scale in which every increase of 1 corresponds to a tenfold increase in ground shaking intensity. That means a 7.0 M quake produces ten times more shaking than a 6.0 M quake. An increase of 1 on the Richter scale also corresponds to a 32-fold increase in released energy. So a 7.9 M quake releases 32 x 32 x 32 times as much energy as a 4.9 M quake. The 1906 San Francisco earthquake was a 7.9 M, a real monster. The 9.2 M quake in Prince William Sound, Alaska in 1964 was even more belligerent. It released the amount of energy equivalent to a billion tons of explosives.

Here in San Benito County, we are used to feeling those little 2.3 or 3.2 M rollers. You may wonder how and where they are recorded. For the answer to this question, we turn to the good folks at the U.C. Berkeley Seismological Laboratory. On their website (click on the link), we can find a map which shows where all the current ShakeAlert stations are located in the state, plus locations where stations are still needed. On the website, you can zoom into San Benito County to see the ones in our area. If you (or someone you know) are a property owner at one of the needed locations, and you wish to participate, you can contact the lab to start that process. Go ahead! Be a citizen seismologist!

To really nerd out on all that is going on in the effort to create an early warning network that may someday save lives and property in advance of a large magnitude earthquake, go to the Northern California Earthquake Data Center. A joint effort between the U.S. Geological Survey and the U.C. Berkeley Seismological Lab, they collect data from all the Nor Cal sensor stations in digital form for scientists to use in their research.

How do seismologists know what they know? Hard work! Built on the work of those who came before them.


Related stories in the San Benito Geology series:

San Benito Geology: The Great San Benito Shakeout

San Benito Geology: Living on the Edge

San Benito Geology: All Your Faults




Jim Ostdick

Jim Ostdick is a retired Earth Science teacher and travel writer. A resident of San Juan Bautista since 2005, Ostdick's primary interests are California geology, energy conservation, outdoor recreation, and human-powered travel. He is the author of Palomino and the Dream Machine: A Retired Dude's Bicycle Tour Around the Lower Forty-Eight United States (, 2015) and Palomino Nation: My 2016 Crazyass Walk Across America (, 2017). Palomino Dream blog Palomino and the Dream Machine Palomino Nation