For anyone living in earthquake country, the concept of “The Big One” is very real. Here on the North Coast, the San Andreas Fault lurks just offshore a few miles. While it has been a long time since the last time it moved with significance, we only need to look back 105 years to see what can happen when it does.

But “big” can be widely interpreted. Is 7.0 big? What about 8.0? Come to think of it, how do they measure earthquakes?

A Little History

Back in 1906, there were no seismographs, and seismology was a hobby more than a field of study. It wasn’t until 1935 that Charles Richter co-developed the measurement scales that bear his name. The original “Richter Magnitude Scale” was created to describe a small local study area along the San Andreas Fault in Central California, and as such came to be abbreviated as M_L (where subscript “L” stands for “local”).

The Richter local scale was suitable during the early days of seismic research; however its design for relatively small and shallow earthquakes soon became a problem. For theoretical events larger than 8.0, the scale saturates and becomes useless. Richter and his partner Beno Gutenberg continued research and created two more scales: surface wave magnitude (M_s) and body wave magnitude (M_b). Both are calibrated to coincide numerically with the traditional Richter local scale. Body wave magnitude is particularly useful for measuring earthquakes at greater distances and depths, but similar to local magnitude saturates around the 6.0-6.5 range.

Seismology Goes Mainstream

By the 60’s a more reliable method of measuring earthquakes was needed. Creation of the “moment magnitude scale” was began in 1966 by Caltech seismologists Thomas C. Hanks and Hiroo Kanamori. Their research was long, arduous and done mostly by hand because computers were not fast enough to keep up. Yet by 1979, the new scale began to be used routinely.

The moment magnitude scale measures the amount of energy released at the moment of the earthquake, and thus was ideal for most large and relatively shallow events. The scale is abbreviated M_w where “w” refers to the amount of “mechanical work” at the moment of the seismic event. The scale is numerically designed to overlap with the Richter local scale for moderate quakes around the 5.0 range to retain consistency. But the benefits of the new scale are many, including the ability for seismologists to compare energy released by earthquakes more directly. The primary downside is that the scale saturates below 3.5, and as such is only used for moderate and larger events.

So Where Did It Happen?

To get the complete picture of an earthquake event, the seismologist uses several criteria. Seismograph measurements differ depending upon their proximity to the earthquake. The more seismograph station measurements reported the more points of view the seismologist has available. By spreading seismographs around earthquake-prone regions, we have a network of sensors that can be monitored and processed by modern computers in real-time.

The first clue the seismologist looks for is the “epicenter” of the quake. The epicenter is the location of the earthquake mapped as a point of latitude and longitude. This point represents the place on the surface directly above the center of the event.

By adding the third dimension of depth, the “hypocenter” can be determined. Surface seismograph data is used to triangulate this point below the epicenter. With the hypocenter determined, the seismologist can move on to determining the actual magnitude of the earthquake.

It Was How Big?

The magnitude is calculated by reexamining the original seismograph data. This is where selection of magnitude scale is initially made based upon initial estimates nearest the presumed epicenter. Note that the first scale used is not always the one used in the final report.

Most moderate or larger earthquakes use moment magnitude. Body wave magnitude can be used for deep or distant quakes. The original Richter local scale (M_L) is still used for small earthquakes recorded by nearby seismographs, particularly here in California where small earthquakes are common.

With the scale determined, the seismograph data is reexamined, and the magnitude is calculated. The seismologist then takes another look at the data, refines the epicenter, hypocenter and recalculates the magnitude, repeating the process until a consistent result is returned.

But It Felt Stronger Than That

The seismologist has one more tool to get the overview of the earthquake, and this one involves the human component. Magnitude is merely a mathematical calculation based on data. This number can only describe so much about the region surrounding the epicenter. This is where the intensity measurement comes to play. On their website the United States Geological Survey (USGS) defines earthquake intensity as,

“…the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment.”

In other words, intensity an attempt to quantify how much shaking was experienced in the region surrounding the epicenter. To specify the intensity of an earthquake, seismologists use the Abbreviated Mercalli Intensity Scale, which employs Roman numerals from I (one) to XII (twelve) for classification.

In addition to seismograph data, the USGS relies on us humans for input regarding local intensity. By visiting the USGS website for the specific earthquake event, we can fill out a short questionnaire describing where we were, what we experienced and what our reaction was. This information is then compiled into a “Shake Map” which describes the intensity over the region using color coding.

A Mercalli intensity of I (one) equates to “instrumental” meaning the quake was only perceived by seismographs. Intensity III reflects the equivalent of a heavy truck passing. By intensity VI (six) the human fear component emerges. And at the top of the scale, XII represents “Cataclysmic, damage total, lines of sight and level are distorted.”

A Little More History

The Tōhoku Japan earthquake of March 2011,  measuring M_w 9.0 generated a Shake Map that holds a wealth of information. Peak intensity was classified at VIII (severe) in inland Miyagi and Iwate prefectures. Most of east coast Honshu felt VII-VIII (moderate to heavy) intensities, and the west coast of Honshu experienced up to VI (strong).

Compare this to the much smaller magnitude M_w 7.9 April 1906 earthquake in San Francisco. While the epicenter is believed to have been offshore from Daly City, Shake Maps generated from perceptions and degree of damage at the time indicate intensities as high as XI (extreme) in Santa Rosa and as high as IX (violent) to X (intense) along the northern third of the San Andreas Fault from Cape Mendocino south.

We cannot forget that Fort Bragg was devastated by the 1906 earthquake and our own ensuing fire. It is well documented that Santa Rosa received significant damage, as did most surrounding cities and towns north and south of San Francisco. Most people simply do not realize that the great quake of 1906 destroyed far more than just the City of San Francisco.

The Scale Speaks For Itself

Earthquake magnitude and intensity are the two primary ways that seismologists measure and classify earthquakes occurring around the globe. By utilizing this data, seismologists can now begin to understand what elements cause fault systems to rupture. And along the way they may discover clues as to how to predict large events in the future.

By adopting several overlapping scales, seismologists can now convey more about an earthquake in a way that is more succinct. The magnitude scale used in specifying an earthquake’s size says a lot more about the earthquake than simply how big it was.

Without specifying a scale, “6.4” doesn’t say much about the quake. On the other hand “6.4 M_w intensity IV” says it was fairly shallow and felt by many.

The Richter scale was so totally last century.