Laurie Dingler | ’92 Cape Mendocino earthquake altered intensity mapping – Times-Standard

When the Cape Mendocino earthquake struck on April 25, 1992, size wasn’t the only factor of interest to seismologists. Until Charles Richter developed a magnitude scale, the primary way to describe the magnitude of an earthquake was the intensity. Intensity is a qualitative measure of vibration strength based on damage, perceptible area, and human perceptions of vibration.

Almost everyone has heard of the Richter and Richter scale. In 1935, Richter published a research paper that suggested using the height of the vibrations on a seismogram to classify the magnitude of an earthquake. When a major earthquake occurs, you are bound to find the “Richter force” used in many media articles. I’m upset we haven’t used Richter’s method of sizing in decades, but the name still stands.

Less well known is Robert Mallet. Mallet, a 19th-century Irish geologist, was the first to develop a seismometer, some 80 years before Richter. Studying the Great Neapolitan Earthquake of 1857, Mallet noted that the pattern of damage and people’s descriptions of the strength or intensity of the shaking varied in a systematic way. The most damaged buildings were concentrated in a small central area and the relative damage and vibration strength decreased in roughly concentric areas moving away from the center. He coined the term “intensity” to describe the relative strength of the vibration, placed a point at the center of his strongest region and named it the epicenter. He was the first scientist to use this term.

Over the next 70 years, several variants of the intensity scale have been proposed. Giuseppe Mercalli, volcanologist and Catholic priest, is the most commonly used. Mercalli mapped the spatial effects of volcanic eruptions including the 1906 eruption of Vesuvius and paid close attention to both the physical properties of the eruption such as the thickness of the ash and the effects on human structures. This work will become the basis for the Volcanic Explosion Index (VEI), which is still used to classify the strength of volcanic eruptions today.

In the late 19th century Mercalli turned his attention to earthquakes, studying the effects associated with vibration. His 10-point scale proposed in 1902 is still the basis for density mapping to this day. By adding two additional levels, it was adopted by the United States Geological Survey (USGS) as the Modified Mercalli Densitometer (MMI) in 1931. Levels were indicated by Roman numerals and described in qualitative terms such as “felt by many people inside, although not always recognized” Immediately as the earthquake “of severity III” and “everyone felt; most frightened most find it difficult to stand or walk” of intensity VII.

The MMI has become the USGS standard for describing earthquake effects. The measure was to send a questionnaire to postal managers in the areas likely to be felt. Questions included perceptions of vibration strength and damage. For major earthquakes, a team of scientists/engineers will be sent to map damage to structures. The information will be aggregated into “equal maps,” which plot areas of equal force of vibration.

I learned about MMI in grad school but didn’t think much about the scale as size seemed like a better scale. Then came the M7.2 Trinidad offshore earthquake in November 1980. I was teaching geophysics at the time and the earthquake gave the students a hands-on field experience. They were directed to interview people and collect density data. We used a survey similar to the USGS Postmaster’s form and ended up with blocks of qualitative descriptions. There were general agreements about what was stronger, or weaker but it varied widely and I found it very unsatisfactory. After similar drills for other North Coast earthquakes, I decided 1992 would be different.

Cathy Molly, a geology student at the time, revised the survey questions so that each answer had a numerical value. The surveys in this paper have been printed and distributed in schools. We had over a thousand responses. The first question was simple: Did you feel it? “Yes” got one and “No” got zero. We had questions about perceptions of vibration force, reaction, whether they hear noises, if heavy furniture has been changed, structural damage, etc. Then we collected the numbers. We wanted to end with values ​​that were roughly the same as the MMI scale – 3 should still be light and 7 relatively strong.

We evaluated, summarized and calibrated the responses by comparing them with USGS values ​​for the same communities. After many attempts, we’ve come up with a system that seems to work just fine. We can perform a numerical calculation of density for a given population that does not include any subjective identification. For the method to work, we needed at least ten responses for a given area.

The USGS was skeptical of our approach at first, but Jim Dewey, a friend of mine at school, was leading their tough group. When the Northridge earthquake hit in 1994, we worked together to test the viability of my approach. Meanwhile, I was working on intensity, Dave Wald of the USGS was developing Shakemap, a quantitative method for mapping Earth’s vibration based on accelerometer data and modeling the propagation of seismic waves. But many areas have few tools and when he became familiar with our survey method, he quickly adapted the methodology to the Internet, and it became the basis for the “Did You Feel It” community internet density that is now used as standard. I think this became the first use of crowdsourcing in the Earth sciences.

Using input from both Shakemap and Do You Feel It, Dave took it a step further in 2010 with the development of PAGER (Instant Global Earthquake Assessment for Response), a method for estimating the effects of earthquakes within minutes of their occurrence. Using population and structure databases, the PAGER system uses ground shaking coefficients to predict potential injuries and economic losses. The purpose is to direct governments and respondents to the areas of worst impact. It’s a far cry from 1992 when it took half an hour to get a reasonable estimate of the volume and weeks to gather damage information. And I like to think that the earthquakes of 1992 played a small role in its development.

If you remember your experience with the 1992 earthquake, did you feel he could still use your input: https://earthquake.usgs.gov/earthquakes/eventpage/nc269151/dyfi/intensity

Laurie Dingler is Cal Poly Humboldt Professor Emeritus of Geology and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of Preparedness magazine “Living on Shaken Earth”? Leave a message at 707-826-6019 or email [email protected].

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