Modern modeling determines the magnitudes of historical earthquakes

The researchers used the modeling approach to study the magnitude of the largest historic earthquakes in California, and found that the 1906 San Francisco earthquake had a magnitude of 7.9.

Written by Lauren Melideo, Ph.D., science writer (lwritesscience)

Citation: Melideo, L, 2020, Modern Modeling Determines Magnitudes of Historic Earthquakes, Temblor, http://doi.org/10.32858/temblor.150

On December 15, 2020, California shook some of its residents slightly when two earthquakes struck near the town of Morgan Hill, south of the San Francisco Bay Area. Scientists were quickly able to manipulate earthquake magnitudes through a comprehensive system of seismometric equipment on the ground, as well as real-time reports from the public, indicating the intensity of the vibration they were exposed to after moments of lulling roars. Planning for these earthquakes – and the largest earthquakes to hit California in the future – relies on knowledge of past and historical earthquake magnitudes and their shaking intensity to understand future earthquakes. But without modern equipment – and without Californians volunteering to get information about whether they “felt it” – how do we know the scale of these historic earthquakes? Newspaper calculations and other records can give scientists an idea, but they do not replace data-based measurements or estimates.

New research explores how scientists can estimate the magnitude of the three largest historic earthquakes in California. The idea for the study crystallized after the 2019 earthquake sequences in Ridgecrest, California, says lead author and USGS geophysicist Susan Hogg. Reports also poured in on the USGS: “Did you feel that?” From the widely felt event, scientists including Hugh and her colleagues noted a strong correlation between crowdsourcing reports of what California residents reported on the ground and device-measured data. After making that observation, Hough says, “I thought, ‘Can you close the historic episode of earthquakes?”

Close loop

The team looked at the three largest known historic earthquakes in California: the Fort Tigon earthquake of 1857, the Owens Valley earthquake of 1872, and the San Francisco earthquake of 1906. To determine the size of each earthquake, the researchers applied a method based on ground motion models. This approach includes three pieces of information: the magnitude of the earthquake itself, the intensity of the vibration at different sites determined from historical calculations using equations developed for this purpose, and the distance of these sites from the epicenter. In historic California earthquakes, researchers knew roughly where the earthquakes occurred and had at least some information about the intensity of the Earth’s shaking gleaned from previously studied historical newspaper reports. They used these two puzzle pieces – the distance from the epicenter and the degree of intensity of the shaking – to determine the third part: the magnitude of the earthquake.

Damage from the San Francisco Earthquake of 1906. Credit: Chadwick, HD, National Archives and Records Administration, Public Domain

The team needed to determine the magnitude that would not only fit their vibration intensity data, but also correlate with the degree of vibration experienced with respect to the distance from the seismic centers. Building on previously published models, Hogg and her colleagues used this data to generate a set of potential magnitude values ​​for each earthquake, then looked at the value that best matched other known parameters, such as the geological data of the site, for each event. In particular, the team used information from the California Historic Density Mapping Project.

Limiting the magnitude of the 1857 Fort Tigon earthquake proved challenging, as little recorded information was available regarding earthquake intensity. Despite similar difficulties, the team determined that the 1872 Owens Valley earthquake was most likely between 7.7 and 7.8. For the 1906 earthquake, Hogg’s team used the data collected in the Historical Severity Mapping Project in California to arrive at an estimated magnitude of 7.9, which is in line with previous estimates of earthquake magnitude that used the geodesic data and scattered tools available at the time.

Confirm the equations

To inform hazard maps, seismologists have developed equations to “predict what the vibration will be like for a given amount and distance,” says Hough. “It’s encouraging that the equations look very good,” she says, matching what other studies using seismic and geodetic data have found, “even for the largest earthquakes we’d expect in California,” she says.

The three earthquakes studied, despite uncertainty about their exact magnitude, are important not only historically, but also for current and future residents of California. “The prehistoric earthquakes are the largest earthquakes on record, so the risk models are entirely driven by these rare events,” says Christy Rowe, a seismic geologist at McGill University who was not involved in the research. She adds that California’s very brief record of earthquakes measured with modern equipment and methods leaves researchers and planners largely dependent on just these three massive earthquakes.

The challenge in creating hazard maps, Hough says, is that although we know that earthquakes at a magnitude of 7.9 are possible in California, we don’t have any such earthquakes that have been measured with modern map reporting tools.

“This paper is really important, because what they did was go back and apply what is modern,” says Rowe [equations] Linking the radiant energy from earthquakes to the shaking that actually occurred and applying that to these prehistoric earthquakes. “

This study “puts the bar around 7.9 as a measure of this very important.” [1906] Hough says. “It says these equations are very good – they run all the way through to the biggest earthquakes we’d expect to see in California.”

Further reading

Hough SE, Page M., Salditch, L., Gallahue, MM, Lucas, MC, Neely, JS, & Stein, S. (2020). A review of previous major earthquakes in California and the rate of long-lasting earthquakes. Bulletin of the American Seismological Society. https://doi.org/10.1785/0120200253

What Are The Main Benefits Of Comparing Car Insurance Quotes Online

LOS ANGELES, CA / ACCESSWIRE / June 24, 2020, / Compare-autoinsurance.Org has launched a new blog post that presents the main benefits of comparing multiple car insurance quotes. For more info and free online quotes, please visit https://compare-autoinsurance.Org/the-advantages-of-comparing-prices-with-car-insurance-quotes-online/ The modern society has numerous technological advantages. One important advantage is the speed at which information is sent and received. With the help of the internet, the shopping habits of many persons have drastically changed. The car insurance industry hasn't remained untouched by these changes. On the internet, drivers can compare insurance prices and find out which sellers have the best offers. View photos The advantages of comparing online car insurance quotes are the following: Online quotes can be obtained from anywhere and at any time. Unlike physical insurance agencies, websites don't have a specific schedule and they are available at any time. Drivers that have busy working schedules, can compare quotes from anywhere and at any time, even at midnight. Multiple choices. Almost all insurance providers, no matter if they are well-known brands or just local insurers, have an online presence. Online quotes will allow policyholders the chance to discover multiple insurance companies and check their prices. Drivers are no longer required to get quotes from just a few known insurance companies. Also, local and regional insurers can provide lower insurance rates for the same services. Accurate insurance estimates. Online quotes can only be accurate if the customers provide accurate and real info about their car models and driving history. Lying about past driving incidents can make the price estimates to be lower, but when dealing with an insurance company lying to them is useless. Usually, insurance companies will do research about a potential customer before granting him coverage. Online quotes can be sorted easily. Although drivers are recommended to not choose a policy just based on its price, drivers can easily sort quotes by insurance price. Using brokerage websites will allow drivers to get quotes from multiple insurers, thus making the comparison faster and easier. For additional info, money-saving tips, and free car insurance quotes, visit https://compare-autoinsurance.Org/ Compare-autoinsurance.Org is an online provider of life, home, health, and auto insurance quotes. This website is unique because it does not simply stick to one kind of insurance provider, but brings the clients the best deals from many different online insurance carriers. In this way, clients have access to offers from multiple carriers all in one place: this website. On this site, customers have access to quotes for insurance plans from various agencies, such as local or nationwide agencies, brand names insurance companies, etc. "Online quotes can easily help drivers obtain better car insurance deals. All they have to do is to complete an online form with accurate and real info, then compare prices", said Russell Rabichev, Marketing Director of Internet Marketing Company. CONTACT: Company Name: Internet Marketing CompanyPerson for contact Name: Gurgu CPhone Number: (818) 359-3898Email: [email protected]: https://compare-autoinsurance.Org/ SOURCE: Compare-autoinsurance.Org View source version on accesswire.Com:https://www.Accesswire.Com/595055/What-Are-The-Main-Benefits-Of-Comparing-Car-Insurance-Quotes-Online View photos