For decades, researchers thought that an October 1843 earthquake on the small Greek island of Chalke caused a powerful tsunami and led to the deaths of as many as 600 people.
But a new analysis of primary accounts of the event by Ioanna Triantafyllou at Hellenic Mediterranean University suggests the truth was much less dramatic and destructive.
As Triantafyllou reports in Seismological Research Letters, evidence from primary sources indicates that the mainshock occurred on Chalke on 17 September 1843, causing rock falls and some damage to poorly constructed houses. There were no deaths and no tsunami reported at the time.
The study demonstrates how secondhand sources of a seismic event can be used to characterize an earthquake incorrectly for years, said Triantafyllou, with impacts on seismic risk calculations for a region.
Without modern instrumentation and data collection, historical seismologists often must sift through accounts of a past earthquake to find clues that allow them to precisely locate the earthquake and estimate its magnitude and intensity.
Triantafyllou has been investigating damaging and deadly earthquakes in Greece since beginning her Ph.D. work in 2017. "I was particularly struck by the 1843 earthquake in Chalke, which remains one of the top 10 deadliest earthquakes in Greece to date. I wondered how a small island could have had 600 casualties at that time," she said. "I began searching for primary, original historical sources to verify whether the number of victims was indeed so high."
Previous researchers had concluded that the magnitude of the 1843 Chalke earthquake ranged from 6.4 to 6.75, and had caused a powerful tsunami. A 1848 study reported the earthquake's death toll to be as high as 600 people.
But when Triantafyllou examined these previous studies, "most of the previous authors either had no access to primary macroseismic information sources or neglected to mention them," she wrote.
Her search for primary accounts about the earthquake led to contemporary reports in Greek newspapers, a German newspaper, and newspapers published in Constantinople (Istanbul). Triantafyllou also used the contemporary testimony of Ludwig Ross, a German archaeology professor fluent in Greek.
"The further back in time you go, the more difficult it is to find primary sources related to earthquakes. This information can be found in newspapers, archives, church codes, and even in travelers' writings, as in the case of Ross," she explained. "In the digital era, the systematic indexing and digitization of relevant information helps researchers gain immediate access to the content."
The primary sources indicated that seismic activity on the island started at the beginning of September 1843, culminating in a strong damaging earthquake on 17 September, with some aftershocks felt into early October.
"The book by Ross is a good example of a reliable author who kept a detailed diary during his travels in the Greek islands. He documented the effects of earthquakes clearly and accurately in terms of time and space," said Triantafyllou.
"His earthquake descriptions provide appropriate information for reconstructing the earthquakes and assign macroseismic intensities based on building damage, ground failures and shaking felt," she added.
Triantafyllou used these macroseismic intensities, or shaking strength, to calculate a new magnitude for the 1843 event, concluding that the mainshock was likely a magnitude 5.93 earthquake.
The magnitudes of historical earthquakes are included in regional earthquake catalogs, which are used to make seismic hazard assessments. "In my study the magnitude of the Chalke 1843 earthquake was drastically reduced in respect to previous estimates," Triantafyllou explained. "Keeping all other factors equal, one may expect that reducing the magnitude will result in seismic hazard reduction as well."
