On 28 December 1908, a magnitude 7.1 earthquake shook Europe, killing more than 80,000 people. The Messina earthquake originated in the Strait of Messina, a narrow graben between the southern toe of Italy and the island of Sicily.
Despite the abundance of mapped faults in the strait, the exact one responsible for the Messina earthquake has been debated by researchers for nearly a century. Now, a team of scientists is merging modeling with detailed geologic mapping to solve the mystery of what caused the deadly event.
Recorded Earthquake History
The 1908 Messina earthquake was one of the first to be measured by instruments in Europe. Although 11 seismograms recorded the event, the instruments were spatially far apart. The resulting seismic coverage was limited and ultimately left the source of shaking a mystery.
Over the past few decades, in an attempt to map the geomorphologic signatures of the earthquake, researchers have searched for ground ruptures, coastal retreat, and slope movements on land and in the ocean. Many earthquake-related features were identified, but no clear surface fault ruptures were identified.
When investigating historical earthquakes, the first thing to do is “look for the faulting source among the officially mapped, capable faults” said Marco Meschis, a geologist at University College London and lead author of the new paper in Scientific Reports.
Because there was no measured focal point from seismograms, Meschis and the team decided to take a new look at the geomorphology around the Strait of Messina and try to determine what fault ruptured in 1908.
“We compared the deformation produced on the ground by the 1908 earthquake and recorded leveling data before and after the earthquake to similar and well-studied earthquakes,” said Meschis. The leveling data measure the “difference in heights recorded for certain known benchmarks before and after the 1908 earthquake,” he said.
“What’s really cool about the leveling data is that it gives you the absolute values on the surface,” said Kimberly Blisniuk, an earthquake geologist at San Jose State University who was not involved with the study.
“What made this study conclusive is that they were able to use the current and up-to-date regional surface geology and geomorphology to then interpret what we can’t see underneath the sea,” said Blisniuk.
She added that geomorphic interpretations allowed the team to remove data points that were influenced by slumping or tsunami deposits. “People could be critical about how they may have interpreted landslides and subsidence,” said Blisniuk, but she said those decisions helped the authors better understand the leveling data before and after the earthquake.
Meschis said researchers compared the profile of leveling data around Messina Strait to that found around normal faulting sources such as the 1983 Borah Peak earthquake in the United States and the 2009 L’Aquila earthquake in Italy. When the areas were compared, they found “an outstanding resemblance for the associated ground deformation,” he said.
“No one ever plotted the only data set available for this earthquake—the leveling data on an E–W oriented profile—to show how the ground deformation developed spatially,” said Meschis. “This simple plot gives important information for both the dip direction of the faulting source and a clearer idea on the dip angle value.”
“It’s an impressive data set,” said Austin Elliott, an earthquake geologist at the U.S. Geological Survey who was not involved in the study. He added that the caveat in the leveling profile was that the data points were more parallel to the fault (north–south trending) rather than a cross section of the Strait of Messina.
“I think that’s probably the most controversial aspects of this,” said Elliott. He noted that the authors are careful about explaining the geological basis of their interpretations, and they back up their claims with modeling based on observed faults.
“Because they go the extra step of modeling to show the data are at least consistent with a very simple case of slip on this offshore fault, it’s a compelling result,” said Elliott.
Modeling a Mysterious Fault
Meschis noted that previous researchers could not agree on the fault source, much less “the dip angle of the faulting source, its dip direction, and how much the fault slipped when the earthquake occurred.” Considering this, the team looked back on their leveling data and determined the fault source was dipping eastward.
The team also wanted to end the debate about the geometry of the fault that caused the Messina earthquake. Specifically, they wanted to know the dip angle and slip amount for the ruptured fault. “We tested different geometries and slip amount for the chosen faulting source in order to reproduce the deformation signal produced by the earthquake,” said Meschis.
The team compared each model to leveling data along the coast and found the best fit for producing an M 7.1 earthquake was a 5-meter slip on a 70°, east dipping fault. They named it the Messina-Taormina fault (formerly the Taormina fault), and it is found in the seafloor in the Strait of Messina.
“I thought the study was very elegant and clever,” said Blisniuk. She said the geologic and geomorphic mapping was really “the selling point for this study because their interpretation ties the entire picture together.”
Homing In on the Hazard
“For the first time, we have unveiled the geometry and kinematics of the faulting source producing the most damaging and powerful earthquake recorded in Europe during the 20th and 21st centuries,” said Meschis.
Meschis added that pairing geological and geomorphologic observations with mapped, earthquake-capable faults can help scientists identify the source of historical earthquakes. And accurately determining which faults have been historically active can be useful to society.
Elliott agreed and said the study shows the value of “reframing the questions that we’ve asked about past events using these archival data sets.”
“It’s really important to go back and revisit data sets that pertain to past earthquakes with newly acquired information about the fault, the tectonic system, mapping, and modeling.”
—Sarah Derouin (@Sarah_Derouin), Freelance Journalist
Derouin, S. (2019), Finding faults in our past: Uncovering the Messina earthquake, Eos, 100, https://doi.org/10.1029/2019EO137630. Published on 19 December 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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