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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Puerto Rico tsunami deposit could have come from pre-Columbian   
   megathrust earthquake    
      
     Date:   
         April 20, 2023   
     Source:   
         Seismological Society of America   
     Summary:   
         Tsunami deposits identified in a coastal mangrove pond in Northwest   
         Puerto Rico could have come from a megathrust earthquake at the   
         Puerto Rico Trench that occurred between 1470 and 1530, according   
         to new research.   
      
      
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   FULL STORY   
   ==========================================================================   
   Tsunami deposits identified in a coastal mangrove pond in Northwest Puerto   
   Rico could have come from a megathrust earthquake at the Puerto Rico   
   Trench that occurred between 1470 and 1530, according to research reported   
   at the Seismological Society of America (SSA)'s 2023 Annual Meeting.   
      
      
   ==========================================================================   
   These Puerto Rican tsunami deposits, along with similar age deposits at   
   other islands in the Caribbean, suggest that the tsunami would have been   
   triggered by a massive magnitude 8.7 or larger earthquake, said Bruce   
   Jaffe of the U.S.   
      
   Geological Survey.   
      
   The deposits could be very important for understanding seismic risk in   
   the region, since written records of tsunamis in the Caribbean extend back   
   only 500 years, and the evidence for magnitude 8.0 or larger earthquakes   
   on the Puerto Rico Trench is sparse.   
      
   Previous research has uncovered Caribbean tsunami deposits along the   
   northern segment of the Lesser Antilles arc in Anegada, St-Thomas   
   (U.S. Virgin Islands), Anguilla, and Scrub Island (British Virgin   
   Islands).   
      
   The researchers estimate that these deposits, ranging in age from 1200   
   to 1500 years before the present, could be the result of a tsunami   
   generated by a magnitude 8.0 to 9.0 earthquake that ruptured at least   
   300 kilometers.   
      
   If the Puerto Rico pond deposits came from the same event, "it could   
   be a large length of fault that ruptured -- 400 kilometers, roughly,"   
   said Jaffe.   
      
   Earthquake modeling for the Lesser Antilles arc deposits suggested   
   a magnitude 8.7 earthquake. Models that include the newly discovered   
   Puerto Rico deposits indicate that the tsunami-triggering earthquake   
   "would have to be quite a bit larger," he added.   
      
   The tsunami itself may have impacted both the Caribbean and Atlantic   
   coasts of Puerto Rico.   
      
   Jaffe and colleagues scouted dozens of locations in Puerto Rico "looking   
   for the right setting for a possible tsunami deposit to form and for it   
   to be preserved," he said. In a coastal mangrove pond at East Bajura   
   near Isabela in Northwest Puerto Rico, they found a likely candidate   
   close to the shore and flanked by ancient dunes that could protect the   
   pond from swells.   
      
   In sediment cores taken from the pond, the researchers found a thin   
   sand sheet about a half a meter deep that blanketed the whole pond. The   
   deposit bears several signatures of a tsunami event, including an eroded   
   basal layer and a unique gradation of sediment particles.   
      
   Sediment swept up in the high-speed flow of a tsunami remains suspended   
   in the water, "because the turbulent eddies are strong enough to mix the   
   sediment from the [sea]bed all the way up through the water column,"   
   explained Jaffe, who has analyzed deposits in the wake of six major   
   tsunamis worldwide. "It creates a very distinctive grading of sediment   
   in the core."  The researchers are examining large cores taken from the   
   pond with CT scans and X-ray fluorescence, among other tools, looking   
   for fine details such as the direction of grass roots and the mineral   
   composition of sediment grains to further constrain the timing and the   
   source of the tsunami.   
      
   One of the researchers, Matthew Baez, a graduate student working with   
   Jaffe and Alberto Lo'pez-Venegas of the University of Puerto Rico,   
   Mayagu"ez is looking for similar ponds along the north coast of Puerto   
   Rico that could corroborate the findings from the East Bajura pond. And   
   Jaffe said there are plans to take a deeper core from the current pond   
   to look for evidence of earlier tsunamis.   
      
   Study co-author Lo'pez-Venegas said there has been a lack of paleo-tsunami   
   research on major earthquakes affecting Puerto Rico's northern coast. The   
   2 May 1787 earthquake that most likely occurred offshore the northern   
   coast of Puerto Rico, likely centered on the Main Ridge within Puerto   
   Rico Trench, is one of the most significant seismic events to affect   
   that part of the island.   
      
   "The bottom line is, we still have a lot of work to do as we do not   
   have a good grasp of which events have occurred along the Puerto Rico   
   Trench, and much more paleo-tsunami work is required to understand   
   better what has occurred in the past and what may happen in the future,"   
   Lo'pez-Venegas said.   
      
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   ==========================================================================   
   Story Source: Materials provided by   
   Seismological_Society_of_America. Note: Content may be edited for style   
   and length.   
      
      
   ==========================================================================   
      
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/04/230420171646.htm   
      
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