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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Ridgecrest faults increasingly sensitive to solid Earth tides before   
   earthquakes    
      
     Date:   
         April 21, 2023   
     Source:   
         Seismological Society of America   
     Summary:   
         Faults in the Ridgecrest, California area were very sensitive to   
         solid earth tidal stresses in the year and a half before the July   
         2019 Ridgecrest earthquake sequence.   
      
      
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   FULL STORY   
   ==========================================================================   
   Faults in the Ridgecrest, California area were very sensitive to   
   solid earth tidal stresses in the year and a half before the July 2019   
   Ridgecrest earthquake sequence, researchers reported at the Seismological   
   Society of America (SSA)'s 2023 Annual Meeting.   
      
      
   ==========================================================================   
   "The signal of tidal modulation becomes extremely strong" after 2018,   
   said Eric Beauce of Lamont-Doherty Earth Observatory, who noted that the   
   signal was identified with seismicity that occurred around the faults   
   that broke in the 2019 magnitude 7.1 earthquake.   
      
   The link does not mean that tidal stresses -- which are very small   
   compared to other tectonic stresses -- triggered the earthquake, however.   
      
   "We don't know if something started to happen in the fault zone,   
   something that is an indicator of the upcoming earthquake," Beauce   
   said. "Maybe that process changed the properties of the crust in a way   
   that made the crust be more sensitive to tidal stresses."  Pulled by   
   the same gravitational forces of sun and moon that create ocean tides,   
   the solid earth also deforms in the same periodic way. People can't feel   
   the changes, but the ground deforms between 10 to 20 centimeters a day.   
      
   These solid tides "induce very, very small stress changes in the crust,"   
   Beauce explains, "which can induce stress changes in all the faults   
   within the crust."  Although researchers have known about these tiny   
   stress changes for more than a century, it has been difficult to extract   
   their signal from the seismic record, and to determine whether they   
   modulate seismicity.   
      
   In the past ten years, however, better earthquake detection and analysis   
   techniques have made it possible to search through earthquake catalogs   
   to find the signal of tidal stresses, Beauce said.   
      
   He and his colleagues built a rich, high-resolution earthquake catalog,   
   using machine learning algorithms along with other techniques, for the   
   past decade of microseismicity in the Ridgecrest area. (Microseismicity   
   usually refers to earthquakes of magnitude 2.0 or smaller).   
      
   They found that "there is suggestive evidence that peak seismicity happens   
   when tidal stresses are maximum," Beauce said, "but this modulation is   
   weak, and because it is weak, it is only suggested."  Other researchers   
   looking at the 2004 Indian Ocean and 2011 Tohoku megathrust earthquakes   
   have detected an increase in modulation of seismicity connected to   
   tidal stresses, decades before the earthquakes, said Beauce. And some   
   scientists have been able to generate similar results in lab-created   
   earthquake experiments.   
      
   The tidal findings do not have direct implications for earthquake   
   forecasting, "as we do not know if we are looking at a general phenomenon   
   or one specific to the Ridgecrest earthquake only, said Beauce, "but I   
   see it as a way of getting new observational constraints on the physics   
   of earthquakes, possibly the preparation and nucleation of earthquakes."   
       * RELATED_TOPICS   
             o Earth_&_Climate   
                   # Earthquakes # Natural_Disasters # Tsunamis # Geology #   
                   Earth_Science # Near-Earth_Object_Impacts # Geography   
                   # Oceanography   
       * RELATED_TERMS   
             o Geologic_fault o 1976_Tangshan_earthquake o   
             1906_San_Francisco_earthquake o Tide o Earthquake   
             o Making_existing_structures_earthquake_resistant o   
             2004_Indian_Ocean_earthquake o Structure_of_the_Earth   
      
   ==========================================================================   
   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/2023/04/230421092025.htm   
      
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