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SUMMARY:Large cold noses and boomerang earthquakes: exploring new tectonic
  frontiers using seafloor seismometers in the Atlantic - Dr. Stephen Hicks
  - Imperial College London
DTSTART:20200129T160000Z
DTEND:20200129T170000Z
UID:TALK137404@talks.cam.ac.uk
CONTACT:Jenny Jenkins
DESCRIPTION:Much of our understanding of tectonic processes on Earth come 
 from studies focussed in the Pacific Ocean and along its margins. The impa
 ct of slower spreading rates in the Atlantic\, and its impact on subductio
 n and earthquake rupture processes has received less attention. My talk wi
 ll focus on new results from two UK-led ocean-bottom seismometer experimen
 ts in the Atlantic during 2016-17: The VoiLA experiment in the Lesser Anti
 lles subduction zone and the PI-LAB experiment on the mid-Atlantic ridge.\
 n\nIn subduction zones\, the volume of water stored in the down-going plat
 e and how it is released into the asthenosphere wedge is key for understan
 ding the mass balance of Earth’s mantle. The Lesser Antilles subduction 
 zone is a global end-member for slow subduction of slow-spread\, hydrated 
 oceanic lithosphere\, which could cause variable hydration and melting of 
 the mantle. Using local earthquakes\, we have generated 3-D images of seis
 mic attenuation\, sensitive to mantle temperature and fluid content. We fi
 nd the asthenosphere wedge does not lie beneath the arc as expected from t
 hermal-mechanical modelling. Melt may pond at the base of the upper plate 
 in the back-arc with lateral migration of melt through the crust towards t
 he arc. The location of the hydrated core of the mantle wedge agrees well 
 with geochemical estimates of water content from erupted magmas at the sur
 face.\n\nThe simple geometry of ocean transform faults that offset spreadi
 ng centres offer a rare opportunity to study how fault zone friction affec
 ts earthquake rupture. We recorded a magnitude 7 earthquake along the Roma
 nche transform fault in the central Atlantic on ocean-bottom seismometers.
  Using these data along with teleseismic waveforms\, we show that the rupt
 ure direction along the fault reversed mid-way through the earthquake. We 
 theoretically expect back-propagating rupture fronts to occur in mature\, 
 low-velocity fault zones\, but we have not observed before such clear evid
 ence of a reversing rupture. This phenomenon is absent in rupture simulati
 ons and unaccounted for in hazard assessments.
LOCATION:Marine/Wolfson Building lecture hall\, Bullard Labs.
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