SEISMIC ATTENUATION IN THE CRUST AT THE MID-ATLANTIC PLATE BOUNDARY IN SOUTH-WEST ICELAND

Very little seismic attenuation occurs in the mid to lower crust of so uth-west Iceland. The lowest path-averaged quality factor for a wave t urning in the mid to lower crust (12-20 km) is Q(P) = 110 for P waves and Q(S) = 250 for S waves, with most of the data having higher values , typically Q(P) = 200-300 and Q(S) = 400-600. Attenuation estimates b ased on a 1-D, layered inversion give correspondingly high values, Q(P ) > 800 and Q(S) = 800-2000. These Q values are inconsistent with ther mal models that predict a broad (100 km wide) region of above-solidus temperatures centred on the volcanic zones. The observed attenuation i mplies an upper limit for mid to lower crustal temperature in the 700- 775 degrees C range (assuming a gabbroic lithology). Much higher atten uation (Q(P) = 60, Q(S) = 100) occurs in the uppermost 4 km of crust. This is most likely apparent attenuation caused by strong near-surface seismic heterogeneity, resulting from fissures, faults and extreme ch anges in porosity (up to 20-30 per cent). The quality factor of the ne ar-surface layer varies regionally, and is lower in volcanic zones tha n in either the Reykjanes Peninsula or the South Iceland Lowland.