Seismic attenuation in the crust at the mid‐Atlantic plate boundary in south‐west Iceland

Abstract

Very little seismic attenuation occurs in the mid to lower crust of south‐west Iceland. The lowest path‐averaged quality factor for a wave turning in the mid to lower crust (12–20 km) is QP= 110 for P waves and QS= 250 for S waves, with most of the data having higher values, typically QP= 200–300 and QS= 400–600. Attenuation estimates based on a 1‐D, layered inversion give correspondingly high values, QP > 800 and QS= 800–2000. These Q values are inconsistent with thermal models that predict a broad (100 km wide) region of above‐solidus temperatures centred on the volcanic zones. The observed attenuation implies an upper limit for mid to lower crustal temperature in the 700–775°C range (assuming a gabbroic lithology). Much higher attenuation (QP= 60, QS= 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 changes in porosity (up to 20–30 per cent). The quality factor of the near‐surface layer varies regionally, and is lower in volcanic zones than in either the Reykjanes Peninsula or the South Iceland Lowland. Copyright © 1995, Wiley Blackwell. All rights reserved