Three-dimensional seismic structure and moment tensors of non-double-couple earthquakes at the Hengill–Grensdalur volcanic complex, Iceland

SUMMARY The volcanic and geothermal areas of Iceland are rich sources of non-double-couple (non-DC) earthquakes. A state-of-the-art digital seismometer network deployed at the Hengill‐Grensdalur volcanic complex in 1991 recorded 4000 small earthquakes. We used the best recorded of these to determine 3-D V P and V P /V S structure tomographically and accurate earthquake moment tensors. The V P field is dominated by high seismic wave speed bodies interpreted as solidified intrusions. A widespread negative (’4 per cent) V P /V S anomaly in the upper 4 km correlates with the geothermal field, but is too strong to be caused solely by the eVect of temperature upon liquid water or the presence of vapour, and requires in addition mineralogical or lithological diVerences between the geothermal reservoir and its surroundings. These may be caused by geothermal alteration. Well-constrained moment tensors were obtained for 70 of the best-recorded events by applying linear programming methods to P- and S-wave polarities and amplitude ratios. About 25 per cent of the mechanisms are, within observational error, consistent with DC mechanisms consistent with shear faulting. The other 75 per cent have significantly non-DC mechanisms. Many have substantial explosive components, one has a substantial implosive component, and the deviatoric component of many is strongly non-DC. Many of the non-DC mechanisms are consistent, within observational error, with simultaneous tensile and shear faulting. However, the mechanisms occupy a continuum in source-type parameter space and probably at least one additional source process is occurring. This may be fluid flow into newly formed cracks, causing partial compensation of the volumetric component. Studying non-shear earthquakes such as these has great potential for improving our understanding of geothermal processes and earthquake source processes in general.

[1]  R. Dowden,et al.  Phase and amplitude perturbations on the NWC Signal at Dunedin from lightning‐induced electron precipitation , 1989 .

[2]  B. Julian,et al.  Non-double-couple earthquakes at the Hengill-Grensdalur volcanic complex, Iceland: Are they artifacts of crustal heterogeneity? , 1993 .

[3]  David Gubbins,et al.  Three-dimensional seismic ray tracing , 1977 .

[4]  J. R. Evans,et al.  Three‐dimensional seismic image of a geothermal reservoir: The Geysers, California , 1996 .

[5]  R. Guza,et al.  Observations of turbulence in the surf zone , 1994 .

[6]  Douglas R. Toomey,et al.  Tomographic inversion of local earthquake data from the Hengill‐Grensdalur Central Volcano Complex, Iceland , 1989 .

[7]  R. E. Long,et al.  Anomalous focal mechanisms: tensile crack formation on an accreting plate boundary , 1984, Nature.

[8]  D. Toomey,et al.  Structure and evolution of the Hengill‐Grensdalur Volcanic Complex, Iceland: Geology, geophysics, and seismic tomography , 1989 .

[9]  Freysteinn Sigmundsson,et al.  Crustal Deformation from 1992 to 1995 at the Mid-Atlantic Ridge, Southwest Iceland, Mapped by Satellite Radar Interferometry , 1997, Science.

[10]  M. Wyss,et al.  The Reykjanes Peninsula, Iceland, earthquake swarm of September 1972 and its tectonic significance , 1977 .

[11]  Urs Kradolfer,et al.  Initial reference models in local earthquake tomography , 1994 .

[12]  Bruce R. Julian,et al.  Non‐double‐couple earthquake mechanisms at the Geysers Geothermal Area, California , 1996 .

[13]  Clifford H. Thurber,et al.  Earthquake locations and three‐dimensional crustal structure in the Coyote Lake Area, central California , 1983 .

[14]  Walter H. F. Smith,et al.  Free software helps map and display data , 1991 .

[15]  F. Sigmundsson,et al.  Magma chamber deflation recorded by the global positioning system: The Hekla 1991 Eruption , 1992 .

[16]  G. Foulger.  HENGILL TRIPLE JUNCTION, SW ICELAND 2. ANOMALOUS EARTHQUAKE FOCAL MECHANISMS AND IMPLICATIONS FOR PROCESS WITHIN THE GEOTHERMAL RESERVOIR AND AT ACCRETIONARY PLATE BOUNDARIES , 1988 .

[17]  J. Garat,et al.  A Petrophysical Interpretation Using the Velocities of P and S Waves (Full-Waveform Sonic) , 1990 .

[18]  A. D. Miller,et al.  Three‐dimensional υp and υp/υs structure of the Hengill Triple Junction and Geothermal Area, Iceland, and the repeatability of tomographic inversion , 1995 .

[19]  G. Foulger.  HENGILL TRIPLE JUNCTION, SW ICELAND 1. TECTONIC STRUCTURE AND THE SPATIAL AND TEMPORAL DISTRIBUTION OF LOCAL EARTHQUAKES , 1988 .

[20]  Páll Einarsson,et al.  S-wave shadows in the Krafla Caldera in NE-Iceland, evidence for a magma chamber in the crust , 1978 .

[21]  M. Grant.  Geothermal Reservoir Engineering , 1982 .

[22]  G. Foulger,et al.  The Krafla spreading segment, Iceland: 1. Three‐dimensional crustal structure and the spatial and temporal distribution of local earthquakes , 1994 .

[23]  A. D. Miller.  Seismic structure and earthquake focal mechanisms of the Hengill volcanic complex, SW Iceland , 1996 .

[24]  F. Birch,et al.  SECTION 7: COMPRESSIBILITY; ELASTIC CONSTANTS (See also Section 9) , 1966 .

[25]  W. Menke,et al.  The crustal magma chamber of the Katla volcano in south Iceland revealed by 2-D seismic undershooting , 1994 .

[26]  A. D. Miller,et al.  Crustal shear‐wave splitting from local earthquakes in the Hengill Triple Junction, southwest Iceland , 1996 .

[27]  B. Julian.  Analysing seismic-source mechanisms by linear-programming methods , 1986 .

[28]  B. Julian,et al.  Earthquake mechanisms from linear-programming inversion of seismic-wave amplitude ratios , 1996, Bulletin of The Seismological Society of America (BSSA).

[29]  H. M. Iyer,et al.  Vp/Vs ratios in the Yellowstone National Park region, Wyoming , 1985 .

[30]  S. Day,et al.  Inversion of local earthquake data for site response in San Diego, California , 1996, Bulletin of the Seismological Society of America.

[31]  G. Foulger,et al.  The Krafla spreading segment, Iceland: 2. The accretionary stress cycle and nonshear earthquake focal mechanisms , 1994 .

[32]  M. Grant.  Chapter 1 – Geothermal Reservoirs , 1982 .

[33]  John A. Hudson,et al.  Source type plot for inversion of the moment tensor , 1989 .

[34]  A. D. Miller,et al.  Non‐double‐couple earthquakes 2. Observations , 1998 .

[35]  A. D. Miller,et al.  Non‐double‐couple earthquake mechanisms at the Hengill‐Grensdalur Volcanic Complex, Southwest Iceland , 1997 .

[36]  A. D. Miller,et al.  Non‐double‐couple earthquakes 1. Theory , 1998 .

[37]  R. S. J. Sparks,et al.  Gas release rates from pyroclastic flows: a assessment of the role of fluidisation in their emplacement , 1978 .

[38]  R. E. Long,et al.  Implosive earthquakes at the active accretionary plate boundary in northern Iceland , 1989, Nature.

[39]  W. Menke,et al.  Seismic attenuation in the crust at the mid-Atlantic plate boundary in south-west Iceland , 1995 .