Task 2: Methods for monitoring crustal wave velocities from microearthquakes

Start: July 1996 (month 5)
End: October 1997 (month 20)
Responsible partner: UUPP.DGEO
Cooperative partner: IMOR.DG

Selected from the SIL seismic database, 6200 local earthquakes have been used in a tomography study of the 3-D P- and S-wave velocities and hypocentral locations beneath SW-Iceland [86,]. The study region is a 224$\times$112 km² rectangular area. Major tectonic features within the study region include the SISZ, the Hengill volcanic system, the Western volcanic zone (WVZ), and the Reykjanes volcanic zone (RVZ). The velocity model block size used was $4\times4$ km horizontally and 1 km vertically, resulting in detailed velocity models. The resulting velocity structures are well constrained down to depths of 10-12 km. Figure 5 shows a horizontal slice through the models between the depths of 6 and 7 km. The main results of the interpretation are:

• A large low-velocity anomaly is found at depths between 4 to 12 km beneath the Hengill volcanic fissure system. Both P- and S-wave velocities are low, but the reduction in the P-wave velocity is larger than in the S-wave velocity, suggesting that the anomaly is not caused by large portions of partial melt. The observed anomaly is interpreted as a heavily fractured volcanic fissure system and the low velocities are caused by supercritical magmatic fluids (CO₂ and H₂O) which are migrating in the fracture system. Theoretical calculations of the elastic moduli for a porous medium, using both magma saturated pores and supercritical water, support this interpretation.
• The SISZ has normal velocities with a tendency towards lower than normal V_p/V_s ratios (1.75). A slight increase in the depth to the lower crust (P-wave velocities of 6.5 km/s) is observed along the SISZ from west to east, from less than 4 km to below 5 km.
• In the depth range 5-7 km, lower than normal P- and S-wave velocities are observed in the center of the RVZ. A tendency to increased V_p/V_s ratios is observed at 4-8 km depth beneath the Krísuvík volcanic system, which might be caused by elevated temperatures.
• The depth to the base of the brittle crust in the study area, defined as the depth above which 90% of the earthquakes occur, varies from about 5 km in young (0-0.7 My) crust near the RVZ and WVZ to about 12 km near the eastern end of the SISZ. This corresponds to the estimated depths of the C isotherm.
• Clusters of earthquakes deeper than the base of the brittle layer occur beneath the Hengill and Eyjafjallajökull volcanoes. These earthquake sequences may be associated with periods of unusually high strain rate in the roots of the volcanic systems.

  

Figure 5: Horizontal slices through the models at 6-7 km for a) the S-wave velocities and b) the P-wave velocities. Model blocks without any ray travelling through them are white. Earthquakes occurring within the layer are shown. The thin stippled lines outline the volcanic zones.