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any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Cover illustration:The illustration is based on the results shown in Figure 7.38. The fault plane solution and seismograms are from an earthquake in California on 12 August 1998 at 14:10. Printed on acid-free paperSpringer is part of Springer Science+Business Media (www.springer.com) of text ! We are grateful to you all. Most of the book was written during one of the authors (JH) sabbatical stay at the Geological Survey of Denmark and Greenland, Copenhagen and the British Geological Survey, Edinburgh with support from the University of Bergen.In our examples we use data from a number of seismic networks, too many to mention them here. However, credit to the data sources is given with the examples in the figure captions. As this book heavily relies on the examples, we very much appreciate the support from the data providers.

The Western Hellenic Subduction Zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches depths of 100 km to 190 km, while the northern continental portion rarely exhibits deep earthquakes. Our study investigates how this oceanic‐continental transition affects fluid release and related seismicity along strike. We present results from local earthquake tomography and double‐difference relocation in conjunction with published images based on scattered teleseismic waves. Our tomographic images recover both subducting oceanic and continental crusts as low‐velocity layers on top of high‐velocity mantle. Although the northern and southern trenches are offset along the Kephalonia Transform Fault, continental and oceanic subducting crusts appear to align at depth. This suggests a smooth transition between slab retreat in the south and slab convergence in the north. Relocated hypocenters outline a single‐planed Wadati‐Benioff Zone with significant along‐strike variability in the south. Seismicity terminates abruptly north of the Kephalonia Transform Fault, likely reflecting the transition from oceanic to continental subducted crust. Near 90 km depth, the low‐velocity signature of the subducting crust fades out and the Wadati‐Benioff Zone thins and steepens, marking the outline of the basalt‐eclogite transition. Subarc melting of the mantle is only observed in the southernmost sector of the oceanic subduction, below the volcanic part of the arc. Beneath the nonvolcanic part, the overriding crust appears to have undergone large‐scale silica enrichment. This enrichment is observed as an anomalously low Vp/Vs ratio and requires massive transport of dehydration‐derived fluids updip through the subducting crust.

Subducting plates release fluids as they plunge into Earth’s mantle and occasionally rupture to produce intraslab earthquakes. It is debated whether fluids and earthquakes are directly related. By combining seismic observations and geodynamic models from western Greece, and comparing across other subduction zones, we find that earthquakes effectively track the flow of fluids from their slab source at >80 km depth to their sink at shallow (<40 km) depth. Between source and sink, the fluids flow updip under a sealed plate interface, facilitating intraslab earthquakes. In some locations, the seal breaks and fluids escape through vents into the mantle wedge, thereby reducing the fluid supply and seismicity updip in the slab. The vents themselves may represent nucleation sites for larger damaging earthquakes.

[1] A moderate size seismic event on 7 May 2001 was strongly felt on platforms in the Ekofisk oil field, in the southern North Sea, but did not cause damage to platforms or wells. We combined near-and far-field observations to develop a consistent source model and to determine whether the event was induced. Seismic data placed the epicenter inside the Ekofisk field and suggested a shallow source depth based on spectral and moment tensor analysis. GPS data from the Ekofisk platforms displayed permanent vertical and horizontal movement due to the event. A topographic bulge in the sea bottom, revealed by differential bathymetry data, and overpressure in the overburden in the northeastern part of the field, detected only after the event, had been caused by unintentional water injection that started in 1999. The injection pressure and rate were sufficient to raise the overburden. Pressure gauge and compaction data ruled out that the event occurred at reservoir level, which was further supported by unaffected production rates and absence of well failure. We therefore conclude that the event occurred in the overburden, at less than 3 km depth. Initially, this appeared unlikely on account of very low shear strength of the overburden clay-rich shale and mud rocks. The seismic event was induced owing to stress changes caused by water injection. The event possibly initiated on the northern flank of the field near the water injector and may have involved flexure of the overburden into the depression bowl in the rest of the field. Moment tensor analysis is consistent with a pure double-couple source. We suggest that slip occurred on the near-horizontal rather than along the near-vertical nodal plane. Stress drop was low, and owing to the low overburden shear strength, the event released less energy than a typical stress drop event with similar source dimensions.Citation: Ottemöller, L