After approximately 60 years of seismic quiescence within Santorini caldera, in January 2011 the volcano reawakened with a significant seismic swarm and rapidly expanding radial deformation. The deformation is imaged by a dense network of 19 survey and 5 continuous GPS stations, showing that as of 21 January 2012, the volcano has extended laterally from a point inside the northern segment of the caldera by about 140 mm and is expanding at 180 mm/yr. A series of spherical source models show the source is not migrating significantly, but remains about 4 km depth and has expanded by 14 million m3since inflation began. A distributed sill model is also tested, which shows a possible N‐S elongation of the volumetric source. While observations of the current deformation sequence are unprecedented at Santorini, it is not certain that an eruption is imminent as other similar calderas have experienced comparable activity without eruption.

No abstract

Well-monitored examples of large-magnitude earthquakes that rupture subduction plate-boundaries reveal that these earthquakes may be preceded by episodes of slow slip, swarm activity, and/or large foreshocks (i.e.,

No abstract

SUMMARY The recently developed precise point positioning (PPP) technique permits to compute instantaneous coordinates of a GPS station relatively to distant reference stations and waveforms of ground displacements during strong motions at the 1 Hz level. This is another application of GPS, different from the computation of static coseismic movements or of accurate monitoring of dynamic displacements of structures using a static receiver and a nearby moving receiver (DGPS). Recently, earthquake ground displacement waveforms using 10‐Hz GPS data have also been calculated, but no independent evidence to assess their quality exists. To overcome this problem, we evaluated the output of 10‐Hz PPP results on the basis of supervised learning experiments. Semistatic and dynamic displacements (damped harmonic oscillations) of known characteristics of the order of a few centimeter were produced and were recorded by GPS, an accelerometer, and a robotic total station. Time series of instantaneous displacements were analyzed using different PPP techniques and were compared with reference (true) values derived from DGPS and the other sensors. Our analysis revealed that the PPP‐derived coordinates are contaminated by long‐period noise but they can display the details of semistatic displacements, while their short‐period component describes well the pattern of waveforms and spectra (at least up to 4 Hz) of dynamic displacements, with up to 20 mm accuracy for isolated points. These results indicate that 10‐Hz PPP‐GPS is useful for earthquake engineering and can safely be used to reconstruct waveforms of deflections of the ground and of various points on structures during strong motions. Copyright © 2014 John Wiley & Sons, Ltd.

The 2011-2012 unrest of Santorini (Thera) volcano (Aegean Sea, Greece) was associated with microseismicity confined to the Kameni Line (KL), a major tectonovolcanic lineament, and has been regarded as a single magmatic episode, produced by a spherical source derived from inversion of GPS data. However, such a source is a few kilometers away from the KL and cannot explain observed microseismicity. For this reason, we divided the unrest episode into five periods based on the fluctuations of seismicity and deformation rates and investigated the connection between seismicity and two spherical magmatic point sources for each period. Based on a new inversion algorithm and consistent GPS data, we recognized during the volcano unrest episode an unstable pattern of intrusions correlating with both the KL and Columbo Line (CL), a second major tectonovolcanic lineament. Intrusions correlating with CL appear relatively persistent, aseismic, small, and shallow, which is consistent with marine geophysical evidence for arrested shallow dykes and geodetic evidence from a previous inflation episode. During the two periods of intense seismicity, sources close to the KL, explaining seismicity, were obtained. This unstable pattern of intrusions explains both the well-observed location and timing of seismicity as well as ground deformation and is consistent with results of an Okada-type inversion for a sill and a dyke. The stress interactions between the two sources agree with Coulomb failure stress models. Santorini appears to be affected by concurrent offset magma pulses, and only recent activity from a magma pulse below the KL produced microseismic swarms.

The importance of splay‐thrust faults in subduction seismogenesis is increasingly acknowledged; however, their elastic interaction with the plate interface remains unclear. Here, we use GPS velocities, constrained by millennial fault slip rates, to study elastic fault‐interactions between the plate interface and its upper‐plate splay‐thrust faults from the southern Hellenic Subduction System (HSS). We find that, despite its largely aseismic character, the HSS plate interface zone is kinematically segmented, with slip rate deficits locally reaching ~85% and ~45% of the plate convergence rate on the western and eastern segments, respectively, and on structures different from those that ruptured historically. Although western Crete has been more active seismically during late Holocene, we find that the eastern HSS has higher seismic potential for large‐magnitude (M > 6) earthquakes and its interface zone is closer to failure. Elastic fault interactions are responsible for both significant intersegment variability in strain accumulation and uniformity in earthquake rupture segmentation along the HSS over millennial timescales.

The 24 May 2014, Mw 6.9, Samothraki‐Gökçeada shallow (depth: 11 km) earthquake along the North Aegean Trough (NAT), at the westward extension of the North Anatolian Fault Zone (NAFZ), is investigated using constraints from seismological and geodetic data. A point source solution based on teleseismic long‐period P and SH waveforms suggests an essentially strike‐slip faulting mechanism consisting of two subevents, while from a finite fault inversion of broadband data the rupture area and slip history were estimated. Analysis of data from 11 permanent GPS stations indicated significant coseismic horizontal displacement but no significant vertical or postseismic slip. Okada‐type inversion of horizontal slip vectors, using the new TOPological INVersion algorithm, allowed precise modeling of the fault rupture both as single and preferably as double strike‐slip faulting reaching the surface. Variable slip models were also computed. The independent seismological and geodetic fault rupture models are broadly consistent with each other and with structural and seismological data and indicate reactivation of two adjacent fault segments separated by a bend of the NAT. The 2014 earthquake was associated with remote clusters of low‐magnitude aftershocks, produced low accelerations, and filled a gap in seismicity along the NAT in the last 50 years; faulting in the NAT seems not directly related to the sequence of recent faulting farther east, along the NAFZ and the seismic gap in the Marmara Sea near Istanbul.