K. I. Konstantinou

Nature, wavefield properties and source mechanism of volcanic tremor: a review

Abstract Volcanic tremor has attracted considerable attention by seismologists because of its potential value as a tool for forecasting eruptions and better understanding the physical processes that occur inside active volcanoes. However, unlike tectonic earthquakes where the dominant source process is brittle failure of rock, the driving mechanism of tremor seems to involve complex interactions of magmatic fluids with the surrounding bedrock. These interactions are responsible for the following distinct characteristics found in volcanic tremor recorded at many volcanoes worldwide: (a) the onset of tremor may be emergent or impulsive, with its amplitude showing in many cases a direct relationship to the volcanic activity; (b) in the frequency domain the spectra consist of a series of sharp peaks in the band 0.1–7 Hz, representing either a fundamental frequency and its harmonics, or a random distribution, while quite often they exhibit temporal variations in their content; (c) the depth of the source can vary considerably from one volcano to another in the range of a few hundred metres to 40 km; (d) tremor may occur prior to and/or after eruptions with a duration that ranges from several minutes to several days or months. The methods used to study tremor include spectral analysis using both the Fast Fourier Transform and the Maximum Entropy Method, polarisation analysis of the wavefield and methods that make use of array data to deduce the backazimuth and type of the seismic waves as well as the location of the source. Visual and/or recorded acoustic observations of the ongoing volcanic activity have assisted in many cases to further constrain proposed physical mechanisms for the generation of tremor. The models suggested as possible sources of tremor can be grouped as follows: (a) fluid-flow-induced oscillations of conduits transporting magmatic fluids; (b) excitation and resonance of fluid-filled cracks; (c) bubble growth or collapse due to hydrothermal boiling of groundwater; (d) a variety of models involving the oscillations of magma bodies with different geometries. It has been proposed by many authors that the source of tremor is not unique and may differ from one volcano to another, a fact that adds more difficulty in the source modelling efforts. As data quality, computer power and speed are improving, it may be possible in the near future to decipher and accurately model tremor source processes at different volcanic environments.

Shear wave anisotropy beneath the Aegean inferred from SKS splitting observations

[1] SKS splitting parameters are measured in the Aegean region using events recorded at a dense temporary network in the south Aegean and the operating permanent networks, especially focusing in the back‐arc and the near‐trench areas of the Hellenic arc. In general, fast anisotropy directions are trench perpendicular in the back‐arc area and trench parallel near the trench. Anisotropy measurements near the volcanic arc mark the transition between these two regions. In the back arc, a gradual increase is observed in delay times from south to north, with a prevailing NE‐SW direction. In Cyclades, this pattern is correlated with GPS velocities and stretching lineations of metamorphic core complexes. Our preferred source of anisotropy in the back‐arc region is the mantle wedge flow, induced by the retreating descending slab. The westernmost termination of the trench reveals directions parallel with the Kefalonia Transform Fault and perpendicular to the convergence boundary. Beneath Peloponnese, the trench‐parallel flow is probably located beneath the shallow‐dipping slab, although scattered measurements may also reflect fossil anisotropy from a past NW‐SE strike of the trench. In western Crete, which may be entering a stage of continental collision, the anisotropy pattern changes to trench perpendicular, with a possible subslab source. Good nulls in central east Crete indicate a change in the anisotropy origin toward the east. At the easternmost side of the trench, fast directions are trench parallel. This reflects a similar subslab flow that may become toroidal around the slab edge beneath western Turkey. This may also produce a trench‐parallel flow within the mantle wedge.

Waveform Relocation and Focal Mechanism Analysis of an Earthquake Swarm in Trichonis Lake, Western Greece

Abstract In early April 2007, a series of moderate earthquakes (M L 4.1–4.8) oc-curred in the area of Trichonis Lake in western Greece. The earthquake activity waswell recorded by the Hellenic Broadband Seismic Network ( HL ) operated by the Na-tional Observatory of Athens. Initial locations for 156 events of the swarm showed adiffuse image of seismicity. Subsequently, 101 events are precisely relocated, calcu-lating source-specific station terms and differential travel times from waveform crosscorrelation. Uncertainties in relocations are estimated with a bootstrap approach byrandomly weighting the original picks and the differential times. Additionally, wave-forms of seven out of the eight largest earthquakes of the swarm were inverted inorder to derive regional moment tensor solutions. The results showed a tight north-northwest–south-southeast cluster located on an offshore extension of a similarlyoriented fault trace mapped onshore. Moment tensor solutions indicate normal fault-ing with a substantial component of left-lateral strike-slip motion. It is possible thatthis identified fault forms part of a link that connects the Gulf of Corinth rift system tothe west-northwest–east-southeast fault zone south of Trichonis basin.Online Material: Event catalog and focal mechanism for the eight largest events.Introduction

Present‐day crustal stress field in Greece inferred from regional‐scale damped inversion of earthquake focal mechanisms

In this study we utilize regional and teleseismic earthquake moment tensor solutions in order to infer the contemporary crustal stress in the Greek region. We focus on crustal earthquakes and select only solutions with good waveform fits and well-resolved nodal planes. A data set of 1614 focal mechanisms is used as input to a regional-scale damped stress inversion algorithm over a grid whose node spacing is 0.35°. Several resolution and sensitivity tests are performed in order to ascertain the robustness of our results. Our findings show that for most of the Greek region the largest principal stress σ1 is vertically oriented and that the minimum principal stress axis σ3 are subhorizontal with a predominant N-S orientation. In the SW Peloponnese the orientation of σ3 axes rotates clockwise and in SE Aegean counterclockwise. These results are in agreement with the generally accepted model that slab rollback combined with gravitational spreading of the Aegean lithosphere are the main causes of the extension. Transitions between different faulting types in NW Greece or in the Aegean occur within narrow zones in the order of tens of kilometers. A visual comparison of the principal horizontal stress axes and the principal strain axes derived from GPS observations shows good agreement, suggesting that the crust in the Greek region behaves largely in an elastic manner.

Moment Magnitude Estimates for Earthquakes in the Greek Region: A Comprehensive Comparison

Abstract Many different agencies and research groups routinely estimate moment magnitudes ( M w ) in the Mediterranean area as a means of offering an unbiased quantitative assessment of earthquake size. In this work, a comprehensive comparison is undertaken of the moment magnitudes estimated for earthquakes in the Greek region by the Global Centroid Moment Tensor (Global CMT) group, Regional CMT group, the Swiss seismological service based in ETH Zurich, Aristotle University of Thessaloniki (AUTH), and the National Observatory of Athens (NOA) Institute of Geodynamics. The orthogonal regression method is applied to obtain the best‐fit line between the NOA and the other magnitude estimates. Results show that NOA moment magnitudes are, on average, smaller than Global CMT/Regional CMT/ETH magnitudes by about 0.12–0.19 magnitude units, whereas they agree quite well with AUTH magnitudes. It is also shown that for the calibration of NOA magnitudes relative to the other magnitude estimates, no scaling correction is needed, but only a variable offset correction should be applied. Taking advantage of the fact that Global CMT and Regional CMT magnitudes are fully compatible, it is possible to calibrate NOA magnitudes relative to Global CMT and then merge the moment magnitudes of the three databases. The new catalog compiled in this way is homogeneous with respect to M w and includes 1966 earthquakes spanning a period from 1976 to 2014. Online Material: Earthquake catalog.

Reduction of randomness in seismic noise as a short-term precursor to a volcanic eruption

Ambient seismic noise is characterized by randomness incurred by the random position and strength of the noise sources as well as the heterogeneous properties of the medium through which it propagates. Here we use ambient noise data recorded prior to the 1996 Gjálp eruption in Iceland in order to show that a reduction of noise randomness can be a clear short-term precursor to volcanic activity. The eruption was preceded on 29 September 1996 by a Mw ~5.6 earthquake that occurred in the caldera rim of the Bárdarbunga volcano. A significant reduction of randomness started occurring 8 days before the earthquake and 10 days before the onset of the eruption. This reduction was observed even at stations more than 100 km away from the eruption site. Randomness increased to its previous levels 160 minutes after the Bárdarbunga earthquake, during which time aftershocks migrated from the Bárdarbunga caldera to a site near the Gjálp eruption fissure. We attribute this precursory reduction of randomness to the lack of higher frequencies (>1 Hz) in the noise wavefield caused by high absorption losses as hot magma ascended in the upper crust.

Hazard assessment of volcanic ballistic impacts at Mt Chihshin, Tatun Volcano Group, northern Taiwan

This study investigates the hazard posed by Volcanic Ballistic Projectiles (VBPs) in the area surrounding Mt Chihshin, Tatun Volcano Group, northern Taiwan. Based on the volcano’s current evolutionary stage, we consider two types of volcanic activity during which VBPs can be generated, namely hydrothermal and vulcanian eruptions. Hydrothermal eruptions may occur after a sudden decompression of water in the hydrothermal system of the volcano, typically due to mass removal processes, while vulcanian eruptions are caused by solidified magma that plugs the eruptive vent and gets blasted when this caprock is no longer able to withstand the pressure in the volcanic conduit. Initial velocities of ejected VBPs were estimated for each type of activity based on physical models and inserted as initial conditions to the equations that describe their motion. A hydrothermal eruption is assumed to occur at the NW flank of Mt Chihshin near the Hsiaoyiokeng fumarole, which is a place prone to flank instability, while a vulcanian eruption is assumed to originate from a central vent at the peak of Mt Chihshin. Modeling results suggest that the radii of the areas impacted by VBPs vary between 0.1 and 1.1 km for a hydrothermal eruption, while they become 1.4–5.1 km for a vulcanian eruption. Within these areas, roads, hiking trails, and public buildings lie within the impact areas; therefore, VBPs may potentially cause damage, injury, and even casualties.

Three-dimensional shallow velocity structure beneath Taal Volcano, Philippines

Based on its numerous historical explosive eruptions and high potential hazards to nearby population of millions, Taal Volcano is one of the most dangerous “Decade Volcanoes” in the world. To provide better investigation on local seismicity and seismic structure beneath Taal Volcano, we deployed a temporary seismic network consisting of eight stations from March 2008 to March 2010. In the preliminary data processing stage, three periods showing linear time-drifting of internal clock were clearly identified from noise-derived empirical Green’s functions. The time-drifting errors were corrected prior to further data analyses. By using VELEST, 2274 local earthquakes were manually picked and located. Two major earthquake groups are noticed, with one lying beneath the western shore of Taal Lake showing a linear feature, and the other spreading around the eastern flank of Taal Volcano Island at shallower depths. We performed seismic tomography to image the 3D structure beneath Taal Volcano using the LOTOS algorithm. Some interesting features are revealed from the tomographic results, including a solidified magma conduit below the northwestern corner of Taal Volcano Island, indicated by high Vp, Vs, and low Vp/Vs ratio, and a large potential hydrothermal reservoir beneath the center of Taal Volcano Island, suggested by low Vs and high Vp/Vs ratio. Furthermore, combining earthquake distributions and tomographic images, we suggest potential existence of a hydrothermal reservoir beneath the southwestern corner of Taal Lake, and a fluid conduit extending to the northwest. These seismic features have never been proposed in previous studies, implying that new hydrothermal activity might be formed in places away from the historical craters on Taal Volcano Island.