Accurate locations of 961 shallow and intermediate depth earthquakes, which occurred between 1956 and 1995 in the Hellenic arc (34–39°N, 19–28°E), are used to define the plate boundaries in this ...area. Reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc. An ocean–continent type of interaction occurs on a curved surface, which is defined by the shallow branch (20–100
km) of the Wadati–Benioff zone. The intersection of this zone with the earth's surface is a curve which follows the convex (outer) side of the sedimentary arc (western Peloponnese–west of Cythera–south coast of Crete–east coast of Rhodes) and dips at low angle (∼30°) to the Aegean sea. Coupling between the subducted oceanic crust and the overriding of the Aegean lithospheric plate takes place on this surface. The deep branch (100–180
km) of the Wadati–Benioff zone is dipping freely (without coupling) at a high angle (∼45°) beneath the south Aegean trough and the volcanic arc. The high shallow seismicity (
h≤20
km) which is observed in the southwestern convex (outer) side of the arc (Ionian sea) is attributed to the fast southwestward motion of the Aegean plate. Location of strong deep earthquakes (
h>100
km) in the fore-arc area of the southwestern part of the Hellenic arc (west of Cythera) indicates that oceanic crust is ‘destroyed’ in this part of the Hellenic trench due to roll-back of the descending lithospheric slab.
Arc volcanoes are underlain by complex systems of molten‐rock reservoirs ranging from melt‐poor mush zones to melt‐rich magma chambers. Petrological and satellite data indicate that eruptible magma ...chambers form in the topmost few kilometres of the crust. However, very few chambers have ever been definitively located, suggesting that most are too short‐lived or too small to be imaged, which has direct implications for hazard assessment and modeling of magma differentiation. Here we use a high‐resolution technology based on inverting full seismic waveforms to image a small, high‐melt‐fraction magma chamber that was not detected with standard seismic tomography. The melt reservoir extends from ∼2 to at least 4 km below sea level (b.s.l.) at Kolumbo—a submarine volcano near Santorini, Greece. The chamber coincides with the termination point of the recent earthquake swarms and may be a missing link between a deeper melt reservoir and the high‐temperature hydrothermal system venting at the crater floor. The chamber poses a serious hazard as it could produce a highly explosive, tsunamigenic eruption in the near future. Our results suggest that similar reservoirs (relatively small but high‐melt‐fraction) may have gone undetected at other active volcanoes, challenging the existing eruption forecasts and reactive‐flow models of magma differentiation.
Plain Language Summary
Arc volcanoes, which mark the curved boundaries between converging tectonic plates, host the most explosive events on Earth. The associated hazard depends on how much mobile magma is currently present shallow beneath a volcano. Standard tomographic methods used so far have relatively low resolution and give a blurred picture of only the largest molten‐rock bodies. In particular, they struggle to distinguish between mobile magma and melt spread between tightly packed mineral grains. This study, a first in volcanology, combines a next‐generation tomographic method with extraordinarily dense seafloor recordings of controlled marine sound sources. This state‐of‐the‐art experiment at Kolumbo volcano, offshore of Santorini allowed us to detect a body of mobile magma which has been growing at an average rate of 4 × 106 m3 per year since the last eruption in 1650 CE. This rate is large enough to counteract the effect of cooling and crystallization. Our results show that Kolumbo poses a serious threat and deserves a real‐time monitoring facility. Despite the excellent data coverage, the small magma body was missed by standard tomography. This suggests that applying next‐generation imaging methods to already‐well‐studied volcanoes may lead to similar discoveries. We envision that small‐volume, high‐melt‐fraction reservoirs may be more widespread than previously thought.
Key Points
A shallow, very strong negative Vp anomaly imaged under the explosive, submarine Kolumbo volcano, Greece, using full‐waveform inversion
The high‐fidelity image and petrologic data indicate the anomaly is a small (∼0.6‐km wide, ∼2‐km deep), magma chamber with ∼42% of melt
The chamber was missed by travel‐time tomography indicating similar reservoirs may have gone undetected at other volcanoes
Volcanic calderas are surface depressions formed by roof collapse following evacuation of magma from an underlying reservoir. The mechanisms of caldera formation are debated and predict differences ...in the evolution of the caldera floor and distinct styles of magma recharge. Here we use a dense, active source, seismic tomography study to reveal the sub-surface physical properties of the Santorini caldera in order to understand caldera formation. We find a ∼3-km-wide, cylindrical low-velocity anomaly in the upper 3 km beneath the north-central portion of the caldera, that lies directly above the pressure source of the 2011-2012 inflation. We interpret this anomaly as a low-density volume caused by excess porosities of between 4% and 28%, with pore spaces filled with hot seawater. Vents that were formed during the first three phases of the 3.6 ka Late Bronze Age (LBA) eruption are located close to the edge of the imaged structure. The correlation between older volcanic vents and the low-velocity anomaly suggests that this feature may be long-lived. We infer that collapse of a limited area of the caldera floor resulted in a high-porosity, low-density cylindrical volume, which formed by either chaotic collapse along reverse faults, wholesale subsidence and infilling with tuffs and ignimbrites, phreatomagmatic fracturing, or a combination of these processes. Phase 4 eruptive vents are located along the margins of the topographic caldera and the velocity structure indicates that coherent down-drop of the wider topographic caldera followed the more limited collapse in the northern caldera. This progressive collapse sequence is consistent with models for multi-stage formation of nested calderas along conjugate reverse and normal faults. The upper crustal density differences inferred from the seismic velocity model predict differences in subsurface gravitational loading that correlate with the location of 2011-2012 edifice inflation. This result supports the hypothesis that sub-surface density anomalies may influence present-day magma recharge events. We postulate that past collapses and the resulting topographical and density variations at Santorini influence magma focusing between eruptive cycles, a feedback process that may be important in other volcanoes.
•There is a shallow low-velocity, high-porosity volume in the north-central caldera.•Vents of the first 3 LBA eruption phases correlate with this inner structure.•Inner collapse involved reverse faults, volcanic deposits, and/or rock fractures.•The low-density volume may have caused 2011-2012 inflation to localize beneath it.•The outer topographic caldera formed by relatively coherent down drop.
In the present work we propose a new seismic zonation model of area type sources for the broader Aegean area, which can be readily used for seismic hazard assessment. The definition of this model is ...based not only on seismicity information but incorporates all available seismotectonic and neotectonic information for the study area, in an attempt to define zones which show not only a rather homogeneous seismicity release but also exhibit similar active faulting characteristics. For this reason, all available seismological information such as fault plane solutions and the corresponding kinematic axes have been incorporated in the analysis, as well as information about active tectonics, such as seismic and active faults. Moreover, various morphotectonic features (e.g. relief, coastline) were also considered. Finally, a revised seismic catalogue is employed and earthquake epicentres since historical times (550 BC–2008) are employed, in order to define areas of common seismotectonic characteristics, that could constitute a discrete seismic zone. A new revised model of 113 earthquake seismic zones of shallow earthquakes for the broader Aegean area is finally proposed. Using the proposed zonation model, a detailed study is performed for the catalogue completeness for the recent instrumental period.Using the defined completeness information, seismicity parameters (such as G–R values) for the 113 new seismic zones have been calculated, and their spatial distribution was also examined. The spatial variation of the obtained b values shows an excellent correlation with the geotectonic setting in the area, in good agreement with previous studies. Moreover, a quantitative estimation of seismicity is performed in terms of the mean return period, Tm, of large (M ≥ 6.0) earthquakes, as well as the most frequent maximum magnitude, Mt, for a typical time period (T = 50 yr), revealing significant spatial variations of seismicity levels within the study area. The new proposed seismic zonation model and its parameters can be readily employed for seismic hazard assessment for the broader Aegean area.
The central area of the Ionian Islands is dominated by the existence of a major tectonic structure called the Cephalonia Transform Fault (CTF). Its main part (Cephalonia segment) has been identified ...by previous work based on the spatial distribution of earthquake foci, fault plane solutions of strong earthquakes, active tectonics, structural studies and geodetic measurements. This part (Cephalonia segment) which exhibits strike-slip motion with a thrust component, strikes in a northeast direction, dips to the southeast and has a length of ∼90 km. In the present paper information concerning new fault plane solutions, orientation of isoseismals, sea bottom topography and recent GPS results are used to further check the properties of this southern part of the CTF and to explore its northeastward prolongation to Lefkada Island. It is shown that the CTF is extended to the western coast of Lefkada. This northern branch (Lefkada segment) of the CTF which is also characterized by strike-slip motion with a thrust component, strikes in a north-northeast direction, dips to the east-southeast and has a length of ∼40 km. These two segments of the CTF form a major kinematic boundary where the slip rate is 2–3 cm/yr.
Recent reliable data are used to study the behavior of seismic activity before 46 strong shallow earthquakes (
M
≥
6.0), which correspond to five complete samples of mainshocks. These samples include ...6 mainshocks (
M
=
6.0–7.1) that occurred in western Mediterranean since 1980, 17 mainshocks (
M
=
6.0–7.2) which occurred in the Aegean (Greece and surrounding area) since 1980, 5 mainshocks (
M
=
6.4–7.5) that occurred in Anatolia since 1980, 12 mainshocks (
M
=
6.0–7.3) that occurred in California since 1980 and 6 mainshocks (
M
=
7.0–8.3) that occurred in Japan since 1990. In all 46 cases, a similar precursory seismicity pattern is observed. Specifically, it is observed that accelerating Benioff strain (square root of seismic energy) release caused by preshocks occurs in a broad circular region (critical region), with a radius about eight times larger than the fault length of the mainshock, in agreement with results obtained by various research groups during the last two decades. However, in a much smaller circular region (seismogenic region), with a radius about four times the fault length, the corresponding preshock strain decelerates with the time to the mainshock. The time variation of the strain follows in both cases a power law but the exponent power is smaller than unit (
m ¯
=
0.3) in the case of the accelerating preshock strain and larger than unit (
m ¯
=
3.0) in the case of the decelerating preshock strain. Predictive properties of this “Decelerating In–Accelerating Out Strain” model are expressed by empirical relations. The possibility of using this model for intermediate-term earthquake prediction is discussed and the relative model uncertainties are estimated.
The validity of the estimation of seismic site response characteristics from ambient noise measurements was investigated in the downtown district of the city of Thessaloniki (Northern Greece), which ...was strongly affected by the 20/6/1978 (
M=6.5) damaging earthquake. For this purpose 250 ‘single site’ ambient noise measurements were performed in a dense grid of points covering the center of the city. The ambient noise
H/
V spectral ratio for each site was calculated and the fundamental frequency (fo) and corresponding
H/
V amplitude level (Ao) were estimated. Contour maps of both, fo and Ao, were compared with results from geological and geotechnical studies as well as with macroseismic data of the 1978 earthquake and were found to be well correlated. These comparisons provide strong evidence that ambient noise measurements properly processed with the (
H/
V) spectral ratio technique can be used as an inexpensive and fast tool for microzonation studies in urban environments.
A new relation is proposed for accurate determination of local magnitudes in Greece. This relation is based on a large number of synthetic Wood-Anderson (SWA) seismograms corresponding to 782 ...regional shallow earthquakes which occurred during the period 2007–2013 and recorded by 98 digital broad-band stations. These stations are installed and operated by the following: (a) the National Observatory of Athens (HL), (b) the Department of Geophysics of the Aristotle University of Thessaloniki (HT), (c) the Seismological Laboratory of the University of Athens (HA), and (d) the Seismological Laboratory of the Patras University (HP). The seismological networks of the above institutions constitute the recently (2004) established Hellenic Unified Seismic Network (HUSN). These records are used to calculate a refined geometrical spreading factor and an anelastic attenuation coefficient, representative for Greece and surrounding areas, proper for accurate calculation of local magnitudes in this region. Individual station corrections depending on the crustal structure variations in their vicinity and possible inconsistencies in instruments responses are also considered in order to further ameliorate magnitude estimation accuracy. Comparison of such calculated local magnitudes with corresponding original moment magnitudes, based on an independent dataset, revealed that these magnitude scales are equivalent for a wide range of values.
We model the macroseismic damage distribution of four important intermediate-depth earthquakes of the southern Aegean Sea subduction zone, namely the destructive 1926
M
= 7.7 Rhodes and 1935
M
= ...6.9 Crete earthquakes, the unique 1956
M
= 6.9 Amorgos aftershock (recently proposed to be triggered by a shallow event), and the more recent 2002
M
= 5.9 Milos earthquake, which all exhibit spatially anomalous macroseismic patterns. Macroseismic data for these events are collected from published macroseismic databases and compared with the spatial distribution of seismic motions obtained from stochastic simulation, converted to macroseismic intensity (Modified Mercalli scale, I
MM
). For this conversion, we present an updated correlation between macroseismic intensities and peak measures of seismic motions (PGA and PGV) for the intermediate-depth earthquakes of the southern Aegean Sea. Input model parameters for the simulations, such as fault dimensions, stress parameters, and attenuation parameters (e.g. back-arc/along anelastic attenuation) are adopted from previous work performed in the area. Site-effects on the observed seismic motions are approximated using generic transfer functions proposed for the broader Aegean Sea area on the basis of V
S30
values from topographic slope proxies. The results are in very good agreement with the observed anomalous damage patterns, for which the largest intensities are often observed at distances > 100 km from the earthquake epicenters. We also consider two additional “prediction” but realistic intermediate-depth earthquake scenarios, and model their macroseismic distributions, to assess their expected damage impact in the broader southern Aegean area. The results suggest that intermediate-depth events, especially north of central Crete, have a prominent effect on a wide area of the outer Hellenic arc, with a very important impact on modern urban centers along northern Crete coasts (e.g. city of Heraklion), in excellent agreement with the available historical information.
Accelerating preshock seismic strain in a broad (critical) region and decelerating preshock seismic strain in a narrower (seismogenic) region constitute a model for intermediate‐term prediction of ...strong main shocks. An effort is made in the present work for a forward test of the Decelerating‐Accelerating Seismic Strain (D‐AS) model by identifying such patterns and estimating the corresponding, probably ensuing, strong main shocks (M ≥ 7.0) in central Asia (20°N–45°N, 42°E–105°E). Five such patterns have been identified, and the origin time, magnitude, and epicentral geographic coordinates of each of the corresponding main shocks have been estimated (predicted). Model uncertainties of the estimated time, magnitude, and space parameters of these probably ensuing main shocks, as well as appropriate statistical tests against a standard Gutenberg‐Richter seismicity distribution, are also presented to allow a future objective evaluation of the model's efficiency for intermediate‐term earthquake prediction.
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