Complex Crustal Structure Research Articles

Crustal thickness map of the Chinese mainland from teleseismic receiver functions

Abstract We present a new crustal thickness map of the Chinese mainland and adjacent areas based on a compilation of previously published data. More than 1900 estimates of crustal thickness from receiver function studies were collected and the weighted averages were used to develop a contour map of crustal thickness beneath the Chinese mainland. The combined dataset provided good coverage, especially over eastern and Central China, and our analysis revealed crustal thickness variations of the Chinese mainland with considerably more detail than in previous models. Our crustal thickness map shows a large variation, from a thickness of less than 30 km in the Mesozoic basin of eastern China to over 80 km in the Tibetan plateau. The thickest crust was found in central Tibet, while the thinnest continental crust was observed beneath eastern China. Normal to slightly thick crust (38–51 km) was found for the stable Precambrian basins and orogenic belts. Comparison between our model and other models reveals that the models share similar large-scale and smaller-scale features over large portions of eastern China where the data coverage is good; however, large differences (up to ± 25 km) in crustal thickness occur in areas with complex crustal structure or poor data coverage. We also evaluated the quality of previous models by comparing them with our newly compiled point observations of crustal thickness.

Influence of continental margin on regional seismic wavefield

Regional seismic analysis is an only way to investigate the source properties of small or moderate-sized seismic events that occur in physically unaccessible regions such as oceanic regions and politically-prohibited regions. The ground motions by earthquakes need to be understood for better mitigation of seismic hazards particularly in continental margins that often incorporate high seismicity. It has not been fully understood how seismic waves interact with complex crustal structures in continental margins. Responses of regional waves to complex crustal structures need to be quantified. Analysis of regional waveforms from controlled sources is desirable for investigation of influence of crustal structures. The influence of continental margin around the Korean Peninsula is investigated in terms of spectral contents, horizontal-to-vertical (H/V) spectral ratios and quality factors using regional waveforms for the 2009 North Korean underground nuclear explosion test that was well recorded by stations in the southern Korean Peninsula. Regional waveforms and spectral amplitudes vary significantly by path. Spectral contents of regional phases are different among stations in common great-circle directions. All regional phases modulate highly in continental margins. Path-dependent seismic attenuation is strong in low frequencies (≤ 3 Hz), and weak in high frequencies (> 3 Hz). Continental margins cause directional energy partition of regional waves depending on the path. Regional waves attenuate highly in passage across continental margin, and then regrow on continental paths. The growth rate is stronger than inherent attenuation rate, causing seismic amplification. The shapes of H/V ratios are similar among various regional phases at common stations. On the other hand, the H/V ratios for common phases vary by station in the same azimuths. Characteristic differences in H/V ratios between two horizontal components suggest directional partition of seismic energy by crustal structures in the paths. • Influence of complex crustal structures in continental margin on regional waveforms • Quantification of regional-phase regrowth rates on oceanic-to-continental paths • Characteristic directional energy partition of regional waves by crustal structures

Seismic discrimination of the 2009 North Korean nuclear explosion based on regional source spectra

Seismic discrimination of an underground nuclear explosion (UNE) based on regional waveforms in continental margins is challenging due to large variations among waveforms. The 2009 North Korean UNE test was conducted in the far eastern Eurasian plate. The UNE was recorded by densely-located regional seismic stations, and regional waveforms exhibit highly path-dependent amplitude and arrival time features due to complex crustal structures. Regional source spectra are calculated by correcting for the path effects on the waveforms. A two-step approach is proposed for stable inversion of source-spectral parameters and path parameters. Characteristic overshoot features are observed in the source spectra, particularly strong in Pn. The path parameter, Q, is determined uniquely regardless of the source-spectral model implemented, which suggests stable separation of path effects from waveform records. The estimated source spectra fit well to a theoretical UNE source-spectral model. The fitness between the estimated and theoretical source-spectral models allows us to discriminate UNEs from natural earthquakes. Also, the P/S source-spectral ratio is observed to be an effective discriminant of UNE.

Mapping the indentation between the Iberian and Eurasian plates beneath the Western Pyrenees/Eastern Cantabrian Mountains from receiver function analysis

In the last decades, active seismic profiling in the northern part of the Iberian Peninsula has evidenced that the Alpine collision between the Iberian and Eurasian plates resulted in a complex crustal structure, with the Iberian crust underthrusting the Eurasian crust and reaching depths of at least 45–50km beneath the Pyrenean chain and the Cantabrian Mountains. In the transition between these two zones the situation is particularly complex, as evidenced in previous wide-angle and passive seismic studies. This contribution focuses in getting new clues on the crustal structure of this transitional zone through receiver function (RF) analysis of teleseismic data recorded at permanent and temporary stations located in both the Spanish and French sides of the Western Pyrenees. Different techniques (H-κ stacking, pseudo-migration, synthetic 2D modeling) have been considered in the analysis. Passive seismic data from previous temporary deployments in the zone have been reworked and added to the discussion. A first order result is that passive seismic data are broadly consistent with the indentation of the Iberian and Eurasian crusts inferred from active seismic profiling, thus providing a completely independent confirmation of this feature. For the first time, an Iberian Moho underlying the Eurasian crust is documented from RF beneath the stations located at the Northern side of the Pyrenean range. Moreover, clear indications of dipping interfaces are observed at some stations. The new RF results suggest that in the crustal indentation beneath the Basque Massifs area, the Eurasian crust extends farther south with respect to the image inferred from active seismic data. This new geometry implies that the Pamplona transfer zone has played a major role in the regional geodynamic history.

Crust-mantle boundary reflectors in Nevada from ambient seismic noise autocorrelations

The depth of reflector layers in the Earth′s crust is usually estimated using controlled sources or earthquake signals. Ambient seismic noise, however, can also be used for this purpose. We develop and apply a new method, based on continuous waveform analysis, to estimate the two-way P-wave reflection component of the Green′s function beneath each station. The Green′s functions are retrieved from continuous record autocorrelation stacks at broad-band sensor locations within the USArray EarthScope Transportable Array in the western Great Basin and the Sierra Nevada, in a region with complex crustal and upper-mantle structure. In this paper, we show evidence of a reflector at the crust-mantle boundary (Moho discontinuity) derived for the first time from ambient noise autocorrelations using short-period (~1 s) data. Our results compare well with earthquake and controlled source investigations, and with tomography findings in the region. Moho depth is difficult to resolve seismically because of the lack of favourable spatial distribution of source and receiver geometries. In contrast, our method can be applied at any desired sensor spacing to estimate Earth reflector depth beneath surface-located sensors, providing unprecedented resolution.

Seismic tomography and anisotropy in the source area of the 2008 Iwate–Miyagi earthquake (M 7.2)

In order to understand the generation mechanism of the Iwate–Miyagi earthquake (M 7.2) that occurred on 14 June 2008 in Northeast Japan, we determined high-resolution 3D seismic velocity (Vp, Vs) and Poisson's ratio ( σ) structures as well as P-wave azimuthal anisotropy in and around the source area using 246,623 P-wave and 189,153 S-wave high-quality arrival times from 1941 aftershocks of the 2008 Iwate–Miyagi earthquake as well as 4878 shallow and intermediate-depth earthquakes in this area recorded by the dense High-Sensitivity Seismic Network (Hi-net) and the seismic stations of Tohoku University. Our results show that hypocenters of the mainshock and three large aftershocks are located in a boundary zone where both seismic velocity and Poisson's ratio change drastically in a short distance. A zone with pronounced low-velocity and high Poisson's ratio is revealed in the lower crust and uppermost mantle under the source area, which may reflect the arc magma and fluids ascending from the upper-mantle wedge. The P-wave azimuthal anisotropy is complex under the study area, which may also indicate the complex crustal structure there. These results indicate that the generation of the 2008 Iwate–Miyagi earthquake was influenced by the ascending arc magma and fluids associated with the dehydration reactions of the subducting Pacific slab under Northeast Japan.

To fault or not to fault

Most of the oceanic crust has a simple layered structure. The discovery that slow-spreading ridges exhibit a comparatively complex crustal structure and some of the largest extensional faults on Earth is leading to the recognition of a new mode of seafloor spreading.

P-wave tomography, anisotropy and seismotectonics in the eastern margin of Japan Sea

To understand the seismotectonics in the eastern margin of the Japan Sea, we determined the first high-resolution 3-D P-wave velocity structure and azimuthal anisotropy under the Japan Sea off Northeast Japan using 175,425 high-precision P-wave arrival times from 2833 local earthquakes recorded by 330 seismograph stations. P-wave arrival times from 145 suboceanic earthquakes relocated with sP depth phase are crucial to determine the structure of the crust and uppermost mantle under the Japan Sea. Our results show that strong velocity variations exist in the crust and uppermost mantle under the eastern margin of the Japan Sea. Many large crustal earthquakes occurred in or around low-velocity zones which may represent weak sections of the seismogenic crust. The P-wave azimuthal anisotropy is complex under the Japan Sea, which may also indicate the complex crustal structures there. In the eastern margin of the Japan Sea, the strong heterogeneities in the crust and upper mantle revealed by seismic tomography may reflect the complicated geologic structures such as the alternate rift zones, ridges, basins, horsts, grabens, volcanics, and continental fragments which were produced during the back-arc spreading, opening of the Japan Sea and the present compressional stage of the Honshu arc.

Seismic imaging of the crust beneath Dronning Maud Land, East Antarctica

SUMMARY With calculations of receiver functions and modelling of a seismic refraction profile we map the depth to the Moho discontinuity beneath Dronning Maud Land (DML), East Antarctica. The crustal converted signal (Ps) is clearly observable for all stations, suggesting that the crust is separated from the mantle by a sharp Moho. We reveal spatial variations in the crustal thickness and for some places the ratio of crustal P -t oS-wave velocity. The crust beneath station Novolazarevskaja in central DML approaches a thickness of about 42 km and is characterized by low V p/V s ratio of about 1.67. This crustal thickness agrees well with previous wide-angle experiments. Further south, the Wohlthat Massif shows thickened crust with depths to Moho between 47 and 51 km indicative of an orogenic root. At station Weigel in the Kottas Mountains in western DML we determine a Moho depth of 44 km using the Ps traveltime. This depth corresponds well with the crustal model retrieved from a seismic refraction experiment across the Kottas Mountains. Moho depths approach values between 44 and 52 km beneath the profile. Underneath the middle of the profile, a remarkable Moho offset of approximately 7 km is observed. It appears to mark a suture between the Mesoproterozoic (1.2–1.0 Ga) Maudheim Province and the Kibarian (1.1–1.0 Ga) arc-related Kottas Mountains. The crustal thicknesses of mobile belts and undeformed areas in southern Africa and DML are remarkably similar. For station SNAA, situated on the Archean Grunehogna Craton, a fragment of the Kalahari–Kaapvaal–Craton, we obtained an ambiguous result that indicates a complex crustal structure. We determined a crustal thickness underneath SNAA of 39 km and a V p/V s ratio of 1.82. The latter higher value corresponds to a gabbroic crust, which was also deduced by former rock investigations. In addition, the presence of Jurassic basaltic intrusions into the predominantly gabbroic crust may contribute to the value.

Collision and subduction structure of the Izu–Bonin arc, central Japan, revealed by refraction/wide-angle reflection analysis

Since the middle Miocene, the Izu–Bonin arc (IBA) has been colliding from the south with the Honshu arc in central Japan associated with subduction of the Philippine Sea plate (PSP). This process is responsible for forming a complex crustal structure called the Izu Collision Zone (ICZ). To obtain direct evidence of the deep structure dominated by collision and subduction, an intensive seismic experiment using explosive and vibroseis sources was conducted in the eastern part of the ICZ. CMP reflection and refraction/wide-angle reflection data were acquired on a 130-km-long seismic line crossing the Kanto Mountains (KM), the Tonoki–Aikawa Tectonic Line (TATL) and the Tanzawa Mountains (TM) from north to south. The TATL is considered to be a collision boundary separating the Honshu arc (KM) from the IBA (TM). The structural model constructed by refraction tomography and forward ray tracing shows remarkable lateral velocity variation across the TATL and some clear reflectors in the deep crust. A north dipping reflector beneath the KM was interpreted to be the deeper extension of the TATL. From the geometry of reflectors, we interpret the Tanzawa block is delaminated from the subducting slab due to the collision to form a wedge-like body thrusting between the upper and lower crust of Honshu. The TM itself shows a velocity structure almost consistent with the upper part of the crust of the Izu intraoceanic arc, south of our study area. The relocated hypocenter distribution using our velocity model shows high seismic activity concentrated around the collision boundary, which is in a marked contrast of low seismicity within the Tanzawa block. These features of seismicity are strongly dominated by the collision of the IBA with the Honshu arc.

Applying InSAR technique to accurately relocate the epicentre for the 1999Ms= 5.6 Kuqa earthquake in Xinjiang province, China

SUMMARY The 1999 M s = 5.6 Kuqa earthquake occurred in a seismically active belt between the Tianshan Mountain and the Traim basin. Because of the sparse seismic network and complex crustal structure, it is very difficult to accurately locate the epicentre for this event using seismic waves. The epicentres located by different research groups vary over a spatial range of 20– 40 km. Interferometric synthetic aperture radar (InSAR) is a geodetic technique with fine spatial resolution, good precision and wide coverage. An interferometric map of the epicentral region constrains the epicentre of this event. The arid climate and sparsely vegetation in the Kuqa region provide excellent conditions for InSAR studies. In this paper, we firstly construct a interferogram to map the coseismic deformation field due to the 1999 Kuqa earthquake using a coseismic interferometric pair of radar images acquired by the ESA ERS-2 satellite. Then, we develop a new geocoding method and apply it to the interferogram. Next, we infer the geometry of the seismogenic fault according to its focal mechanism and tectonic setting. To model the interferogram, we assume a dislocation buried in a uniform elastic half-space. Finally, we infer the epicentre of this earthquake to be located at (82.80 ◦ E, 41.92 ◦ N), which is close to the results of the National Earthquake Information Center of USGS and China Earthquake Administration. The epicentre location inferred from InSAR falls in the six-level isoseismal contour described by Xinjiang earthquake administration using the field investigations.

Comprehensive Geophysical Research on the Deep Structure of Northeastern South China Sea

AbstractThe complex crustal structure of northeastern South China Sea and neighboring areas, especially the structure of the land‐sea transition zone, has been a hot subject of the lithospheric structure research of South China Sea. In this paper we construct the density model along two profiles based on gravity data and constraints from Pn wave tomography and deep seismic sounding. The result of 2.5D density modeling supports the point of view that a high velocity layer exists in the lower crust of the area from Qiongyue Uplift to Luzon island arc; and the fitting result to the Taixinan Basin indicates that the crust in the southern depressed region should belong to the transitional type. Although the tectonic feature of the northeastern shelf of South China Sea is generally of volcanic type, the structures of profile AA' and BB' are different, which indicates the lateral variation and the complexity of crustal structure in the northeastern margin of South China Sea.

Crustal structure of the ultra-slow spreading Knipovich Ridge, North Atlantic, along a presumed amagmatic portion of oceanic crustal formation

The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated. The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading (5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers.

Three‐dimensional gravity and magnetic modeling of crustal indentation and wedging in the western Pyrenees‐Cantabrian Mountains

The complex crustal structure of the boundary zone between the Iberian and European plates, from the western Pyrenees to the Cantabrian Mountains, is probed by three‐dimensional (3‐D) gravity and magnetic modeling constrained by deep seismic profiles. The 3‐D modeling results support the presence of a continuous Iberian crustal root and suggest that the geometry of the orogenic belt that formed in latest Cretaceous‐Tertiary times along the northern margin of Iberia was conditioned by oblique structures separating areas characterized by different tectonic styles. In the western Pyrenees and the Basque‐Cantabrian basin (a thick Mesozoic basin presently incorporated to the Pyrenean‐Cantabrian belt), the relatively narrow thinning of the crust inherited from the Mesozoic rifting stage conditioned a structural style in which portions of the southward indenting European lower crust are interpreted to be back thrusted toward the north and uplifted to shallow depths, promoting the appearance of significant potential field anomalies. In the Basque‐Cantabrian basin, the strongest aeromagnetic anomaly of the whole Iberian Mainland is superimposed on the eastern part of a well‐defined positive gravity anomaly, similar in amplitude and wavelength to those located along the North Pyrenean Zone. These observations suggest that the eastern part of the dense, lower crustal causative body is strongly magnetized and may correspond to a gabbroic cumulate originated in the axis of the ancient Mesozoic rift. To the west, Tertiary compression affected the North Iberian (Cantabrian) passive continental margin, whose geometry and inherited structures conditioned the formation of a double crustal delamination and the uplift of the Cantabrian Mountains.

Cenozoic tectonic and geodynamic evolution of the Kyrgyz Tien Shan Mountains: A review of geological, thermochronological and geophysical data

The complex crustal structure of the Tien Shan has a strong impact on the distribution of strain induced by the India–Eurasia collision, with intracontinental deformation in Eurasia’s interior as a distant effect. The northward propagation of the India–Eurasia deformation front is suggested by the rejuvenation of mountain ranges and intermittent intramontane basins. The Tien Shan basement is formed by the rigid, heterogeneous Precambrian blocks (microcontinents) of Tarim, Issyk-Kul (or Central Tien Shan) and Aktyuz-Boordin, surrounded by a ‘soft’ matrix of Paleozoic accretion–collision belts. The Kyrgyz Tien Shan Mountains are situated between the active structures of the Tarim Plate and the Pamir indenter (south), and the stable Kazakhstan Shield (north). Underplating by the Tarim Plate and thrusting by the Pamirs are responsible for the building of the Cenozoic Tien Shan, the reactivation of its inherited structural fabric and the tectonic layering of the upper lithosphere underlying the area. Large earthquakes ( M > 6) delineate the northern and southern margins of the Issyk-Kul microcontinent, indicating that crustal heterogeneity influenced the location of active structures in the northern Kyrgyz Tien Shan.

A tomographic image of Indian lithosphere break-off beneath the Pamir-Hindukush region

SUMMARY The P and S seismic anomalies in the upper mantle beneath the Pamir‐Hindukush collision zone and surrounding areas are investigated using more than 4 × 10 5 traveltimes from 5775 events located in the study region, recorded by the stations of the worldwide seismic network and reported in the ISC catalogue. All available epicentral distance ranges of seismic rays are considered. All sources are relocated using an algorithm, which includes the double-difference method. In addition, source parameters are corrected in the main inversion step, simultaneously with the velocity models. The high reliability of the resulting P-velocity images is confirmed by various tests. The S model provides much smoother patterns, which are, however, generally consistent with P anomalies. Our model shows a complex image of lithospheric plate under the study region. Under SW Hindukush, the northwards subducting Indian plate is imaged throughout the upper mantle and transition zone. Further to the NE, within a 300-km-wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at ∼250 km depth. The tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern Tien Shan down to at least 250 km depth. To the north of Tarim, we observe a northward-dipping of the Tarim plate down to ∼500 km depth. At the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults, which indicate complex crustal structure in this region.

Aftershock distribution and 3D seismic velocity structure in and around the focal area of the 2004 mid Niigata prefecture earthquake obtained by applying double-difference tomography to dense temporary seismic network data

Abstract A destructive large earthquake (the 2004 mid Niigata prefecture earthquake) sequence occurred in the central part (Chuetsu district) of Niigata prefecture, central Japan on October 23, 2004. We have deployed a temporary seismic network composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month. Using travel time data from the temporary seismic network and surrounding routine stations, we obtained precise aftershock distribution and 3D seismic velocity structure in and around the fault planes of the earthquake and four major (M ≥ 6) aftershocks by double-difference tomography. The results clearly show three major aftershock alignments. Two of them are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond to the fault plane of the mainshock and that of the largest aftershock (M6.4), respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds to the fault plane of the M6 aftershock on October 27. General feature of the obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall (eastern part of the focal area) has higher velocity. Major velocity boundary seems to shift westward in comparison to in northern and southern parts at a location near the central part of the focal area, where the main shock rupture started. Some parts of the fault planes were imaged as low velocity zones. This complex crustal structure would be one of possible causes of the multi-fault rupture of the 2004 mid Niigata prefecture earthquake sequence.

Crustal Structure and Comparison of Different Tectonic Blocks in North China

The North China paleocontinenal block has undergone reconstruction through multi-stage tectonic movements, resulting in distinct characteristics of crustal subdivision. Based on data and study results of nearly 30 profiles of deep seismic sounding with a total length of about 20000km, we make a further study on the fine structures of the sub-blocks of the crust beneath North China and comparison of their differences. According to the structural features, the crust of North China can be classified into 3 types. The first is a simple crustal structure in the Ordos basin in the western part of the region, with an integrated basement structure, showing the crustal structure feature of stable paleocontinent. The second is a relatively simple crustal structure in the Taihang mountain located in the middle part, and the Yinshan mountain and Yanshan mountain in the northern part. These are uplifted zones where slight velocity reversal exists in local areas of the middle and lower crusts which is possibly related to medium decoupling deformation within the crust caused by the regional crustal thickening and uplifting. The third is a complex crustal structure in the rift-depression basin in the eastern part of North China, with a sinking and fragmentized basement, loosen media within the crust, low velocity, uplifted Moho, thinned crust, and obvious lateral structural differences, showing tectonic features of the newly reconstructed crust. Based on these analyses, we discuss the crustal block-subdivision tectonics and related dynamic evolution in North China in a comprehensive manner.

The heterogeneous characteristics of crustmantle structures and the seismic activities in the northwest Beijing region

In this paper, the abnormal characteristics of the crustal structures in the seismic active region, Yanqing-Huailai and Zhangbei-Shangyi, are obtained by means of comprehensively interpreting and studying the data of deep seismic sounding profiles passing through the northwestern part of Zhangjiakou-Bohai seismic zone. The results show that the fluctuation of crystalline basement in the study region is obvious and that there exist considerable differences in depth in different geological units. The locally abrupt variation of crystalline basement depths may be regarded as a mark of existence of crystalline basement faults. These crystalline basement faults and deep crustal faults provide a pass for the magma upwelling, resulting in the strong inhomogeneity of crustal structures. These phenomena of the complex seismic reflected waves and locally discontinuous reflection zones with different energy indicate that the intensive squeeze and deformation of crust took place, which have led to the complex crustal structures and offered the dynamic source for the earthquake occurrence in this region. The low velocity bodies in different depths of crust and the local interface C1 in Zhangbei-Shangyi region may result from repeated magmatic activities. The certain stress accumulation in the brittle upper crust can cause the occurrence of earthquake under the action of local tectonic activity.

A new seismic tomography of Aigion area (Gulf of Corinth, Greece) from the 1991 data set

A new three-dimensional delay traveltime tomography is performed to image the intermediate structure of the western Gulf of Corinth. A large data set, collected in 1991 during a two-month passive tomographic experiment, has been reanalysed for the reconstruction of detailed Vp and Vs images. An improved tomography method, based on an accurate traveltime computation, is applied to invert simultaneously delayed P and S first-arrival traveltimes for both velocity and hypocentre parameters. We perform different synthetic tests to analyse the sensitivity of tomography results to the model parametrization and to the starting 1-D model selection. The analysis of the retrieved Vp and Vs models as well as deduced Vp/Vs and Vp·Vs images allows us to interpret and delineate the distribution of lithological variation, porosity/crack content and fluid saturation in the upper 9–11 km of the crust beneath the gulf. The tomographic models image a rather complex crustal structure, which is characterized by a vertical change in both velocity features and seismicity distribution. We identify a shallower zone of the crust (0–5 km depth), in which velocity distributions seem to be controlled by the still active N–S extensional regime and a deeper zone (7–11 km depth), which matches the seismogenic zone. The correlation between this latter and a specific unit of the Hellenic mountain structure (the Pyllite–Quartzite series) allows us to suggest a possible explanation for seismicity concentration in a narrow band at 7–9 km depth. Finally, the occurrence of clusters showing low-angle normal fault mechanisms in areas characterized by high Vp/Vs values indicates a possible role of fluids in triggering brittle creep along the identified low-angle normal faults.