Earthquake Tomography

In order to delineate the tectonic elements and active faults at the most seismically active part of the Hellenic arc we conducted a microseismicity study using a local seismic array. This consisted of 17 land and 17 marine seismic stations. In a two months period we recorded 3504 seismic events that were located by the Geiger’ s classical method based on 1D velocity model. A 3D velocity model was then developed using 2D active seismic lines that constrained 2D and 3D gravity modeling. The density model developed by this procedure was converted into a 3D velocity model applying empirical relationships between velocity and density. Seismic events were relocated using this 3D velocity model and tomographic inversion based on the SIMULP algorithm. This procedure significantly improved the relocation accuracy of the seismic events. RMS values of nearly all recoded seismicity are less than 0.2s, with a maximum concentration of relocated events at 0.07s. Most of the shallow seismicity is ...

We investigate 3-D local earthquake tomography for high-quality travel time arrivals from aftershocks following the 2010 M7.0 Haiti earthquake on the Léogâne fault. The data were recorded by 35 stations, including 19 ocean bottom seismometers, from which we selected 595 events to simultaneously invert for hypocenter location and 3-D V p and V s velocity structures in southern Haiti. We performed several resolution tests and concluded that clear features can be recovered to a depth of 15 km. At 5 km depth we distinguish a broad low-velocity zone in the V p and V s structure offshore near Gonave Island, which correlate with layers of marine sediments. Results show a pronounced low-velocity zone in the upper 5 km across the city of Léogâne, which is consistent with the sedimentary basin location from geologic map. At 10 km depth, we detect a low-velocity anomaly offshore near the Trois Baies fault and a NW-SE directed low-velocity zone onshore across Petit-Goâve and Jacmel, which is consistent with a suspected fault from a previous study and that we refer to it in our study as the Petit-Goâve-Jacmel fault. These observations suggest that low-velocity structures delineate fault structures and the sedimentary basins across the southern peninsula, which is extremely useful for seismic hazard assessment in Haiti.

The aim of this study was to study the diagnostic parameters in
brain tuberculosis (meningeal and parenchymal)
Material and Methods: This study was conducted in the
department of Neurosurgery and Neurology SKIMS for a period
of two years. A total of 61 patients presenting with brain
tuberculosis admitted at skims during these two years were
included in the study.
Results: The most presenting symptom in our study was headache
found in 95.10% followed by vomiting found in 86.90% of
subjects, fever in 78.70%, altered sensorium in 49.20%, seizures in
19.70% and diplopia in18%. Out of 61 patients cranial nerve
involment was found in 34(55.73%) with 11 having more than two
cranial nerves involved. The most common cranial nerve involved
were 3rd and 6th. ADA was positive in 36 of 53 patients of TBM
with a sensitivity of 67.9% and a specificity of 75%. PCR proved
to be highly specific CT scan of head was abnormal in 56 out of
61 patients (91.8%). 12 (19.70%) were in stage I (meningeal
involvement only), 29 (47.50%) were in stage II (parenchymal
involvement only) and 15 (24.60%) were in stage III (both
parenchymal and meningeal invlolvement). The most common
finding in CT head was meningeal enhancement in 43 patients,
hydrocephalus in 37 patients and tuberculomas in 14 patients. The
most common sites of tuberculomas were frontal lobe (n=6;
42.8%), parietal lobe (n=4; 28.5%), followed by cerebellum in 2
patients and occipital in two. Nine patients had single and five
multiple tuberculomas. Of the 14 patients with tuberculomas,
hydrocephalus on CT was seen in 6 patients.
Conclusion: CT scan is a useful diagnostic tool even in very early
stages of TBM. Abnormalities reported on CT scan done are
hydrocephalus, infarcts, basal enhancement, and tuberculomas.
Normal study is reported in up to 20% of the cases.

In Fall 2006, in the frame of the European Community SEAHELLARC project, we deployed an “amphibious” seismic array consisting of 17 3-channel stand alone stations onshore and 17 4-channel Ocean Bottom Seismographs in the Kyparissiakos Gulf and surrounding area of western Peloponnese. We observed the microseismic activity for a period of two months. The geometry and location of the array were
designed to particularly observe the offshore activity and define the continuation of known onshore active faults offshore. Data evaluation and location of the seismic events were accomplished in three steps, successively improving the location accuracy. First we used the standard procedure for foci location with a 1D velocity model and
the Hypoinverse algorithm. Locations were then improved by 3D passive tomography, using P and S arrivals. The optimum solution of the seismic foci location was finally achieved by using a 3D velocity model developed from active seismic observations and 3D gravity modelling. More than 3500 earthquakes over a threshold magnitude
of 0.3 Md were identified, using arrivals from a minimum of six stations per event.
Shallow seismicity is confined mainly onshore Zakynthos Island, at the continentocean
transition to the west, and in the area of Pylos - Messinia. These zones are
deforming by westward thrusting of the Alpine napes and collision of the continental
backstop of pre-Apulia with the Ionian oceanic lithosphere at the Mediterranean Ridge.
Deeper seismicity follows the subduction of the Ionian lithosphere below western
Peloponnese with successively increasing hypocentre depths eastwards. A cluster of
subcrustal events extending to 90 km depths is located in a pull apart basin NE of
Strofades Island in the forearc. These events also generated T-phases. We attribute this
“unusual” seismicity to lateral deformation and fracturing of the oceanic lithosphere
during its subduction below a continental crust of laterally varying thickness.

In Fall 2006, in the frame of the European Community SEAHELLARC project, we deployed an "amphibious" seismic array consisting of 17 3-channel stand alone stations onshore and 17 4-channel ocean bottom seismographs in the Kyparissiakos Gulf and surrounding area of western Peloponnese. We observed the microseismic activity for a period of two months. The geometry and location of the array were designed to particularly observe the offshore activity and define the continuation of known onshore active faults offshore. Data evaluation and location of the seismic events were accomplished in three steps, successively improving the location accuracy. First we used the standard procedure for foci location with a 1D velocity model and the Hypoinverse algorithm. Locations were then improved by 3D passive tomography, using P and S arrivals. The optimum solution of the seismic foci location was finally achieved by using a 3D velocity model developed from active seismic observations and ...

A combined onshore -offshore network of 30 portable seismographs (23 land-stations and 7 OBS) was deployed in N. Evoikos Gulf area, Central Greece, to record the microseismic activity for a 4-month period, in the summer of 2003. We located more than 2000 earthquakes ranging from 0.7 to 4.5 ML (with a 6 records per event limit).

A combined onshore - offshore network of 30 portable seismographs (23 land-stations and 7 OBS) was deployed in N. Evoikos Gulf area, Central Greece, to record the microseismic activity for a 4-month period, in the summer of 2003. We
located more than 2000 earthquakes ranging from 0.7 to 4.5 ML (with a 6 records per event limit). The velocity model used for the initial location of the events was a formerly obtained one from deep seismic profiles.
From the initial dataset we selected the most reliable events depending on the number of phases. We set a minimum of 10 records and GAP< 180o per event and we selected 481 events (see figure below) .
Both P and S wave arrivals were inverted initially with the algorithm Velest (Kissling, 1995) to calculate the minimum 1D velocity model and afterwards they were inverted with the 3D linearised tomography code Simulps14 in order to
determine the 3D velocity structure of the study area. The recorded seismicity of the area was relocated with the velocity model resulted.
The Velest algorithm simultaneously inverts the hypocentral parameters, the velocity model and the corrections of the seismographic stations, in order to calculate the model which gives the optimum solutions, the so-called “1D minimum
model”. Although the usual application of this algorithm is the hypocentral relocation in the present study is being implemented for the determination of the optimum initial model for the tree-dimensional inversion.
The initial and final model of the 1D inversion of Velest is presented in the figure on the right with the black and red line respectively. For the design of the initial model we took into account the results of the seismic surveys, previously
contacted in the area (Makris et . al, 2001). The model was compared with the 1D model implemented by Papoulia et al. (2006) and others and was found to be substantially better.

A combined onshore -offshore network of 30 portable seismographs (23 land-stations and 7 OBS) was deployed in N. Evoikos Gulf area, Central Greece, to record the microseismic activity for a 4-month period, in the summer of 2003. We located more than 2000 earthquakes ranging from 0.7 to 4.5 ML (with a 6 records per event limit).

A combined on-offshore seismic network consisting of 17 4C ocean bottom seismographs and 15 3C land stations deployed in the surrounding area of Kyparissiakos gulf (western Peloponesse) and recorded the microseismic activity for a 2 months period. More than 3500 earthquakes ranging from 0.3 to 4.0 Md) were located by using a limit of 6 arrivals per event and taking into account a local velocity model obtained from active seismic profiles in the region. Up to 2800 epicentres of these events have been estimated within the circular like array and almost the 65% of them cover depths vary between 30 and 90km.
By selecting the most reliable of them we created a homogeneous distributed data set covering the interior of array which is used as an input to velest algorithm code for the calculation of the minimum 1D velocity model. Subsequently three dimensional simultaneous inversion is performed by using the simulps14
code in order to determine the 3D velocity structure of the investigated region. From the 3D linearised tomography analysis on a large number of intermediate depth events new details of the deeper crust structure are coming out. Interpretation of these results are crucial for the understanding of the seismotectonic
status in the broader area due to the subduction of the Ionian lithosphere under western Peloponesse. Relocation of the initial data is also accomplished for the improved velocity model resulting from the inversion process.

New findings on the deep origin of the geothermal field and volcanic centres at the North Euboean Gulf (or North Evian Gulf), Central Greece, were obtained by combining a three-dimensional traveltime inversion of microseismic data recorded by an on/offshore local seismic network with a Curie Point Depth analysis based on aeromagnetic data.
A magma chamber was detected from low seismic P-wave velocity values and high Poisson ratios at depths below 8 km also coincident with a Curie surface estimated at 7–8 km depth.
Furthermore, it was also observed that local geothermal anomalies are generated by hydrothermal flux facilitated by NW–SE and NE–SW oriented faults. Microseismic activity is also associated with these fault systems.