N. Voulgaris

Subcrustal microearthquake seismicity and fault plane solutions beneath the Hellenic Arc

Subcrustal seismicity recorded in the southern Aegean sea during a 7-week microearthquake study was low compared with shallow seismicity. Most intermediate depth seismicity occurred beneath the western and eastern ends of the Hellenic arc. This distribution confirms that a slab of lithosphere is being subducted at a very shallow (<15°) angle for 200 km beneath the western end (Peloponnese) but more steeply beneath the eastern end (Dodecanese). We could locate only one intermediate depth event beneath the central pan of the arc, where teleseimically located intermediate depth earthquakes also are infrequent. T axes for most of the 22 focal mechanisms of subcrustal earthquakes are roughly parallel to the local dip direction of the seismic zone. Between depths of 40 and 80 km, the mechanisms are more confused than at greater depth, perhaps because some of these earthquakes did not occur within the downgoing slab. Earthquakes deeper than 80 km, and within the subducted slab, have nearly horizontal P axes that trend NNE-SSW in the eastern part and NNW-SSE in the western part of the arc. These deeper mechanisms show horizontal P axes along strike, perhaps in response to the contortion of the slab or to the westward motion of Turkey, as well as lengthening downdip, probably in response to gravity acting on excess mass in the slab. Thus the short slab, both downdip and along strike, subducting beneath the Aegean is subjected to a more complex set of forces than the long slabs of the Pacific.

The Mw = 6.0, 7 September 1999 Athens earthquake

On 7 September 1999 at 11:56 GMT a destructive earthquake (Mw = 6.0) occurred close to Athens (Greece). The rupture process is examined using data from the Cornet local permanent network, as well as teleseismic recordings. Data recorded by a temporary seismological network were analyzed to study the aftershock sequence. The mainshock was relocated at 38.105°N, 23.565°E, about 20 km northwest of Athens. Four foreshocks were also relocated close to the mainshock. The modeling of teleseismic P and SH waves provides a well-constrained focal mechanism of the mainshock (strike = 105°, dip = 55° and rake = -80°) at a depth of 8 km and a seismic moment M0 = 1.01025 dyn·cm. The obtained fault plane solution represents normal faulting indicating an almost north-south extension. More than 3500 aftershocks were located, 1813 of which present RMS < 0.1 s and ERH, ERZ < 1.0 km. Two main clusters were distinguished, while the depth distribution is concentrated between 2 and 11 km. Over 1000 fault plane solutions of aftershocks were constrained, the majority of which also correspond to N–S extension. No surface breaks were observed but the fault plane solution of the mainshock is in agreement with the tectonics of the area and with the focal mechanisms obtained by aftershocks. The hypocenter of the mainshock is located on the deep western edge of the fault plane. The relocated epicenter coincides with the fringe that represents the highest deformation observed on the differential interferometric image. The calculated source duration is 5 sec, while the estimated dimensions of the fault are 15 km length and 10 km width. The source process is characterized by unilateral eastward rupture propagation, towards the city of Athens. An evident stop phase observed in the recordings of the Cornet local stations is interpreted as a barrier caused by the Aegaleo Mountain.

OBSERVATIONS ON THE 3-D CRUSTAL VELOCITY STRUCTURE IN THE KOZANI-GREVENA (NW GREECE) AREA

Abstract Three-dimensional velocity structure of the upper crust was determined by inversion of P-waves travel time in the area of the Aliakmon river basin, where a strong earthquake of Ms = 6.6 occurred on May 13, 1995. The area is located at the northwestern part of the Greek mainland. To investigate the 3-D crustal velocity structure of the region, a two-step tomographic procedure has been applied. The data set consists of the travel time residuals of 366 very well located earthquakes, recorded by at least seven stations of a portable network, which was installed at the Kozani-Grevena broader region by the Institute of Geodynamics, immediately after the shock of 13 May, 1995. In order to improve the initial (reference) velocity model, before the inversion of the travel times, the “minimum 1-D” initial velocity model was obtained. The results show that at shallow depths low velocities are predominant in the whole region and there is no sharp horizontal velocity variation. The velocity at shallow depths seems to be affected from the surface geology. Despite the absence of large scale seismotectonics features in the region there is a remarkable vertical increase of the velocity below the depth of 15 Km, supporting the existence of a second order discontinuity in the crust of the region.

The May 13, 1995, Kozani-Grevena (NW Greece) earthquake: Source study and its tectonic implications

Abstract At 08:47 GMT, on May 13, 1995, a strong earthquake of Ms = 6.6 occurred in the NW part of Greece (Western Macedonia) and caused serious damage in the Kozani and Grevena prefectures, but fortunately no fatalities. The maximum observed macroseismic intensity was IX + of the Modified Mercalli scale. The main shock was preceded by several foreshocks and followed by intense aftershock activity lasting several months. The Institute of Geodynamics of the National Observatory of Athens, in order to monitor and study the aftershock activity, installed a seismic network of nine (9) stations operated for a period of 50 days. Thousands of aftershocks were recorded. Based on the analysis of recorded data, a NE-SW trending zone dipping NW is defined. In the field a surface rupture of normal slip was observed, following a NE-SW direction for a length of 8 km with a 4 cm down throw of the NW area. This break was located along a pre-existing minor normal fault, while a main fault system exists 10 km to the SE. The focal mechanism of the main shock shows normal faulting, which is in agreement with the field observations. Moreover focal mechanisms of several well defined aftershocks were computed, showing various types of faulting.

Seismic site characterization at the western Cephalonia Island in the aftermath of the 2014 earthquake series

BackgroundThe site response during a strong earthquake event may be proven crucial for earthquake hazard assessment and risk mitigation. Two moderate magnitude earthquakes that occurred in early 2014 in Cephalonia produced the largest ground motion values ever recorded in Greece, highly exceeding the provisions of the effective seismic code implying for local effects. This motivated the investigation of site response in the epicentral area presented herein.MethodsWe applied the HVSR method on free-field ambient noise measurements obtained during an in situ survey. 68 measurements were adopted for site characterization after their validation using earthquakes and geotechnical data. The site response was approximated by the peak frequency and the amplification ratio of the HVSR curves.ResultsThe majority of measurements exhibit smooth lateral variations in the frequency range 0.7–17 Hz, at a factor up to 7 and they are clearly classified in two bands, a low (0.7–4 Hz) and a high one (5–17 Hz). Some discrepancies that are observed between microtremor measurements and earthquake recordings for peak frequencies <2 Hz and overall underestimated ambient noise HVSR amplification are likely explained by near-source, radiation pattern and/or nonlinear soil effects.ConclusionsHigh frequencies combined with low amplification correlate with damage in the hardest hit areas. Low frequencies are aligned in a NNE-SSW direction in the epicentral area, similar to the strike of the activated fault, indicating that the properties of rocks along the fault zone have possibly been affected by slippage and/or dynamic effects.

A Study of Site Effects on Strong Motion Records Obtained from Local Network Around Gulf of Corinth, (Central Greece)

In July 1991, a strong motion network consisting of 8 SSA-1 accelerometric stations, was installed around the western part of gulf of Corinth, (C. Greece), in order to provide the basis for strong ground motion attenuation and site effect studies. Until now almost 550 3-component accelerograms from 130 events with epicentral distances ranging from 1 to 100 km having local Richter magnitudes ranging from 2.5 to 5.3 R, with maximum horizontal peak-to-peak acceleration values ranging from 2 to 360 milli-g, were recorded. Three of the stations were installed in the southern part of gulf of Corinth on Plio-quaternary sediments while the other five were installed on the northern part on alpine formations. The eighteen events which provided satisfactory record quality at most of the stations since 1991, have been analysed in order to define the importance of the site effects in a very geologically complex area like the gulf of Corinth.

Qualitative Study of Site Effects of Seismograms. A Case Study in the Area of Eastern-Central Greece

From January 1983 up to June 1985 a nine station telemetric seismographic network was in operation in the Volos-Almiros-Atalandi (Eastern Greece) area. This network, called VOLNET, was part of a multidisciplinary effort to study the potentiality of the w~ll known Atalandi fault (Richter, 1958) and the broader area of Eastern-Central Greece. Within this project an experiment was performed aiming at examining the effects of station geology on the measurements from earthquakes of this area. This was accomplished by installing another five, portable, geologically representative sites Almiros basins. seismographic stations at in the area of Volos and By eliminating the effects from source. travel path, and instrument response. the spectral analysis of several events has shown a consistent relation between the amplitude and site geology of each station.