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Val d'Aran seismic station (CARA)

Code: CA.CARA Name: Val d'Aran 

Latitude: 42.7076 N Longitude: 0.8181 E Height: 1627 m

Local depth: -27 m

Owner: Institut Cartogràfic i Geològic de Catalunya

Subsoil features

Lithology

Period

Vs30 (m/s)

Classe EC8

F0 (Hz)

Talus deposits

Quaternary

438 (0 m)

>1000 (-27 m)

B (0 m)

A (-27 m)

5 (-5 m)

11 (-27 m)

Instrumentation by channels and periods

Channels

Start date

Sensor type

Digitizer type

Instrumental response

HHZ, HH1, HH2

16/11/2011

Broadband velocimeter

24 bits

[StationXML]

Geophysical characterisation

H/V Spectral ratio from seismic noise

H/V Spectral ratio using seismic noise records from Aran seismic station, with sensor installed 27 meters deep. A low amplitude peak around 11 Hz is observed, which can be associated with the fundamental frequency of the soil, and so, it is expected to encounter an acoustic impedance contrast in the subsoil.

H/V Spectral ratio curve using seismic noise records from Aran seismic station

P-waves velocity model

The P-waves velocity model, obtained by seismic tomography technique, shows a low velocity layer (Vp < 2000 m/s) extended up to 25 meters deep (ripables materials). Up to around 10 meters deep the velocity is lower than 1500 m/s, often related with sedimentary unconsolidated materials. Around 30-35 meters deep the velocity raises almost 2500 m/s, value supossed as transition between non-ripable bedrock and fractured or weathered rock. The bedrock is not clearly detected.

P-waves velocity model of Aran seismic station, obtained by seismic tomography technique.

Wave velocity model S obtained by refraction tomography

In the model of S-wave velocity obtained by seismic tomography, a layer of low velocity materials (Vs <1000 m / s) with a thickness of more than 25 meters is observed. It is necessary to bear in mind that the most superficial layers, up to about 5 meters deep, have low velocity values (Vs <300 m / s), related to soft sedimentary materials. The depth of the bedrock (Vs> 1000 m / s) varies along the profile between 30 and 35 meters, deepening as we move from west to east. In the sector of the seismic well the depth of the bedrock is about 35 meters.

S-Waves velocity model of Aran seismic station, obtained by seismic tomography technique.

 

Wave velocity model S obtained by the analysis of surface waves

From the analysis of the dispersion of the Love surface waves (MASW) it has been possible to calculate the shear-wave velocity profile (Vs). Three layers are identified. In the sector of the seismic well a superficial layer is observed with a velocity of about 250 m / s of about 5 meters of thickness. Below this the speed increases slightly to about 600 m / s. At about 36 meters there is a high speed layer, with values close to 1500 m / s. This last layer is interpreted as the bedrock.

S-Waves velocity model of Aran seismic station, obtained from the analisi of surface waves (MASW).

Images

3 multimedia

eneral view of the different elements: antenna VSAT, geodetic pillar, instrumental house with the seismometer located at 30 m depth of the casing borehole, lightning rod (PDC) and security fence..