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La Rabassa seismic station (ARBS)

Code: CA.ARBS Name: La Rabassa 

Latitude: 42.4344 N Longitude: 1.5337 E Height: 2145 m

Local depth: -30 m

Owner: Institut d'Estudis Andorrans

Subsoil features

Lithology

Period

Vs30 (m/s)

Classe EC8

F0 (Hz)

Slates

Upper Ordovician

476 (0 m)

>1014 (-27 m)

B (0 m)

A (-27 m)

1.5 (0 m)

2.5 (-27 m)

Instrumentation by channels and periods

Channels

Start date

Sensor type

Digitizer type

Instrumental response

HHZ, HH1, HH2

01/10/2009

Broadband velocimeter

24 bits

[StationXML]

Geophysical characterisation

H/V Spectral ratio from seismic noise

H/V Spectral ratio using seismic noise records from La Rabassa seismic station, with sensor installed 30 meters deep. A peak around 2.5 Hz is observed that can be related to the fundamental frequency of the ground. Therefore, an acoustic impedance contrast is expected.

H/V Spectral ratio of seismic noise records from La Rabassa seismic station.

P-waves velocity model

The P-waves velocity model, obtained by seismic tomography technique, is characterized by significant lateral variations along the seismic profile. A package of low-velocity materials (VP ​​<2000 m/s) is observed that extends to almost 30 meters deep and is located only 5 meters deep on the northwest side of the profile. The depth of investigation of the seismic profile, with a maximum of 30 meters, is not sufficient to detect materials with velocities greater than 2500 m/s, which are usually associated with unweathered non-ripable rock and which can be interpreted as the bedrock.

P-waves velocity model of La Rabassa seismic station.

S-waves velocity model

In the shear wave propagation velocity (VS) model obtained by seismic refraction tomography it is characterized by an extensive layer of low velocity materials (VS <1000 m/s) with variable power. The depth of the isolines that is usually interpreted as the bedrock (VS = 1000 m/s) varies between about 40 meters on the southeast side of the seismic profile and less than 15 meters at the northwest end of the profile. Even, a superficial layer of unconsolidated materials (VS <500 m/s) of a 10-meter thickness is observed. In the seismic well area, the bedrock (VS = 1000 m/s) is located at a depth of approximately 40 meters.

S-waves velocity model of La Rabassa seismic station.

Images

4 multimedia

Detail of the fence with a record box of the ground cable