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Seismic anisotropy in the Alto Tiberina area

People:

Marina Pastori (marina.pastori@ingv.it)

Lucia Margheriti (lucia.margheriti@ingv.it)

Davide Piccinini (davide.piccinini@ingv.it)

The aim of this Task is the study of crustal anisotropy (CA).

Since cracks are preferentially aligned with their flat faces oriented in the direction of minimum compressive stress, CA can be used both to determine the state of stress in the crust and to look for changes in the preferred orientation with time, thus indicating a rotation of the stress field.

When a seismic shear wave travels into an anisotropic medium, its energy is split into two components with orthogonal polarisation direction, called fast () and slow component, travelling at different velocities. The measure of the lag between the two orthogonal components is called delay time (t). If the S-wave is originally polarized in the fast or slow direction it does not split and we get a null measurement. In this case we can identify the null direction (which is either the fast or the slow direction of the anisotropic medium).

There are two man sources of crustal anisotropies: 1) stress-aligned crack-induced anisotropy (a) and 2) structural alignments due to rock or mineral fabric (b).

In the first hypothesis (1), the is typically oriented parallel to the direction of maximum horizontal stress, as suggested by the Extensive-Dilatancy Anisotropy model (EDA; c) and the t is a measure of the intensity and/or thickness of the fracture field. It is also possible to study the spatio-temporal changes in the direction of open micro-fractures as well as that state and/or amount of fluid present in the fractures in response to changes in the local active stress field. This type of study is well interpreted by the model Anisotropic Poro-Elasticity (APE; d).

If the anisotropy is instead caused by structural alignments (2), the is parallel to the strike of the fracture or the fast axis of the anisotropic minerals and is not related to the active stress field. In this case the t is a measure of the fabric strength (e).

We will focus on the analysis of the propagation of seismic waves in the upper crust and how they vary in space and time. Such investigations will help us to define the characteristics of the fractured medium and the active stress field near the main faults pertaining to the ATF system.

References:

  1. Crampin S., 1993; Arguments for EDA. Can. J. Explor. Geophys., 29, 18-30.
  2. Brocher T.M. and Christensen N.I.; 1990. Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska. Geology, 18, 737-740.
  3. Crampin S.; 1978; Seismic wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic. Geophys. J. R. Astron. Soc. 53, 467–496.
  4. Zatsepin S.V. and Crampin S.; 1995. Rock deformation: A non-linear anisotropic poro-elastic theory (APE) for pre- stressed fluid-saturated rock. In: 21° General Assembly IUGG, Boulder, CO, USA, July 2-14.
  5. Zinke J.C. and Zoback M.D.; 2000. Structure-related and stress-induced shear-wave velocity anisotropy: Observations from microearthquakes near the Calaveras Fault in Central California. Bull. Seismol. Soc. Am., 90, 1305-1312.