Geological data

March-April, 2018, Mauro Alberti

Last run: 2019-06-16

Developement code:



In [1]:

    
%load_ext autoreload
%autoreload 1

1. Fault data vs. focal mechanisms

Structural investigations of brittle regimes rock volumes deal with fault data derived from field measurements of meso-fault surfaces. Seismic events, generated by active faults, are generally represented as focal mechanisms.

Due to their common physical origin, meso-fault data and focal mechanisms share common aspects, even if it is useful to treat them as two distinct type structures, also from an algorithmic point of view.

In gsfpy fault data types are stored in the faults submodule:



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from pygsf.geology.faults import *

We use also matplotlib inline and mplstereonet for plotting stereonet:



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%matplotlib inline

2. Fault data

Fault geometry characteristics can be defined using the concept of a geological plane, approximating the local fault surface, and of a slickenline, basically a vector that is parallel to the movement orientation. A slickenline parameters are its orientation and, when recognised, a movement direction.

In the algorithms, slickenlines are represented by the Slick class, that stores the slickenline orientation and movement direction. A meso-fault datum is represented by the Fault class, that is composed of a Plane instance (i.e., the geological plane), and a Slick instance.

2.1 Slickenlines

A slickenline is created via a Slick instance. The input parameters are: trend, plunge and whether movement direction is known or not.



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slick_k = Slick(140, 15)  # slickenline with known, downward movement direction



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slick_k









    Out[5]:





Slick(az: 140.00°, pl: 15.00°, known_dir: True, time: 0.0)

In the above example, the slickenline movement direction is towards N140°, with a positive dip angle value of 15°, i.e., pointing downward. In the print output, True indicates that the movement sense is known.



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slick_k.hasKnownSense









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True

We can create a slickenline object with unknown movement sense by explicitely setting the known flag to False (default value is True), as in the following example:



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slick_u = Slick(210, 60, known=False)  # slickenkline with unknown movement sense



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slick_u









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Slick(az: 210.00°, pl: 60.00°, known_dir: False, time: 0.0)



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slick_u.hasKnownSense









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False

It is possible to convert an unknown movement to a known movement or viceversa, and, when the movement is known, to invert it, with the setKnownSense() and setUnknownSense() methods.

2.2 Fault planes with slickenlines

Having described slickenlines, we can now consider how to represent fault planes presenting slickenlines.

An example is:



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flt = Fault(90, 45, slickenlines=[Slick(90, 45, known=False)])

A fault instance is created by using the Fault class, initialized with two parameters:

the geological plane: a Plane instance
the slickenlines: zero, one or more Slick instances Note that there is no required 1 : 1 relationship between a fault plane and a slickenline. A fault plane can contain no slickeline at all, or even more than a single slickenline, for instance because the movements on it were curvilinear or because the history of movements is polyphasic.



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type(flt)  # we check the type









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pygsf.geology.faults.Fault

We check if this fault instance has a known movement sense:



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flt.knownSense









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False

Since the slickenline orientation was defined via a Axis instance, the movement sense is not known.

3. Focal mechanisms and P-T-B axes

Classes for P-T-B axes are defined in the ptbaxes submodule:



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from pygsf.geology.ptbaxes import *

P-T axes instances can be created using the PTBAxes class.

We can initialize an instance in four ways:

by providing a couple of Axis, representing the T and P axes
by giving a Fault instance as input parameter
defining a couple of Cartesian vectors, representing the P and T axes
by providing a quaternion.

Creation of a P-T axes instance from a couple of Axis:



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ptbx_from_axes = PTBAxes(p_axis=Axis.fromAzPl(0, 0), t_axis=Axis.fromAzPl(90, 0))



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print(ptbx_from_axes)









    



PTBAxes(P Axis(az: 360.00°, pl: -0.00°), T Axis(az: 90.00°, pl: 0.00°))

Creation of a P-T axes instance from a FaultSlick instance:



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ptbx_from_fltsl = PTBAxes.fromFaultSlick(Fault(90, 45, slickenlines=[Slick(90, 45)]))



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print(ptbx_from_fltsl)









    



PTBAxes(P Axis(az: 360.00°, pl: -90.00°), T Axis(az: 90.00°, pl: -0.00°))

An example of creating a P-T axes instance from a couple of Vect instances is:



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ptbx_from_vects = PTBAxes.fromVects(t_vector=Vect(1,0,0), p_vector=Vect(0,1,0))



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print(ptbx_from_vects)









    



PTBAxes(P Axis(az: 360.00°, pl: -0.00°), T Axis(az: 90.00°, pl: -0.00°))

It is possible to compare two PTBAxes instances for being almost equal:



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ptbx_from_vects.almostEqual(ptbx_from_axes)









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True



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ptbx_from_vects.almostEqual(ptbx_from_fltsl)









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False

A PTBAxes instance can be converted to a matrix or to a quaternion, for being further processed (e.g., rotated)



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quat = ptbx_from_vects.toQuatern()



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print(quat)









    



Quaternion(1.00000, 0.00000, 0.00000, 0.00000)