Copyright 2005-2009 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise subject to inclusion of this copyright notice and my World Wide Web URL: users.monash.edu.au/~weinberg. We would very much appreciate an email stating how this material will be used. Thanks, RW. |

*This section explores the diffusion
of pore
pressure through a FLAC calculation. Because of compressibility of the
pore fluids, as measured by the bulk modulus ( K_{w}),
an increase in pore pressure in part of the modelled volume, for
example through fluid influx (discharge), will gradually propagate
through the model. Here, this propagation is explored by studying the
evolution of pore pressure in two models with different fluid bulk
moduli (K_{w}),
one with the value of water K_{w}=2.e9,
and another three orders of magnitude higher, for comparisson.*

**MODELS**

**THE ROLE OF FLUID BULK MODULUS**

The role of the bulk modulus of the pore fluid here is explored by varying its value in two calculations.

*These
two images show the results after the same number of calculation steps
for two models with different fluid bulk modulus: a) calculation using
the bulk modulus of water K_{w}=2.e9
Pa; b) calculation where the fluid bulk modulus is three orders of
magnitude higher than water K_{pore}=2.e12
Pa. The results are identical, the only difference is a three order of
magnitude faster time step for b)
due to the higher value of K_{pore}.
This is because the time step is inversely proportional to the K_{pore}
(see FLAC manual).*

**IMPLICATIONS AND EFFECTS**

*The
implications are best exemplified by the models presented in
wherea rapid increase in pore
pressure at the base of the model due to
fluid influx, causes yielding of the lower part of the model and
releases the stresses in the upper part of the model, leading to
fracturing at the base and inhibiting yielding at the top.*

*
If the bulk modulus of the pore fluids used were to be higher, pore
pressure would propagate more rapidly across the model, leading to a
crustal wide yielding to a generalized pore pressure increase, such as
happens late in the model shown above (movie).*

* *