 |
| Titel |
Friction coefficient of faults inferred from earthquake focal mechanisms |
| VerfasserIn |
Alfio Vigano, Giorgio Ranalli, Daniele Andreis, Silvana Martin, Riccardo Rigon |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078522
|
|
|
|
|
|
| Zusammenfassung |
| In earthquake mechanics and structural geology the static friction coefficient is usually
assumed to have the laboratory value (μ = 0.6–0.8) according to the Coulomb-Byerlee’s law.
Estimates from deep boreholes and/or natural faults generally confirm this hypothesis
but in some cases friction coefficients can be significantly lower, suggesting the
existence of weak faults able to be activated by lower effective stress than theoretically
expected.
We apply a modified version of the method proposed by Yin and Ranalli (1995, Journal of
Structural Geology, vol. 17, pp. 1327–1335), where the average friction coefficient of a set of
n faults is estimated. This method is based on minimization of the sum of squares of the
misfit ratios, where the misfit ratio of each fault is given dividing the misfit stress
difference (i.e. the misfit between normalized stress difference and average normalized
stress difference) by the average normalized stress difference. The normalized stress
difference is defined as the critical stress difference divided by the effective overburden
pressure, while the average stress difference is obtained considering the entire fault
dataset. Input data are (i) the orientation of faults, (ii) the stress field orientation, and
(iii) the stress ratio. The latter two must be independently estimated. A uniform
stress field and a similar normalized critical stress difference for the fault dataset are
assumed.
The procedure has been extended to apply to fault plane solutions by considering both
nodal planes of a set of n focal mechanisms and estimating the range of acceptable average
friction coefficients from all possible combination of planes (2n number of combinations).
The amount of calculation can be considerably reduced if independent information makes it
possible to select which one of the nodal planes of each focal mechanism is the true fault
plane (for example when aftershocks delineate the fault geometry at depth), resulting in only
n combinations.
We present an initial application of this method by applying it to a set of focal
mechanisms from the Giudicarie region in northern Italy, and analyzing various subsets. The
data were obtained from literature data and earthquake fault plane solutions computed by
seismological agencies. Preliminary results show that best-fitting average friction coefficients
are sometimes within the expected range, but occasionally well below it (μ about 0.4 or less). |
|
|
|
|
|
|