Quantifying and characterizing crustal deformation The geometric moment Brittle strain The usefulness of the scaling laws

Quantifying and characterizing crustal deformation: the geometric moment The geometric moment for faults is: where U is the mean geologic displacement over a fault whose area is A f. Similarly, the geometric moment for earthquakes is: where  U is the mean geologic displacement over a fault whose area is A e. Thus, the geometry moment is simply the seismic moment divided by the shear modulus.

Quantifying and characterizing crustal deformation: brittle strain Brittle strains are a function of the geometric moment as follows [Kostrov, 1974]: Geologic brittle strain: Seismic brittle strain:

Quantifying and characterizing crustal deformation: brittle strain To illustrate the logic behind these equations, consider the simple case of a plate of brittle thickness W* and length and width l 1 and l 2, respectively, that is being extended in the x 1 direction by a population of parallel normal faults of dip . The mean displacement of the right- hand face is: which may be rearrange to give:

Quantifying and characterizing crustal deformation: brittle strain Geodetic data may also be used to compute brittle strain:

Quantifying and characterizing crustal deformation: brittle strain Geologic brittle strain: Advantages: Long temporal sampling (Ka or Ma). Disadvantages: Only exposed faults are accounted for. Cannot discriminate seismic from aseismic slip. Geodetic brittle strain: Advantages: Accounts for all contributing sources, whether buried or exposed. Disadvantages: Short temporal window.

Quantifying and characterizing crustal deformation: brittle strain Seismic brittle strain: Advantages: Spatial resolution is better than that of the geologic brittle strain. Disadvantages: Short temporal window. Owing to their contrasting perspective, it is interesting to compare:

Quantifying and characterizing crustal deformation: brittle strain Ward (1997) has done exactly this for the United States:

Quantifying and characterizing crustal deformation: brittle strain What are the implications of these results? For Southern and Northern California: For California:

Quantifying and characterizing crustal deformation: fault scaling relations Displacement versus fault length What emerges from this data is a linear scaling between average displacement, U, and fault length, L: The use of scaling relations allows one to extrapolate beyond one’s limited observational range.

Quantifying and characterizing crustal deformation: fault scaling relations Cumulative length distribution of faults: Normal faults on Venus San Andreas subfaults figure from Scholz Faults statistics obeys a power- law size distribution. In a given fault population, the number of faults with length greater than or equal to L is: where a and C are fitting coefficients.

Quantifying and characterizing crustal deformation: fault scaling relations These relations facilitate the calculation of brittle strain. Recall that the geometric seismic moment for faults is: and since: the geometric seismic moment may be written as: This formula is advantageous since: 1. It is easier to determine L than U and A; and 2. Since one needs to measure U of only a few faults in order to determine  for the entire population.

Furthermore, recall that the geologic brittle strain is: Using: one can write: Quantifying and characterizing crustal deformation: fault scaling relations

Quantifying and characterizing crustal deformation: earthquake scaling relations Gutenberg-Richter relations: Similarly, in order to calculate the brittle strain for earthquake, one may utilize the Gutenberg-Richter relations and the scaling of co- seismic slip with rupture length.

Quantifying and characterizing crustal deformation: earthquake scaling relations Seismic moment versus source radius What emerges from this data is that co-seismic stress drop is constant over a wide range of earthquake sizes. The constancy of the stress drop, , implies a linear scaling between co- seismic slip,  U, and rupture dimensions, r:

Quantifying and characterizing crustal deformation: brittle strain Further reading: Scholz C. H., Earthquake and fault populations and the calculation of brittle strain, Geowissenshaften, 15, Ward S. N., On the consistency of earthquake moment rates, geological fault data, and space geodetic strain: the United States, Geophys. J. Int., 134, , 1998.