1 Earthquake location (Determination of the hypocenter)
- Grid search
* A very simple method that performs a grid search over all possible locations.
* For each point in the grid there will be n equations...
* and the hypocentral location would be the point with the best agreement between the observed and calculated times...
* Location by grid search is up to 1000 times slower than location by iterative methods....
* Probabilistic earthquake location uses a variation of the grid search method.... performing a so called directed random walk
within the x,y,z volume which means that the whole volume does not have to be searched....(
Lomax 2000)
-Location by Iterative Methods
This method is based on linearizing the problem...
First step is to make a guess of hypocenter and origin time (x0, y0, z0, t0)..... In order to linearise the problem
it is now assumed that the true hypocenter is close enough to the guessed value.
Geiger method for earthquake location.
1.2 Error Quantification and Statistics
2 Earthquakes and friction laws
2.1 Seismic coupling and seismic styles
The linear measure of earthquake size is seismic moment, Mo = GuA, where u is the mean slip in the earthquake, A is the rupture area and G the shear modulus. Three stability states result in three seismic styles:
1) regions with the stable field (outer parts of accretionary prisms .... totally aseismic,
2) Faults in the unstable field characterised by infrequent large earthquakes separated by long interseismic periods of quiescence
and 3) Faults in the conditionally stable regime, such as the creeeping section of the San Andreas, are characterised by high steady rates of small-event activity and NO LARGE events ('large' events are earthquakes that rupture the entire seismogenic thickness). These small events together contribute very little to the total moment release, which is primarily aseismic (
Amelung 1997,
Amelung 1997). Small events are found to occur repeatedly with a high repetition rate at the same spots
Nadeau 1995 . The spots may mark small geometric irregularities where the normal stress is higher, causing a transition to the unstable field.
3. Large-scale tectonic deformation inferred from small earthquakes (Amelung 1997)
Q: focal mechanisms of small eq can be used to provide info about deformation on a regional scale???
...good agreement between the seismic strain patterns in the lower magnitude intervals (M = 0.33-3.94) and the strain patterns from the plate motion and from geodesy clearly shows that the deformation by the small earthquakes is in direction identical to the applied deformation....
4. Cluster analysis
Grouping events with highly correlated waveforms works well for events in very close proximity .....BUT breaks down at larger separations, as waveform correlations decreases systematically with increasing source separation distance (
Schaff 2004)
Cluster analysis has also been applied to group events by similarity in the moment tensor (
Cesca 2013)
hypoDD a double difference earthquake location algorithm that takes as input differential times...