6 Conclusions
We use shallow borehole recordings of 4537 earthquakes at Parkfield,
California, to investigate scaling, and spatio-temporal variation of
stress drop. We find:
- Correction for spatio-temporal variation in material properties is
needed to improve the accuracy of stress drop estimations. In the
Parkfield region, corrections for variations in attenuation and
rupture velocity with depth provide the most significant improvements
to the stress drop estimates.
- The frequency bandwidth of the signal is important. Insufficient
bandwidth can lead to systematic underestimation, and increased
scatter of corner frequency and hence stress drop estimates.
Interpretation of stress drop distributions should only include events
within the resolution limit of the available dataset.
- Apparent depth and magnitude dependence can be caused by poor
correction for material properties and the limited frequency range. We
see neither dependence at Parkfield.
- At Parkfield we observe strong, stable spatial variability at small
scale, with no significant dependence on the nature of fault slip
(creeping versus locked) or the slip distribution of the 2004 M6
earthquake.
- Temporal variations of stress drop following the 2004 M6 earthquake
are smaller than the spatial variations, but can be observed in
regions with sufficient number of events and well-resolved
coseismic/post-seismic slip.