| Movement of the tectonic plates leads to strain build-up in the Earth, which can be released
during earthquakes when one side of a seismic fault suddenly slips with respect to the other
one. The amount of seismic strain release (or “strain drop”) is thus a direct measurement of a
basic earthquake property, i.e. the ratio of seismic slip over the dimension of the ruptured
fault. SCARDEC, a recently developed method, gives access to this information through the
systematic determination of earthquakes source time functions (STFs). STFs describe the
integrated spatio-temporal history of the earthquake process, and their maximum value can
be related to the amount of stress or strain released during the earthquake. Here I
analyse all earthquakes with magnitudes greater than 6 occurring in the last 20
years, and thus provide a catalogue of 1700 STFs which sample all the possible
seismic depths. Analysis of this new database reveals that the strain drop remains
on average the same for all earthquakes, independent of magnitude and depth. In
other words, it is shown that, independent of the earthquake depth, magnitude 6
and larger earthquakes keep on average a similar ratio between seismic slip and
dimension of the main slip patch. This invariance implies that deep earthquakes are
even more similar than previously thought to their shallow counterparts, a puzzling
finding as shallow and deep earthquakes should originate from different physical
mechanisms. Concretely, the ratio between slip and patch dimension is on the order of
10-5-10-4, with extreme values only 8 times lower or larger at the 95% confidence
interval. Besides the implications for mechanisms of deep earthquake generation,
this limited variability has practical implications for realistic earthquake scenarios. |