Аннотация
The earthquake size distribution follows, in most instances, a power
law with the slope of this power law, the 'b value', commonly used
to describe the relative occurrence of large and small events (a
high b value indicates a larger proportion of small earthquakes,
and vice versa). Statistically significant variations of b values
have been measured in laboratory experiments, mines and various tectonic
regimes such as subducting slabs, near magma chambers, along fault
zones and in aftershock zones. However, it has remained uncertain
whether these differences are due to differing stress regimes, as
it was questionable that samples in small volumes (such as in laboratory
specimens, mines and the shallow Earth's crust) are representative
of earthquakes in general. Given the lack of physical understanding
of these differences, the observation that b values approach the
constant 1 if large volumes are sampled was interpreted to indicate
that b = 1 is a universal constant for earthquakes in general. Here
we show that the b value varies systematically for different styles
of faulting. We find that normal faulting events have the highest
b values, thrust events the lowest and strike-slip events intermediate
values. Given that thrust faults tend to be under higher stress than
normal faults we infer that the b value acts as a stress meter that
depends inversely on differential stress.
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