Abstract
Recent studies on seismological data indicate that earthquake magnitude
scales with either the dominant period or the peak amplitude in the
seismogram's first few seconds. At first sight, this may indicate
that the earthquake's final size is somehow related to the way rupture
starts. One working hypothesis is that strong radiation from the
initial phase of rupture is indicative of a triggering asperity releasing
a consistent amount of elastic energy, with the potential to drive
the fracture to large extents. We tested this concept with a number
of numerical simulations, but within the models investigated, scaling
was found only for ruptures extending up to about four times the
size of the initial asperity; at larger distances the correlation
was lost. Alternatively, a careful kinematic analysis of the earthquake
source radiation shows that the initial signal recorded at any station
does not necessarily correspond to the rupture initiation but may
represent an extended portion of the radiating source. Using the
concept of isochrones, we show that the apparent scaling may be explained
by a simple kinematic model respecting causality, up to a given magnitude
threshold where the scaling relation saturates. The saturation level
is in agreement with that observed in some, but not all, of the real
seismicity catalogs. 10.1785/0120070246
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