We tested the hypothesis that, on the scale of a decade, earthquakes deeper
than 60 km are produced at an approximately constant, that is, stationary
Poissonian rate. We selected earthquakes with depths greater than 60 km to
avoid the ubiquitous clusters present in shallow seismicity, without declus
tering the data set. Six of the 90 randomly positioned volumes contained ob
vious aftershock sequences and, thus, were excluded from the scoring. We sh
owed that magnitude shifts of 0.2 to 0.4 units are present in some catalogs
. Since we measure the seismicity rate above a minimum magnitude, these shi
fts introduce artificial fluctuations in the rate. Therefore, we modeled th
e influence of the magnitude shifts on the test. Measuring the goodness of
fit to a stationary Poisson process model by the chi (2)-test for samples o
f 500 earthquakes each, we found that the hypothesis cannot be rejected at
the 95% confidence levels based on the data in the 84 volumes without obvio
us aftershock sequences. We propose that this constancy of production rate
is due to the effect of a constant loading rate by tectonic plate motion on
an asperity assembly with a uniform distribution of current loading stress
. Although this result cannot be obtained for shallow seismicity because of
the presence of numerous clusters, it seems reasonable to assume that the
shallow background seismicity is also produced at a stationary Poissonian r
ate.