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This the push request #219 created by Laurentiu with some small fixes on the documentation. Tests are green: https://ci.openquake.org/job/zdevel_oq-hazardlib/148/
This is a magic number. Usually you would define a constant on top of the module, something like GRAVITY_ACCELERATION = 981  # cm/s^2

Alternatively, one can use the constants define in scipy:
from scipy.constants import g  
(this is 9.80665 m/s^2)