This may have several origins : (1) the source intersects the ground surface, because the its depths is too shallow (depths are positive upward and the reference is sea level) or because its geometry is not appropriate (radius too large, fracture dip too small) ; (2) Inconsistency between the eruptive fissure point spacing, which is too small, and the source mesh interval which is too large. Try a larger point spacing or a smaller source mesh interval.