Geographical and vertical variation of the earth's seismicity

Information concerning a total number of 13700 instrumentally recorded eart hquakes is used to study the geographical and the vertical distribution of the Earth's seismicity. From these earthquakes, which form four complete sa mples of data (M greater than or equal to 7.0, 1894-1992;M greater than or equal to 6.5, 1930-1992; M greater than or equal to 6.0, 1953-1992; M great er than or equal to 5.5, 1966-1992), 11511 are shallow (h less than or equa l to 60 km), 2085 are of intermediate focal depth (61 less than or equal to h less than or equal to 300 km) and 564 are deep focus earthquakes (301 le ss than or equal to h less than or equal to 720 km). The parameters a and b of the frequency-magnitude relationship were calculated in a grid of equal ly spaced points at 1 degrees by using the data of earthquakes located insi de circles centered at each point. The radius of the circles increased from 30 km with a step of 10 km until the information for the earthquakes locat ed inside the circle fulfil three criteria which concern the size of the sa mple used to compute these parameters at each point of the grid. The result s are given in a qualitative way (epicenter maps) as well as in a quantitat ive way (mean return periods).