The primary purpose of the passage is to describe

Answer this question based on the following passage.

            That tectonic earthquakes are closely 
            connected with the formation of faults 
            seems now established beyond doubt. 
            They occur far from all traces of recent
(5)        volcanic action. Their isoseismic lines 
            are elongated in directions parallel to 
            known faults. Indeed, when several 
            isoseismics are carefully drawn, it is 
            possible from their form and relative 
(10)      position to predict the position of the 
            originating fault.The initial formation and 
            further spreading of the rent may be the 
            cause of a few earthquakes, but by far the 
            larger number are due to the subsequent 
(15)      growth of the fault. The relative displacement
            of the rocks adjoining the fault, which may 
            amount to thousands of feet, occasionally 
            even to miles, is the result, not of one great
            movement, but of innumerable slips taking 
(20)      place in different parts of the fault and spread
            over vast ages of time. With every fault-slip,
            intense friction is suddenly brought into action
            by the rubbing of one mass of rock against the
            other; and, according to the modern view, it is 
(25)      this friction that gives rise to the earthquake 
            waves.
            In most earthquakes, the slip takes place at a 
            considerable depth, several miles down, and 
            the vertical slip is so small that it dies out 
(30)      before reaching the surface. But, in a few 
            violent earthquakes, the slip is continued up 
            to the surface and is left visible there as a 
            small cliff or fault-scarp. In these cases, the 
            sudden spring of the crust may increase and 
(35)      complicate the effects of the vibratory shock.