If a disturbance is horizontal, and has a direction parallel to the length of a wall, the wall is carried forward at its foundations. This motion is opposed by the inertia of the upper portion of the wall and the various loads it carries. The wall being elastic, distortion takes place, and cracks, which are widest at the top, will be formed. In a uniform wall the two most prominent fissures ought to be near the ends.
If the horizontal backward and forward movement has a direction oblique to the plane of the wall, the wall will be either overthrown, fractured, or have a triangular fragment thrown off towards the origin from the end last reached.
Should the wave emerge steeply, diagonal fissures at right angles to the direction of transit will be formed, or else triangular pieces will be projected.
The accompanying figures are reduced from Mallet’s ‘Account of the Neapolitan Earthquake of 1857.’
Fig. 18.—Cathedral Church, Potenza (Mallet).
Taking a b as the general direction of the fractures in fig. 18, then c d will represent the direction in which the shock emerged, which is at an angle of 23°·20′ to the horizon. It might be argued that the direction of these fractures was due to the direction in which surface undulations had travelled, or to the relative strengths and proportions of different portions of the building. The directions of cracks in a building are undoubtedly due to a complexity of causes, but for buildings situated in the region of shock the impulsive effect of the shock is probably the most important function to be considered. The method of applying the directions of emergence, deduced from observations on fractures, to determine the origin of a disturbance will be referred to in Chapter X.
Fig. 19.—The Cathedral, Paterno (Mallet). Neapolitan Earthquake of 1857.
Mallet observed that, although two ends of a building might be nearly the same, the fissures and joints do not occur at equal distances from the ends, nor are they equally opened.