 Also, construction codes and methods have evolved, making much of built environment stronger; these make a given intensity of ground shaking seem weaker.And it is now recognized that some of the original criteria of the most intense degrees (X and above), such as bent rails, ground fissures, landslides, etc., are "related less to the level of ground shaking than to the presence of ground conditions susceptible to spectacular failure".The categories "catastrophe" and "enormous catastrophe" added by Cancani (XI and XII) are used so infrequently that current USGS practice is merge them into a single category "Extreme" abbreviated as "X+".