The strength of the seismic ground motion ordinarily decreases with the greater the distance from the focal region. In some locations, however, distance from the hypocenter alone does not determine the strength of the ground motion. The ground near the earth's surface is also a major influence. Generally, structures on ground with a thick alluvium or on weaker ground, such as land created by landfills, undergo more violent motion and suffer greater damage. Therefore, ground of this type is suspected to be a factor when unexpected destruction occurs in locations some distance from the hypocenter. The strength of the ground motion depends on the direction of the progression of the fault movement, the distribution of the slip, and other factors. The region's underground structure is also a factor. For example, strong ground motion is sometimes generated in the direction of rupture propagation due to the interaction of the rapid progression of the rupture and the diffusion of the seismic waves. In the 1994 Earthquake far off the Coast of Sanriku (M 7.5), the fault movement began directly west of the Japan Trench. It moved in a westerly direction and reached off the east coast of Hachinohe (Hatinohe) City (Fig.2-13A). It seems likely that the fault movement progressing in the general direction of the city was one factor behind the strong ground motion at Hachinohe (seismic intensity 6 in JMA scale). In the 1995 Southern Hyogo Prefecture Earthquake, fault movement progressed from the vicinity of the Akashi Channel (Akasi Kaikyo) to both the Awaji Island (Awazisima) and toward Kobe City (Fig.2-13B). Factors believed to account for the extensive destruction in the outskirts of Kobe City are the progression of the fault movement in the direction of the city and the local amplification of the earthquake wave by the underground structure.

The circumstances of the destruction become more complex when we consider how the effects of ground motion combine with other natural factors.

For example, the phenomenon of liquefaction frequently occurs due to strong ground motion in soft ground with a sandy stratum and substantial water content. Liquefaction tends to occur when strong motion loosens the bonds between the grains of sand all at once, and the entire stratum becomes fluid. At this time, the fluid soil sometimes rises to the earth's surface (see Fig.2-25, Sand Blow Phenomenon). As a result, the ground loses the ability to support structures. Buildings and bridges with a high specific gravity sink, and buoyancy causes underground embedded pipes, manholes, and other objects with a low specific gravity to float to the surface, resulting in serious damage. When liquefaction occurs, the ground does not merely lose the capacity to support structures; the liquefied stratum also shifts extensively in a lateral direction as if in a landslide. This results in damage when embankments collapse.

Seismic ground motion also causes slope failures and landslides. This destroys buildings on sloping surfaces and causes destruction in surrounding areas. Sometimes, debris flow occurs as a result of a large slope failure. In the 1984 Western Nagano Prefecture Earthquake (Nagano-ken Seibu Earthquake) (M 6.8), a large slope failure took place south of the summit of Mt. Ontake (Ontakesan). In addition, the large volume of dirt that collapsed started an avalanche and flowed about 10 km downstream, leading a wide path of destruction. When slope failures and debris flows occur, rivers are blocked and secondary disaster results from collapsing embankments. Smaller collapses also occur frequently, such as the caving in of dirt and rock from ground motion. This results in small pockets of localized destruction. Sometimes, ground motion triggers slow landslides over a wide area on gently sloping land. During the 1995 Southern Hyogo Prefecture Earthquake, localized damage was sustained in the hilly area of Kobe due to the cracking that accompanies landslides.

Ground motion and rainfall are some of the factors that cause slope failures and landslides. Underlying these phenomena, however, are natural factors as the local geology, topography, and ground water. Slope failures and landslides also occur due to aftershocks after a main shock and rainfall.