The word "earthquake" in daily use usually refers to a perceptible shaking of the earth. When used in the term "earthquake distribution", however, the word has a different meaning. "Earthquake" in the latter sense means the source of the shaking of the earth, and refers to an event that occurs underground. In this report, the term "earthquake" will be used in the latter sense. We will use the term "seismic ground motion" in the sense of a general shaking of the earth.

The shaking of the earth (ground motion) results when a seismic wave is transmitted from the site where the earthquake occurs, and this wave shakes the earth. The source that causes this wave, or the essence of the earthquake itself, has been accurately identified only recently, during the 1960s. This essence is the fault movement that releases the strain energy accumulated in the bedrock (rock mass) underground.

Let's review the process in the order in which this happens. First, a strong force is exerted on the rock mass underground for some reason. This rock mass gradually becomes deformed. At the same time, energy accumulates in the form of strain in the rock mass. To visualize this, think of a rock being compressed by a giant press. When, the rock mass can no longer withstand the continually mounting pressure, a rupture occurs. The strain energy that has built up is violently released in the form of a seismic wave. Earthquakes are what happen when this phenomenon occurs underground. If it had no support, the rock under stress would crumble and fly apart at the instant of rupture. The rupture of rock mass underground, however, is considerably different from this sort of situation and will generally occur as slip along a certain plane. This may be more difficult to visualize than the more ordinary conception of rupture, but the rupture that occurs underground and causes an earthquake consists of a rapid slipping movement along a plane between giant slabs of rock mass. Planes of this type within the rock mass are thought to be existing weak planes (fault planes) that caused slipping in the past. Hereafter, we will refer to the portion of the rock mass slabs in the weak plane that slip as the "area of rupture". Also, the slipping movement of the rock mass on both sides of this plane will be termed "fault movement". A fault refers to a condition in which the fault movement on the originally continuous strata and rock mass results in the slipping with a certain plane as a boundary. The basic types of faults are classified by the type of slip, and are known as "normal faults", "reverse faults", and "lateral faults".

The velocity of rupture propagation - the speed of the expansion of the rupture zone - is extremely fast. In locations that are not deep underground (in the earth's crust), this speed is about 2 or 3 km per second. (In fact, seismic waves are transmitted at an even faster speed.) The speed at which both sides of the rock mass slip against each other is slow, between 10-100 cm per second (Fig.2-9). We will refer to the range of maximum expansion of the rupture zone - the entire range over which fault movement occurs - as the "focal region".

From the proceeding, we can see that earthquakes do not occur at a single point, but spread along a plane. It is easy to picture in the mind's eye that an earthquake's magnitude will increase as the extent of the fault movement grows (and the extent of the rupture grows) where rock mass is alike. The amount of slipping volume and the breadth of the area over which the fault movement occurs determine the extent of the fault movement. If the fault is viewed as a rectangle, this breadth can be expressed by the length in a horizontal direction (fault length) and the length in an angular direction (width) (Fig.2-10).

Next, we'll use an example to examine fault movement. The 1995 Hyogo-ken Nanbu Earthquake (Southern Hyogo Prefecture Earthquake) (M 7.2) that caused the Great Hanshin-Awaji Earthquake Disaster was an earthquake of M 7 class. An analysis of all the data shows that the fault length of this earthquake was about 40-50 km, and the width was about 15 km. The value of the slip was about 1 to 2 m. Recent research has enabled us to make a rather specific estimate of the distribution of slip on the fault plane, as shown in Fig.2-11. A look at these figures shows areas with substantial slip, and areas with very little slip. Also, the Kanto Earthquake of 1923 (M 7.9) that caused the Great Kanto Earthquake Disaster was nearly M 8. The fault length of this earthquake was about 100 km, and the width was about 50 km. The slip is estimated to have been about 5 to 7 m.

The hypocenter announced by the Japan Meteorological Agency (JMA) immediately after the earthquake occurred is the location where slip first occurred, i.e., the point where the rupture started (Fig.2-9). The hypocenter of an earthquake is determined by the time required for the first earthquake waves (P waves and S waves) to reach a set of observation points. The hypocentral region of a large earthquake is very large, so points at a distance from the hypocenter will be subject to strong ground motion if the fault movement has extended to a nearby location.

Fig.2-1, Fig.2-2, Fig.2-3, Fig.2-5, and Fig.2-6 show the location of earthquakes using the hypocenter. We are convinced that for large earthquakes, however, fault movement occurs over a rather large area, including the hypocenter. Fig.2-12 uses current observations to show the focal regions for the primary destructive earthquakes of the past 100 years. Fig.2-12 shows the fault model for earthquakes that occurred on land and the source area of tsunami for earthquakes that occurred at sea. Most of the faults in the focal region of the earthquakes that occurred on land are roughly vertical. Therefore, as the figures shows, the actual focal region appears smaller than it actually is, when projected at ground level. In contrast, the source area of tsunami indicates the range in which a tsunami is generated, and tends to appear larger than the actual underground focal region.

Generally, when a large earthquake (main shock) occurs, many somewhat smaller earthquakes follow. These are known as aftershocks. Most of these aftershocks occur in the focal region of the main shock. The distribution of aftershocks - in particular those that occur immediately after the main shock (from several hours to one day later) - indicate very clearly the location of the focal region of the main shock. Therefore, the focal region of the main shock (the area in which the fault movement occurred) can be determined by the aftershock distribution (Fig.2-13). Many aftershocks immediately follow the main shock, but these decrease with time. It is now understood that the manner of this decrease is quite regular. Most aftershocks are of a magnitude of 1 or smaller than the main shock - even aftershocks of the largest magnitude. However, sometimes there are aftershocks of a magnitude approaching that of the main shock.