What is a normal fault? A normal fault is a common geological structure in the earth's crust. It is a fracture phenomenon caused by the horizontal tensile stress on the crustal rocks. During the formation of a normal fault, the rocks in the upper part of the crust are vertically displaced relative to the rocks in the lower part along a fracture plane. After the fault is formed, the upper plate drops relatively and the lower plate rises relatively. This type of fault usually appears as a cliff or steep slope in the topography.

　　1. Characteristics of a normal fault

　　1.1 The fault plane is inclined and the upper plate rises relatively.

　　1.2 The inclination of the fault plane is usually steep, generally above 45 degrees.

　　1.3 The rocks on both sides of the fault may be dislocated due to fracture and displacement.

　　1.4 Normal faults are often associated with extension areas of the crust, such as rift valleys, basins and other geological structures.

　　The inclination of the cross section of a normal fault is steep, generally about 45 to 90 degrees, often greater than 45 degrees; the space occupied by the two plates of rock in the horizontal direction perpendicular to the fault direction is elongated. The rock fragmentation in the normal fault zone is relatively weak, breccias are mostly angular, ultra-fractured rocks are less obvious, and there are usually no complex small folds formed by strong compression.

　　Most normal faults are believed to be formed under the action of gravity and horizontal tension, so they are also called gravity faults. It can develop alone, or it can form a specific combination of a series of faults within a certain range and on a specific geological background. Common combinations include:

　　1.1 Graben: mainly composed of two normal faults with basically the same strike and tilted in opposite directions, with a common descending plate between the two normal faults;

　　1.2 Horst: mainly composed of two normal faults with basically the same strike and opposite tilt directions, with a common ascending plate between the two normal faults;

　　1.3 Step fault: composed of several normal faults with basically the same strike, and the upper plates of each fault fall in the same direction in turn, forming a step shape;

　　1.4 Ring fault: several arc-shaped semi-ring faults are arranged around a center or in concentric circles;

　　1.5 Radial fault: several faults are arranged toward a center or radially;

　　1.6 En echelon fault: several nearly parallel normal faults are staggered in an oblique manner;

　　1.7 Block fault: Two groups of large and medium-sized normal faults with different directions cut each other to form rectangular grid-shaped or diamond-shaped fault blocks.

　　Normal faults in nature vary in size, ranging from the dislocation of rock layers to the formation of mid-ocean ridges across the ocean. Some large-scale normal fault combinations are the result of regional extension, reflecting tectonic extension. In terms of landforms, large normal faults often form rifts, gullies, river valleys and lakes. For example, the Great Rift Valley of East Africa was formed in a tensile environment caused by plate movement, and normal faults are on both sides of it. In addition, in the Basin and Range region of the western United States, there are also landforms with alternating basins and mountains formed by normal faults.