The geologic fault, like the geologic formation, is equally basic to the science of geology. In Malibu, as elsewhere in California, it is especially important because of its relation to earthquakes and related damage to developed property. However, before considering faults in Malibu as hazards, the fundamental character of the geologic fault should be understood.

DEFINITION
First of all, a fault is an incorporeal surface. It isn't a thing. Rather, it is a place - a place where separate masses of rock, hereinafter "blocks," rub against each other. The operative word is "rub," because the shearing it implies introduces a certain kind of force the analysis of which is vitally important in understanding certain earth processes. Further, this rubbing due to shear movement serves to distinguish a fault from a fissure. On geologic maps, the presence of a fault is indicated by a line called a fault "trace" whereas in the field it is referred to as a fault "contact." A fault contact in the field or a fault trace in a map is the line where the fault surface intersects the ground surface.

FAULT CLASSIFICATION
Faults are classified according to the orientation of the fault surface with respect to the horizon and the directions the blocks move along that surface. The surface may be vertical or at either a steep or a shallow angle with respect to the horizon. Where the surface is at an angle, the face of the upper block is called the "hanging wall" and the face of the lower block is called the "foot wall." Where the movement is normal to the trace and the hanging wall moves downward with respect to the foot wall, the fault movement is called "normal," and where it moves upward it is called "reverse." A reverse fault along a surface with a shallow angle is called a "thrust," although some geologists reserve "thrust" for very large faults. Interestingly enough, there is no consensus on what to call a normal fault acting on a surface with a shallow angle.

In instances where the blocks move along either a vertical or sloping surface without a relative change in elevation, i.e., horizontally, the fault called "strike-slip." When standing on one side of a strike-slip fault and it known that the block on the other side had moved either leftward or rightward, the fault is referred to as either a left-lateral or a right-lateral strike-slip fault. Simple enough. Where matters get a little complicated is trying to determine exactly how, spatially, two points that prior to faulting were adjacent have become separated due to faulting. Different components of such movement on the fault surface are called "slip." The horizontal component is called "strike-slip," the component down the surface in the steepest direction is called "dip-slip," and the component between the two is called the "net slip." Some refer to net slip movement as "net-slip." Some geologists refer to net-slip as "oblique-slip." For the more interested, old tried-and-true Billings (1942) is still the best basic reference on the subject. I have an extra copy I'll let go for a fair price. But I digress.

The reason this stuff is of more than passing interest is because Malibu's major geologic structure, the Malibu Coast fault, is an oblique thrust. This means that if one stands on the block south of it, say at Bluffs Park, the block north of it where Pepperdine campus is located, has moved upward and to the west. Furthermore, this doesn't necessarily mean that only the Pepperdine block has moved upward and to the west. It is possible that both blocks have done so, but that the Pepperdine block has moved farther than the southern Bluffs Park block. All of which comes down to the fact that the Malibu Coast fault is regarded as a left-lateral oblique thrust. Nothing is easy.

RELATION OF FAULTS TO EARTHQUAKES
Not so long ago, say about 60 years, before plate tectonics had been discovered, geologists weren't sure whether faulting causes earthquakes or simply that faulting may accompany earthquakes which are caused by something else such as some sort of thermal phenomenon at depth such as a change in volume accompanying physiochemical alteration.

Now we do know that the sudden release of crustal stress induced by plate movement causes many faults which results in earthquakes. Thus the term "earthquake fault" is reasonably applied to a fault movement along which an earthquake has occurred, although some apply it to faults for which it is only assumed has generated earthquakes. When a fault occurs, it results in the release of energy in the form of waves called seisms. Many small seisms, referred to as "microseisms," occur each day in California, but most are too small to be sensible to humans or other animals. Evidence indicates that some seisms are sensible to some animals but not humans, and it is anyone's guess how frequently seisms occur that are not sensible at all or even recordable. Generally, the term "earthquake" refers only to a seismic event sensible to humans.

SOME IMPLICATIONS OF LARGE FAULTS
A "large" fault is one with a relatively great contact length. Some fault contacts can be recognized for many miles. One of the most well known is the San Andreas fault which extends at least as far south as the northern end of the Gulf of California and at least as far north as Cape Mendocino, a distance of about 650 miles. However, there are other faults in California, of much less length that are important as known sources of earthquakes. Just ask anyone who was in Long Beach in 1934, or San Fernando in 1971, or Northridge in 1994. Similarly, faults of sufficient length are regarded as potential earthquake sources. Ostensibly in this latter category are faults in a zone along the southern base of the Santa Monica Mountains, of which the Malibu Coast fault is one.

Faults in Malibu are of interest not only because they might generate earthquakes, but also in relation to property development. For one thing, it is regarded as prudent to avoid building across a fault contact because renewed movement along would induce structural damage due to shearing. Most structures are not designed to withstand much shear stress. How close to a fault trace a permanent structure may be built is a matter for the local public Building Official to decide. The Building Official acts generally as dictated by the Alquist-Priolo Special Studies Zone Act of 1972, which is an enabling act. Alquist-Priolo, hereinafter the "Act," requires the establishment of a special zone of prescribed width along fault where there is evidence of historic fault movement. Application of the Act is treated in detail in Special Publication 42 originally issued by the California Division of Mines and Geology, now called the California Geological Survey. This is significant for two reasons. One is that the Act requires a seller of property known to be located in a Special Studies Zone to disclose this to a prospective buyer. Another is that a developer is required to show through special geological study that the fault does not present an especially hazardous risk to the proposed construction.

The Act excludes single-family residences, but in Malibu that exclusion is not adopted. As a result, geologic reports required as one element of a building permit application in Malibu must include data from a "seismic trench" which is an exploratory excavation commonly oriented more or less north-south on the assumption that any fault contacts in the site are oriented more or less east-west. In the past, even sites where evidence in adjacent areas demonstrates that no fault of concern could exist in the site, the seismic trench, and the not inconsiderable additional cost it requires, has been mandatory. After all, it's not the City's money.

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