FOCAL MECHANISM STUDIES: AN EXPLANATION

By

ROGER W. SHERBURNE and CHRIS CRAMER

California Division of Mines and Geology

Many of our readers may not be familiar with earthquake mechanism studies or the significance of the "beach balls" used in graphic illustrations. The followtng article provides a simplified explanation of earthquake mechanism studies and fault plane solutions......Editor.

INTRODUCTION

Most geologic processes are slow, so slow that usually they cannot be easily observed but must be carefully measured. The upward movement of mountains is caused by the action of forces within the earth's crust and mantle; occasionally, the motions will be sudden-an earthquake. Earthquakes provide the seismologist with an opportunity to view the force system which is responsible, at least in part, for the mountains and valleys of the earth's surface. The seismological study of the displacements and forces at the focus of an earthquake are commonly referred to as focal (or earthquake) mechanism studies.

INTERPRETATION

The cosnmon approach to focal mechanism studies of shallow earthquakes is to use the first motion of the P-wave which is either up or dawn and probably the most easily determined feature of a recorded earthquake. The process is to project data written by seismographs at the earth's surface backward to the source area of the earthquake. The data are usually displayed upon a focal sphere, an imaginary sphere that surrounds the earthquake focus (the point on the fault where rupture initiates).

In Figure 1 the zones of compressional (P-wave up) and dilatational (P-wave dawn) first motion that result from an earthquake on a right lateral strike slip fault are shown on a model Earth. A P-wave recorded by a seismograph located in a stippled area has an up (compressional) first motion and those seismographs located in the unstippled areas record a down (dilatational) first motion. Note that there are four quadrants which are alternately compression and dilatation. These quadrants are separated by two perpendicular planes (nodal planes), one of which is the fault plane and the other is called the auxiliary plane

The result of a first motion study, often refered to as a fault plane solution, is displayed as a stereographic projection of the lower hemisphere on the equatorial plane of the focal sphere. This is simply a flat representation of the lower half of the focal sphere as viewed from directly above the focus (at the epicenter). In the fault plane solution of a vertical strike slip fault the nodal planes form two perpendicular lines intersecting at the center of the projection (Figure 2).

The first motion studies provide two possible solutions for the fault plane. For example, the distribution of compressional and dilatational quadrants for a right lateral strike slip fault trending west of north (Figure 2) is the same as a left lateral fault trending north of east. Geological information, such as rupture of the earth's surface, is needed to determine which of the two nodal planes is the fault plane and which is the auxiliary plane.

If the motion is not pure strike slip but includes a component of dip-slip motion, the auxiliary plane, which is Perpendicular to the direction of movement, is tilted toward the circumference (Figure 3). For pure dip-slip motion on a vertical fault the auxiliary plane is the equatorial plane (Figure 4).

In the foregoing only a vertical fault plane has been considered and its representation in the lower hesnlsphere of a stereographlc projection is a line passing through the center of the diagram. For a non-vertical fault such as a normal fault (Figure 5) or a reverse fault (Figure 6) the nodal plane wtuch corresponds to the fault plane is tilted toward the circumference.

An example ofa focal mechanism study appeared in CALIFORNIA GEOLOGY, December 1983, Figure 3, p. 273. The fault mechanism illustration may be explained by applying the generalizations above (Figure 7, Table 1).

Figure 1. The zones of compression and dilatation for a right lateral strike slip fault are shown on a model Earth. A P-wave recorded by a seismograph located in a stippled area will have an up (compressional) first motion while a P-wave recorded in an unstippled area will have a down (dilatational) first motion. A close-up of the epicentral area is shown.

Figure 2. Fault plane solution for a vertical strike-slip fault

Figure 3. Fault plane solution of a vertical oblique-slip fault (part strike slip and part dip slip).

Figure 4. Fault plane solution for a vertical dip-slip fault

Figure 5. Fault plane solution of an east-dipping normal fault.

Figure 6. Fault plane solution of an east-slipping reverse fault.

Figure 7. Fault plane solutions for six earthquakes along the Tanlu fault in China. From B.A. Bolt and R.B. Darragh, "The Tanlu Fault System," CALIFORNIA GEOLOGY, December, 1983, Figure 3, p. 273.

Table 1. Explanation of fault plane solutions in Figure 7.