The relative motion of blocks of rock on either side ofa fault zone can be determined by mapping the way thepen on a seismograph moved (up or down on theseismogram) when P-waves first arrived at variousseismic stations adjacent to the fault. This pen motionis called first motion and represent the reaction of theP-wave to dilation (pulling rocks apart) andcompression (squeezing rocks together), as observedon seismograms (Figure 1).

If the first movement of the P-wave was up on aseismogram, then that location (where the seismogramwas made) experienced compression during theearthquake. If the first movement of the P-wave wasdown on a seismogram, then that location wasdilational during the earthquake.

What was the first motion at all of the seismicstations in Figure 2?

Physical GeologyRelative Motion Of Rocks Along A Fault

Adapted from Lab Manual In PhysicalGeology (AGI and NAGT)

NAME___________________________

PERIOD_____

FIGURE 2 Seismograms recorded at stations X, Y, and Zfor a strong (Richter Magnitude 4.6) aftershock of theLoma Prieta, CA, earthquake (USGS). During the quake,little damage occurred at X, but significant damage tohouses occurred at Y and Z.

FIGURE 1 Sketch of typical seismograms forcompressional first motion (first p-wave motion isup) compared with dilational first motion (first p-wave motion is down).

ANSWER: the first movement of the pen wasup for each P-wave, so the first motion at allthree sites was compressional.

By plotting the first motions observed at seismicstations on both sides of a fault that has experiencedan earthquake, a picture of the relative motions of thefault emerges. For example, notice that in Figure 3the first motions observed at seismic stations oneither side of a hypothetical fault are plotted inrelation to the fault. The large open arrows drawn oneither side of the fault show that the motion along thefault proceeded AWAY from the stations wheredilation (D) was recorded (those rocks werestretched), and TOWARD the stations wherecompression (C) was recorded (those rocks weresqueezed). The diagram therefore reveals that BlockX has moved in a southeasterly direction relative toBlock Y as indicated by the half-arrows drawn alongthe fault.

Faults such as this one, where the motions arehorizontal (with little or no vertical displacement) arecalled strike-slip faults. These faults are furthercharacterized by the direction of movement. If you imagine yourself standing on one of the faultblocks and facing the fault, a fault in which the block opposite you moves to the right is called aright lateral strike-slip fault. If the block opposite moves to the left, the fault is classified as a leftlateral strike-slip fault. What type of strike slip fault is drawn in Figure 3?

(ANSWER: The relative motion of the opposite block is to the left, so it is a left lateral strike slip fault.)

A REAL EXAMPLE

The New Madrid Fault System is located withinthe Mississippi Embayment, a U-shapedgeological feature filled with Mesozoic andCenozoic rocks and surrounded by Paleozoicand Precambrian rocks (Figure 4). Faults of theNew Madrid System are not visible at thesurface of the earth because they occur in oldPaleozoic and Precambrian rocks that areburied by younger Mesozoic and Cenozoicsediments. Such faults are called blind faults.

The main fault of the New Madrid System isdrawn on Figure 4. It is well know because aseries of strong earthquakes occurred along it in1911 and 1912. One of these earthquakes wasthe strongest earthquake ever recorded in NorthAmerica, and the potential for more strong

FIGURE 3 Map of a region showing a fault along whichan earthquake has occurred, and the P-wave first motions(C=compresssional, D=dilational) observed for theearthquake at seismic stations adjacent to the fault.Stress moves away from the field of dialtion and towardthe field of compression on each side of the fault (largeopen arrows), so the relative motion of the fault is asindicated by the smaller half-arrows.

FIGURE 4. Map of the Mississippi Embayment showing thelocation of the main fault of the New Madrid (Blind) FaultSystem, numbered seismic stations (as in Figure 5), stateboundaries, and the city of Memphis (M).

earthquakes here is a lingering hazard. The locations of thirteen seismic stations are also plotted onFigure 4. Seismograms from those stations from an earthquake along the fault are provided inFigure 5.

FIGURE 5. Seismograms from the 13 numbered seismic stations in the Mississippi Embayment after an earthquakethat occurred in the New Madrid Fault System.

PROCEDURE

Use the seismograms in Figure 5 to determine whether the first motions at each station weredilational or compressional, and plot this information on the enlarge copy of Figure 4 below bywriting either “D” (for dilational) or “C” (for compressional) beside each station.

Use the data you’ve plotted to draw the appropriate half arrows, in their proper places, showing thedirections of motion along the fault.

What type of fault is this?__________________________________

In terms of plate tectonics, what is unusual about the location of this fault?_____________________

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