GEOL 106: Earthquake Country Activity 06: Focal Mechanisms
Figure 1 shows the “beachball” representation of a fault. Seismologists refer to the direction of slip in an earthquake and the orientation of the fault on which it occurs as the focal mechanism or fault plane solution. They use information from seismograms or from measurements of fault slip to calculate the focal mechanism and typically display it on maps as a "beachball" symbol. Figure 1. The three main categories of faulting and the associated “beachball” plot. The beachball projection is constructed by imagining a small sphere, called the focal sphere, around the hypocenter and imagining about how the fault slip will deform the sphere. Some areas of the sphere will be compressed or squeezed by the offset. Other areas will be stretched or extended (figure 2). Figure 2. Cross‐section view of focal spheres for normal and reverse faults. Imagine a small sphere around the earthquake hypocenter (star). When slip occurs on the fault, the shaded zones will be extended or stretched and the white zones will be squeezed. The points T show the direction of maximum tension and the P shows the maximum pressure or compression. 1 | P a g e GEOL 106: Earthquake Country Activity 06: Focal Mechanisms
Understanding beachballs requires visualizing them in three dimensions. Figure 2 shows cross sections, looking at the faults from the side. Now imagine that you are looking down on the imaginary sphere from the top. The middle diagrams in figure 3 attempts to show what this looks like. The top of the sphere is a little dome and the fault cuts across it. The light areas of the dome are regions where the sphere is compressed, the point P is the point of maximum pressure. The dark areas of the dome are under tension and the T marks the point of maximum tension or stretching. The diagram on the right shows the final beachball, flattening the dome so it becomes a simple representation of the areas of compression and tension. Figure 3. Going from the focal sphere to beachball for normal faulting (top) and thrust faulting (bottom). Left: cross section. Middle: view of the focal sphere looking down from the top. Light areas show regions of compression and dark areas show tension. Right: final beachball, red line is the fault, P is direction of maximum pressure and T is maximum tension. The boundaries between the zones of compression and dilatation are called nodal planes. One of the nodal planes always corresponds to the true fault plane. The other nodal plane, perpendicular to the fault plane is called the auxiliary plane. Figure 4: Nodal planes. There are always four quadrants when a fault rupture – two compression and two tension. The boundaries between them are nodal planes. 2 | P a g e GEOL 106: Earthquake Country Activity 06: Focal Mechanisms
Strike‐slip faults Strike‐slip faults involve fault motion parallel to the earth’s surface along nearly vertical faults. The auxiliary plane is also vertical. The beach ball pattern shows four equal quadrants – two areas of compressions (shaded area) and two areas of dilatations (white areas). The example shown at the right is a right lateral strike‐slip fault – looking across the fault the rock on the other side appears to move to the right. Arrows on the beachball show the fault motion. Note that the arrows point from a dilation quadrant to a compression quadrant. Strike‐slip fault If we did not know which nodal plane was the fault, it would not be possible to distinguish the fault plane from the auxiliary plane from the focal mechanism alone. Normal faults Normal faults involve extension and fault slip along a fault usually dipping at an angle of about 60° relative to the ground surface. The footwall slides down relative to the hanging wall and the focal mechanism looks like the side view of a beach ball with zones of compression on the outside and a zone of dilatation in the middle (figure below left). Normal fault Reverse faults Reverse faults involve shortening of the crust. The hanging wall moves up and over the footwall. The focal mechanism is the opposite pattern as the normal fault – areas of dilatation on the outside and compression in the inside (above right) Reverse fault 3 | P a g e GEOL 106: Earthquake Country Activity 06: Focal Mechanisms
Determining focal mechanisms In the 1960s, seismologists realized thatthe direction of the initial P‐wave was an indication of whether the slip on the fault was pushing the wave toward or away from the station. If an earthquake was recorded by a number of stations surrounding the epicentral region, the first motion of the P‐wave, whether it moved the ground up or down, could be used to estimate the focal mechanism. The figure below shows the initial P‐wave motion on seismograms recorded in the region of a strike‐slip earthquake. Note how the first motions are up in the shaded quadrants because the fault motion stretched the ground outward towards these stations. In the white quadrants, the first motion is down. Focal mechanisms today are routinely determined by analyzing the surface wave signal as recorded on broadband seismographs. More about this in the next set of notes on intensity and magnitude. Figure 8. Focal mechanism solution for 4 December 2008 crustal earthquake with seismograms showing dilatational (stars) and compressional (circles) picks plotted on focal sphere. (Mulcahy et al., 2013)
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Name (Last, First): __________________________________ Date: _______________ For each of the LOWER‐hemisphere, focal mechanisms, draw the sectional view (below the horizontal line) as indicated by the straight line with arrows: hint the side view. 1. What type of mechanism does this represent? Hint (tensional, compressional, etc) Enter your answer here ______________________. a) What are the directions and approximate dips of the faults? b) On what type of boundary would you expect to find this mechanism ? 2. What type of mechanism does this represent? Hint (tensional, compressional, etc) Enter your answer here ______________________. a) What are the directions (strikes) and approximate dips of the faults? b) b. On what type of boundary would you expect to find this mechanism ? 5 | P a g e GEOL 106: Earthquake Country Activity 06: Focal Mechanisms
3. What type of mechanism does this represent? Hint (tensional, compressional, shear) Enter your answer here ______________________. a) What are the directions (strikes) and approximate dips of the faults? b) On what type of boundary would you expect to find this mechanism? c) What type of fault has a NW‐SE trend? 4. Below are some first arrivals plotted on a blank beachball diagram (Ozacar and Beck, 2004). Use your knowledge of focal mechanisms to draw a beach ball that fit the first motion data. 6 | P a g e GEOL 106: Earthquake Country Activity 06: Focal Mechanisms
5. Below is a map (Ozacar and Beck, 2004) that shows the regional faulting information for the earthquake that created the first motion data in #4. Focal mechanisms (and moment tensors) provide enough data to plot two possible fault plane solutions (a primary fault plane and an auxiliary one). Geologists must use their knowledge of the regional tectonics to help interpret which fault plane solution is the correct one. a. Given the information in the map, which is the correct fault plane in your focal mechanism plot? (i.e. the north‐south fault or the east‐west fault) Enter your answer here ______________________. Label the primary and auxiliary fault planes in your focal mechanism diagram above. b. What type of fault is the Kunlun fault? ______________________ References: Mulcahy, P., Chen, C., Kay, S.M., Brown, L.D., Isacks, B.L., Sandvol, E., Heit, B., Yuan, X., and Coira, B.L., 2013. Central Andean mantle and crustal seismicity beneath the Southern Puna plateau and the northern margin of the Chilean‐Pampean flat slab in Tectonics, v. 33, doi: 10.1002/2013TC003393 Ozacar, A.A. and Beck, S.L., 2004. The 2002 Denali Fault and 2001 Kunlun Fault Earthquakes: Complex Rupture Processes of Two Large Strike‐Slip Events in BSSA, v. 94, no. 6B, p. S278‐S292. 7 | P a g e