Figure G-59 contains a map showing the position of the Mendocino triple junction off the coast of northern California. The Mendocino triple junction is the point of intersection of the Pacific, North American, and Juan de Fuca Plates. Two earthquakes (event A and event B) occurred near the triple junction. They could have been centered on either of two transform boundaries or on the convergent boundary between the Juan de Fuca and North American Plates. The table presents data for the two earthquakes recorded at 25 seismic stations. Using two sheets of tracing paper (one sheet for each event) and your equal-area net (Fig. G-11), do the following: for each earthquake: 

(1) Construct a focal-mechanism plot, 

(2) Determine the plate boundary along which the earthquake occurred, 

(3) Determine the attitude of the fault plane, 

(4) Use arrows to show the slip sense along the fault, and 

(5) Plot the P (shortening) and T (extension) axes.

Figure G-59

Fig. G-11

Pacific Plate transform- Juan de Fuca Plate triple junction San Andreas Fault (transform) subduction zone Oregon California North American Plate Data recorded at twenty-five seismic stations for two earthquakes with epicenters near the Mendocino triple junction. Azimuth Incidence Station (degrees) (degrees) 1 234in 2 3 5 6 67 7 8 9 10 11 12 13. 269 262 255 256 233 254 258 244 218 148 25 0 347 66 290 46 271 275 261 266 116 101 232 5 48 4 31 157 161 45 44 36 21 14 17 21 19 90 40 31 73 51 14 85 61 64 50 34 51 First motion First motion Event A Event B 56 55 88 14 15 16 17 18 19 20 21 22 N.C. 23 D 24 25 N.C. C, compression first motion; D, dilational first motion; N, nodal; N.C., nodal (weakly compressional); N.D., nodal (weakly dilational) CDDNDDUUZUAUUAUZZUUD D N.D. N.D. D N.D. N.C. N.D. CO3Q SCUDDUAAAAC