Geol 344: Earthqu akes and Seismic Hazards Spring 2008
Home work Set 4: Foca l Mechanism Solut ions: Answe rs
1. The figure below shows focal mechanism solutions for three earthqua kes. The sha ded areas represent the
compressional regions in first arrivals at seismograph stations. The letters P and T refer to the compressional
and e xtensional strain axis orientations, respectively.
a) Draw the lines onto the fault plane solutions to show the orientations of the two fault planes in each
double-couple solution. Don’t forget, fault plane s always intersect the edges of the circle at two points
that are exactly on opposite sides of the circle, 180º apart. (6)
Red lines drawn below represent the double couple solution for each focal mechanism solution. All lines
intersect the edge of the circle at points 180º apart and reach the ir maximum curvature at the midway point
along the line. The fault pla nes represented by each dou ble couple must be oriented 90º a part in 3D space.
(6)
b) If there is any strike-slip motion on any of the faults, indicate this using arrows along the fault lines (i.e.,
indicate whether left-lateral or right-lateral). (4)
In A, there is a left-lateral fault and a right-lateral fault. In B, there are no strike-slip motions. In C, both faults
show a lateral component of motion (one left-lateral and one right-lateral). (4)
