Directivity Plot for Petrolia M4.3
02/24/2012 21:17 PST
The PGA and PGV directivities are limited (ν ≡ 0.92β) and poorly constrained because there are no stations to the west, offshore of the earthquake. Both rupture directions are mostly horizontal and strongly favor the EW striking nodal plane as the fault plane, despite the large uncertainties. The earthquake occurred on the Mendocino Fracture Zone, where it extends to depth east of the Triple Junction. The nodal planes are projected on the upper hemisphere. All the stations used are within 120 km of the fault.
The residual PGAs and PGVs are plotted on whole stereonets. Symbols indicating the amplitude of the residual peak ground motions are plotted at the azimuths and vertical takeoff angles of the S-waves. Orange circles indicate amplified ground motions; green crosses indicate deamplified ground motions. Because we restrict the station distance, most takeoff angles are vertically upward, but the takeoff angles to the more distant stations can be slightly downward. The thick magenta lines show the rupture direction. The longer the line, the more horizontal the rupture direction. Upward rupture directions end in arrows; downward rupture directions end in stars. The thin magenta lines around the rupture directions enclose the 2σ uncertainty areas: these uncertainty areas can extend into both the upper and lower hemispheres. The blue lines show the nodal planes of the Berkeley Seismological Laboratory Moment Tensor, which are projected onto the upper or lower hemisphere, depending on the rupture direction. The sets of three numbers indicate the strike and dip of the nodal plane and the angle between the rupture direction and the nodal plane. If you are unfamiliar with nodal planes and stereoscopic projections, there is an excellent explanation titled Fault Plane Solutions. Remember that nodal planes are determined from P-wave first motions, while the directivity is determined from S-wave peak motions.