LARGE SLIP VELOCITY OF THE SURFACE RUPTURE ASSOCIATED WITH THE 1990 LUZON EARTHQUAKE

We found more than fifty people who provided important information on coseismic fault motion of the 1990 Luzon, the Philippines, earthquake. About thirty of them ''witnessed'' coseismic fault motion close to (less than 30 m) the surface rupture. The average slip duration reported was about 1 s, and some reported much less than 1 s, which is surprisingly short compared with the predicted value of about 10 s for its size (e.g., fault length: 120 km, slip: 5 m) according to scaling laws obtained from teleseismic studies. Teleseismic studies only obtain spatially and temporally averaged values of fault motion, but actual coseismic fault slip has a high degree of spatial heterogeneity. The large slip velocity over 10 m/s obtained by our study suggests a very high local stress drop (> 100 MPa). Other important information from the eyewitness accounts is that the slip velocity seems to have been nearly constant, implying that the fault slip is well described by a ramp function proposed by Haskell.