Geodetic constraints on postseismic deformation following the 1990 M-s 7.8Luzon earthquake and implications for Luzon tectonics and Philippine Sea plate motion

[1] We have obtained three epochs of high-quality dual-frequency GPS data a t 15 stations in the Luzon, Philippines, region since the July 16, 1990, M- s 7.8 earthquake, from which we generate horizontal velocity fields at the Earth's surface for the 1993-1996 and 1996-1998 periods. We use these veloc ities, plus 1996-1998 GPS velocity results reported elsewhere from 13 addit ional stations, to show that present-day deformation in Luzon is dominated by strike-slip motion along the Philippine fault system. The measured strik e-slip rate across central Luzon is faster than the expected long-term slip rate on the fault, which we attribute to postseismic deformation following the 1990 earthquake. The 1993-1996 and 1996-1998 deformation patterns cann ot be statistically distinguished within the similar to 10% uncertainties o f the data. If the observed deformation is interpreted using an elastic hal f-space model with uniform slip below a subsurface locking depth, we find a fault slip rate of 40 mm/yr and a locking depth of similar to 15 km. This is too fast to be a steady interseismic rate, as it disagrees with what is known from paleoseismic and other evidence about earthquake recurrence and long-term slip rate on the Philippine fault system in central and northern Luzon. Also, a 15 km locking depth for steady interseismic slip is probably too shallow compared to the depth of rupture in the 1990 earthquake. If th e deformation is instead interpreted with two types of viscoelastic model, we find that the observed velocities can be fit well for a range of values of lower lithosphere viscosity and long-term slip rate. Using a two dimensi onal (2-D) viscoelastic coupling model, the minimum allowable lower lithosp here viscosity is 0.5 x 10(19) Pa s, with an associated long-term slip rate of 15-22 mm/yr. Faster long-term slip rates also fit well, with correspond ingly higher values of viscosity. With a possibly more realistic model that includes 3-D viscoelastic effects we find preferred long-term velocities i n the range 20-35 mm/yr and viscosities in the range 2-6 x 10(19) Pa s. Our preferred viscosities correspond to Maxwell times on the order of 7-20% of the typical earthquake recurrence interval and are several times higher th an values found for a number of Californian strike-slip earthquakes. Other implications of our 1993-1998 surface velocities are that the Philippine Se a-Eurasia (PH-EU) Euler vector of Seno et al. [1993] is a better descriptio n of the PH-EU convergence rate than a Euler vector recently estimated from GPS velocities in the northern PH and that the great majority of the norma l component of PH-EU plate convergence is taking place west of Luzon, presu mably largely at the Manila Trench. We also find that Luzon is rotating cou nterclockwise relative to PH at 1 degrees -2 degrees /Myr, only similar to 25% of the rate recently suggested by other workers on the basis of limited GPS data.