OBSERVATIONAL CONSTRAINTS ON EARTHQUAKE SOURCE SCALING - UNDERSTANDING THE LIMITS IN RESOLUTION

I examine the resolution of the type of stress drop estimates that hav e been used to place observational constraints on the scaling of earth quake source processes. I first show that apparent stress and Brune st ress drop are equivalent to within a constant given any source spectra l decay between omega(1.5) and omega(3) (i.e., any plausible value) an d so consistent scaling is expected for the two estimates. I then disc uss the resolution and scaling of Brune stress drop estimates, in the context of empirical Green's function results from recent earthquake s equences, including the 1992 Joshua Tree, California, mainshock and it s aftershocks. I show that no definitive scaling of stress drop with m oment is revealed over the moment range 10(19)-10(25); within this seq uence, however, there is a tendency for moderate-sized (M 4-5) events to be characterized by high stress drops. However, well-resolved resul ts for recent M > 6 events are inconsistent with any extrapolated stre ss increase with moment for the aftershocks. Focusing on corner freque ncy estimates for smaller (M < 3.5) events, I show that resolution is extremely limited even after empirical Green's function deconvolutions . A fundamental limitation to resolution is the paucity of good signal -to-noise at frequencies above 60 Hz, a limitation that will affect ne arly all surficial recordings of ground motion in California and many other regions. Thus, while the best available observational results su pport a constant stress drop for moderate- to large-sized events, very little robust observational evidence exists to constrain the quantiti es that bear most critically on our understanding of source processes: stress drop values and stress drop scaling for small events.