Implications of the Scaling of Dynamic Strains for Earthquake Triggering

Understanding the significance and the underlying physics of dynamic triggering requires knowledge of the characteristics of the causative triggering deformations. Thus, I examine the scaling of dynamic strains with source-site distance and the source size using peak ground velocities (PGV), which serve as proxies for dynamic strains, measured for earthquakes with magnitudes from M4.4 to M7.9 at distances ranging from 0.1 km to 5300 km. The square root of the rupture area provides a useful measure of rupture dimension, and at distances less than this, PGVs increase more and more gradually as the fault is approached, converging to ~150 cm/sec regardless of magnitude. At large distances from the source PGVs vary approximately as the inverse of the distance-squared. Notably, the PGV dependence on magnitude vanishes when the distance is scaled by the inverse of the rupture dimension for each earthquake. All these observations may be explained simply, by considering a constant stress drop per unit fault area, so that at large distances the velocities scale with the total rupture area, and as the fault is approached, the effective source area gets smaller. The scaled observations imply that peak dynamic strains within aftershock zones (within a few rupture dimensions), where triggering is obvious, are no different than those that caused clear triggering out to remote distances (many rupture dimensions) following the M7.1 Hector Mine, M7.3 Landers, and M7.9 Denali earthquakes. These results also suggest that remote triggering may require exceptionally large dynamic deformations, perhaps due to strong directivity, thus explaining its apparent rarity. Another implication of the invariance of the scaled distance dependence of PGVs is that the frequency content of the triggering deformation is at best, of second order importance. For example, the PGVs measured within the aftershock zone of a M4.4 earthquake are nearly the same as those for the aforementioned three, M7.1 and greater, earthquakes at a site of remotely triggered seismicity despite the large differences in their frequency contents.