A Comparison of Geodetic and Geologic Rates Prior to Large Strike-Slip Earthquakes: A Diversity of Earthquake-Cycle Behaviors?

Abstract

Comparison of preevent geodetic and geologic rates in three large-magnitude (Mw = 7.6–7.9) strike-slip earthquakes reveals a wide range of behaviors. Specifically, geodetic rates of 26–28 mm/yr for the North Anatolian fault along the 1999 MW = 7.6 Izmit rupture are ∼40% faster than Holocene geologic rates. In contrast, geodetic rates of ∼6–8 mm/yr along the Denali fault prior to the 2002 MW = 7.9 Denali earthquake are only approximately half as fast as the latest Pleistocene-Holocene geologic rate of ∼12 mm/yr. In the third example where a sufficiently long pre-earthquake geodetic time series exists, the geodetic and geologic rates along the 2001 MW = 7.8 Kokoxili rupture on the Kunlun fault are approximately equal at ∼11 mm/yr. These results are not readily explicable with extant earthquake-cycle modeling, suggesting that they may instead be due to some combination of regional kinematic fault interactions, temporal variations in the strength of lithospheric-scale shear zones, and/or variations in local relative plate motion rate. Whatever the exact causes of these variable behaviors, these observations indicate that either the ratio of geodetic to geologic rates before an earthquake may not be diagnostic of the time to the next earthquake, as predicted by many rheologically based geodynamic models of earthquake-cycle behavior, or different behaviors characterize different fault systems in a manner that is not yet understood or predictable.