How Does Seismic Rupture Accelerate? Observational Insights From Earthquake Source Time Functions

Observation of the seismic process for a large earthquake population is of key interest to detect potential magnitude-dependent behaviors and, more generally, to quantify how seismic rupture develops. In contrast with studies focusing on the first radiated waves, we here propose to characterize the growing phase leading to the main seismic moment release episode(s), which we refer to as the development phase. Our analysis uses the 2,221 teleseismic source time functions (STFs) of shallow dip-slip earthquakes provided by the global SCARDEC database and consists in measuring the moment acceleration during the development phase at prescribed moment rates. This approach is therefore insensitive to hypocentral time uncertainties and aims at quantifying how seismic ruptures accelerate, independently of when they accelerate. Our results first show that rupture acceleration does not exhibit any magnitude-dependent signal emerging above the intrinsic measurements variability. We thus use the full STF catalog to characterize the moment rate. M d of the development phase and show that, on average,. M d (t) ∝ t n d with n d equal to 2.7. This time evolution therefore does not follow the steady t 2 growth expected for classical circular crack models, which indicates that stress drop and/or rupture velocity transiently vary during the development phase. We finally illustrate with a synthetic STF catalog that, due to initial rupture variability, approaches based on hypocentral time are not expected to fully characterize the behavior of the development phase.