Source parameter scaling and radiation efficiency of microearthquakes along the Irpinia fault zone in southern Apennines, Italy

Abstract

We analyzed the P and S wave displacement spectra of 717 microearthquakes in the moment range 4 × 109 - 2 × 1014 N m recorded at the dense networks operating in southern Apennines (Italy) and deployed along the 1980 Ms 6.9 Irpinia earthquake fault zone. Source, attenuation, and site parameters are estimated by using a parametric modeling approach, which is combined with a multistep, nonlinear inversion strategy. We found that in the analyzed frequency band, an attenuation model with constant Q has to be preferred to frequency-dependent Q models. Consistent estimates of the median P and S quality factors Q̃P=167 (90; 296) and Q̃S=226 (114; 417) are obtained from two different techniques and relatively high values of QS/QP (median value 1.3, (0.8; 2.1)) are found in the same depth range where high VP/VS and a peak in seismicity distribution are observed. This is the evidence for a highly fractured, partially, or completely fluid-saturated medium embedding the Irpinia fault zone, down to crustal depths of 15-20 km. A nearly constant stress drop (Δσ̃=1.4MPa, (0.4; 5.0)) and apparent stress (τã=0.1MPa, (0.03, 0.4)) scaling of P and S corner frequencies and seismic energies is observed above a seismic moment value of about 10 11 N m. The measured radiation efficiency is low (ηSW̃=0.06, 0.030.13), e.g., the radiated energy is only a small fraction of the whole energy spent by friction and fracture development. A large positive dynamic overshoot (high dynamic shear strength) can be the dominant mechanism controlling the microearthquake fractures along the Irpinia fault zone. ©2014. American Geophysical Union. All Rights Reserved.