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

We analyzed the P and S wave displacement spectra of 717 microearthquakes in the moment range 4 x 10(9) - 2 x 10(14) N m recorded at the dense networks operating in southern Apennines (Italy) and deployed along the 1980 M-s 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 (Q) over tilde (P) = 167 (90; 296) and (Q) over tilde (S) = 226 (114; 417) are obtained from two different techniques and relatively high values of Q(S)/Q(P) (median value 1.3, (0.8; 2.1)) are found in the same depth range where high V-P/V-S 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 ((Delta sigma) over tilde = 1.4 MPa, (0.4; 5.0)) and apparent stress ((tau(a)) over tilde = 0.1 MPa, (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 ((eta(SW)) over tilde = 0.06; (0.03; 0.13)), 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.