Modeling high frequency waves in the slab beneath Italy

Tomographic images indicate a complicated subduction system beneath Italy. Gaps in fast velocity anomalies within the subducting lithosphere are interpreted as deep slab tears. The detailed shape and location of the tears are important for kinematic reconstructions and evolution of the subduction system. However, the tomographic images which are produced by smoothed, damped inversions will underestimate the sharpness of the structures. Here, we use the records from Italian National Seismic Network (IV) to study the detailed slab structure. The waveform records for stations at southern Italy show large amplitude, high frequency (f > 5 Hz) late arrivals with long coda after relatively low-frequency onset for both P and S waves. High frequency arrivals are the strongest from events of approximately 300 km depth and correlate spatially within the slab inferred from fast P-wave velocity perturbations. This behavior agrees with previous studies from other tectonic regions, suggesting the high frequency energy is generated by small scale heterogeneities within the slab which act as scatterers. Using a 2-D finite difference (FD) code, we calculate synthetic seismograms to search the scale and velocity perturbation of the heterogeneities. The 2-D FD code can simulate seismic wave propagation to frequencies as high as 10 Hz accurately for our model setup with grid size of 0.05 km and time steps of 0.002 s using a Graphics Processing Unit (GPU). Our preferred model of the slab heterogeneity beneath Tyrrhenian Sea has lamella structure parallel to the slab dip and can be described by a von Karmann function with a downdip correlation length of 10 km and 0.5 km in thickness provided the standard deviation of Vp fluctuations within the slab is about 3-4%. These scatterers could originate in many ways. Their slab-parallel orientation suggests they activate structures formed within the oceanic lithosphere before subduction, as the orientation of the stress field and the pathways of fluid and melt flow during subduction are unlikely to be slab-parallel. During formation of oceanic upper mantle lithosphere in a mid-ocean ridge environment, several mechanisms are available to create strong lithologic heterogeneities of appropriate scale and aspect ratio, including reactive melt migration, shear-band driven melt segregation and deep crystal fractionation. All such structures, if formed near a ridge, would be transposed to slab-parallel orientation by corner flow. Although this idea predicts that these structures are present everywhere beneath oceanic plates, it is plausible that the reactions associated with subduction (pressure increase, dehydration, partial melting) are required to activate them and generate seismic velocity contrast. Formation of subcontinental lithosphere by stacked emplacement of slabs could then be a unifying explanation for high-frequency scatterers beneath continents and in active subducting slabs.