Unified Scaling Law for Earthquakes: Space-Time Dependent Assessment in Friuli-Venezia Giulia Region

Anastasia Nekrasova,Antonella Peresan

doi:10.3389/feart.2020.590724

Abstract

The concept of the Unified Scaling Law for Earthquakes (USLE), which generalizes the Gutenberg-Richter relationship making use of the fractal distribution of earthquake sources in a seismic region, is applied to seismicity in the Friuli-Venezia Giulia region, FVG (Northeastern Italy) and its surroundings. In particular, the temporal variations of USLE coefficients are investigated, with the aim to get new insights in the evolving dynamics of seismicity within different tectonic domains of FVG. To this purpose, we consider all magnitude 2.0 or larger earthquakes that occurred in 1995–2019, as reported in the catalog compiled at the National Institute of Oceanography and Applied Geophysics (OGS catalog), within the territory of its homogeneous completeness. The observed variability of seismic dynamics for three sub-regions of the territory under investigation, delimited based on main geological and tectonic features, is characterized in terms of several moving averages, including: the inter-event time,τ; the cumulative Benioff strain release, Ʃ; the USLE coefficients estimated for moving six-years time intervals, and the USLE control parameter,η. We found that: 1) the USLE coefficients in FVG region are time-dependent and show up correlated; 2) the dynamical changes ofτ, Ʃ, andηin the three sub-regions highlight a number of different seismic regimes; 3) seismic dynamics, prior and after the occurrence of the 1998 and 2004 Kobarid (Slovenia) strong main shocks, is characterized by different parameters in the related sub-region. The results obtained for the FVG region confirm similar analysis performed on a global scale, in advance and after the largest earthquakes worldwide. Moreover, our analysis highlights the spatially heterogeneous and non-stationary features of seismicity in the investigated territory, thus suggesting the opportunity of resorting to time-dependent estimates for improving local seismic hazard assessment. The applied methods and obtained parameters provide quantitative basis for developing suitable models and forecasting tools, toward a better characterization of future seismic hazard in the region.

Highlights

Capturing the main features of seismicity in a region is not an easy task
We investigate the changes in seismic regime during the time period from 1995 to 2019 for the three sub-regions (A1, Area 2 (A2) and Area 3 (A3)) of the Friuli-Venezia Giulia (FVG) territory (Figure 1A)
The space variability of the Unified Scaling Law for Earthquakes (USLE) coefficients, namely the seismicity rate (A), the earthquake magnitude exponent (B), and the fractal dimension of epicenter loci (C), was examined in some detail, comparing the values obtained within three sub-regions defined grouping seismic districts with similar tectonic features (Bressan et al, 2019)

Summary

Introduction

While average properties of earthquakes occurrence over large areas can be defined in pretty robust way, when zooming to a smaller territory, the specific features from limited data sets may prevail, and space and time regularities may be lost. The smaller is the region and data time span, the less representative (and verifiable) is the resulting seismicity “portrait,” with obvious consequences on earthquake hazard assessments and forecasting. In this study we aim to analyze, within the Friuli Venezia Giulia region (Northeastern Italy) and surrounding areas, a set of quantitative measures of seismic activity, which have been investigated in other regions worldwide, so as to verify their possible similarity/generality and identify their peculiar features. The instrumental seismic activity recorded since prevalently consists of low to moderate earthquakes, only sporadically exceeding magnitude 4.0, and with the largest recorded event having magnitude 5.6

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Methods

Conclusion