How geology can influence scaling relations

Abstract

This study presents an attempt to determine the influence of the environment (through lithology and mechanical properties) on source parameters and, subsequently, scaling relations. This aim is pursued using data collected from a gas field during a 3-year-long period. The induced seismicity is recorded by a local permanent network, allowing the precise location of 149 earthquakes with local magnitudes ranging from 1.0 to 4.2. Moreover, geology of the site is defined by 70 bore-holes up to 5 km deep. Such information allows a geological correlation of seismic events. Then, a spectral analysis is performed to compute the source parameters of earthquakes, using a dynamic circular fault model. Seismic moments, source radii, and post-rupture stress drops are evaluated, taking into account source, propagation and site effects. However, it is shown from our results, that relative uncertainties remain, especially for seismic moments and stress drops. The diagram of seismic moment versus source radius, for the whole data set, indicates relatively constant stress drops (around 10 bar), confirming the self-similarity assumption even for microearthquakes. In contradiction with this result, scaling relations in each geological unit do not verify self-similarity This feature implies that second order factors, as the mechanical properties of the rock, influence source characteristics and bring new complications in scaling relations. However, even in this case of a very differentiated medium, these parameters cannot explain a global breakdown in scaling laws.