← Back to Sessions

The Combination of Long- and Short-Period "Deformation" Signals in Meter-scale Hydraulic Fractures Provides New Insights Into the Cause of Induced Seismicity

Session: Mechanisms of Induced Seismicity: Pressure Diffusion, Elastic Stressing and Aseismic Slip I

Type: Oral

Date: 4/19/2021

Presentation Time: 02:45 PM Pacific

Description: 

During decameter-scale high-pressure injection experiments at Äspö Hard Rock Laboratory broadband seismometers installed in the near-field of the fractured rock volume (at distances of ~20 m and ~60 m, respectively) recorded peculiar long-period signals. A clear correlation among long period signals and pressure and flow rate time series and the lack of long period signals on the vertical component support the interpretation as tilt-induced signals generated by the aseismic growth and opening of the hydraulic fractures. Similar signals on broadband seismometers were often reported in volcanic environments and attributed to tilt resulting from inflations or deflations of dykes or magma chambers. We find that the amplitude of the tilt signal correlates with the volume of water injected during the experiments, while the tilt duration correlates with the duration of the injection process. We are able to forward model a first-order approximation of the observed tilt signals by two different approaches: (1) tensile rectangular faults of constant opening (Okada solution in elastic full space) and (2) the theoretical response of a seismometer to an acceleration step to simulate the tilt-induced excursion on the horizontal components, a technique used in our laboratory for the calibration of seismometers. We show that the tilt signals provide an independent constraint on the the fracture orientations and aseismic growth for each individual injection and re-injection experiment. The orientation and the extent of the fracture planes in the Okada models are in agreement with orientations inferred indirectly from the clustering of acoustic emissions and their migration, which has been estimated by using a network of 11 piezoelectric sensors surrounding the injection borehole. The simultaneous measurement of deformation, tilt and seismicity, therefore, provides new insights in understanding hydraulic-fracturing-induced seismicity.

Presenting Author: Peter Niemz

Student Presenter: Yes

Authors