Event couple spectral ratio &amp;lt;i&amp;gt;Q&amp;lt;/i&amp;gt; method for earthquake clusters: application to northwest Bohemia

Marius Kriegerowski,Frank Krüger,Simone Cesca,Torsten Dahm,Matthias Ohrnberger

doi:10.5194/se-10-317-2019

Abstract

Abstract. We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q−1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high-frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wave-field seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to northwest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Qp<100. The application to S phases fails due to scattered P-phase energy interfering with S phases. The Qp anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in northwest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations.

Highlights

The intrinsic and scattering attenuation of the amplitudes of seismic waves is described by the dimensionless factor Q
We apply the event couple spectral ratio method to northwest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth
The mapping of spatiotemporal changes of Q is an important step in seismology, since Q is controlled by temperature, rock porosity, fluid saturation and rock composition (Toksöz and Johnston, 1981)

Summary

Introduction

The intrinsic and scattering attenuation of the amplitudes of seismic waves is described by the dimensionless factor Q. The mapping of spatiotemporal changes of Q is an important step in seismology, since Q is controlled by temperature, rock porosity, fluid saturation and rock composition (Toksöz and Johnston, 1981). This factor may help to unravel the possible causes of fluid-induced earthquakes, or thermal anomalies in crustal regions affected by magmatic intrusions. The attenuation structure in the source region of the earthquake swarms is scarcely discussed. The main aim of this study is to test whether the method developed here can enhance the resolution of near-source Q and enable more robust conclusions on source dynamics and the role of fluids in the swarm cycle

Objectives

Methods

Discussion

Conclusion