Low-frequency Seismic Signals Research Articles

Amplification of moment and strain energy release due to interaction between different size fault slip zones

Previous work of Rudnicki and Kanamori, which used collinear crack solutions to examine quantitatively the effects of fault slip zone interaction on moment, stress drop, and strain energy release, is extended by considering the effects of interaction between different size slip zones. The calculations demonstrate that the presence of a large preexisting slip zone can substantially amplify the low frequency seismic signal due to slip on a nearby smaller slip zone. For example, if the length of the larger slip zone is l, that of the smaller is l/10, and the distance between the nearest ends of the zones is l/100, the seismic moment due to slip on the larger zone induced by slip on the smaller zone is about 5 times that due to slip on an isolated zone of length l/10. Furthermore, because of the interaction between slip zones, the moment due to slip on the smaller zone is about 2.5 times the value for an isolated zone of the same size and subjected to the same effective stress. Conversely, estimates of stress drop based on the measured value of the moment and the total length on which slip occurs substantially underestimate the actual stress drop. Although the stress drop on the larger slip zone is zero, the induced slip there does contribute to WO, the difference between the strain energy change and the frictional work. In fact, it is demonstrated that WO = τeM/2μ where τe is the effective stress, M is the actual value of the seismic moment, and μ is the shear modulus.

Transient Water Flow in Subglacial Channels

AbstractA hydraulic mechanism is postulated as the source of certain low-frequency (1-5 Hz) seismic signals that are recorded in the vicinity of temperate glaciers. To illustrate that the mechanism is a plausible seismic source, the equations of motion and continuity for transient flow in a subglacial conduit are developed. The solution to these equations is obtained for the case of the flow of water being terminated in a glacial conduit. Using reasonable values of physical parameters taken from the literature, the displacement of the conduit walls is determined as a function of time and distance along the tunnel. The spectral character of the proposed source and the period of time over which it operates is compared with seismic records of typical type II signals. The agreement in terms of these parameters is good.To further examine the validity of the proposed seismic source a preliminary energy calculation is made which demonstrates that the proposed mechanism is energetically reasonable. Evidence of hydraulically generated seismic waves from a dam site is introduced to support the theory of a hydraulic source.Based on the evidence presented, it is suggested that the flow of water from glaciers is modulated by an internal flow regime that is intermittent in time and space.The substance of this paper is included in a report by the present authors entitled “Hydraulic transients: a seismic source in volcanoes and glaciers”, published in Science, Vol. 203, No. 4381, 1979, p. 654–56.

Transient Water Flow in Subglacial Channels

Abstract A hydraulic mechanism is postulated as the source of certain low-frequency (1-5 Hz) seismic signals that are recorded in the vicinity of temperate glaciers. To illustrate that the mechanism is a plausible seismic source, the equations of motion and continuity for transient flow in a subglacial conduit are developed. The solution to these equations is obtained for the case of the flow of water being terminated in a glacial conduit. Using reasonable values of physical parameters taken from the literature, the displacement of the conduit walls is determined as a function of time and distance along the tunnel. The spectral character of the proposed source and the period of time over which it operates is compared with seismic records of typical type II signals. The agreement in terms of these parameters is good. To further examine the validity of the proposed seismic source a preliminary energy calculation is made which demonstrates that the proposed mechanism is energetically reasonable. Evidence of hydraulically generated seismic waves from a dam site is introduced to support the theory of a hydraulic source. Based on the evidence presented, it is suggested that the flow of water from glaciers is modulated by an internal flow regime that is intermittent in time and space. The substance of this paper is included in a report by the present authors entitled “Hydraulic transients: a seismic source in volcanoes and glaciers”, published in Science, Vol. 203, No. 4381, 1979, p. 654–56.

Applying microtremor analysis to identify hydrocarbon reservoirs

R. Holzner, P. Eschle, H. Zürcher, and M. Lambert of Spectraseis, R. Graf of Proseis, and S. Dangel and P. F. Meier of University of Zurich describe an innovative passive technology for identifying the presence of geological structures containing hydrocarbon by analyzing low frequency seismic signals. They illustrate the method with results from a pilot study in Brazil.