Maddy Corbin and Jürgen Neuberg
Introduction
The precise cause of low frequency seismic events is not fully understood. It is generally accepted however that the low frequency coda is produced by resonance in a fluid, although the shape and character of the fluid is still under debate. In the 'Leeds model' low frequency seismic events are explained by resonance in the volcanic conduit, triggered by shear fractures of viscous magma at the conduit wall.
Moment tensors - A quantitative representation
The trigger mechanism for low frequency earthquakes has been derived through a conceptual model. Moment tensors can be used to quantitatively represent the source mechanisms of earthquakes. For a given earthquake, its moment tensor describes the equivalent forces, which, if acting on a point source at the earthquakes hypocentre, would produce the same displacements at a seismic station as the earthquake.
Moment tensor inversion
In order to obtain the moment tensor of an earthquake and thus have a quantitative representation of its source mechanism a technique called 'moment tensor inversion' is employed: several seismograms produced by a single earthquake are inverted for the source time function and moment tensor components.
The displacement recorded at the seismogram is a convolution of 3 components: the source mechanism (S), the greens function (G) and the response of the seismometer (R) (see diagram).
In order to solve for the source mechanism, the other components must be known. The response of the instrument is usually known and can be easily deconvolved. The greens function is the impulse response of the earth which contains all path effects. This can be obtained through forward modelling which requires a good knowledge of the velocity structure of the earth.
With the greens functions calculated the equation can then be solved for the source mechanism and the components of the moment tensor can be defined.
Application to low frequency earthquakes
There are distinctly two parts to low frequency earthquakes recorded on Montserrat a high frequency, low amplitude onset (the trigger mechanism), followed by a low frequency harmonic coda. The source mechanisms of the two parts are thus thought to be different and so it is intended to invert the two sections separately to obtain a moment tensor for the trigger mechanism and for the harmonic coda. This will give further understanding of the processes which generate low frequency events, helping to constrain the conceptual model, and will eventually help us to quantify magma ascent rates.