Overlain traces showing one multiplet from the 12/04/1997. Note the fact that although the waveforms are extremely similar (see normalised traces) the amplitude variation is extremely large (see overlain non-normalised traces)
Low-frequency earthquakes are a common feature of the ongoing eruption of the Soufriere Hills volcano, Montserrat. These earthquakes are believed to be caused by a resonance of the magma conduit, setting up waves at the interface between a solid and a fluid phase.
A feature of these events is that they cluster into groups with extremely similar waveforms (Green and Neuberg, 2003) known as multiplets. Multiplets have been observed at a number of previous volcanic eruptions and the main use of these groups has been to produce extremely good relative locations of events (e.g. Fremont and Malone 1987, Got et. al. 1994).
Overlain traces showing one multiplet from the 12/04/1997. Note the fact that although the waveforms are extremely similar (see normalised traces) the amplitude variation is extremely large (see overlain non-normalised traces)
At Montserrat within a single multiplet the events can have variable amplitude yet retain almost exactly the same waveform shape. As the waveforms are so similar the source location for each of the events within a multiplet must be extremely localised, for a regular brittle failure earthquake this would correspond with the quarter-wavelength hypothesis.
This data is taken from station MBGA for the swarm on the 19th/20th May 1997, starting at 15:00 on the 19th. The figure depicts 12 hours, including a low-frequency earthquake swarm, split into 4 segments of 3 hours each starting with the top panel. Each multiplet, or waveform family is represented by a different colour and each event by a narrow vertical bar.
Viewed in this manner it is difficult to distinguish any patterns, however when plotted with the tilt the families are seen to be well ordered. The waveform families start at approximately the same time as the maximum in derivative of tilt and then evolve, with new families being triggered throughout the swarm, until families stop when the derivative of tilt reaches a minimum. If the derivative of tilt is taken to be a proxy for the rate of pressurisation this leads us to conclude that the low-frequency earthquakes trigger, or are a consequence of, depressurisation within the volcanic conduit.
At Leeds we are using these multiplets to improve source locations of low-frequency earthquakes, and linking their occurence to different phases of the ongoing eruption with the help of accurate observational records taken alongside seismic data collection (see Watts et. al. 2002).
References:
Fremont, M.-J. and Malone, S. D., 1987. High Precision Locations of Earthquakes at Mount St. Helens, Washington. JGR 92(B10) 10233-10236.
Got, J.-L., Frechet, J. and Klein, F. W., 1994. Deep Fault plane geometry inferred from multiplet relative relocation beneath the south flank of Kilauea. JGR 99(B8) 15375-15386.
Green, D. and Neuberg, J., 2004. Waveform classification of low-frequency earthquake swarms and its implication. submitted to JVGR.
Green, D.N. and Neuberg, J., 2003. Waveform analysis of low-frequency earthquakes, with examples from prior to the July 12th 2003 dome collapse, Montserrat. Working group of the European Seismological Commission Annual Workshop 2003 " Seismic signals related to volcanic unrest".
Watts, R.B., Herd, R.A., Sparks, R.S.J. and Young, S.R., 2002. Growth patterns and emplacement of the andesitic lava dome at Soufriere Hills Volcano, Montserrat. from The Eruption of Soufriere Hills Volcano, Montserrat, from 1995 to 1999. Geological Society Memoirs 21, 115-152.