Robert
RobertI had to ask a friend, Dr. Gillian Norton, who has been working at the Montserrat Volcano Observatory. She isn't a seismologist either, but here is her answer: Hi Scott Mmmm Not being a seismologist, I'll give it my best shot!! A hybrid earthquake is a mixture (hence the name hybrid) between a long period earthquake (usually indicative of fluid movement) and a volcano-tectonic earthquake (high frequency signal, usually indicative of rock fracture). So a hybrid earthquake has a wide range of frequencies. Long period events, VTs and hence hybrids generally have a sudden onset. A hybrid is tentatively interpreted as magma forcing its way to the surface, having to break its way through rock (?solidifying dome) and then flowing through the pathway created. (But really the seismologists are only hypothesising!!). Most of the hybrids on Montserrat occur within the top 1-2 km beneath the volcano, and therefore they are thought to be earthquakes within the dome or the conduit close to the dome, whereas VTs sometimes occur deeper (approx up to 8 km, near the supposed magma chamber). A swarm is really just a collection of a large number of earthquakes within a certain time period. I'm not sure what the threshold is for a swarm precisely, but we usually term it a swarm when there is maybe more than 1 event per 30 minutes. I have to admit that MVO has changed its terminology of earthquakes with time, due to the recognition that what were called VTs early in the eruption did have significant low freuqency components. As I'm sure you can appreciate there is a whole range of frequency spectra and our terminology is an attempt to simplify the counts. And in fact what we call hybrids may be called other things at other volcanoes!! That is her answer. I hope it is helpful.
Sincerely,
Scott Rowland
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