Absolute and relative locations of earthquakes at Mount St. Helens, Washington, using continuous data: Implications for magmatic processes

This study uses a combination of absolute and relative locations from earthquake multiplets to investigate the
seismicity associated with the eruptive sequence at Mount St.
Helens between September 23, 2004, and November 20, 2004.
Multiplets, a prominent feature of seismicity during this time
period, occurred as volcano-tectonic, hybrid, and low-frequency earthquakes spanning a large range of magnitudes and
lifespans. Absolute locations were improved through the use
of a new one-dimensional velocity model with excellent shallow constraints on P-wave velocities. We used jackknife tests
to minimize possible biases in absolute and relative locations
resulting from station outages and changing station configurations. In this paper, we show that earthquake hypocenters shallowed before the October 1 explosion along a north-dipping
structure under the 1980-86 dome. Relative relocations of
multiplets during the initial seismic unrest and ensuing eruption showed rather small source volumes before the October 1
explosion and larger tabular source volumes after October 5.
All multiplets possess absolute locations very close to each
other. However, the highly dissimilar waveforms displayed by
each of the multiplets analyzed suggest that different sources
and mechanisms were present within a very small source
volume. We suggest that multiplets were related to pressurization of the conduit system that produced a stationary source
that was highly stable over long time periods. On the basis
of their response to explosions occurring in October 2004,
earthquakes not associated with multiplets also appeared to be pressure dependent. The pressure source for these earthquakes
appeared, however, to be different from the pressure source of
the multiplets.