Magma system recharge of Mount St. Helens from precise relative hypocenter location of microearthquakes

Abstract

[1] Four hundred forty-seven well-recorded earthquakes at Mount St. Helens during the late 1990s are relocated with high precision using a combination of a cross-correlation technique and then a procedure to obtain a one-dimensional velocity structure solution together with station corrections. The resulting high-resolution pattern of the spatialtemporal evolution of the earthquake activity, along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5.5-10 km, connected to the surface by a thin vertical conduit. Moreover, the distribution of the events below 5.5 km yielded a much clearer view of the structures that generate the seismicity at that depth. Some of the deeper events occur along a sequence of faults under the influence of a radial stress field; however, a large number of the events occur on a NNE-SSW striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault.

Attenuation and scattering tomography of the deep plumbing system of Mount St. Helens

“…We do not use absolute amplitude information to obtain exact measurements of direct attenuation ( Q ) and scattering ( g ): we simply obtain variations of these two parameters with respect to an average. The exponential decrease of the coda envelopes, the energy ratios, and, more generally, the shape of the envelopes in the considered time windows (see supporting information (SI)) are similar if we use either the two horizontal or the vertical components, as shown by both Musumeci et al [] and Waite et al []. As this is true for different source types, we assume that the high‐scattering characteristics of the heterogeneous volcano are sufficient to justify the use of vertical sensors for our application, even if those are mainly composed of S waves [ Sato et al , , chapter 2.4.1].…”

Multiplets: Their behavior and utility at dacitic and andesitic volcanic centers

“…[3] Studies including multiplets on volcanoes have largely leveraged the similarity between events to calculate highly precise locations and illuminate structures at depth [e.g., Got et al, 1994;Musumeci et al, 2002]. Fewer studies have studied the dynamic behavior of multiplets to assess conditions within a single volcano Grêt et al, 2005;Petersen, 2007;Umakoshi et al, 2008].…”

Mount Hood Earthquake Activity: Volcanic or Tectonic Origins?

“…10), that these earthquakes are being generated within a different stress regime than earthquakes elsewhere beneath the volcanic edifice and that the stress regime is less uniform than for other earthquake groups. These events, similar to shallow events directly under Mount Rainier (Giampiccolo et al, 1999) and the shallow events at Mount St. Helens (Musumeci et al, 2002) have distinct properties that are different than events away from their respective volcanoes: their shallow depths, nonuniform focal mechanisms, and diffuse distribution suggest that these earthquakes are likely generated by volcanic processes within and under Mount Hood.…”

“…It has been used extensively to relocate earthquakes in volcanic regions. Examples include Rowe et al (2002) for Montserrat, West Indies, Rubin et al (1998) for Kilauea, and Musumeci et al (2002) and Fremont and Malone (1987) for Mount St. Helens. The method works because earthquakes with similar locations and source mechanisms produce similar seismograms at a given station.…”