Imaging the magmatic system of Newberry Volcano using joint active source and teleseismic tomography

Abstract

In this paper, we combine active and passive source P wave seismic data to tomographically image the magmatic system beneath Newberry Volcano, located east of the Cascade arc. By using both travel times from local active sources and delay times from teleseismic earthquakes recorded on closely spaced seismometers (300–800 m), we significantly improve recovery of upper crustal velocity structure (<10 km depth). The tomographic model reveals a low‐velocity feature between 3 and 5 km depth that lies beneath the caldera, consistent with a magma body. In contrast to earlier tomographic studies, where elevated temperatures were sufficient to explain the recovered low velocities, the larger amplitude low‐velocity anomalies in our joint tomography model require low degrees of partial melt (∼10%), and a minimum melt volume of ∼2.5 km3. Furthermore, synthetic tests suggest that even greater magnitude low‐velocity anomalies, and by inference larger volumes of magma (up to 8 km3), are needed to explain the observed waveform variability. The lateral extent and shape of the inferred magma body indicates that the extensional tectonic regime at Newberry influences the emplacement of magmatic intrusions. Our study shows that jointly inverting active source and passive source seismic data improves tomographic imaging of the shallow crustal seismic structure of volcanic systems and that active source experiments would benefit from longer deployment times to also record teleseismic sources.

Composition of Magma and Characteristics of the Hydrothermal System of Newberry Volcano, Oregon, From Magnetotellurics

“…Using this m and the 28‐Ωm resistivity used above, we find that a 28‐Ωm magma reservoir would have 11.2% melt with m = 1.5. This gives a range very close to the 8–12% melt inferred seismically (Heath et al, ), demonstrating how jointly interpreting resistivity and velocity can provide constraints on magma reservoir properties. A smaller melt fraction would be achieved using the slightly higher resistivity in our model, but we will show that our modeling has little sensitivity to the exact magma reservoir resistivity once it is more resistive than 25 Ωm.…”

“…Deeper within the caldera we examine the resistivity of the low‐velocity region interpreted as partial melt in previous seismic work (Heath et al, ). The magnitude of the velocity reduction is ~10%, a strong enough anomaly to require partial melt.…”

“…A melt fraction of 8–12% was inferred from a 10% reduction in P wave velocities (Heath et al, ). A larger melt fraction was found to be allowable in the waveform modeling of Beachly et al ().…”

“…R1 = resistive unaltered surface material; R2 = west flank pluton; C1 = conductive hydrothermal alteration zone; C2 = conductive caldera rim; C3 = conductive south rim fluid conduit. Dotted line outlines low‐velocity zone interpreted as magma reservoir by Heath et al (). Vertical lines mark cross section intersection point.…”

Structure, Seismicity, and Accretionary Processes at the Hot Spot‐Influenced Axial Seamount on the Juan de Fuca Ridge

“…The distribution of melt (volume and depth of the reservoir) present beneath Axial can be put into context by comparing it to some other magmatic systems worldwide. Magma at Axial Seamount is stored within a shallower but larger reservoir than at the 9°N overlapping spreading center on the East Pacific Rise (3.6–5.3 km 3 , depth 1.65–2.2 km; Arnulf, Singh, et al, ), Newberry volcano in the Cascade Range (2.5–8 km 3 , depth 3–6 km; Heath et al, ), or Soufrière Hills in the lesser Antilles volcanic arc (~4 km 3 , depth 5.5–7.5 km; Paulatto et al, ), but the volume of melt stored beneath Axial is much smaller than at the Yellowstone supervolcano magmatic system, where two overlapping magma reservoirs (~900 km 3 each, depth 7+ and 22+ km) with a combined volume of melt of ~1,820 km 3 have been imaged (Huang et al, ). In other words, melt storage at Axial Seamount is large compared to typical estimates for plate boundary volcanoes, but relatively modest compared to prominent active hot spot‐dominated intraplate volcanoes.…”