High Rates of Inflation During a Noneruptive Episode of Seismic Unrest at Semisopochnoi Volcano, Alaska in 2014–2015

Abstract

Magma intrusion rate is a key parameter in eruption triggering but is poorly quantified in existing geodetic studies. Here we examine two episodes of rapid inflation in this context. Two noneruptive microseismic swarms were recorded at Semisopochnoi Volcano, Alaska in 2014-2015. We use differential SAR techniques and TerraSAR-X images to document surface deformation from 2011 to 2015, which comprises island-wide radial inflation totaling~25 cm (+/−1 cm) line of sight displacement in 2014-2015. Multiple source geometries are tested in an inversion of the deformation data, and InSAR data are best fit by a spheroid trending to the northeast and plunging to the southeast, with a major axis of~4 km and minor axes of~1 km, directly under the central caldera of Semisopochnoi. In 2014, a modeled influx of 0.043 km 3 of magma caused line of sight displacement of~17 cm. This magma was stored at a depth of~8 km, until 2015 when 0.029 km 3 was added. Along with the definition of inflation source parameters, the recorded seismic events are relocated using differential travel times. These relocated events outline a linear aseismic area within a larger zone of shallow (<10 km) seismicity. This aseismic region aligns with the centroid of the deformation model. Based on these geodetic and seismic models, the plumbing system at Semisopochnoi is interpreted as a spheroidal magma storage zone at a depth of˜8 km below a linear feature of partial melt. The observed deformation and seismicity appear to result from rapid injection into this main storage region.

Monochromatic Long-Period Seismicity Prior to the 2012 Earthquake Swarm at Little Sitkin Volcano, Alaska

“…The largest earthquake (M2.7) did not occur until November 15 and there were 22 earthquakes with magnitude greater than two; however, none of those happened on the first two days of the swarm, when the rate was highest. The rates, magnitudes, and durations of the swarm at Little Sitkin are similar to other recent non-eruptive swarms at nearby Semisopochnoi volcano (DeGrandpre et al, 2019).…”

Structure of the magma plumbing system beneath Semisopochnoi Island (Aleutian Arc) inferred from seismic tomography

“…Several thousand recorded volcano-tectonic events may have marked magmatic intrusion; however, none resulted in a magmatic eruption. Interferometric synthetic aperture radar (InSAR) observations were used to evaluate ground deformations during this unrest, and DeGrandpre et al 24 reported more than 25 cm of total inflation during 2003 and up to June 26, 2014, of which more than half occurred in the ten days between June 15 and 26, 2014. In June, the inflation rate reached 6 mm per day 24 , with inflation occurring as a simple radial pattern centered inside the caldera with a slight northeast–southwest oblique skew.…”

“…Interferometric synthetic aperture radar (InSAR) observations were used to evaluate ground deformations during this unrest, and DeGrandpre et al 24 reported more than 25 cm of total inflation during 2003 and up to June 26, 2014, of which more than half occurred in the ten days between June 15 and 26, 2014. In June, the inflation rate reached 6 mm per day 24 , with inflation occurring as a simple radial pattern centered inside the caldera with a slight northeast–southwest oblique skew. After June 26, 2014, the ground deformation almost ceased until resuming during another episode of seismic activity from January to May 2015 23 , 24 .…”

“…In June, the inflation rate reached 6 mm per day 24 , with inflation occurring as a simple radial pattern centered inside the caldera with a slight northeast–southwest oblique skew. After June 26, 2014, the ground deformation almost ceased until resuming during another episode of seismic activity from January to May 2015 23 , 24 . Although the total number of events was considerably lower compared to 2014, some of the events had relatively large magnitudes between M 2.0 and 2.8, with depths of up to 10 km 22 .…”

“…Although the total number of events was considerably lower compared to 2014, some of the events had relatively large magnitudes between M 2.0 and 2.8, with depths of up to 10 km 22 . Based on the observed InSAR measurements and relocated seismicity, DeGrandpre et al’s 24 inverse modeling estimated the locations of the deformation sources for both episodes. They associated the observed ground deformations with magma influx from a depth of ~ 8 km directly below the caldera.…”