Microseismicity around Orca submarine volcano, Bransfield Strait,
Antarctica, during 2019-2020 from OBS data
Abstract
Orca seamount is located in the Bransfield Strait, between the South
Shetland Islands and the Antarctic Peninsula. The volcano developed on
an extensional rift produced by a combination of slab rollback at the
South Shetland trench and transtensional motions between the Scotia and
Antarctic plates. From January 2019 to February 2020, the BRAVOSEIS
project deployed a dense amphibious seismic network in the Bransfield
region, comprising both land and ocean bottom seismometers (OBS), as
well as moored hydrophones. We perform an analysis of the seismicity
recorded in the area of Orca volcano using a subnetwork composed of 15
OBS around Orca seamount and its SW rift, covering a region of about 20
km x 10 km, with inter-station distances of ~4 km. OBS
data are organized, visualized and analyzed using the SEISAN software
package. Earthquake detection was achieved through an STA/LTA algorithm.
We use a visual procedure including spectral analysis and filtering to
discriminate local earthquakes from other types of signals. For
earthquake location, we use P and S phase arrivals and a layered model
derived from previous geophysical studies in the region. In this way, we
identify and locate around 3000 earthquakes with magnitudes in the range
from -1 to 3. There is a continuous background level of microearthquake
activity, although a large part of the earthquakes occurred during a
swarm in June-July 2019. Source depths are mostly concentrated in the
first 10 km (within the crust). The epicentral distribution covers the
whole area around the volcano, but it is clearly densest in the NE
flank, where an intense seismic series started in September 2020.