Detecting Deformation

Sunday August 23, 2015
By Rachel Teasdale

Weather 
Bright but overcast skies with calm wind and seas.

What’s happening today? 
The Jason pressure dive continues for the second day. This morning the SCPR (Self Calibrating Pressure Recorder) was recovered, after two years on the seafloor. This afternoon AUV Sentry will be recovered after nearly 24 hours of mapping in the caldera.

Bathymetric map of the Axial Seamount caldera (horseshoe shape open to the south) with north and south rift zones and lavas of the April 2015 eruption outlined with black lines.

Detecting Deformation
As previously discussed, the goal of the pressure dive is to use the array of the Bottom Pressure Recorders (BPRs) to precisely measure the depth of the seafloor at Axial Seamount over time. As magma moves up from deeper sources, the seafloor rises, in a process referred to as inflation. When magma erupts (or moves laterally in the volcano), the seafloor lowers, known as deflation. Following a series of pressure dives in 2011 and 2013, Bill Chadwick, Scott Nooner and their colleagues used their high precision pressure data to determine that since the 2011 eruption, Axial Seamount had inflated approximately 60 cm/yr (1.9 ft/yr).

As pressure surveys have continued to monitor the movement of magma at Axial Seamount, colleagues at the Monterey Bay Aquarium Research Institution (MBARI) have used sonar instruments to map the surface of the seafloor. Multibeam sonar mapping like that described for AUV Sentry (< link to Sentry blog 16 August), works by emitting sound waves that are reflected from the ocean floor and returned to receivers. The time between the emitted and received reflected pulses is used to determine the distance the sonar traveled to the seafloor and back. Maps of the seafloor are color coded, usually with warm colors representing shallow (or high seafloor topography) and deeper (lower) seafloor topography represented by cool colors (see map above).

Image of MBARI’s AUV D. Allan B in 2006.

Axial Seamount is one of the few places where high resolution mapping has been completed prior to and after an eruption. Multibeam sonar surveys before and after the 2011 eruption were completed by MBARI. Their AUV, D. Allan B (see photo left) collected sea floor depth data with 20 cm (0.6 ft) vertical resolution. By subtracting the pre-eruption bathymetry from that collected after the 2011 eruption, they were able to detect depth changes on the sea floor that are greater than 20 cm, including the dimensions of new lava flows and locations of eruptive fissures (see maps below).

Maps of 2011 Axial Seamount lava flows (c) defined from differencing of pre- and post-eruption bathymetry. Right side (d) is 2011 bathymetry with interpreted flow margins (black lines), eruptive fissures (red) and lava flow channels (blue). Ashes is a hydrothermal vent field. Topography is color coded, in m below sea level. Figure modified from Caress et al., 2011.

Given their success with high resolution mapping and improved data processing, David Caress and David Clague of MBARI had the idea that their multibeam sonar mapping might be able to capture the same inflation and deflation measured by Chadwick and Nooner. To test this hypothesis, Caress and the AUV, D. Allan B re-mapped Axial’s surface in 2014. They then subtracted the 2011 survey from their new 2014 mapping and found there had been 1.8 m (5.8 ft) of uplift at the center of the caldera in the three years between surveys, which was consistent with the deformation observed by Chadwick and Nooner from 2011-2013 using pressure sensors. Thus two independent methods for measuring the deformation of the seafloor yielded the same rates of inflation. These two methods are now being used in tandem for the first time during this expedition, because they are complimentary and together can provide new insights into the extent of deformation.

Based on the continuous inflation rates observed from 2011-2013 by the pressure sensors and the continued inflation measured by the MBARI AUV into 2014, Chadwick and Nooner were able to successfully forecast that the next eruption at Axial Seamount would occur sometime in 2015. The April 2015 eruption proved their forecast correct. We will explore more about eruption forecasting at Axial Seamount in an upcoming blog.