Myth-Busting

Thursday August 20, 2015
By Rachel Teasdale

Weather:
Winds and seas have been gradually easing and are expected to be 15-20 knots with waves at 4-6 ft.

What’s happening today?
CTD casts continued overnight in the northern part of Axial’s caldera; AUV Sentry was launched and recovered today to test some operational systems and for preliminary work along the caldera wall. We completed another CTD cast in the International District hydrothermal field, and Jason was launched this afternoon for a 24-hour dive in the caldera.

Myth-busting:
When news of the April 2015 eruption became widely known, questions emerged regarding the possible relationship of the eruption to other geologic events. Here we address some of the questions we’ve been asked. Be sure to submit your questions to the blog using the “Send us your Questions” link at right, or here. Read on to see some of the themes that have come up already:

1.    Will the eruption of Axial Seamount have any effect on geologic activity on the west coast, for example does it make a large earthquake more likely?

Map of Axial Seamount on the Juan de Fuca Ridge and the tectonic plates in the NE Pacific.

Response: The eastern boundary of the Juan de Fuca plate is subducting beneath the North American plate off the coast of British Columbia, Washington, and Oregon and has the capacity to generate large earthquakes. Earthquakes occur as the rocks of the two plates get locked against each other, building stresses over hundreds of years that eventually get released during an earthquake. Axial Seamount is located on the western boundary of the Juan de Fuca plate (see map at right), approximately 400 km (250 mi) west of the subduction zone, which is so far away that any stresses imposed on the west side of the plate would not be transmitted to the east side (see cross section below). Thus, volcanic activity at Axial Seamount does not directly influence the earthquake hazard in the Cascadia subduction zone and does not make an earthquake there any more or less likely than before.

Cross section (side view) of Axial Seamount, the Juan de Fuca spreading ridge, and the Cascadia subduction zone. Image courtesy of the Center for Environmental Visualization, University of Washington.

2.    When Axial erupts, could it generate a tsunami?
Response:  Tsunamis result when a large vertical movement of the ocean floor displaces the water above it suddenly, usually associated with large subduction earthquakes. The resulting wave of seawater, the tsunami, can travel across the ocean rapidly and grow to such large sizes when they hit the shore that the waves are catastrophic, as was the case in Japan after the magnitude 9.0 earthquake in 2011. Earthquakes associated with the movement of magma below Axial Seamount are generally much smaller (usually only up to magnitude 4) than any that would generate a tsunami. The vertical movements associated with inflation and deflation at the volcano is too gradual to cause a sudden displacement of water required to generate a tsunami. The elevation change of the surface of Axial Seamount during eruptions is about 2 to 3 m (6 – 10 ft), which occurs over the course of a few days, so this is too slow a movement to generate tsunami waves.

3.    Is there any connection between the eruption at Axial Seamount and the “Warm Blob” of water in the northeast Pacific?
The unusually warm sea surface temperatures that have characterized the northeast Pacific Ocean this year have been referred to as the “Warm Blob,” and it has disrupted the food chain off the west coast of North America since late 2013. This anomalously warm water extends from Mexico to Alaska (see map below).  Since the “Warm Blog” pre-dates the eruption at Axial Seamount (which occurred in April 2015), the eruption is clearly not the cause.  The sea surface temperature anomaly of 1-4 °C (2-7 °F) is also much greater than any change in water temperature that could be associated with the eruption. Heat escaping from the lava flows does heat the water near the seafloor, but it can only rise a few hundred meters above the bottom. The warm water venting from the new lava flows is buoyant and so will rise, but as it does it mixes with cold seawater and is diluted until it becomes neutrally buoyant and no longer rises. Thus, it is almost impossible for the heat from seafloor hot springs at Axial Seamount to make it to the ocean surface.

Map of sea surface temperature (SST) anomaly showing the “Warm Blob” in the NE Pacific. Image from NOAA.

4.    Would someone on a boat above Axial Seamount know it was erupting?
A ship sailing over Axial Seamount during an eruption would not likely have any indication that anything was happening on the seafloor. The caldera is approximately 1500 m (4800 ft) below the surface and the 2015 lava flows were erupted at depths ranging from 1400-1700 m (4480-5440 ft). If the ship happened to have instruments that it could lower down to near the seafloor (like our CTD instrument package, or perhaps a hydrophone), that would be the only way to detect an eruption.  Otherwise, the volcano is just too deep to detect the activity at the surface.

For more information on earthquakes in the Pacific Northwest see:
USGS Hazards and Pacific Northwest Science Network: http://pnsn.org/
New Yorker: http://www.newyorker.com/magazine/2015/07/20/the-really-big-one

For more information on how tsunamis are generated, see:
USGS (United States Geological Survey) Tsunami Information and animations: http://walrus.wr.usgs.gov/tsunami/ 
NOAA (National Oceanographic and Atmospheric Administration) Tsunami information: http://www.tsunami.noaa.gov/index.html

For more information on the northern Pacific Ocean “Blob” of warm sea surface temperature, see:
NOAA Sea Surface Temperature maps and information at:
https://www.ncdc.noaa.gov/teleconnections/enso/indicators/sea-temp-anom.php