Underwater Earthquake

This past Sunday, Cyndi, Andrew, Jonathan, and our friends Ken, Beth took the Rod Bender up to Double Reef for an afternoon on the water. I brought my speargun, and while we were there I did a little spearfishing. While I was stalking some Mu, and was almost laying on the bottom at about 40 feet, all the fish scattered. A second later, I was startled out of my wits by the loudest sound I have ever heard underwater. It was a very violent sound, like something was exploding. At first I thought that it was a large boat or ship. But I was very close to the reef and it came on very suddenly. The sound was so intense, I could feel it in my chest. I looked up and all around and didn't see anything. I surfaced very slowly, looking in each direction to make sure I was not about to get hit by a boat. I surfaced and there was nothing in sight. No boats, no explosions, nothing but my boat and family about 200 yards away. It was at that point that I figured that it was an earthquake. I asked everyone else if they had experienced the loud vibrations, and with the exception of Ken, nobody else had heard or felt anything. When we got home later that evening, I checked the news online, and what da ya know, there was a 5.1 earthquake off the coast of Guam. Truly a unique experience.

Comments

[Capt David Williams]

Wow . . . Randy, you almost became our first "beached scuba diver." I'd like to know just how close you were to the epicenter. Can you furnish me with the lat/long numbers of Double Reef where you were. (david07@deafwhale.com)

The quake you cite was a stike/slip event, (side to side motion) hypocentered at a depth of 37.5 km deep and located at 13.507°N, 144.987°E Had it been a thrust event and shallow, say 5 km deep, you and your family would likely not be around.

My group has been working on underwater earthquakes as a cause of whale beachings.
http://www.deafwhale.com/stranded_whale/

Our theory goes like this: Feeding on squid along seismically-active mid-oceanic ridge systems exposes deep diving whales to an almost continuous barrage of underwater earthquakes. The motion in the seafloor in seventy percent of these events is side-to-side and relatively slow. Such motion does not generate potent pressure waves. On the other hand, when the hard seafloor around the epicenter dances up and down rapidly, the quick vertical thrusting generates potent pressure changes in the water column that could cause barotraumatic injury in the head sinuses and middle ear cavities of the entire pod.

When the vertical motion is relatively slow, the water has time to flow to the sides and prevent the pressure from building too high. However, when the seafloor jerks up and down violently at a rapid speed, the rock bottom becomes like the faceplate of a gigantic sonar transducer, pushing and pulling the water, generating ambient pressure changes that might exceed 14,500 pounds per square inch one meter off the bottom (280 decibels re 1 micro PA).

Surprisingly, the intensity of the pressure change in the water is not so much related to the magnitude of the earthquake, rather the level of danger faced by a pod of diving whales is determined by the speed of the vertical thrusting.

The pressure quickly dissipates as the waves move toward the surface but still may exceed five atmospheres 500 meters above the epicenter.

A pod of whales above such an event would experience changes in pressure similar to diving back and forth from the surface to a depth of ~450 feet several times per second for as long as the vertical quaking continued.

Whales would be specially vulnerable near the end of a long dive because their muscles, bones, and blood would be supersaturated with dissolved nitrogen.

At the slightest hint of negative pressure, the nitrogen pops out of solution to form thousands of bubbles resulting in decompression sickness. The bubbles would also cavitate violently during rapid changes in pressure; thus, a seaquake following closely by potent aftershocks would be especially dangerous. In addition, the small air sacs that surround each cochlea helping the whale to sense direction of sound underwater could be ruptured resulting in echo-navigation failure.

No research has ever been published about how a marine mammal might endure rapid and intense pressure changes produced by an undersea quake.

In summary, the SEAQUAKE SOLUTION developed by CAPT Williams indicates that exposure of the entire pod to seismically-induced pressure changes answers the centuries old mystery of why whales and dolphins mass strand on beaches around the world.