potent pressure changes generated by seaquakes

SUDDEN PRESSURE CHANGES during seaquakes

by Captain David Williams

Deafwhale Society, Inc

PO Box 319, Dumaguete City

6200 Oriental Negros

Philippines

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There is much to learn about how undersea earthquakes generate sudden pressure changes in hydrospace.

Seaquakes occur mainly in belts coinciding with the margins of tectonic plates (see map on the left. Since eighty percent of these seismic margins meander along the ocean floor, the Earth releases eighty percent of its seismic energy under the surface of the oceans in the backyard of the whales. In fact, over the last one hundred years, more than seven million seaquakes of magnitude four or greater have occurred where whales roam.

Seaquakes erupt when strain in the seabed accumulates to a point where the resulting stresses exceed the strength of the rocks, and sudden fracturing results. As a fault rupture progresses, rock masses are flung in opposite directions and thus spring back to a position where there is less strain. At any one point this movement may take place not at once but rather in irregular jerks; these sudden stops and restarts give rise to alternating pressure changes in the water.

Not all earthquakes are hazardous to marine mammals. The reasoning is simple. Water will not transmit shear (side-to-side) movement; thus, marine mammals directly above the epicenter during a strike-slip earthquake in which the seafloor shifts horizontally might not be in much danger. That is. . . providing the event does not generate potent Raleigh waves (see below).

On the other hand, water is exceedingly proficient at transferring compressional waves; therefore, it seems only reasonable to believe that changing ambient pressures might encircle a pod of diving whales when the seafloor suddenly shifted vertically during a reverse thrusting (dip-slip) earthquake. It also seems reasonable to conclude that the intensity of any sudden change in surrounding water pressure is directly related to the speed at which the bottom moves in the vertical plane, not to the magnitude of the earthquake. This is so because the speed of the vertical movement in any given area is the generator of the pressure. The scientific principal is similar to easing into a tub filled with water without making a ripple verses jumping in and splashing water everywhere. The angle (dip) of the vertical movement and the shape of the seabed would also be critical in determining the degree of pressure change.

Some vertical shifting can be rather surprising. In the 1999 Taiwan earthquake, the Chelung-pu Fault slipped vertically for eight meters. Some rare records show vertical movement of the seafloor exceeding seventy meters. Of course, the larger the magnitude, the greater the distance of movement, but not necessarily speed. Since speed is the generator of the pressure changes, it would be a mistake to look only at earthquake magnitude to determine the potential injury to marine mammals. Magnitude is important, but should not be relied on exclusively. Of course, major earthquakes are more likely to generate potent Raleigh waves (see below), which in turn might generate potent pressure changes. To repeat the opening statement in a different way, much is not clear about the actual generator of potent pressure changes.

Reverse-thrusting submarine earthquakes occur most often in subduction zones where the ocean floor is descending back into the molten earth. Reverse thrusting also occurs along the inside of mid-oceanic ridge systems is areas where the axis of the ridge is bending in one direction (compressional stress builds on the inside of the bend). These events can also occur at the junction between the ridge and a transform zone in which the stresses are trying to force one side of the fault over the opposite side

There are two types of body waves that travel within the solid earth (see graphic on right). The P-wave is a compressional waves that move as a series of pressure oscillations at various speeds from 7 kilometers per second to as low as 2.5 kilometers per second depending on the density and elasticity of the rock in which it travels. P-waves can leave the solid earth and enter the water. In general, the amount of energy that will transfer across the interface will depend on the speed of the P-wave in the solid earth and the angle of approach to the hydrospace. If the speed is 7 km per second when it encounters the water interface, more energy will be reflected and channeled back into the solid earth. On the other hand, if the P-wave speed in the rock is closer to the speed of sound in water (1.5 kilometer second), more energy will enter the water.

The intensity of the compressions and rarefactions of P-waves (longitudinal waves) in water are stated by most scientists in decibels (referenced to 1 micro Pa). Decibels are confusing when it comes to understanding barotrauma in marine mammals since rapid ambient pressure change above a seaquake epicenter include both a positive pressure phase and negative pressure phase. In addition, seaquakes often generate shockwaves. For this reason, most folks will find it easier when thinking about ambient pressure changes to reason in pounds per square inch (psi) and not in decibels.

The topic of seaquakes is not very well researched so it is difficult to determine whether P-waves are more dangerous to a pod of whales or whether Raleigh waves present a greater hazard. Then again, maybe the greatest hazardous is from shockwaves?

A Raleigh wave is formed on the surface of the seabed by the combination of a P-wave and a S-wave. These waves move across the seafloor in the same fashion as a water wave on the surface of the ocean and can generate potent pressure changes in the water column above. The speed of Raleigh waves is determined by their frequency. Higher frequency waves are proportionally faster than lower frequency waves; however, of the two, lower frequency Raleigh waves might present a greater danger to whales because the seafloor moves up and down at a much great degree and will thus generate powerful disturbances when these waves pass under a pod of whales. But then again, confusion exists since high frequency Raleigh waves travel faster and might generate a greater percentage of pressure change in the water due to their faster vertical component. These waves are also continuous during the undersea event and might subject a pod of whales to repeated ambient pressure changes since each crest and valley represents a separate change.

Encounters with Raleigh waves have been reported throughout history by ships at sea (see seaquake history below) but no research has even been done to determine the potential to injury marine mammals.

Fractures in the seabed propagate rapidly through the rock, usually tending in the same direction and sometimes extending many kilometers along a local zone of weakness (an old fault). In 1906, for instance, the San Andreas Fault slipped along a plane 430 km (270 miles) long. Along this line the ground was displaced horizontally as much as six meters. Thus, P and S waves might combine to form potent Raleigh waves that can spread out from anywhere along the length of the fault and might generate pressures changes potent enough to injury whales several hundred kilometers from what is commonly referenced as the earthquake focus.

S-waves (aka shear waves) and Love waves concern us the least because they involve horizontal side-to-side shearing motion, which is not transmitted by water.

Fault rupture starts at the earthquake focus, a spot that in many cases is 2–200 km below the epicenter. The depth of focus plays a large part in determining the intensity of any sudden pressure changes in the water since the deeper the focus, the more energy is absorbed and dispersed in the solid earth before reaching the rock-water interface. On the other hand, the closer the focus is to the rock-water interface, the greater the pressure changes will be in the water and the greater the potential injury to whales. In general, seaquakes deeper than 15 kilometers can be ignored by those trying to associate these events with whale strandings. On the other hand, magnitude three and four earthquakes should not be overlooked if their are hypocentered less than 5 kilometers deep.

Sorting out the danger zone of a seaquake can be complicated because the rupture propagates in both directions over the fault plane until stopped or slowed at a barrier. Sometimes, instead of being stopped at the barrier, the fault rupture recommences on the far side; at other times the stresses in the rocks break the barrier, and the rupture continues.

During a normal dip-slip fault often encountered along a mid-oceanic ridges, the sides of the fault move away from each other creating a tension, not a compression; thus, normal dip-slip fault should not be as dangerous as the reverse did-slip fault. However, as mentioned throughout this section, much is unknown about this entire process and general concepts should be used with caution.

One reason not to make general assumptions is that the seafloor is seldom flat and smooth near mid-ocean ridges. Instead, the area where whales feed is usually mountainous with very rough terrain. What could start out as tension of a normal dip-slip fault could quickly change into a reverse trusting event under different terrain.

Seaquake vibrations that get trapped in the deep sound channel are known as T-Phase Waves. Several T-phase arrivalswere recorded on hydrophones during the FRAM II experiment by Ruth Keenan (1991). Fromthe measured acoustic levels, a transmission loss prediction between sourceand receiver, and the scattering conversion losses, the acousticsource level in the water from micro earthquakes was estimated to beabout 458 pounds per square inch (250 dB re 1 µPa) at one meter from the source. Dr. Keenan indicated that the pressure changes were transmitted at near vertical angles through the seafloor to the water above the epicenter.

Scientists studying these waves have figured out that volcanic mountains acts as transducers and help reflect these waves into a horizontal plane. Thus, seaquakes at or near the base of undersea volcanic mountains might be extremely dangers since certain whales and dolphins are known to feed along the sides of these mountains.

The depth of water is also critical. If the water is 3,000 meters deep, the pressure will dissipate and arrive at the surface rather weak. The most likely danger presents in water under 1,000 meters.

The actual faulting associated with an earthquake may be complex, and it is often not clear whether in a particular earthquake the total energy issues from a single fault plane; thus, most everything within this theory should be considered in general terms.

There are also many unique features about certain mid-oceanic ridge systems just now coming to light. For example, scientists recently increased their estimates for the number of probably volcanoes along the mid-ocean ridges to ten-times greater than earlier estimates. Extrapolating these results for all of the North Atlantic suggests there are as many as 85 million seamounts on the ocean floor; 2.5 million of these are over 600 feet (200 m) tall. We are also learning that the style in which new oceanic crust is created is different at slow- and intermediate-spreading ridges relative to fast-spreading ridges, because large numbers of small seamounts form the crust along the axis of the ridge. The amount of magma is not enough to generate the large fissure eruptions which occur at fast-spreading mid-ocean ridges like the East Pacific Rise. (outside link to more on slow seafloor spreading along the Mid-Atlantic Ridge System)

The cross-section in the link above shows the crustal structure of the slow-spreading Mid-Atlantic Ridge. The crust is made of seamounts and fissure-fed flows (area above magma chamber). Normal dip-slip faults bound the edges of the ridge's inner valley. Small separate magma bodies feed individual volcanoes. The solidified magma bodies make up the lower oceanic crust. The affect this has on generating dangerous pressure changes in hydrospace is unknown. Could these mini-volcanoes serve to funnel and aim seaquake energy in a more vertical direction than would be possible without such funnel-shaped structure? For example, if a seaquake occurred in the base of a 600-foot high volcano, the upward traveling P-waves in the solid earth might be channeled toward the apex of the seamount and thus far more P-wave energy could enter the water column.

Or, could these areas pose more risk because pods of whales and dolphins dive closer to these volcanic structures to feed on smaller fish and squid? It might be that many pods are injury within 100 meters of the actual rupture in the seafloor.

Although the species that consistently mass strand will feed at any opportunity, these pods primarily eat squid, especially during the breeding season when the squid gather into tightly packed groups. Squid, for mysterious reasons, prefer to lay their eggs and breed in the warm water along the seismically active mid-oceanic ridge systems in a seasonal pattern usually lasting about three months. The pods follow the squid into these seaquake-prone danger zone and, when the squid move off the ridge system three months later, the pods follows them into less dangerous waters. Since the pods are much more likely to encounter a seaquake during the time they are over the mid-oceanic ridges, most major stranding sites around the world all have a corresponding three-month high season for strandings. This migration also moves the pods in and out of major oceanic currents that often travel in opposite directions. Thus, the area where a stranding might occur depends on the time of year, the ocean currents and other predictable variables.

One other variable looks important. It appears to this researcher that dangerous seaquakes are seasonal, coming at a period of time when high winds and waves dominate the surface waters. It could be that internal pressure changes generated by the high surface waves participate the release of more seaquakes. In other words, high seas tend to cause more seaquakes and could play a minor role in the seasonal patterns of strandings.

ENERGY RELEASED DURING A SEAQUAKE

On 28 August 1997, the front page of the Washington Times reported that seismic stations around the world had detected an underwater explosion offshore near a Russian nuclear test facility on a small island in the Kara Sea. The first impression of many scientists was that the Russians had exploded an underwater nuclear bomb in violation of the Comprehensive Test Ban Treaty. CIA geologists insisted the explosive nature of the event was proof of a nuclear explosion, while others felt the seismic spectrum was that of a magnitude 3.5 seaquake that had erupted in the upper crust of the bottom with explosive characteristics. The argument went back and forth in the newspapers and was finally resolved three months later when two Columbia University seismologists released evidence that indeed a shallow-focused seaquake had ruptured through the brittle layer of the seafloor near the Russian test site.

This quibbling back and forth by experts shows just how tough it is to tell the difference between an explosive seaquake and a small nuclear device. The difficulty is easy to understand since the energy released during a shallow magnitude 3.5 undersea quake is just a little shy of that released by a 1-kiloton nuclear explosion.

HISTORY OF SEAQUAKE ENCOUNTERS

The word seaquake was first coined in the 1880's by Eberhart Rudolph, Professor of Geophysics at the University of Strassburg in Germany.

Before the 1900's, most geologists insisted that earthquakes could not occur at sea because the ocean floor was too water-logged to support the buildup of stress, but Professor Rudolph believed differently. He'd heard many sea captains relate stories of strange encounters in which their vessels suffered severe damaging vibrations and believed these stories to be the true accounts of seaquakes.

His peers disagreed. They concluded that these strange reports were just the drunken tales of men trying to impress the crowds who commonly gathered at the taverns along the docks to hear wild sea adventures. But Rudolph knew that many of the captains telling of these horrifying events were God-fearing teetotalers and honest to the bone. He believe their stories and launched a massive effort to prove what they had experienced were actually seaquakes. He visited every major port, large library, and all the meteorological offices in Europe, going over ship's logs and newspaper accounts of weirdness at sea, locating thousands of seaquake/vessel encounters.

In those days, ship's logs were religiously kept by the captain and first mate. Each noteworthy event, the exact time it occurred, the lat/long locations, the weather condition, the direction of the wind and the sea, and any pertinent comments were religiously recorded. Captains and mates were proud of their record keeping and wrote down exactly what they witness. In the days of wooden ships and iron men, having a reputation as a man of truth was an honor most sea professional sought. Time has shown that few were liars.

Rudolph, comparing the seaquake-like reports by time and date from many different ports, found that often the same seaquake had struck different ships at different locations at the same time. Crewmen did not know of other vessels in the area as they were traveling in different directions, over the horizon and out of sight of each other. Combining this information, he was able to triangulate the epicenters of many large seaquakes and show proof of their occurrence and location at sea. He would sometimes have as many as 7-8 vessels and 100 witnesses reporting the same event.

He published over six-hundred pages, documenting many hundreds of accounts from captains who depicted these encounters as violent shuddering events that completely unnerved their crew. He repeatedly describes broken mask and other severe damage, and instances in which unrestrained objects and persons were flung up from the deck into the air, as well as rare cases in which an entire ship was witnessed being raised from the water.

Many of these events were accompanied by explosive sound like "distant thunder or cannon fire."

One rather odd sighting occurred on 12 January 1878. Captain Garden on board the Northern Monarch, while at 12.4 S by 84.4 E, "observed the sea thrown up to a great height, possibly 80 feet or more, in a column; this occurred three - four times, each upheaval lower than the one before; effect similar to that produced by a torpedo."

It appears by description that the energy had been focused into a narrow beam, likely by some geological formation on the bottom. Maybe the energy had radiated to the surface from the crest of a small underwater seamount?

Captain Stiven, the master of Arethusa, experience two events less than a year apart. The first lasted 10 seconds and occurred on 9 June 1882, while at 32.4 N by 39.5 W. "It shock the ship so violently from stem to stern that all hands came running out to see what had happened. It was accompanied by a rumbling noise like distant thunder, but seemed close to us." Captain Stiven noted in his log that he had later spoken to the captain of another vessels that was 20 miles east of the Arethusa, and this captain indicated that they had not experienced the event.

The second seaquake encounter occurred on 10 May 1883, while at 29.5 N by 41.4 W. "The motion was vertical, as if the ship was grating over the ground, and was accompanied by a low rumbling sound, as if some heavy object were being dragged over the deck. This time the vertical motion seems more decided than my last encounter on 9 June. The crockery and other loose articles shook and rattled. I judge it lasted 10 seconds. Two and one-half minutes later another shock without noise, lasting say five seconds. No unusual disturbance in the sea."

Rudolph's work, published in German, even includes a section that compares seaquakes to underwater explosions.

In their famous book, The Oceans, Professors Sverdrup, Johnson, and Fleming report that upon reaching the surface, such longitudinal oscillations would be felt on board a ship as a shock that could violently rock the vessel. They state that the shock may be so severe as to cause the sailors to believe their vessel has struck hard bottom. They stress many ship reports dealing with shock waves, particularly from regions in which seismological records show that undersea earthquakes are frequent.

According to Frank Rossi, the retired editor of the Mariner's Weather Log, on 15 April 1941, somewhere near 18 N by 103 W, a seaquake interacted with a vessel loaded with steel assembly causing "some pieces weighing 6 tons, to shift about six inches and to jump as much as five to six inches up and down from its blocks."

Rossi also reports that on 15 June 1966, the captain of the M/V Ninghai, while at about 10 S by 161 East in the Solomon Islands, reported being shaken repeatedly for over two hours by seaquake activity. The damage report read as follows: "The cathode ray tube shattered, the capillary tube in the barometer was smashed, valves were shaken out of their sockets in the wireless transmitter, the suspension wire on the gyro snapped and the azimuth mirror on the monkey island gyro repeater feel off. In addition we made some water in No. 3 double bottoms and after peak; also the main engine fuel line was broken and the sanitary tank on the monkey island was holed. No water was made after the tremors, which suggest that as the ship was being shaken water was entering these tanks through various rivets and seams which had started and opened, but only for the duration of these tremors. The mast whipped about a great deal, and the funnel rattled alarmingly."

Rossi also reflects on the varied ways many ships, located at random distance from the epicenter, are effected by reviewing ship reports from the large shallow-focused Mexican earthquake of 3 June 1932. Although the epicenter was located 30 miles inland, the S/S Solana, steaming through a smooth sea with light variable winds in water over 4,800 feet deep, 60 miles from the epicenter, experienced strong violent shaking for about seven seconds. Only 10 miles away, the M/V Sevenor experienced less severe vibrations but lasting nearly a minute. Conditions aboard the M/V Northern Sun were entirely different. Although the vessel was 115 miles from the epicenter, vibrations lasting for three minutes became so violent that the engines were stopped. Before the earthquake, the sea was smooth with a slight westerly swell, but after the event the sea had become confused and the swell pattern had changed. Further to the North, 130 miles from the epicenter, the S/S Arizona commenced to vibrate and continued to do so for about 75 seconds.

The aftershocks continued for many days. Ship reports indicated that during the next 36 hours several strong seaquakes were experienced in the area. The M/V Silerwillow began to vibrate dangerously in every part and at the same time began an uneven short pitching motion followed by heavy rolling. The disturbances commenced at 0530 GMT on 4 June, and the rolling continued 15 minutes. Seven hours later at 1245 GMT the crew aboard the S/S Talmanca heard a loud noise like distant gunfire, then experienced severe vibrations, and at 1337 GMT two similar gunfire reports were heard again about 10 seconds apart but there were no apparent vibrations. However, 20 minutes later the sea surface was littered for five or six miles with small dead fish. Several hours later, the S/S Hanover reported violent shocks that "rocked the ship as a nearby explosion might." Fifteen minutes later two more shocks were felt.

Aftershocks were reported by ~50 ships within a 500 mile radius of the epicenter during the Great Alaska Earthquake of 1964. These events were similar to those reported by Rudolph. Crew members mentioned "running aground, hitting a reef, being hit by another ship, or losing part of a ship's propeller."

Harold Harding, on the fishing vessel Roald, reported the initial shock felt hard like "hitting rocks." He said he felt many hard jolts during the night and could hear "booming sounds even above the engine noise." He described the jolts as like "explosive depth charges." Joe Clark, on the fishing vessel Quest, reported shocks felt as if the "boat were going aground." The captain of the Little Purser said it felt "like the boat was being pounded on the rocks."

On 29 March, two days after the main event, the US Coast Guard Cutter Sedge was 10 miles offshore when crew members heard a sound like an underwater explosions, "similar to that of a depth charge or torpedo." During the next few minutes they experienced three minor tremors. The engineer reported that it felt exactly like going over a reef and asked if the captain intended to go back over the reef when he was ordered to back the engine to full astern. The Sedge later reported that during one aftershock, sensations were felt similar to the propeller cavitation in a smooth sea.

On 15 April 1979, a major earthquake struck the Adriatic Sea area and caused severe shell plating damage to the Italian freighter Carso while she set comfortably berthed at the Yugoslav port of Bar. Before she could sink to the bottom, her master beached her in the harbor where she was declared a total loss and sold to Italian ship breakers for scrap.

On 23 October 1964, in the North Atlantic, a magnitude 6.4 seaquake occurred not far from a well equipped research vessel: Scientists from Woods Hole experienced the event on board the R/V Chain. Birch, the scientific team's leader, stated: "The quake shook the ship violently; the motioned seemed mostly vertical. Immediately the bridge rang the general alarm and the ship was stopped. Opinion on the cause of the disturbance varied. Some people thought the ship had run aground or hit a submerged object; others, that a shaft or screw had broken."

The Chain had an array of hydrophones strung over her stern shooting seismic profiles, and was able to record the event on tape. But the intensity of the initial seaquake waves over-modulated the recording equipment. Unfortunately, the intensities for the five aftershocks could not be ascertained either because when the ship made its panicked stop, the array of geophones sank to an unknown depth, obscuring their sensitivity. The event, 52 kilometers from the Chain's position, had a focus depth of 25 kilometers and its epicenter was layered with a few hundred meters of sediment.

Four years earlier, two Scripps' Institution vessels, the R/V Argo and Malita, had encounter distant seaquakes during a two-ship seismic refraction operation in the Flores Sea (8). While shooting shots of 100 pounds of TNT from the forward ship, three crystal hydrophones strung out over the stern of the trailing vessel began picking up seaquake activity. The distance between the two ships was not noted; still the shooting had to be stopped because the larger seaquakes, epicenter up to 200 kilometers from the ships, generally overloaded the recording system masking completely the energy from the 100 pounds of TNT. Twenty-seven events were recorded in a 12 hour period. Most of the energy was center at ~10 hertz, but no reliable measurements were available due to the equipment overload. Since the sensitivity gain on the hydrophones were set to measure the arrivals from an underwater blast of 100 pounds of TNT from a ship usually no more than ~10 miles away, it can be concluded that the pressure waves from the seaquakes, epicenter distance up to 100 miles, were much greater than 100 pounds of explosives. In addition, these were not explosives sounds with a single peak of energy, several of the seaquakes lasted for four minutes (average duration ~1 1/2 minutes).

Only one of these events was recorded on the land-based seismic network. But this was 1960 and seismographs did not provide good coverage of the world's oceans during this period. Things have improved, but there is still a doubt of whether or not there are good recordings on present day oceanic events.

It is common knowledge that hydrophones installed as part of the missile impact tracking system off the coast of California have picked up numerous distant submarine earthquakes of medium magnitude that were otherwise unreported by seismic stations. This adds an ambiguous factor to coordinating undersea events with the strandings of whales or with vessel encounters. It seems that seaquakes occurring in the upper five km of the crust and rupture through the seafloor to alter the terrain, generate the most potent pressure waves in the water column, but are the most nebulous to land-based instruments.

But two seaquakes noted by scientists from Goddard Space Center certainly were not difficult to record.

Eighteen minutes after a 7.5 magnitude seaquake off the Coast of Mexico on 29 April 1970, Goddard Space Flight Center scientists noticed that the infrared equipment onboard the ITOS-1 spacecraft recorded a 60 km circular area directly above the epicenter in which the sea surface had experienced a temperature enhancement of +3 degrees Kelvin. Four months later, on 11 August 1970, a 7.6 magnitude seaquake occurred in the region near New Hebrides Island in the South Pacific. This time infrared equipment onboard the Nimbus 4 spacecraft recorded a 2 degree Kelvin increase in temperature. Goddard Center scientists calculated that both anomalous increases in temperatures had been caused by a shock wave of ~100,000 psi generated by the seaquake (9).

Certainly, a 100,000-psi change in ambient pressure would kill the whales. But "shock waves" from seaquakes and events above magnitude 7 are extremely rare and would result in the death of the pod, not in a stranding. In trying to solve the enigma of why whales mass strand, the researcher should be most interested in the pressure waves generated by events of 3 - 5 magnitude.

RECENT VESSEL ENCOUNTERS

The most documented seaquake encounter in modern times occurred on 28 February 1969 when a magnitude 7.8 erupted ~20 kilometers from the position of a 32,000 ton tanker sailing in ballast from Lisbon to the Persian Gulf. Ambraseys reports that in the wheelhouse of the Ida Knudsen, compasses and other permanent instruments, including the radio station binnacles were torn loose from their mountings and collapsed. Doors and fixtures where broken from their hinges and mountings. The radar mast was broken. Damage to the superstructure was more serious amidships than at the aft peak. From eyewitness accounts it appears that the ship was lifted up bodily, the bow moving up faster than the bridge, and then the whole ship was slammed back with violent vibrations, the whole event lasting about 10 seconds. After hours of drifting helplessly, the ship's engines were restarted, and with a bent propeller shaft vibrating horribly, the ship was able to ease back into Lisbon where it was surveyed and declared a total loss.

The surveys proved that the hull, machinery and other equipment had sustained great damage and, because of the permanent deformation and breaks, the ship had lost a substantial part of her longitudinal strength. The complete surface of the vessel's skin from cofferdam to cofferdam buckled in places with permanent sets of four cm and the hull was twisted 18 cm. Bulkheads, hull frames and girders were buckled or torn apart and all wing tanks leaked. The bottom parts of the side platings were torn away from the girders, by as much as five cm, "effects resembling those of underwater mine explosions."

Although there was not enough data to calculate the response of the ship and a good deal of uncertainty about the duration of the event, Ambraseys did rough calculations indicating a pressure wave as high as 17 atmospheres (~250 PSI). Even though Ambraseys indicated that this was ample energy to cause the damage to the Ida Knudsen, according to US Navy reports, the severe hull damage reported would not be expected unless shock pressure reach 1,000 to 2,000 psi.

Even the M/S Toubkal, 180 km away, reported experiencing violent vibrations for about 1 minute, and several vessels out 190 km experienced "severe vertical shocks."

Ambraseys stated that in many cases of damaging seaquakes the available information is too incomplete to be of value in determining the energy in the water. He said that the earthquake of 23 July 1894 in the Lofoten Islands of magnitude 6 was felt by a number of vessels in the region. One of them in calm weather experienced such vibration that it sprung a serious leak, sinking 14 hours later. He states, "...cases of serious structural damage during some of the large Japanese earthquakes are known, but details are lacking."

WHY NOT REPORT SEAQUAKE DAMAGE?

For reasons of resale value and banking, ship's owners are naturally secretive about any earthquake encounter with their ships. No owner in his right mind would want it known, prior to a offer for sale or an attempt to secure a loan or an attempt to get new insurance, that the vessel in question had sustained severe structural damage in a seaquake!

Ambraseys reported that the absence of well-documented cases of damage does not mean that such incidents are rare. He said, "Access to ship's logs, which are the sources for such information, is extremely difficult particularly when the inquiry concerns details of previous damage. Moreover, such logs are never kept by ship owners for more than six to nine years, after which they are invariably destroyed."

"Lloyd's casualty records," Ambraseys wrote, "do not include a section for vessels reportedly damaged by earthquake activity and they show no evidence of damage caused to the Ida Knudsen by earthquake shock or other phenomenon of this nature, the loss of which was settled on a compromise basis." He pointed out that many marine policies exclude the "peril of earthquake" and it could well be that "policy wordings have kept earthquake damage to vessels out of casualty list under this heading."

If the insurance policy excluded seismic activity, which many do, one would naturally expect the owners to report a seaquake loss to be the result of some other type of covered peril. Most likely, seaquake damaged is reported as: a collision with a submerged object, an encounter with a rouge wave, an accidental grounding, a cargo shift due to rough seas, an accidental pipe rupture, or a lost propeller, etc.

OTHER REASONS NOT TO REPORT SEAQUAKE ENCOUNTERS

At 22:35 UTC on March 6, 1988, a 6.7 shallow focused event occurred in the Gulf of Alaska ~60 km from the location of the Exxon North Slope, the Exxon Boston, and the Exxon New Orleans. Each super tanker reported ~$5,000 in damage (was the true damage more?). This report never reached the press. Obviously Exxon did not want this information broadcast to an oil-spill concerned public.

CRUISE SHIP INDUSTRY ALSO CONCERNED

The Norwegian luxury liner Bergensfjord was plowing its way along the western coast of South America off Ecuador, one day out of Lima, Peru, heading towards the canal zone with 250 pampered passengers. Suddenly, at 11:35 PM they were awakened by a rough shaking of the ship, a loud, bumping, rasping sound, as if the ocean liner had hit solid bottom and was skidding over rocks. The ship listed and then just a quickly righted herself. The violent shaking woke the chief engineer, O. Olsen. He looked out of his cabin window at a completely white-topped and confused sea reflecting the vessels lights back to him. He thought that the engine was exploding or that they had grounded. Dressing quickly, he then ran toward the engine room. On the way, he noticed that the engine rpm increased as if the propeller was spinning freely. When he got there, three minutes later, the quaking had stopped.

Captain Fasting ordered the engines stopped and all passengers to the lifeboat stations, ready to abandon ship. Now the sea was calm and as black as before. No serious damage could be found. Furniture had been thrown around, crockery and glassware had been shatter, and one passenger was dead due to a heart attack. Later, after the entire vessel was inspected, Captain Fasting announced over the deck speakers that the ship had struck a rather large submerged object, but had apparently suffered no serious damage. He knew he had not hit bottom, the water was 3,600 feet deep.

The next afternoon, they heard a CBS radio broadcast telling of the 7.4 earthquake that had hit Peru at the same time as their encounter. The ship's position from the center of the quake was 70 nautical miles.

ARE PROFESSIONAL MARINERS AWARE?

Hove, et al, wrote about the Ida Knudsen and displayed many pictures of the damage. He also lists 10 different vessels that had suffered serious seaquake damage, including cruise ship above. He stated that, although the unlucky tanker was the most severe record know to them, even more reliable reports of many similar less severe occurrences did exist. "There are reasons to believe, however, that many severe cases have not been reported as earthquake generated simply because of the fact that most sailors and other people--including engineers--are not aware of the fact that seismic waves can cause effects on vessels at sea. It is believed that such incidents most often are explained by grounding on unknown shallows or rocks, collision with 'floating wreck', or even underwater explosions."

Besides the hundreds of ships every year that are reported to strike an unidentified submerged objects, many ships suffer from unexplained rupture of pipes in the engine room, mysterious engine room fires and explosions, and sudden shifts in cargo. The momentarily severe shaking a vessel experiences during a seaquake, even if only for a few seconds, could easily cause a ruptured water pipe or cracked fuel line in the engine room, a boiler explosion or an electrical short in an explosive area, or even a broken cargo strap resulting in a cargo shift. Such damage could cause a serious leak or fire or explosion or cause the ship to turn turtle and sink. The incident would occur quick and unexpected, in fair weather or storm. The crew could be caught by such surprise and shock that they would not know what had hit them.

Captain David Williams

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