Faced with the onrushing water of a tsunami wave, Jose "Pepe" Martinez of La Manzanilla, Mexico, stood his ground and captured the event on film. Personal accounts, photos, and data recorded by newly installed offshore sensors are testimony to how vulnerable flat lands near river inlets are to tsunami attacks and show that even moderate tsunamis can produce large currents inside harbors.
by Jose Borerro, University of Southern California, Los Angeles; Modesto Ortiz, CICESE, Oceanology Division, San Diego;Vasily Titov and Costas Synolakis, University of California, Los Angleles
On October 9, 1995, an earthquake of Mw = 8.0 shook the Pacific coast of central Mexico. Centered offshore of the states of Jalisco and Colima, the earthquake caused extensive damage near Manzanillo and was felt strongly in Mexico City. Approximately 40 people were killed and about 100 injured, and many tall buildings were destroyed or severely damaged.
The earthquake set off a moderate tsunami that affected approximately 200 kilometers of coastline and sent tsunami run-up of from 1 to 5 meters from north of Playa de Cuyatlán to south of Tenacatita Bay. Where the shoreline rises steeply from the sea, the damage was minor, but along the shallowest beaches, the damage was severe.
Although the Manzanillo tsunami did not produce extreme 30-m runup heights observed in Flores, Indonesia, in 1992 or in Okushiri, Japan, in 1993, it was an interesting event for several reasons. Eyewitnesses observed the tsunami and were able to describe the sequence of motions of the sea's surface well. By serendipity, a week before the tsunami, two moorings of bottom-mounted pressure sensors had just been installed offshore of La Manzanilla for a study of internal wave motions. This array of sensors recorded a tsunami train propagating to shore at about 50 meters water depth, a measurement not contaminated by the effects of harbor resonance on tidal gages inside harbors. Experiments using these data will allow modelers to refine their inundation codes to better match freefield tsunami data with seafloor displacement scenarios.
Two international scientific survey teams measured horizontal inundation distances and vertical runup heights and interviewed eyewitnesses. The measurement locations were recorded with hand-held Global Positioning System units and corrected for tidal elevation differences. The survey teams gathered data and observations at locations from the town of Paraiso, about 50 kilometers south of Manzanillo Bay, north along the coast to Punta Chalacate, 50 kilometers north of Boca de Iguanas (see figure, below).
A map of the area affected by the tsunami. Runup heights are on the graph above.
Most of the inland inundation was in Tenacatita Bay; the Port of Manzanillo also suffered severe damage when a 12-knot tsunami-induced current eroded the banks of the port. This event provided data on the freefield offshore profile of a tsunami. The incoming wave was measured for internal wave motions by two bottom pressure transducers deployed off of La Manzanilla in 50 m of water; eyewitness observations and photographs documented the height of the leading-depression wave as it rushed ashore.
The most spectacular tsunami damage of this event was observed in Tenacatita Bay. La Manzanilla, a small town on the southern end of this bay, was flooded by 2 meters in most areas, 4 meters above mean water level. The water advanced through the town, stopping only when it reached a steep hill 200 meters inland. The eyewitnesses reported a leading-depression wave, with the water in the bay retreating several hundred meters, as seen in this figure.
Tenacatita Bay minutes before a tsunami struck. Normally, the entire area is covered with water, but the bay was sucked dry by a leadling-depression tsunami. [Courtesy of Josef and Helga Lehemen] A family eating breakfast on their patio felt the earthquake and reported that 15 minutes later, the bay began to empty. The son, who frequently dives for shellfish near the exposed rocks, reported that the "normal" depth at the point of largest withdrawal is 5–6 m.
When the water began to return and advance, it did so like "a fast rising tide." Fortunately, the people on the beach were able to outrun the water. Jose "Pepe" Martinez took a sequence of photos as the water advanced through his town. The photos below show the first and last shots of the three-shot sequence taken of the advancing wave.
On the northern end of Tenacatita Bay, there is a small river inlet and a camping area known as Boca de Iguanas, where the land is flat and low. After the initial withdrawal, the water came in with much greater force than in La Manzanilla. The beach-facing walls of several houses were blown away in a manner reminiscent of Playa Popollo in Nicaragua during the 1992 tsunami or Rajekwesi in east Java during the 1994 event. Debris was strewn in a classic tsunami debris wake behind the buildings. A small pickup truck flipped over and rested against a palm tree. A camper was pushed backward about 30 meters and pinned against a row of palm trees.
The beach area here is backed by marshy wetlands. The wave penetrated up the river and into the marsh, and overturned boats were deposited over 500 meters inland. No one was killed in Boca de Iguanas, but several houses were destroyed by the wave. Run-up in Boca de Iguanas was measured in several locations and varied between 3.5 and 5 meters.
Runup heights decreased from 4.75 to 1.75 meters along the long, sandy beach from Manzanillo southward. Witnesses in the southernmost survey point of Paraiso reported strong shaking, followed a few minutes later by a recession of the sea surface, suggestive of a leading-depression wave. The sea withdrew to about 50 meters offshore before advancing to an elevation of 1.76 m above sea level; the wave was described as "slow" and did not cause damage or flooding.
In Manzanillo Bay, near the entrance to Manzanillo harbor, there was some extensive damage from the tsunami currents, which local current meters showed had surged up to 12 knots. This part of Manzanillo Bay is a central market area with a well-developed and picturesque waterfront. The strong current scoured away the banks, undermining a road, a local monument, and a house, all of which collapsed into the ocean. An eyewitness reported seeing the ships in the bay rise above the horizon, and one large ship was seen floating above the sea wall. When the wave began to recede, eyewitnesses saw a whirlpool form just outside the entrance of the harbor.
Manzanillo Bay is divided by a rock isthmus, with the southern part known as Manzanillo Bay and the northern part as Santiago Bay. On the extreme north end of Santiago Bay is an area of steep cliffs. At the Vida del Mar resort, maintenance workers felt the earthquake and noticed the wave about 15 minutes later. From their vantage point at the 10-meters-high cliff's edge, they watched as the water withdrew several hundred meters offshore and then rushed in and up the cliff, violently splashing over the edge. The witnesses reported seeing 4 to 5 waves, with the first being the largest. This locale was the point of highest measured run-up (10.9 m), probably because the cliff is located behind a narrow, steep-sided indentation in the coastline.
North of Manzanillo Bay, measurements were taken at Playa de Oro, a long, sandy beach that sits in front of the international airport that serves the area. Run-up at Playa de Oro was measured at 4.08 m and confirmed by an eyewitness. North of Playa de Oro, there are two bays—Navidad Bay to the south, and Barra de Navidad to the north. Both were inundated with fairly high run-up. Run-up ranged from 3.7 meters in Barra de Navidad, to 5.7 m in the middle of the bay, to 4.5 meters at Melaque on the northern end of the bay. The only casualty of the tsunami occurred when a fisherman wading close to the shore at Barra de Navidad was unable to outrun the rising water. Eyewitnesses reported that a 100-ton, 54-foot-long sunken shrimp boat was shifted underwater horizontally about 100 meters, but we were unable to confirm their observation.
The survey continued further north at El Tecuan, Punta Chamela, San Mateo, Perula, and ended at Punta Chalacate (about 50 kilometers north of La Manzanilla), where run-up varied between 2.5 and 5 meters. Runup values decreased as the survey progressed northward from Boca de Iguanas, except for a noticeable variation at San Mateo, where run-up was 5 meters. Damage due to flooding was not observed or reported in any of these spots, as this area is sparsely populated.
Tsunami waves were reported as far away as Puerto Vallarta, 300 kilometers to the north. An article in the November issue of Latitude 38, a sailing magazine, described how pleasure sailors in the port felt the earthquake and witnessed "shock waves" that raced across the water surface. A few minutes after the quake, water began to flow out of the marina as if "some one had tilted the basin." Water flowed out of the marina at speeds estimated to be 12 knots. After the first withdrawal, the water flowed back into the marina. Several yachts were damaged as they were knocked together by the waves.
The Manzanillo earthquake was the largest event in 60 years along the Northern Middle America Subduction Zone. The earthquake exhibited extremely shallow faulting, with maximum displacement of 5 meters at 15 kilometers depth and 2 meters at 8 kilometers depth, which caused widespread coastal subsidence. The fault had a dip angle of 160° and ruptured along 200 kilometers of strike length.
Source: Eos, February 25, 1997, p. 85.
Ever since elementary school I was interested in science, and I took part in the science fair competitions throughout junior high. But in high school, I became more interested in"other" things: cars, surfing, messing around. That path continued into college, where I started on what I thought was the easiest route possible to get through the University of Florida in four years— journalism and communications. However, I was enlightened a little more than halfway through school by hanging around and going surfing with several friends who were engineering majors. They were just regular guys, but their classes and homework assignments seemed so much more interesting than what I was doing.
At that point I decided I was going to be an engineer, and I was accepted into the University of Southern California (USC) in the engineering program; I initially wanted to be an electrical engineer, but that changed in the summer between Florida and USC, when I went on a surfing expedition to Mexico. I drove a beat-up pickup truck with two friends and 11 surfboards from Florida to Acapulco and back. That trip made me realize that my true love was the ocean, and that I had to move my studies in that direction.
So here I am, about to finish my masters in ocean and coastal engineering. I'm in the Ph.D. program, and I should finish that in three years or so. My advice to anyone interested in science is to set out to be the best. There is a lot of competition, and the way to get above the crowd is to somehow set yourself apart. In college, if you are in a science or engineering major, find a professor who is researching a project you're interested in and beg him/her to let you hang around the lab. Work for free in your spare time; you never know, it may lead to something. But most of all, don't be afraid of the work. I was afraid of working hard when I first went to college. However, once I got into engineering, I found it was a lot of work, but I enjoyed it—so it didn't seem that hard.
My current research deals with the numerical (computer) simulations of tsunami waves. We are planning a project to assess the tsunami hazard on the southern California coastline, and I will probably work on this topic throughout my Ph.D. course of study. I am also interested in any other topics that have to do with waves or beaches. I am a member of a team of scientists and engineers who are constructing the world's first artificial surfing reef in El Segundo, California. I surf every chance I get, and I study the rest of the time.
Be cool, stay in school. It's easier than working; trust me.
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