I am unseasoned when it comes to earthquakes, but on Wednesday, September 5, at 8:40 am, I encountered my first ground surfing experience due to a 7.6 magnitude earthquake off the Pacific coast of the Nicoya Peninsula in Costa Rica. I felt awkwardly inexperienced, while watching my neighbors calmly balance their babies and young children in their arms, waiting patiently for the deep waves of the temblor to pass. I stood ..uneasily.. in awe of these expert ground surfers, for they have experienced many earthquakes and volcanic eruptions on Ometepe Island.
Unfettered by the newness of the experience, I was eager to explore and share my feelings about the earthquake. I wondered what kind of Teutonic plate collision was the cause of my uneasiness. I wondered why I still had an eerie feeling of a loss of equilibrium 30 minutes after the 20 second quake; I wondered if it would happen again.
I found an interesting article by geoscientists, who retold the story of one man’s experience during the 1950 Nicoya Peninsula earthquake.
The sky dawned dark and cloudy on the morning of October 5, 1950. It was rainy season along the Pacific coast of Costa Rica’s Nicoya Peninsula. In scattered coastal villages, farmers and ranchers gathered along the shoreline waiting for the cargo launch to arrive from the port city of Puntarenas. Their ox-carts were full of freshly harvested corn, ready for shipment to the mainland. Near the village of Garza, Don Daniel Ruíz Matarita was riding on horseback along the beach with several other men. The horses were skittish, and the nearby rainforest echoed with the roar of nervous howler monkeys. Suddenly, without warning, the ground heaved violently, trees and branches toppled over, and the beach exploded with geysers of water and sand. “Earthquake! We’re done for!” shouted one of the men. Their horses bolted in terror, throwing the riders to the ground. Huge chunks of rock toppled off of nearby cliffs, crashing into the water with a tremendous splash. Certain they were doomed, the men prayed for salvation. When the great earthquake finally subsided, Don Daniel and his companions were amazed and thankful to be alive. As they stood up and looked around, they saw that the ocean curiously had withdrawn from the bay, leaving a wide expanse of barren rocks, seaweed, and flopping fish. Seizing the moment, the men snatched up handfuls of sea bass, content at least that their bellies would be full in this time of disaster. In the days following the earthquake, Don Daniel recalls that the sea did not return as they had expected. He heard stories from others that the same thing had happened all along the central Nicoya coast. Don Daniel remembers one place where the drop in sea level was particularly obvious, a rocky headland known to local fisherman as “La Raspa Nalgas” (The Butt Scratcher). Prior to the earthquake, it had been impossible to get around this rocky point on foot, as it was under water at even the lowest tides. But, after the quake one could walk around the headland without entering the water, indicating a drop in tidal levels near a grown man’s height. Don Daniel recalls that it took nearly four decades for the ocean to reclaim its former level, quickly during the first few years, then slowly thereafter. High tides now reach further inland in many places than they did before the 1950 earthquake. (Marshall, 1991)
Just like the 1950 earthquake, the tide rolled out and the beaches were expanded by at least 30 meters. Maybe the man in the photo is gathering the flopping fish for his dinner.
In my search for the type of earthquake which caused my uneasiness, I encountered many new terms such as: subduction Megathrust, subduction trench, tsunami generating earthquake, and sudden geomorphic changes.
The Nicoya Peninsula is unique because it is one of the few landmasses along the Pacific Rim located directly above the seismogenic zone of a subduction megathrust. Due to its proximity to the subduction trench, the Nicoya Peninsula is particularly sensitive to vertical movements related to the earthquake cycle. (Marshall,J., Cal Poly Pamona University, 1991)
Can someone explain in laymen terms what this means?
Since I am a visual learner, this simple illustration explains how the subduction trench (area where the plates are stuck together) ruptures when one plate slides over the other ( a Megathrust), releasing pent-up energy, and causing major expansions of beaches and tsunamis ( sudden geomorphic changes). Simple, right? There were several tsunami watches broadcast, but they were canceled later that day.
Now that I understand the dynamics of a Megathrust earthquake, I am still curious about the feeling I experienced..that of a loss of equilibrium and a minor balance disorder. Standing outside our house for the 20 second duration..which, by the way felt like 20 minutes, I felt a deep wave rolling beneath my feet. I spread my legs apart to gain a sense of balance. I was slightly nauseated and dizzy. This was ground surfing in all its horrific glory. I wasn’t afraid, only disoriented and awed by the power of the swaying ground waves rolling gently below me at a depth of 23 ft. Why did this annoying dizziness continue for about 30 minutes after the quake?
Even we humans are affected with disorientation, giddiness, nausea, uneasiness and feelings of impending calamity prior to and during a quake. Scientists suggest that this is the result of human sensitivity to ground waves, and to electrostatic effects (including the Serotonin Irritation Syndrome or Serotonin Hyperproduction Syndrome) and electromagnetic forces. In other words, observations have shown that we humans are sensitive to the Earth’s nervous system impulses, too.( Pasichnyk, R. M., http://www.livingcosmos.com/earthquakes.htm)
So, I discovered there is a name for my weird feelings, a syndrome! Some reports indicate that people have an aura before another earthquake…a feeling of unease, a little nausea, and dizziness. I awoke this morning, two days after the earthquake, with these same symptoms. Maybe I’m answering my third question…Will it happen again? There is a “green alert” issued for Nicaragua today. See report here: Green Alert
I’m not sure exactly what a “green alert” means, but if my wooziness is any indication, I may have become a more experienced and seasoned ground surfer. Expect the unexpected….I’m moving my fragile treasures to the floor.
Important update to the Green Alert: I knew it wasn’t my imagination! Nicaragua’s Capital Prepares for Major Earthquake