The Great Wave
Long ago during the golden reign of the Greek gods and goddesses the shores of the oceans were calm, and peaceful, for Poseidon had no reason to disturb the beach dwellers. Then there came a time that Poseidon created the tsunami, though not on a whim or a fancy, to learn of his reasons we must first go to the beginning of the story.
Every night as Artemis drew her pale blue sleigh across the sky; her reindeer leading in their silver harnesses, the immaculate moon trailing behind with a splash of golden radiance left following, Poseidon watched her with caring eyes, watching so as to be assured that she was safe. Artemis was not only Zeus's most favored child, she was also Poseidon's favorite niece, when asked why he watched her night after night he would claim that the moon's tender, embracing light soothed him, and helped him to fall asleep. While everyday when Apollo flew overhead in his brilliant, flaming chariot pulling the great, blazing sun, Poseidon slept. Whenever Apollo wanted to land to visit Poseidon he found that he was sound asleep. While Apollo was Poseidon's favorite nephew, Poseidon didn't worry about his safety like he did Artemis. When Apollo started to get fed up with not being able to visit with Poseidon anymore; he asked his sister to talk to him about it.
One night Artemis landed near where Poseidon sat on the beach and walked over to him. "Poseidon," Artemis called. "We need to talk. Apollo has noticed that you're never awake during the day anymore. He wants to know why, I know but he wants me to ask. He wants to visit with you again."
"Oh, yeah I have been sleeping in pretty late recently," Poseidon hesitantly began, he didn't admit it but he also missed visiting with Apollo. "It's just I have trouble getting to sleep at night…" he lied.
"You do not! You are always watching me, I know why! I'm able, I won't get hurt, now, go to sleep!" Artemis exclaimed this in a very menacing voice.
The next day Apollo saw Poseidon lying on the shore under some palm trees, so he landed; to Apollo's displeasure Poseidon was dead asleep. As Apollo started to get back into his chariot he saw Pan playing his pipes and dancing in a nearby meadow with some of his fauns and nymphs. Walking over to Pan he thought up a plan to wake Poseidon up the next morning. Having reached Pan Apollo presented his plan to Pan who eagerly joined in on the scheme. That very night Artemis ignored Poseidon, as Apollo had instructed her to do, and then as dawn approached Poseidon stood up, stretched, and walked down the shore and under the water to his palace under the waves to go to bed. As the morning sun began its long journey over the horizon Pan leapt onto the shore with his fauns and they began to play their pipes loudly so as to wake Poseidon.
Having been woken from his sleep, so suddenly and much too soon, Poseidon threw off his covers, jumped out of his bed, and ran through the halls of the palace. Once outside of the palace Poseidon began to run faster and faster towards the shore to stop the pipes playing so he could go back to sleep, unaware that as he was running he was causing huge waves to swell and follow his every step on his very straight trail. As Poseidon ran up the shore to Pan and the fauns, so too did the waves, crashing and shoving on everything and everyone in their path, drowning some, throwing others, displacing all.
Poseidon stood over the sodden shape of Pan's body laying on the beach at his feet, his chest heaving with fury, face blazing red, teeth bared into a snarl, fists at his sides ready to swing if he would only let them, Poseidon proceeded to glare around with a piercing danger like gaze. Looking around Poseidon saw that he had made the waves crash and roar over the hapless creatures that frolicked there upon the beach. Poseidon's jaw dropped as he witnessed his creations turmoil, he had only meant to quiet them, never to hurt them. Pan coughed painfully lungs full of salt water, causing Poseidon to look down in anguish, appalled with his own masterpiece. Pan chuckled, "Well, you're awake as Apollo so wished."
Poseidon knelt down to help Pan his very being guilt ridden and seeking forgiveness. As Apollo approached, Poseidon looked up at him. "Is this your idea of a good time? This…t-this tidal wave! I don't know what to call it! I could have killed all of them! I could have killed Pan!" Poseidon snarled at Apollo for a while longer, and then calming down Poseidon listened to Apollo's plea for forgiveness. Through much discussion Poseidon finally agreed to sleep for part of the day and part of the night, for his favorite niece and nephew only though. Apollo felt horrible at his indirect hand in the harm of nymphs and fauns, and proceeded to promise to never try to force Poseidon to do anything he didn't want to do again.
Now Poseidon does not make tsunami on accident, he chooses when and where very carefully, weighing the risks and benefits. This is also why you see the tide following the phases of the moon, Poseidon still watches Artemis at night but also visits with Apollo during the day. The Japanese named Poseidon's great "tidal wave" tsunami, the Romans and Greeks had no actual name for it.