The Orphan Tsunami of 1700: Japanese Clues to a Parent Earthquake in North America

Introduction

Introduction ~i t; 3fJ~: OUTSIDERS SCARCELY KNEW of northwestern North America in the year 1700. Leading European geographers of the time left that part of the map blank. Not until 1741 would Russians land in Alaska. From there to Oregon's Cape Blanco, the coast would remain uncharted until Spanish and English expeditions of the 1770s. Across the Pacific Ocean in Japan, unusual seas ran ashore in 1700. People wrote of the effects: flooded fields, wrecked houses, a fire, a shipwreck, evacuation, fright. Having felt no earthquake beforehand, some writers called the flooding a "high tide" and most resisted calling it a tsunami. None could have known that a seismic shift on a North American fault had set off a train of trans-Pacific waves. Far from its parent earthquake, the tsunami of 1700 was an orphan. The 1700 tsunami in Japan would remain an orphan for nearly three hundred years. The North American fault at its source would go unnoticed until the last decades of the 20th century. Today the fault is charted, and an earthquake on it is regarded as the orphan's parent. This kinship gives the earthquake an exact date (January 26, 1700) and an estimated size (magnitude 8.7-9.2) that spur precautions against future earthquakes and tsunamis in the United States and Canada. THE INDIAN OCEAN TSUNAMI of December 26, 2004, reminded the world of what an earthquake of magnitude 9 can do. Earth rarely provides such reminders; only three 20th-century earthquakes reached or exceeded magnitude 9.0 worldwide. The Indian Ocean disaster, by affecting areas from southeast Asia to Africa, raised concern about earthquake and tsunami hazards around the planet. The disaster reminded North Americans of such hazards not only in Alaska, struck in 1964 by an earthquake of magnitude 9.2, but also at Cascadia-the region west of the Cascade Range from southern British Columbia to northern California. Cascadia is home to a gently inclined boundary between two of the moving tectonic plates that make up Earth's outer shell. The shallow, mostly offshore part of the boundary is the fault that ruptured in 1700. What losses will Cascadia sustain the next time it breaks? A scenario printed in 2005, several months after the Indian Ocean disaster, gives an idea of what to expect. The scenario begins with an earthquake of magnitude 9.0. Strong shaking lasts for minutes along the Pacific coast in British Columbia, Washington, Oregon, and California. The main coastal highway, U.S. 101, becomes largely impassable, and landslides "sever highway travel between the coast and inland areas." Thus isolated, coastal residents "have to do much of the work of rescuing those trapped in the rubble." Introduction 3 The expected damage extends inland to Vancouver, Seattle, and Portland. In this urban corridor, "utilities and transportation lines in some areas could be disrupted, perhaps for months." Damage to tall buildings "could lead to significant fatalities in downtown areas." These risks used to be unthinkable. Cascadia has no written records of homemade earthquakes larger than magnitude 7.5, nor of transoceanic tsunamis generated in its backyard. However, the region does have geologic records of great earthquakes-shocks of magnitude 8 or larger-and of tsunamis they spawned. It is the most recent of these Cascadia tsunamis that entered written history in Japan. RECOGNIZING A HAZARD is just the first step toward dealing with it. Next, the hazard must be defined well enough for practical precautions to be devised and put into effect. Discoveries about the orphan tsunami of 1700 helped drive this process at Cascadia. Earth science in North America revealed earthquake and tsunami hazards that Japanese history sharply defined. The findings spurred precautionary steps like the mapping of areas that future Cascadia tsunamis may flood and the posting of evacuation signs. The safeguards also include teaching schoolchildren the basics of tsunami survival: If you feel a strong earthquake, run to high ground. If the sea recedes strangely, run to high ground. If a tsunami ensues, stay on high ground; its first wave probably won't be the last-or the highest. If only such precautions could have been taken around the Indian Ocean before its 2004 disaster. Most of the victims experienced the earthquake, which was felt even in Thailand and Sri Lanka. Many saw the sea withdraw before the first damaging wave. Some thought the first wave would be the last. Almost everyone was surprised by the earthquake's magnitude and by the tsunami's height...