EQNEWS / January, 1997

Cover Photograph:
The Great Cascadia Earthquake, January 27, 1700
Cover: This month's earthquake is prehistoric in North America, but its probable effects in Japan are historic. Coastal field work over the past ten years by many scientists from universities and federal, state, and provincial geological surveys in northern California, Oregon, Washington, and British Columbia has uncovered overwhelming evidence of a great (moment magnitude larger than 8) earthquake or series of earthquakes about 300 years ago.

The earthquake (or earthquakes) was caused by a sudden slip of the Pacific plate beneath the North America plate along the Cascadia subduction zone, a 1000-km (600-mi.)-long fault that marks the boundary between the two tectonic plates off the coast of western North America. Some of the most convincing and best-preserved evidence of the earthquake(s) are sand layers that cover the peaty soils of coastal lowlands. These layers are believed to have formed when a tsunami generated by the powerful earthquake inundated the coastal lowlands.

The above photo shows such a sand layer in an exposure near the mouth of the Salmon River along the central Oregon coast about 8 km (5 mi) north of Lincoln City. One of the series of tsunami surges that probably followed the earthquake by 20 minutes to several hours picked up sand from the beach or dunes as it came ashore and deposited the sand as it moved up the river valley. At the site of the photo, the sand bed covers the remains of two fire pits dug by Native Americans, perhaps not long before the tsunami. The layers are well preserved partly because much of this part of the Oregon coast permanently subsided about 0.5-1.0 m (2-3 ft.) during the earthquake. The rise in sea level produced by the subsidence allowed tidal sediments to quickly bury the sand layers, protecting them from later erosion.

An exciting new discovery about this earthquake is that its tsunami was probably recorded in Japan on 27 January AD 1700. Extensive, unusually precise radiocarbon dating of geologic evidence of the earthquake(s) at estuaries in northern California, Oregon, Washington, and British Columbia suggested that the earthquake(s) occurred during a twenty-year period centered at about AD 1710.

Recent study of written records from this period in Japan by Japanese historians and seismologists shows that a large tsunami was recorded in Japan on 27 January AD 1700. Because written records from other coasts that experience great earthquakes, such as western South America, suggest that no earthquake large enough to produce this type of tsunami in Japan occurred during this period, the Japanese researchers concluded that the tsunami was probably produced at the Cascadia subduction zone. By making reasonable assumptions about the size and speed of the tsunami from the records of tsunami inundation along the Japanese coast the researchers concluded that the tsunami was produced by an earthquake larger than magnitude M8.5, probably about 9 P.M. on January 26, 1700, (Pacific Standard Time). This conclusion agrees not only with the geologic evidence, but with Native American legends of a large earthquake and/or tsunami that occurred one winter night about three hundreds years ago.

Image and text by:

Alan R. Nelson

U.S. Geological Survey