# 6: Shaky Ground and Big Waves


Up to this point, we have discussed earthquake sources: where earthquakes are likely to strike, how large they might be, and how often they might be expected. From the preceding chapter, you might conclude that we are not very far along in our ability to forecast the time when an earthquake might occur, although we have devised some fairly elaborate statistical procedures to describe our uncertainty.

It is also important to describe the geologic setting at the Earth’s surface, in particular, the response of the ground to an earthquake. Most of us are concerned less about the strength of the earthquake itself than we are about its effects where we are at the time, or where we live, or own property, or work. As has been said about politics, all earthquakes are local.

I am continually amazed at the apparently random damage of a major earthquake. The Nisqually Earthquake, with its epicenter close to Olympia, did major damage in Seattle, but Tacoma, much closer to the epicenter, got off fairly easily. After the 1994 Northridge Earthquake, I visited the Fashion Square Mall, in which several major stores and a large parking garage were demolished. Nearby, other shopping malls had hardly been damaged at all. This was not necessarily due to the distance from the epicenter of the earthquake. Interstate 10, connecting Santa Monica and downtown Los Angeles, on the south side of the Santa Monica Mountains and far from the Northridge epicenter, suffered severe damage, including the collapse of a major interchange. But condominiums and houses perched high in the Santa Monica Mountains, closer to the epicenter, were not severely damaged.

We have now come to recognize certain geologic settings where built structures are likely to suffer much more earthquake damage than others. Liquefaction maps of Seattle and Olympia were prepared prior to the 2001 Nisqually Earthquake, and liquefaction tended to be limited to those areas where those maps predicted it would occur. The Oregon Department of Geology and Mineral Industries has published maps of Portland, Salem, and Eugene locating the more hazardous environments with respect to construction. Similar maps have been prepared for Victoria, B. C. Although no two earthquakes will produce the same damage pattern in a given region, certain sites can be recognized as hazardous in advance of decisions to develop them.

Some of the greatest losses of life and property resulting from the dislodging of great masses of earth as landslides and rockfalls. These are not the normal mudslides that plague the Pacific Northwest in a rainy winter. In some cases, they are much larger volumes of rock and soil that would not move even during very heavy rainfall.

For example, an M 7.9 earthquake on the subduction zone off the coast of Peru on May 31, 1970, caused a slab of rock and ice hundreds of feet across to break off a near-vertical cliff high on Mt. Huascarán, the highest mountain in Peru. The mass of rock and ice fell several thousand feet, disintegrated, slid across a glacier, then overtopped low ridges below the glacier and became airborne. After falling back to the ground, the rock mass swept down the valley of the Shacsha River, entraining the water of the river as it did so. This flow of mixed debris and water reached velocities of one hundred and twenty miles per hour. Seven miles from its source, this rapidly moving mass separated into two streams of debris, one of which rode over a ridge and buried the town of Yungay. The other stream of debris obliterated the city of Ranrahirca. Nearly 80,000 people lost their lives in this single landslide, the greatest recorded natural disaster in the Western Hemisphere prior to the Port-au-Prince earthquake of January 2010. Most of the residents of these overwhelmed cities died instantly, without warning. The entire time from the first collapse high on Huascarán to the destruction of these cities was less than four minutes!

Closer to home, the Oso landslide in March 2014 in Snohomish County, Washington, on the western slope of the Cascades, killed 43 people and was categorized as a national disaster emergency. The area had previously been determined to be a landslide hazard— a previous landslide had been described by the USGS in 2006. Yet local government took no responsibility for warning people about the hazard, and in fact not only allowed additional houses to be built in the landslide area but also permitted logging that further destabilized the slope.

But landslide danger is not limited to steep slopes. Nearly flat areas underlain by clean, water-saturated sand may fail by liquefaction of the sand, which bursts to the surface as fountains and causes the land itself to move like a gigantic snowboard, snapping utility lines. During the Northridge Earthquake, a mass of land along Balboa Boulevard slid along a very gentle slope, rupturing a buried water line and a gas line. Escaping gas led to a fire that destroyed many homes in the vicinity. Television newscasts showed the odd combination of flames leaping above the roadway combined with torrents of water from the ruptured water line.

People living on the coast face another hazard: tsunamis. Tsunamis have produced catastrophic losses of life in the tens of thousands. The earthquake generating a tsunami may be thousands of miles away, across the ocean. The Pacific Northwest had its own deadly tsunami on the Easter weekend of 1964, after the great Alaskan earthquake.

One of the main reasons our risk is increasing is that we are building in increasingly unstable and dangerous areas. The demand for housing has expanded urban development into river floodplains like the Duwamish River in Seattle, steep hillslopes like Salmon Beach in Tacoma, and sandbars such as Seaside on the Oregon coast. These environments pose hazards other than earthquakes, as shown in the drowned-out homes in the floods of February 1996, and in the recent mudslides of Portland and Puget Sound, all unrelated to earthquakes.

I was astounded to read an Associated Press article on December 23, 1996, stating that the demand for building sites in the Portland metropolitan area is so strong that builders hire professional scouts to look for owners of undeveloped land who might be persuaded to sell, if not now, perhaps two or three years in the future. Land hunters may call up a title company and request information on any land parcel two acres or larger in a particular area. Armed with that information, they start calling landowners. Some land in Washington County, Oregon, is reported to be selling for more than $150,000 an acre. The article did not mention that some of these building sites around Portland, as well as Seattle and other cities in the Northwest, are dangerously flawed by their geology, with possibilities of landslides, flooding, earthquake shaking, and liquefaction. I know of no automatic legal provision that a potential homeowner in these newly developed subdivisions (as well as in neighborhoods long since built up) must be fully informed of these geologic hazards before purchasing a lot or a home. I recall the Keizer homeowner who had lived in his new house only a few months when he was flooded out by the Willamette River in February 1996. Said he on the TV evening news: “The county said it was OK.” Neither the landowner, who may get more than$100,000 an acre for the family farm, nor the developer wants to be the one to enlighten the unwary buyer.

California now has legislation that requires inspection of building sites with respect to earthquake hazards as well as other geologic hazards. Protection of this sort is available in Washington and Oregon only in a few communities such as Seattle and King County, where grading ordinances have been passed.

In all these cases, it is possible to assess the geologic hazards to construction and, in most cases, to “engineer” around them, although strengthening a building site against earthquakes increases the cost of development. The person building on a particular site (or moving to an already-built house on such a site) must weigh the risk of an unlikely but potentially catastrophic earthquake against the possibility that the house could remain safe for a lifetime. In the two chapters that follow, I consider these hazards and conclude that we know quite a lot about predicting how a particular site will respond, even though we do not know when the earthquake will strike that will put the site and the people living and working there at risk. We know enough that we could put teeth into laws requiring that a buyer be made aware of geologic hazards before investing in a piece of property. We could make sure that local grading ordinances require inspection of building sites against possible geologic hazards, in addition to inspection of the building itself. I will return to such ordinances in Chapter 14.

6: Shaky Ground and Big Waves is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert S. Yeats via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.