The Sangre de Cristo Fault & Crestone’s role in earthquake studies

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by James P. McCalpin

Crestone played a pivotal role in the development of the science of studying large earthquakes and the hazards posed by them. It continues to play an important role by example even today. This odd situation resulted from the chance events described below.

Geologists have long known that the Sangre de Cristo Mountains are Colorado’s youngest mountain chain, raised up in geologically recent time (past 20 million years) by slip on the Sangre de Cristo fault. Prior to 1978, most of the uplift was assumed to have happened 10-20 million years ago. Since historic records began (1851) there had been no historic earthquakes on the Sangre de Cristo fault, so the fault was thought to be “dead.” However, in the mid-1970s the U.S. Geological Survey in California began developing methods for recognizing if historically-quiet faults had moved during large earthquakes in prehistoric times, such as the past 10,000 years (the Holocene epoch). Coincidentally, I worked as an entry-level USGS geologist during this time.

The new “paleoseismic” method looked for places where prehistoric large earthquakes had caused the fault to break upward all the way to the ground surface, causing displacement of the ground surface and all the underlying young geologic deposits. At such sites a deep trench would be dug across the fault to expose the fault plane in vertical cross section on the trench walls. The trench walls were meticulously cleaned using methods borrowed from archeology. From the cleaned walls it was apparent which strata had been offset and how large the offset was. Back in California, I was asked to help log USGS’s first paleoseismic trench (1977) on the Raymond fault in suburban Los Angeles, because the previous summer I had learned trench wall logging techniques from archeologists on a dig in Alaska. USGS wanted to see if archeo-style trench logging would work for interpreting fault displacement histories, and I was the guinea pig.

Although getting in on the ground floor of a new science was exciting, I did not look forward to spending the next 20 years in the smog of the LA Basin trenching all those urban faults. The mountains of Colorado beckoned me, and I was sure paleoseismic techniques could be applied to some Colorado fault. So in 1978 I resigned from USGS and returned to Colorado to begin a PhD at the Colorado School of Mines (CSM), studying the activity of the Sangre de Cristo fault. The summer of 1979 was spent hiking all the glaciated valleys of the west Sangres and mapping the glacial moraines and their connection with the alluvial fans that make up the piedmont zone in which Crestone and the newly-created Baca Grande subdivision are located.

I had been introduced to Maurice Strong in 1978, when he gave the commencement address at CSM. He invited me to visit him in Crestone when I started working the Crestone-Baca area. So in 1979 I met him at his new home (Baca Ranch headquarters) and surprisingly he gave me the keys to the ranch gates, so I could drive along the fault trace between Crestone and the Great Sand Dunes. But more importantly, he offered me free use of the ranch backhoe and an operator to dig two trenches across fault strands in the Baca Grande development. This offer elevated the whole study to a higher state-of-the-art, because the trenches showed that the Sangre de Cristo fault was the youngest and most active fault in Colorado, and in fact, in the entire 600-mile-long Rio Grande rift. Trenches showed that the fault has ruptured in Magnitude 7+ earthquakes about once every 10,000-15,000 years for the past several hundred thousand years at least, with the latest such event about 7,500 years ago. During these earthquakes the east side of the fault had jumped up 5-6 feet and offset the ground surface, creating a vertical “fault scarp” (see photo of the 1983 Borah Peak scarp, Idaho).

Meanwhile, major changes were happening in the world that influenced paleoseismology. Nuclear power was on the rise worldwide, and the most sophisticated studies were for those done for nuke plants. English became the de-facto global language and foreign geological surveys all looked to America for the most modern techniques for fault hazard analysis. In 1989 I took the techniques to the USSR in the first years of Gorbachov’s “glasnost”, and was in a trenching field camp in Siberia in August of 1991 when the KGB arrested him and tried to take over the government in Moscow. That was the end of the USSR, the beginning of modern Russia, and the impetus for me to write the first reference book on Paleoseismology (published in 1996). That 600-page tome included many examples from the Sangre de Cristo fault and Crestone-Baca area, and won major awards from scientific societies in 1999 and 2000. It soon became the international standard reference work, and was adopted by many countries around the globe as the handbook on how to do active fault studies. Geologists from Beijing to Santiago were looking at examples of fault scarps in the book, many from the Baca Grande subdivision, to apply to their work.

Since 2000 the field of paleoseismology has continued to develop and grow, attracting many of the best and brightest young geologists to analyze fault hazards affecting their country’s critical infrastructure. To honor the rapid progress in techniques, I revised the Paleoseismology book in 2009 (in my office in Crestone), and it was translated into a 2-volume Russian set in 2011. A translation into Chinese is pending, reflecting concern in the Far East after the 2010 Japan earthquake and Fukushima nuclear power plant meltdown. The examples and case histories from the Crestone area are still included in the book, now being examined by an audience of young geologists to mid-level hazard analysts to senior policy officials. The recent energy boom in oil and natural gas has led to many planned pipelines and coastal oil & LNG terminals, all of which have to be carefully studied for potentially active faults. Since 2010 I have been spending each summer in Alaska and northern Canada, mapping active faults from a helicopter using the latest LiDAR imagery, and then dropping down to trench the faults while the bear guards stand watch nearby.

In 2011 USGS-Denver and the Colorado Geological Survey collected high-resolution LiDAR imagery over the entire San Luis Valley. LiDAR “sees through the trees” and has revealed many more fault traces along the Sangre de Cristo than were previously mapped from aerial photos. So the spatial probability of being near an active fault trace has increased for some Baca residents. But the temporal probability of the Big One occurring in the near future still remains very low, because we are only 7500 years through the 10,000-15,000 year-long seismic cycle between M7+ earthquakes. The probability of the Big One happening in the next 100 years is less than 1%.

Since 2011 I have given several keynote lectures at international conferences, suggesting that fault maps based on airphotos may have missed many active strands, especially in forested areas, and that to see all the strands we need to switch to LiDAR-based fault mapping. The foreign audiences sat captive while I showed them LiDAR images and fault maps of Crestone and the Baca. This year I will talk at the international paleoseismology conference in South Korea; visit a Japanese nuclear site with suspected faults; and fly to Vietnam to help their geological survey set up an active fault trenching program. And it all started in Crestone.

It is a cliché in geology that you never know where your career will take you. Certainly, when I was 29 years old hiking the Sangres and camping in Crestone in 1979-80, I had no idea that 20 years later I would be living here, and influencing the path of global hazard science. As they say, “what a long strange trip it’s been.”

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