OSO — A new paper published by a team of researchers working on the Oso mudslide site reveals a growing understanding of the slide’s history, and points to further avenues for exploration.

When the slide struck March 22, 2014, a hillside in the valley of the North Fork Stillaguamish River was known to have slid periodically over the decades, occasionally shifting the course of the river.

The 2014 slide was orders of magnitude larger, wiping out the Steelhead Haven neighborhood, killing 43 people and forever altering how people of the valley view the steep slopes over their heads.

Even if a definitive cause of the slide has not been identified — and may never be — each successive study narrows the field of research a little more.

Meanwhile, the legal battle to determine whether anyone should be held responsible for the disaster has brought to light more information researchers have discovered about the area’s geology.

Drilling on the site has found evidence that the 2014 slide broke loose deep within a thick layer of clay that formed on the bottom of an ancient glacial lake, according to court papers. It was in a part of the hill that hadn’t fallen before.

One key piece of research to date was the publication of the July 2014 report from the Geotechnical Extreme Events Reconnaissance (or GEER) group, engineers and geologists who gathered as much raw data as possible from the slide area before the land began to be obscured by natural processes.

The new paper distills the GEER data and has been accepted by the peer-reviewed journal Geomorphology, with an expected publication date early next year.

The new article, simply titled “The March 22, 2014 Oso Landslide, Washington, USA,” reinforces the earlier report’s key conclusion that the intense rainfall the region experienced in the three weeks leading up to the slide played a major role in its triggering.

The paper posits a two-stage collapse at Oso. The first stage reactivated the older slide people already knew about, which the geologists call the Hazel slide. The last time it collapsed was in 2006. The destabilized mass merged with and incorporated parts of an ancient landslide, which was forested over. That created a deadly slurry of mud and trees.

The slide’s first phase was followed about two minutes later by the collapse of a portion of the Whitman Bench, which had lost support from below and slumped down the hillside.

In this interpretation of events, virtually all of the debris that crossed the river and slammed into Steelhead Haven was from the older landslides. The newer debris from the bench remained mostly within the area on top of the 2006 Hazel slide.

J. David Rogers is a landslide expert from the Missouri University of Science &Technology retained by the state for the Oso litigation. He also has been working with state drilling crews boring into the slide mass and the surrounding hillside. Prior to that effort, much of what people believed about the hillside came from educated guesses.

In a declaration filed with the court, Rogers said experts now know at what depth and in what soils the hill broke loose in March 2014. It was different from prior events at the Hazel slides and occurred in soil that hadn’t moved since the Ice Age.

“These two landslides appear to have different footprints, different soil compositions, different groundwater regimes and different causal mechanisms,” Rogers wrote.

Rogers has yet to opine on what he thinks triggered the deadly mudslide, but in a recent filing he concluded one theory — that a cribwall built at the toe of the 2006 Hazel slide somehow played a role — simply isn’t supported by the evidence in the dirt.

The cribwall was designed to minimize silt entering the river from the Hazel slide material. Lawyers for those harmed by the more recent Oso mudslide contend that groundwater piled up behind the cribwall and in adjacent settling ponds, making the hillside more ripe for a fall.

Rogers says that theory can’t be true, because the dirt shows the ground ruptured at a point three stories higher than the cribwall, and the sandy soils below the rupture wouldn’t have held the water.

The GEER report from 2014 listed 10 possible contributing factors to the Oso landslide, including the possibility that logging on the Whitman Bench in 2004 may have influenced the groundwater recharge system in the hillside and caused it to become more saturated.

The new paper in Geomorphology narrows the list of possible contributing factors to six, and logging isn’t among them.

Joseph Wartman, of the University of Washington Department of Civil and Environmental Engineering, was the lead author of the new paper and a leader of the GEER team in 2014. He said the Geomorphology article derives its hypotheses from collected data rather than from analysis.

In the case of logging, the data simply isn’t there, Wartman said. In addition, it was outside the scope of the initial GEER report, as well as the new paper, to evaluate land use policies and procedures.

“I think that to further explore the land use and logging question really requires some analysis and some field data,” Wartman said.

“There is a lack of evidence, because people weren’t monitoring the amount of groundwater recharge.”

“That doesn’t exist, but it doesn’t mean that cannot be collected,” he said.

A trial in the mudslide lawsuits is now scheduled for June. Attorneys for the plaintiffs have lined up experts to testify that timber harvests over the years made the hillside more prone to slides. Loss of tree cover allows more water to reach deeper ground, something experts call “throughfall.”

“An increase in the recharge to groundwater is especially dangerous to a deep-seated landslide complex as it not only increases the instability of the landslide but it also keeps the landslide in an unstable condition for longer periods of time,” Hugo Loaiciga said in one declaration. Loaiciga is a geomorphologist and engineer at the University of California, Santa Barbara.

Were it not for alterations in the landscape brought on by logging, “I believe it is very unlikely the Oso Landslide, with its highly mobile and extensive debris flows, would have occurred,” he wrote.

While the Hazel slide has been active over several decades, most of the sliding has been comparatively minor and had little visible lasting impact on the landscape, even if it might have helped set the stage for the Oso slide, Wartman said.

The North Fork Stillaguamish Valley shows geologic evidence of many large-scale slides that have reshaped the contours of the valley over thousands of years. Aerial lidar mapping has identified several in the vicinity of Oso, including the Hazel slide, that are older than industrial civilization.

“Detailed consideration of land use practices (most notably, timber harvesting) was beyond the scope of our investigation; however, essentially all of the large landslides in the Stillaguamish River Valley visible in the LiDAR shaded relief images pre-date logging,” the authors of the Geomorphology paper wrote.

The Oso slide is on the same geologic scale as other ancient slides, Wartman said.

Samples of bark from long-buried trees that were uncovered by the Oso slide were radiocarbon-dated to about 6,000 years ago.

“There probably have been repeated episodes of landsliding and a lot of the deposits are probably mixed together,” he said.

Chris Winters: 425-374-4165; cwinters@heraldnet.com. Twitter: @Chris_At_Herald.