13 new acres of prime landslide derived beachfront property available now!

Figure 1: Map showing location of the Mud Creek slide. Before and after photos are present showing the spectacular movement of material. Also present are other similar landslides and instrumentation

Geologic time is slow. Very slow. When geologists talk about mountains rising, such as the Himalayas, they speak on the millimeter per year scale. Every so often, however, drastic movement occurs. On May 17^th2017, the Mud Creek landslide occurred off of Highway 1 in Big Sur (fig.1). The slide was so large, it added 13 new acres to the coastline of California in an instant (fig. 2); amazingly, no one was injured. Since then, scientists have been able to research the slide to figure out how and why the slope failed so dramatically. 

Figure 2: Oblique time lapse view of the slope before failure, after, and the process of rebuilding the road.  

This event was what’s known as a bedrock landslide, which occur at large scales and often move very large, deep masses of earth. Bedrock landslides are highly dependent on groundwater rather than surface water. If the water table rises high enough into the soil that overlies the bedrock, it will effectively cause the overlying rock and soil to float. Water finds its way between individual grains and exerts a pressure on each grain, pushing them away from each other. This force destabilizes the rock, leading to rapid and catastrophic slope failure. 

It takes time for groundwater to move and cause the water table to rise. Groundwater flow isn’t like a raging river of water moving through the subsurface: in reality, groundwater saturates deeper rocks, sort of like a sponge, and only slowly creeps along if there is sufficient water pressure. Because of this, bedrock landslides can occur on seemingly innocuous, sunny days often months after a large rainfall event. In the case of the Mud Creek landslide, the last large rainfall events occurred in February, March, and April (fig. 3), but the slide didn’t occur until mid-May.

Figure 3: Hydrograph showing discharge of the Big Sur River. Discharge can be used as a proxy for rainfall events by looking for spikes in discharge. The landslide occurred in May when there was no rainfall occurring. 

 The Mud Creek landslide is interesting because it was actually moving steadily before the catastrophic slide occurred. Using remote sensing satellites, researchers were able to track the motion of the Mud Creek landslide for the past several years before the big slide occurred. Before total failure, the slope was moving at a stable rate, creeping along at about .24 to .43 m/yr (fig. 4). During this time (2009-2017), California was in a historic drought. Small rainfall events during the winters would cause the velocity of the slide to increase slightly, but it wasn’t until the historically wet winter of 2017 that the slide started to accelerate and deviate from past years’ motion (fig.4).

Researchers attribute the sudden catastrophic failure of the Mud Creek landslide to rapid changes in climate. The fast transition from a drought to very intense rain events caused the Mud Creek landslide to become unstable very quickly. Other similar, slow moving slides in the area remained stable, likely due to their smaller slope angles. 

Figure 4: Top: Downslope velocity of 3 creeping landslides. Notice rapid acceleration and subsequent failure of the Mud Creek slide. Bottom: Solid lines are precipitation values. The dotted line is the calculated pore fluid pressure. Notice how pore fluid pressure rapidly increases before the point of catastrophic failure.  

            California is currently experiencing another historically wet winter after a small drought, and Sierra snowpack currently stands at about 146% of what it was last year. What will this mean for catastrophic slope failure in California? If the Mud Creek landslide is any indication, it will likely mean more landslides during the spring months as snow melts and percolates through the subsurface. Keep a close eye on the coastal ranges of California this coming year and hopefully we’ll see some geology in action! 

Handwerger, A.L., Huang, M., Fielding, E.J., Booth, A.M., and Bürgmann, R., 2019, A shift from drought to extreme rainfall drives a stable landslide to catastrophic failure: Scientific Reports, p. 1–12, doi:10.1038/s41598-018-38300-0.

https://earthobservatory.nasa.gov/images/144552/a-strong-start-to-sierra-snowpack?utm_source=TWITTER&utm_medium=NASA&utm_campaign=NASASocial&linkId=63616203

Buried liquid water on Mars?

The question of whether there is water on Mars has been hotly debated for decades. It has generally been proven that water exists on Mars, though it’s mostly trapped in its ice form. For life to exist as we know it, liquid water is a vital component. Because Mars has such a thin atmosphere, liquid water on the surface isn’t stable and will immediately boil off. This makes it very tricky to even begin to try to observe liquid water. Recently however, using radar instruments, Italian researchers have discovered evidence of a large, liquid water lake beneath the glaciers at the southern pole of Mars.

Aboard the EU’s Mars Express spacecraft is a special instrument called a radar sounder. Radar sounders work by shooting specific frequencies of radar pulses at the surface of a planet and then observing the reflected signal. This can be thought of like clapping in a cave and listening to the echo that comes back to you. Depending on what the echo “sounds” like, scientists can gain a wealth of information about whatever surface they aimed the instrument at. Differing frequencies used by the instrument contain information about the surface rock, ice, or even what lies beneath the surface.

Figure 1: Radargram of the subglacial lake. In this figure, the x-axis is horizontal distance on the planet, and the y-axis is the time is takes the radar signal to bounce back to the spacecraft, which can be interpreted to understand subsurface topography. This figure can be thought of as a cross section. The subglacial lake can be seen at the basal reflection label. The SPLD label shows the South Polar Layered Deposit, which is an area of the south pole where pure water ice is formed in layers. Figure 2A from Orosei et al. (2018).

Using a radar sounder named MARSIS, Italian scientists made several flybys of the south pole region, which is mostly covered in water-ice glacier year round. What they found were bright subsurface radar reflections where, ordinarily, there should be nothing (fig.1). When you shoot a radar wave at a material, the target’s atoms will react by becoming electrically polarized to varying degrees (the atoms will have one side become more positive than the other, or vice versa). This degree of polarization can be unique for different materials, so it’s a great way to identify materials hidden in the subsurface. This quality of materials is known as dielectric permittivity. By analyzing the MARSIS radar reflections, the scientists discovered that the dielectric permittivity of the anomalous reflection matched that of water-bearing materials. By making several flybys of the area, the scientists were able to map out the subglacial lake and found it to be about 20 km wide and approximately 1.5 km beneath the surface (fig.2) 

Figure 2: This image shows the estimated size of the lake. Radar tracks are colored depending on subsurface elevations with warmer colors corresponding to higher elevations and cooler corresponding to lower elevations. The perceived lake is marked by the bold, black line. Figure 3A from Orosei et al. (2018).

Does this confirm that there’s a subglacial lake at Mars south pole? It’s very likely. (SHARAD, another radar sounder orbiting Mars, didn’t see the lake, though this may be to due to the different set of frequencies SHARAD uses). Does this confirm that there’s life on Mars? Not yet. For all we know, the lake could be completely inhospitable, or might not even be there in the first place! What this study does provide is compelling evidence that the secrets to extraterrestrial life in our solar system might be hiding under some ice, just one planet away!

Orosei, R., Lauro, S. E., Pettinelli, E., Cicchetti, A., Coradini, M., Cosciotti, B., … Seu, R. (2018). Radar evidence of subglacial liquid water on Mars. Science, 361(6401), 490 LP-493. Retrieved from http://science.sciencemag.org/content/361/6401/490.abstract

Intermittent Streams Across Land Use Gradients: Talk Summary

Speakers: Dr. Sarah Godsey and Dr. Rebecca Hale

Why should we care about temporary streams? Temporary streams include both intermittent streams (streams that periodically cease to flow, usually seasonal) and ephemeral streams (streams that only flow in immediate response to a rainstorm). A third of the US population relies on temporary streams for a portion of their water supply, and half of the stream lengths in the US are temporary, though the number of temporary streams is predicted to increase with climate change. This is important to consider when thinking about water management. Although many water policy decisions are based off of the national hydrography dataset (NHD), which maps out stream networks in the United States, one study has shown that the NHD is incorrect ~50% of the time, often underestimating the extent and permanence of headwater streams.

Styles of partially intermitted stream networks. Blue indicates running water, orange dashes represent intermittently dry areas. Image from Rebecca Hale.

But how do temporary streams dry? While it may seem logical to think of streams contracting from their tips, there are actually multiple possible modes of drying. Non-stable streams may contract from their tips, but they also may have short or long gaps of flow between the headwaters and the outlet. Where, why, and for how long do these sections dry up?

In an effort to start answering these questions, Rebecca Hale has been conducting a case study in the Gibson Jack watershed here in Pocatello. Currently, the NHD models this watershed as one that contracts from its tips. She has used direct field observations as well as temperature loggers (relating temperature fluctuations with presence of water) to determine flow regimes through time. She found that even in a relatively small watershed, like Gibson Jack, there were stable sections, sections that retracted from the tips, sections with various sized gaps in flow, and even sections that retracted from both ends. Even on a relatively small scale, variable, dynamic flow regimes were observed. To better understand of the ecological response to intermittency in a network context, Rebecca is also studying organic matter decomposition and primary productivity in Gibson Jack, and relating data collected to the flow regime.

Urban intermittent streams are another foci of Rebecca’s work. Her research is focusing on infrastructure use across climate gradients, impacts of city design on runoff, and decomposition rates and mechanisms within these impacted systems. She argues that currently available models may not be accurate across all regions and thus more research is needed to elucidate the mechanisms at work in urban intermittent streams.

This body of work will improve understanding of intermittent systems in both natural and human-impacted environments.