Ancient lake contributed to past San Andreas fault ruptures

Ancient lake contributed to past San Andreas fault ruptures
San Andreas fault area. Credit: Rebecca Dzombak

The San Andreas fault, which runs along the western coast of North America and crosses dense population centers like Los Angeles, California, is one of the most-studied faults in North America because of its significant hazard risk. Based on its roughly 150-year recurrence interval for magnitude 7.5 earthquakes and the fact that it’s been over 300 years since that’s happened, the southern San Andreas fault has long been called “overdue” for such an earthquake. For decades, geologists have been wondering why it has been so long since a major rupture has occurred. Now, some geophysicists think the “earthquake drought” could be partially explained by lakes—or a lack thereof.

Today, at the Geological Society of America’s 2020 Annual Meeting, Ph.D. student Ryley Hill will present new work using geophysical modeling to quantify how the presence of a large lake overlying the fault could have affected rupture timing on the southern San Andreas in the past. Hundreds of years ago, a giant lake—Lake Cahuilla—in southern California and northern Mexico covered swathes of the Mexicali, Imperial, and Coachella Valleys, through which the southern San Andreas cuts. The lake served as a key point for multiple Native American populations in the area, as evidenced by archaeological remains of fish traps and campsites. It has been slowly drying out since its most recent high water mark (between 1000 and 1500 CE). If the lake over the San Andreas has dried up and the weight of its water was removed, could that help explain why the San Andreas fault is in an earthquake drought?

Some researchers have already found a correlation between high water levels on Lake Cahuilla and fault ruptures by studying a 1,000-year record of earthquakes, written in disrupted layers of soils that are exposed in deeply dug trenches in the Coachella Valley. Hill’s research builds on an existing body of modeling but expands to incorporate this unique 1,000-year record and focuses on improving one key factor: the complexity of water pressures in rocks under the lake.

Hill is exploring the effects of a lake on a fault’s rupture timing, known as lake loading. Lake loading on a fault is the cumulative effect of two forces: the weight of the lake’s water and the way in which that water creeps, or diffuses, into the ground under the lake. The weight of the lake’s water pressing down on the ground increases the stress put on the rocks underneath it, weakening them—including any faults that are present. The deeper the lake, the more stress those rocks are under, and the more likely the fault is to slip.

What’s more complicated is how the pressure of water in empty spaces in soils and bedrock (porewater) changes over both time and space. “It’s not that [water] lubricates the fault,” Hill explains. It’s more about one force balancing another, making it easier or harder for the fault to give way. “Imagine your hands stuck together, pressing in. If you try to slip

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The Explosive Hazard Hiding in an African Lake | Science

Lake Kivu is one of Africa’s strangest bodies of water. An unusual set of properties make it an intriguing subject for scientists, as well as a potential source of peril and prosperity for the millions of people living nearby.

Kivu doesn’t behave like most deep lakes. Typically, when water at the surface of a lake is cooled — by winter air temperatures or rivers carrying spring snowmelt, for example — that cold, dense water sinks, and warmer, less dense water rises up from deeper in the lake. This process, known as convection, generally keeps the surfaces of deep lakes warmer than their depths.

But at Lake Kivu, circumstances have conspired to block this mixing, giving the lake unexpected qualities — and surprising consequences.

Straddling the border between Rwanda and the Democratic Republic of the Congo, Kivu is one of a string of lakes lining the East African Rift Valley where the African continent is being slowly pulled apart by tectonic forces. The resulting stresses thin the Earth’s crust and trigger volcanic activity, creating hot springs below Kivu that feed hot water, carbon dioxide and methane into the lake’s bottom layers. Microorganisms use some of the carbon dioxide, as well as organic matter sinking from above, to create energy, producing additional methane as a byproduct. Kivu’s great depth — more than 1,500 feet at its deepest point — creates so much pressure that these gases remain dissolved.

This mixture of water and dissolved gases is denser than water alone, which discourages it from rising. The deeper water is also saltier due to sediment raining down from the upper layers of the lake and from minerals in the hot springs, which further increases the density. The result, says limnologist Sergei Katsev of the University of Minnesota Duluth, is a lake with several distinct layers of water of sharply different densities, with only thin transition layers between.

The layers can be separated roughly into two regions: one of less-dense surface water above a depth of about 200 feet and, below that, a region of dense saline water that is itself further stratified, says Alfred Wüest, an aquatic physicist at the Swiss Federal Institute of Technology in Lausanne. There is mixing within each layer, but they don’t interact with each other. “Just think of the entire water mass sitting there for thousands of years and doing nothing,” says Wüest, author of a 2019 article in the Annual Review of Fluid Mechanics surveying convection in various lakes of the world, including weird outliers like Lake Kivu.

But Lake Kivu is more than just a scientific curiosity. Its unusual stratification and the carbon dioxide and methane trapped in its deeper layers have researchers worried that it could be a disaster waiting to happen.

Lake Kivu Graphic
The unique makeup of Africa’s Lake Kivu prevents the mixing typically seen in other deep lakes, leading to unusual stratification of the waters. There are distinct density differences between each layer. The sharp transition between two of those layers is shown here, with
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