Seismic activity of New Zealand’s alpine fault more complex than suspected

New Zealand
Credit: CC0 Public Domain

A rupture along the full length of the fast-slipping Alpine Fault on New Zealand’s South Island poses the largest potential seismic threat to the southern and central parts of the country. But new evidence of a 19th century earthquake indicates that in at least one portion of the fault, smaller earthquakes may occur in between such large rupture events.

The findings published in the Bulletin of the Seismological Society of America suggest that some places along the fault, particularly around the towns of Hokitika and Greymouth, could experience strong ground shaking from Alpine fault earthquakes more often than previously thought.

The best paleoseismic evidence to date suggests the southern and central sections of the Alpine Fault, at the boundary separating the Australian and Pacific tectonic plates, typically rupture during very large full-section earthquakes of magnitude 7.7 or larger. The last such earthquake took place in 1717.

After trenching along the fault at the Staples site near the Toaroha River, however, Robert Langridge of GNS Science and colleagues uncovered evidence of a more recent earthquake along the northeastern end of the fault’s central portion. Radiocarbon dating places this earthquake between 1813 and 1848.

“One of the real challenges with the Alpine Fault—because it is so bush-covered—is actually finding sites that have been cleared and therefore can be studied,” said Langridge. “Once we started working there [at the Staples site] the story really grew in large part because of the richness of dateable organic material in the trenches.”

The four most recent earthquakes uncovered by the researchers at the site range in dates from 1084 to 1848. The events were confirmed by data collected from other nearby trenching sites and from geological deposits called turbidites, which are sediments shaken loose into a body of water by seismic activity, in lakes along the central section of the Alpine fault.

The most recent earthquake could represent a “partial-section” rupture of only the central portion of the Alpine fault, a rupture of the fault’s northern section that continued southwest into the central segment, or even triggered slip from a rupture along the nearby Marlborough Fault System. Langridge and colleagues said that there isn’t enough evidence yet to favor one of these scenarios over the others.

However, the findings do suggest that seismic activity on the Alpine Fault is more complex than suspected, particularly along its northern reaches where the plate boundary transitions into another fault zone.

“One of the outcomes of this study is that you should expect a shorter recurrence interval of strong shaking at fault section ends,” Langridge said. “Because of the recurrence times of earthquakes though, you obviously have to wait a long time to see the effects of such fault behavior.”

“That’s why paleoseismology is a vital tool in understanding faults,” he added, “because otherwise we’d have only short insights into the past.”

The Alpine Fault is sometimes compared with California’s San Andreas Fault, being another fast-moving strike slip fault near a plate boundary. Langridge said researchers

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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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