Drilling Reveals Fault Rock Architecture in New Zealand’s Central Alpine Fault

            Figure 1: Location map of study by Virginia Toy et al. Image Credit: GSA
Boulder, Colo., USA - Rocks within plate boundary scale fault zones become fragmented and altered over the earthquake cycle. They both record and influence the earthquake process. In this new open-access study published in Lithosphere on 4 Feb., Virginia Toy and colleagues document fault rocks surrounding New Zealand's active Alpine Fault, which has very high probability of generating a magnitude 8 or greater earthquake in the near future.

Descriptions already suggest that the complex fault rock sequence results from slip at varying rates during multiple past earthquakes, and even sometimes during aseismic slip. They also characterize this fault before rupture; Toy and colleagues anticipate that repeat observations after the next event will provide a previously undescribed link between changes in fault rocks and the ground shaking response. They write that in the future this sort of data might allow realistic ground shaking predictions based on observations of other "dormant" faults.

The first phase of the Deep Fault Drilling Project (DFDP-1) yielded a continuous lithological transect through fault rock surrounding the Alpine fault (South Island, New Zealand). This allowed micrometer- to decimeter-scale variations in fault rock lithology and structure to be delineated on either side of two principal slip zones intersected by DFDP-1A and DFDP-1B. Here, we provide a comprehensive analysis of fault rock lithologies within 70 m of the Alpine fault based on analysis of hand specimens and detailed petrographic and petrologic analysis. The sequence of fault rock lithologies is consistent with that inferred previously from outcrop observations, but the continuous section afforded by DFDP-1 permits new insight into the spatial and genetic relationships between different lithologies and structures. We identify principal slip zone gouge, and cataclasite-series rocks, formed by multiple increments of shear deformation at up to coseismic slip rates. A 20−30-m-thick package of these rocks (including the principal slip zone) forms the fault core, which has accommodated most of the brittle shear displacement. 

This deformation has overprinted ultramylonites deformed mostly by grain-size-insensitive dislocation creep. Outside the fault core, ultramylonites contain low-displacement brittle fractures that are part of the fault damage zone. Fault rocks presently found in the hanging wall of the Alpine fault are inferred to have been derived from protoliths on both sides of the present-day principal slip zone, specifically the hanging-wall Alpine Schist and footwall Greenland Group. This implies that, at seismogenic depths, the Alpine fault is either a single zone of focused brittle shear that moves laterally over time, or it consists of multiple strands. Ultramylonites, cataclasites, and fault gouge represent distinct zones into which deformation has localized, but within the brittle regime, particularly, it is not clear whether this localization accompanies reductions in pressure and temperature during exhumation or whether it occurs throughout the seismogenic regime. These two contrasting possibilities should be a focus of future studies of fault zone architecture.

Source:GSA

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The Network for tracking earthquakes exposes glacier activity: Accidental find offers big potential for research on Alaska's glaciers

Alaska’s seismic network records thousands of quakes produced by glaciers, capturing valuable data that scientis ts could use to better understand their behavior, but instead their seismic signals are set aside as oddities. The current earthquake monitoring system could be “tweaked” to target the dynamic movement of the state’s glaciers.
Credit: Chris Larson

Alaska's seismic network records thousands of quakes produced by glaciers, capturing valuable data that scientists could use to better understand their behavior, but instead their seismic signals are set aside as oddities. The current earthquake monitoring system could be "tweaked" to target the dynamic movement of the state's glaciers, suggests State Seismologist Michael West, who will present his research today at the annual meeting of the Seismological Society of America (SSA).

"In Alaska, these glacial events have been largely treated as a curiosity, a by-product of earthquake monitoring," said West, director of the Alaska Earthquake Center, which is responsible for detecting and reporting seismic activity across Alaska.

The Alaska seismic network was upgraded in 2007-08, improving its ability to record and track glacial events. "As we look across Alaska's glacial landscape and comb through the seismic record, there are thousands of these glacial events. We see patterns in the recorded data that raise some interesting questions about the glaciers," said West.

As a glacier loses large pieces of ice on its leading edge, a process called calving, the Alaska Earthquake Center's monitoring system automatically records the event as an earthquake. Analysts filter out these signals in order to have a clear record of earthquake activity for the region. In the discarded data, West sees opportunity.

"We have amassed a large record of glacial events by accident," said West. "The seismic network can act as an objective tool for monitoring glaciers, operating 24/7 and creating a data flow that can alert us to dynamic changes in the glaciers as they are happening." It's when a glacier is perturbed or changing in some way, says West, that the scientific community can learn the most.

Since 2007, the Alaska Earthquake Center has recorded more than 2800 glacial events along 600 km of Alaska's coastal mountains. The equivalent earthquake sizes for these events range from about 1 to 3 on the local magnitude scale. While calving accounts for a significant number of the recorded quakes, each glacier's terminus -- the end of any glacier where the ice meets the ocean -- behaves differently. Seasonal variations in weather cause glaciers to move faster or slower, creating an expected seasonal cycle in seismic activity. But West and his colleagues have found surprises, too.

In mid-August 2010, the Columbia Glacier's seismic activity changed radically from being relatively quiet to noisy, producing some 400 quakes to date. These types of signals from the Columbia Glacier have been documented every single month since August 2010, about the time when the Columbia terminus became grounded on sill, stalling its multi-year retreat.

That experience highlighted for West the value of the accidental data trove collected by the Alaska Earthquake Center. "The seismic network is blind to the cause of the seismic events, cataloguing observations that can then be validated," said West, who suggests the data may add value to ongoing field studies in Alaska.

Many studies of Alaska's glaciers have focused on single glacier analyses with dedicated field campaigns over short periods of time and have not tracked the entire glacier complex over the course of years. West suggests leveraging the data stream may help the scientific community observe the entire glacier complex in action or highlight in real time where scientists could look to catch changes in a glacier.

"This is low-hanging fruit," said West of the scientific advances waiting to be gleaned from the data.

Source: Seismological Society of America

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The Tibetan Plateau was larger than previously thought, geologists say

Known as the "Roof of the World," the Tibetan Plateau covers more than 970,000 square miles in Asia and India and reaches heights of over 15,000 feet. Hoke's discovery not only makes the plateau larger than previously thought, but also suggests that some of the topography is millions of years younger. Credit: Image courtesy of Syracuse University

Earth scientists in Syracuse University's College of Arts and Sciences have determined that the Tibetan Plateau -- the world's largest, highest, and flattest plateau -- had a larger initial extent than previously documented.

Their discovery is the subject of an article in the journal Earth and Planetary Science Letters (Elsevier, 2014).

Gregory Hoke, assistant professor of Earth sciences, and Gregory Wissink, a Ph.D. student in his lab, have co-authored the article with Jing Liu-Zeng, director of the Division of Neotectonics and Geomorphology at the Institute for Geology, part of the China Earthquake Administration; Michael Hren, assistant professor of chemistry at the University of Connecticut; and Carmala Garzione, professor and chair of Earth and environmental sciences at the University of Rochester.

"We've determined the elevation history of the southeast margin of the Tibetan Plateau," says Hoke, who specializes in the interplay between Earth's tectonic and surface processes. "By the Eocene epoch (approximately 40 million years ago), the southern part of the plateau extended some 600 miles more to the east than previously documented. This discovery upends a popular model for plateau formation."
Known as the "Roof of the World," the Tibetan Plateau covers more than 970,000 square miles in Asia and India and reaches heights of over 15,000 feet. The plateau also contains a host of natural resources, including large mineral deposits and tens of thousands of glaciers, and is the headwaters of many major drainage basins.

Hoke says he was attracted to the topography of the plateau's southeast margin because it presented an opportunity to use information from minerals formed at Earth's surface to infer what happened below them in the crust.

"The tectonic and topographic evolution of the southeast margin has been the subject of considerable controversy," he says. "Our study provides the first quantitative estimate of the past elevation of the eastern portions of the plateau."

Historically, geologists have thought that lower crustal flow -- a process by which hot, ductile rock material flows from high- to low-pressure zones -- helped elevate parts of the plateau about 20 million years ago. (This uplift model has also been used to explain watershed reorganization among some of the world's largest rivers, including the Yangtze in China.)

But years of studying rock and water samples from the plateau have led Hoke to rethink the area's history. For starters, his data indicates that the plateau has been at or near its present elevation since the Eocene epoch. Moreover, surface uplift in the southernmost part of the plateau -- in and around southern China and northern Vietnam -- has been historically small.

"Surface uplift, caused by lower crustal flow, doesn't explain the evolution of regional river networks," says Hoke, referring to the process by which a river drainage system is diverted, or captured, from its own bed into that of a neighboring bed. "Our study suggests that river capture and drainage reorganization must have been the result of a slip on the major faults bounding the southeast plateau margin."

Hoke's discovery not only makes the plateau larger than previously thought, but also suggests that some of the topography is millions of years younger.

"Our data provides the first direct documentation of the magnitude and geographic extent of elevation change on the southeast margin of the Tibetan Plateau, tens of millions years ago," Hoke adds. "Constraining the age, spatial extent, and magnitude of ancient topography has a profound effect on how we understand the construction of mountain ranges and high plateaus, such as those in Tibet and the Altiplano region in Bolivia."

Source: Syracuse University

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More Earthquakes for Chile? Seismic gap has not been closed

GPS measurements of the displacement vectors. Credit: GFZ

After the strong earthquake that struck Chile on April 2 (CEST), numerous aftershocks, some of them of a considerable magnitude, have struck the region around Iquique. Seismologists from the GFZ German Research Centre for Geosciences doubt that the strong earthquake closed the local seismic gap and decreased the risk of a large earthquake. On the contrary, initial studies of the rupture process and the aftershocks show that only about a third of the vulnerable zone broke.

This vulnerable area is referred to as the seismic gap of Iquique and a strong earthquake is expected to strike here. The Pacific Nazca plate meets the South American plate at South America's west coast. "In a subsea trench along the coast, the Pacific Ocean floor submerges beneath the continent building up tension that is released in earthquakes," explains Professor Onno Oncken of the GFZ. "In the course of about 150 years the entire plate boundary from Patagonia in the South to Panama in the North breaks completely with a segmented series of strong earthquakes." This cycle has been completed except for a last segment west of Iquique in northern Chile. As expected, the strong earthquake of April 2 took place exactly at this seismic gap.

No All-Clear
Initial analyses conducted by GFZ seismologists have shown that there is no sign that tension in the earth' crust has significantly decreased: "So far tension has been released only in the central section of this vulnerable zone," Oncken further explains. The series of earthquakes began on March 16 with a 6.7-magnitude earthquake. Although the main earthquake with a magnitude of 8.1 broke the central section of the seismic gap of a length of some 100 kilometres, two large segments further north and south remain intact, and these segments are able to cause strong earthquakes with a high risk of ground shaking and tsunamis.Oncken: "This means that the risk of one or even several earthquakes with a magnitude clearly above 8 still exists." Furthermore, the location and magnitude of the aftershocks suggest such a scenario.

Since the main quake struck, hundreds of aftershocks have been registered, the strongest that of April 2 (CEST) of a magnitude of 7.6. This earthquake struck about 100 kilometres south of the main earthquake's epicentre. Together with the its associated aftershocks, it forms a second rupture zone.

Scientists getting ready for a field trip
For such extreme events, the GFZ has a task force called HART (Hazard and Risk Team) that will travel to the area affected to conduct further studies. The assignment aims at gaining a better and more detailed understanding of the rupture process based on the aftershocks, and defining the rupture surface more precisely based on the distribution of the aftershocks. Currently 25 seismometers are being prepared for air transport. Early next week a team of eight GFZ scientists will fly to Chile. The 25 portable seismometers will be used to expand the existing observatory network IPOC (Integrated Plate Boundary Observatory Chile) in order to be able to determine the earthquake epicentres more precisely. In addition highly precise surface displacements will be measured at 50 GPS measuring points. Two new additional continuous GPS stations will be installed to determine how the earthquake has deformed the earth' crust.

The Helmholtz Centre for Ocean Research Geomar in Kiel intends to support the measuring campaign. Ocean floor seismometers will supplement land-based seismic data by providing measurements of the aftershocks on the seafloor.

The Plate Boundary Observatory IPOC in Chile
The GFZ initiated the setup of an observatory directly within the seismic gap in northern Chile in order to be able to precisely measure and capture tectonic processes before, during and after the expected strong earthquake. The observatory called Integrated Plate Boundary Observatory Chile (IPOC) is a European-American network of institutions and scientists. Together with several Chilean and German universities, German, French, Chilean and American non-university research institutions operate a decentralized instrumentation system located at Chile's convergent plate boundary to gather data on earthquakes, deformations, magmatism, and surface processes.

The mission succeeded in the case of the April 2 earthquake: "All our instruments survived the quake and aftershocks unscathed. We now have a set of data that is unique in the world," says GFZ seismologist Günter Asch with a smile, who was responsible for checking the instruments on site right after the earthquake and who is once again on his way to the region. "We believe that these data will help us understand the entire earthquake process -- from the phase that tension builds up to the actual rupture, and also during the post-seismic phase." This understanding will provide insights into earthquake risks in this part of the world as well as elsewhere.

The IPOC will further expand. To this day more than 20 multi-parameter stations have been set up. They comprise broadband seismographs, accelerometers, continuous GPS receivers, magneto-telluric probes, expansion measuring devices and climate sensors. Their data is transferred to Potsdam in real time. The European Southern Observatory on Cerro Paranal is now also part of the observatory network

Source: Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences

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