Horton River Delta, Arctic Canada

A river flowing down a steep slope follows a pretty straight path, with gravity exerting a tremendous pull on the water. But a river flowing over a flat landscape can meander left and right, occasionally abandoning river channels to become oxbow lakes or to take a shorter route to the sea.
In the past few centuries, the Horton River in northwestern Canada stopped wandering and assumed a more direct route to the sea. Situated about 420 kilometers (260 miles) east of the Alaska border, the modern Horton River empties into Franklin Bay; but as this image shows, it once followed a different path. The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite acquired this image on July 31, 2012.
The Horton River Delta forms a fan-shaped interruption to the otherwise straight coastline. North of the delta, lakes fill abandoned meanders. Centuries ago, the Horton River continued another 100 kilometers (60 miles) or so northward, draining into Harrowby Bay. In a 1989 study, researchers from the University of British Columbia used multiple lines of evidence to estimate a time frame for when the Horton River abandoned its old channels and adopted a shortcut to the sea.
One line of evidence comes from historical records. Dr. John Richardson of the second Franklin expedition mapped the mouth of the Horton River sometime between 1825 and 1827, and his colleague E.N. Kendall drew a sketch of it. Obviously the river had taken its shortcut to the sea by then.
Next, the researchers turned to tree-ring dating of driftwood to identify the earliest possible date for the shortcut. Near the sea, the Horton River flows through treeless tundra; but about 200 kilometers (125 miles) upstream, spruce trees grow. When pieces of wood wash into the river, they can be carried far downstream, either washing into the ocean, or getting deposited somewhere along the riverbed. That means driftwood lying in the Horton River’s abandoned channels must have been carried there before the river changed course. Analysis showed that the most recent driftwood in the old meanders was deposited before 1640.
The third line of evidence for the delta’s age comes from studies of the rock and sediment layers, including observed rates of delta growth since the area was first mapped. The researchers concluded: “If rates of delta and fan development are assumed constant, then the breakthrough on geomorphic evidence would have occurred about 1750.”
Although the land around the Horton River is flat, it is not at sea level but instead sits about about 75 meters (250 feet) above sea level, and cliffs line the coastline. So once it stopped meandering and broke through a barrier, the Horton River took a steep trip downward to the Canadian coast. The river’s shortcut formed a delta that continued growing for decades.

A comparison between Richardson’s mapping efforts and aerial photographs acquired later indicates that the Horton River Delta expanded between the early 19th century and the mid-20th century. But subsequent aerial photos showed the delta shrinking later in the 20th century. A study published in 1998 concluded that the ocean eventually began to eat away at the delta faster than river sediments could build it.

A Voyager Far From Home

On September 5, 1977, NASA’s Voyager 1 spacecraft lifted off from Cape Canaveral, Florida, aboard a Titan-Centaur rocket. Thirty-five years later, the planetary probe is now an interstellar traveler, having traveled farther from Earth than any manmade object in history. As of 21:00 Universal Time on September 4, 2012, Voyager 1 was 18.21 billion kilometers (11.31 billion miles) from home, or 121 times the distance from the Earth to the Sun. Light takes 33 hours and 44 minutes to travel the distance from Voyager 1 to Earth.

The images above were taken at a time when Voyager 1 was much closer to home. The top image of a crescent-shaped Earth and Moon was captured on September 18, 1977, when Voyager was a mere 11.66 million kilometers (7.25 million miles) from Earth and directly above Mount Everest (on the night side of the planet at 25 degrees north latitude).

The lower image and inset—often referred to as “the Pale Blue Dot” image—was acquired on February 14, 1990, when the spacecraft was 6.4 billion kilometers (4 billion miles) from Earth and 32 degrees above the ecliptic plane. Earth is a mere point of light, just 0.12 pixels (picture elements) in size when viewed from that distance. The fuzzy light in the images is scattered sunlight because Earth was very close to the Sun (from the perspective of Voyager). The image was part of a series of 60 images collected to make the first-ever mosaic portrait of our solar system.

Both images were assembled from data from the Imaging Science Subsystem on Voyager 1, a modified version of the slow-scan vidicons used in the Mariner spacecraft and similar to early television cameras. The wide-angle camera had a field of view comparable to a 200 millimeter lens with an aperture of f/3, while the narrow-angle camera had the field of view of a 1500 mm, f/8.5 lens.

Having long since passed its primary targets of Jupiter and Saturn, Voyager 1 has been cruising for decades toward the edge of the solar system. In fact, researchers have analyzed data from the probe’s particle detectors, cosmic ray detectors, and magnetometer and found evidence that they have passed the termination shock and into the heliosheath—the outer edge of influence for solar wind plasma and energy from our Sun. The probe is now in an area similar to the windless “doldrums” found in tropical seas on Earth. The solar wind has calmed, the magnetic field has piled up due to pressure from outside the solar system, and high-energy particles appear to be leaking out into interstellar space. The Voyager science team expects the spacecraft itself to pass out into that space sometime in the next year or so.

“Voyager tells us now that we're in a stagnation region in the outermost layer of the bubble around our solar system,” said Ed Stone, Voyager project scientist at the California Institute of Technology, at a December 2011 press conference. “Voyager is showing that what is outside is pushing back. We shouldn't have long to wait to find out what the space between stars is really like.”

For more information about the spacecraft, visit the Voyager web site at the Jet Propulsion Laboratory.

Earthquakes near Tabriz, Iran

Earthquakes with magnitudes 6.4 and 6.3 struck northwestern Iran on August 11, 2012. The earthquakes occurred midway between Lake Orumiyeh and the Caspian Sea. Separated by only 11 minutes and 10 kilometers (6 miles), both quakes were shallow, occurring less than 10 kilometers below ground. The quakes leveled mud-brick and concrete homes in about 230 villages, Al Jazeera reported, and by August 13, the death toll had passed 300. More than 3,000 people had been injured, and more than 16,000 people had been left homeless by the disaster.

This image shows the region where the earthquakes struck. Rings mark earthquake locations, and bigger, thicker rings indicate higher magnitudes. Note that this image only shows the large quakes that occurred on August 11, and aftershocks with a magnitude of 4.0 or greater that occurred through the afternoon of August 13. The red lines indicate faults. The earthquake locations and faults are superimposed on a digital elevation map, made from the ASTER Global Digital Elevation Model Version 2 (GDEM2). Lighter colors correspond with higher elevations.

The U.S. Geological Society (USGS) explained that the earthquakes occurred in the crust of the Eurasian Plate. In this region, the Arabian Plate moves roughly northward with respect to the Eurasian Plate, at about 26 millimeters (1 inch) per year, but the USGS stated that these earthquakes occurred about 300 kilometers (200 miles) east of the plate boundary. The event likely occurred as a result of an oblique strike-slip fault, where blocks of the Earth slide past each other, the USGS said, but because of the location was so far from a plate boundary, “precise identification of the causative fault(s) is difficult at this time.”

1. References

2. Al Jazeera. (2012, August 13) Thousands left homeless after Iran quakes. Accessed August 13, 2012.
3. Al Jazeera. (2012, August 13) Iran earthquake death toll continues to rise. Accessed August 13, 2012.
4. The Big Picture. (2012, August 13) Iran earthquakes. Boston Globe. Accessed August 13, 2012.
5. CBS/Associated Press. (2012, August 13) Iran earthquake death toll passes 300 day after search for survivors called off. Accessed August 13, 2012.
6. U.S. Geological Survey. (2011, August 11) M6.4 – 23km SW of Ahar, Iran. Accessed August 13, 2012.
7. U.S. Geological Survey. (2011, August 11) M6.3 – 32km WSW of Ahar, Iran. Accessed August 13, 2012.

Seismotectonics of the Middle East and Vicinity

No fewer than four major tectonic plates (Arabia, Eurasia, India, and Africa) and one smaller tectonic block (Anatolia) are responsible for seismicity and tectonics in the Middle East and surrounding region. Geologic development of the region is a consequence of a number of first-order plate tectonic processes that include subduction, large-scale transform faulting, compressional mountain building and crustal extension.

Mountain building in northern Pakistan and Afghanistan is the result of compressional tectonics associated with collision of the India plate moving northwards at a rate of 40 mm/yr with respect to the Eurasia plate. Continental thickening of the northern and western edge of the India subcontinent has produced the highest mountains in the world, including the Himalayan, Karakoram, Pamir and Hindu Kush ranges. Earthquake activity and faulting found in this region, as well as adjacent parts of Afghanistan and India, are due to collisional plate tectonics.

Beneath the Pamir-Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Shallower crustal earthquakes in the Pamir-Hindu Mountains occur primarily along the Main Pamir Thrust and other active Quaternary faults, which accommodate much of the region's crustal shortening. The western and eastern margins of the Main Pamir Thrust display a combination of thrust and strike-slip mechanisms.

Along the western margin of the Tibetan Plateau, in the vicinity of southeastern Afghanistan and western Pakistan, the India plate translates obliquely relative to the Eurasia plate, resulting in a complex fold-and-thrust belt known as the Sulaiman Range. Faulting in this region includes strike-slip, reverse-slip and oblique-slip motion and often results in shallow, destructive earthquakes. The relatively fast moving left-lateral, strike-slip Chaman Fault system in southeastern Afghanistan accommodates translational motion between the India and Eurasia plates. In 1505, a segment of the Chaman Fault system near Kabul, Afghanistan ruptured causing widespread destruction of Kabul and surrounding villages. In the same region, the more recent 30 May 1935, M7.6 Quetta, Pakistan earthquake, occurred within the Sulaiman Range, killing between 30,000 and 60,000 people.

Off the south coast of Pakistan and southeast coast of Iran, the Makran trench is the present-day surface expression of active subduction of the Arabia plate beneath the continental Eurasia plate, which converge at a rate of approximately 20 mm/yr. Although the Makran subduction zone has a relatively slow convergence rate, it has produced large devastating earthquakes and tsunamis. For example, the November 27, 1945 M8.0 mega-thrust earthquake produced a tsunami within the Gulf of Oman and Arabia Sea, killing over 4,000 people. Northwest of this active subduction zone, collision of the Arabia and Eurasia plates forms the approximately 1,500-km-long fold and thrust belt of the Zagros Mountains, which crosses the whole of western Iran and extends into northeastern Iraq. Collision of the Arabia and Eurasia plates also causes crustal shortening in the Alborz Mountains and Kopet Dag in northern Iran. Eastern Iran experiences destructive earthquakes that originate on both strike-slip and reverse faults. For example, the 16 September 1978 M7.8 earthquake, along the southwest edge of the Dasht-e-Lut Basin killed at least 15,000 people.

Along the eastern margin of the Mediterranean region there is complex interaction between the Africa, Arabia and Eurasia plates. The Red Sea Rift is a spreading center between the Africa and Arabia plates, with a spreading rate of approximately 10mm/yr near its northern end, and 16mm/yr near its southern end (Chu, D. and Gordon, R. G., 1998). Seismicity rate and size of earthquakes has been relatively small along the spreading center, but the rifting process has produced a series of volcanic systems across western Saudi Arabia.

Further north, the Red Sea Rift terminates at the southern boundary of the Dead Sea Transform Fault. The Dead Sea Transform is a strike-slip fault that accommodates differential motion between the Africa and Arabia plates. Though both the Africa plate, to the west, and the Arabia plate, to the east, are moving in a NNE direction, the Arabia plate is moving slightly faster, resulting in the left-lateral, strike-slip motion along this segment of the plate boundary. Historically, earthquake activity along the Dead Sea Transform has been a significant hazard in the densely populated Levant region (eastern Mediterranean). For example, the November 1759 Near East earthquake is thought to have killed somewhere between 2,000-20,000 people. The northern termination of the Dead Sea Transform occurs within a complex tectonic region of southeast Turkey, where interaction of the Africa and Arabia plates and the Anatolia block occurs. This involves translational motion of the Anatolia Block westwards, with a speed of approximately 25mm/yr with respect to Eurasia, in order to accommodate closure of the Mediterranean basin.

The right-lateral, strike-slip North Anatolia Fault, in northern Turkey, accommodates much of the westwards motion between the Anatolia Block and Eurasia Plate. Between 1939 and 1999, a series of devastating M7.0+ strike-slip earthquakes propagated westwards along the North Anatolia Fault system. The westernmost of these earthquakes was the 17th August 1999, M7.6 Izmit earthquake, near the Sea of Marmara, killed approximately 17,000 people.

At the southern edge of the Anatolia Block lies the east-west trending Cyprian Arc with associated levels of moderate seismicity. The Cyprian Arc represents the convergent boundary between the Anatolia Block to the north and the Africa Plate to the south. The boundary is thought to join the East Anatolia Fault zone in eastern Turkey; however no certain geometry or sense of relative motion along the entire boundary is widely accepted.

M6.3 - 32km WSW of Ahar, Iran

Event Time

1. 2012-08-11 12:34:35 UTC
2. 2012-08-11 17:04:35 UTC+04:30 at epicenter
3. 2012-08-11 17:04:35 UTC+04:30 system time

Nearby Cities

1. 32km (20mi) WSW of Ahar, Iran
2. 49km (30mi) ENE of Tabriz, Iran
3. 86km (53mi) E of Marand, Iran
4. 97km (60mi) NNW of Hashtrud, Iran
5. 282km (175mi) SE of Yerevan, Armenia

Tectonic Summary

The August 11, 2012 M 6.4 and M 6.3 earthquakes in northwestern Iran occurred as a result of oblique strike-slip faulting in the shallow crust of the Eurasia plate, approximately 300 km east of the plate boundary between the Eurasia and Arabia plates. The two earthquakes are separated by just 10 km in an east-west direction. Focal mechanisms, describing the style of faulting for the earthquakes, suggest slip on either fault planes striking roughly east-west, or those striking roughly north-south. Because these earthquakes are intraplate events, away from the main plate boundary structures in the region, precise identification of the causative fault(s) is difficult at this time, though their offset suggest they may be associated with an east-west striking structure.

On a broad scale, the seismotectonics of this region are controlled by the collision of the Arabia and Eurasia plates; at the latitude of the earthquakes, the Arabia plate moves almost due north with respect to the Eurasia plate at a rate of approximately 26 mm/yr. To the south of today’s earthquakes, towards Iraq and the Persian Gulf, tectonics are dominated by the Zagros fold and thrust belt. To the west, in Turkey, tectonics are dominated by strike-slip faulting on the East (in southern Turkey) and North (in northern Turkey) Anatolian fault zones, accommodating the westward motion of the Anatolian block as it is being squeezed by the converging Arabian and Eurasian plates. The August 11, 2012 earthquakes occurred in the broad, elevated Turkish-Iranian Plateau region between these regimes and the Alborz Mountains further east. The events are consistent with the distributed, dominantly strike-slip mechanisms of historic earthquakes nearby, and with the orientation of mapped faults in the region.

Over the past 40 years, seven earthquakes of M 6 or greater have occurred within 300 km of today’s events. The closest was a M 6.1 earthquake in February of 1997, approximately 100 km to the east, which caused 1100 fatalities. In October of 2011, a M 7.1 earthquake struck the region of Van in eastern Turkey, near the Iranian border 300 km to the west of the August 11, 2012 earthquakes, resulting in over 500 fatalities.

NPP's 'Blue Marble'

A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's most recently launched Earth-observing satellite - Suomi NPP.

The Visible/Infrared Imager Radiometer Suite or VIIRS is the primary imaging instrument onboard NPP, and it acquires data in 22 spectral bands covering visible, near-infrared, and thermal infrared regions of the electromagnetic spectrum.

The Eastern, Western, and Australian views were created by NASA scientist Norman Kuring. They are composite images using a number of swaths of the Earth's surface taken on January 4, 2012. The Arctic composite was collected on May 26, 2012 from the OceanColor group at Goddard/NASA.

Ouarkziz Impact Crater, Algeria

The Ouarkziz Impact Crater is located in northwestern Algeria, close to the border with Morocco. The crater was formed by a meteor impact less than 70 million years ago, during the late Cretaceous Period of the Mesozoic Era, or “Age of Dinosaurs.”

Originally called Tindouf, the 3.5-kilometer wide crater (image center) has been heavily eroded since its formation; however, its circular morphology is highlighted by exposures of older sedimentary rock layers that form roughly northwest to southeast-trending ridgelines. From the vantage point of an astronaut on the International Space Station, the impact crater is clearly visible with a magnifying camera lens.

A geologist interpreting this image to build a geological history of the region would conclude that the Ouarkziz crater is younger than the sedimentary rocks, as the rock layers had to be already present for the meteor to hit them. Likewise, a stream channel is visible cutting across the center of the structure, indicating that the channel formed after the impact had occurred. This Principal of Cross-Cutting Relationships, usually attributed to the 19th century geologist Charles Lyell, is a basic logic tool used by geologists to build relative sequence and history of events when investigating a region.