The temblor that shook Russia's Kamchatka Peninsula last year was one of only six supersonic earthquakes ever identified [image] Jul 14, 2014 |By Becky Oskinand LiveScience • • A comparison between the 2013 Okhotsk earthquake and the 1994 Northridge earthquake. One of the world's deepest earthquakes was also a rare supersonic quake, upending ideas about where these unusual earthquakes strike. Only six supersonic (or supershear) earthquakeshave ever been identified, all in the last 15 years. Until now, they all showed similar features, occurring relatively near the Earth's surface and on the same kind of fault. But last year, a remarkably super-fast and super-deep earthquake hit below Russia's Kamchatka Peninsula, breaking the pattern. "This was very surprising," said Zhongwen Zhan, lead author of the study, published today (July 10) in the journal Science. "It's not only deep, it's supershear, and it's also quite small." The weird earthquake struck May 24, 2013, about 398 miles (642 kilometers) beneath the Sea of Okhotsk offshore of the Kamchatka Peninsula. The magnitude-6.7 quake was an aftershock to the largest deep earthquakeon record, a magnitude 8.3 that also hit May 24. [ Image Gallery: This Millennium's Destructive Earthquakes] The shaking provided the first sign that this was a strange quake. Earthquakes of similar size, such as the 1994 Northridge quake in Los Angeles, shimmy for seven to eight seconds. But this magnitude-6.7 temblor lasted for just two seconds. After dredging up all the available seismic recordings, Zhan and his co-authors realized the earthquake was extremely short because it was extremely fast. An earthquake occurs when two sides of a fault rip apart, opening up like a zipper. Faultscan slide side-by-side or up-and-down, or a combination of both directions. The event unleashes waves of seismic energy. Certain types of waves called shear waves usually travel faster than the rupture unzips, but in supershear earthquakes, the rupture catches the shear waves. When the rupturing fault moves faster than the shear waves, the waves of energy pile up like the Mach cone surrounding a jet flying faster than the speed of sound, creating a phenomenon akin to a seismic sonic boom. The Okhotsk quake's rupture speed clocked in at a zippy 5 miles per second (8 km/s), said Zhan, a seismologist at the Scripps Institution of Oceanography in La Jolla, California. Regular earthquakes, at shallower depths, break loose at about 2.2 miles per second (3.5 km/s), he said. 'U' is for unique Until now, seismologists had never documented a super-fast earthquake at such extreme depths. Nor have they seen supershear earthquakes on this kind of fault. Previously, the super-fast quakes were on strike-slip faults, where two slabs of the Earth slide past each other with no up-and-down motion. But the Okhotsk earthquake was in a subduction zone, where a fault thrusts one of Earth's tectonic plates down below another plate.

Supersonic Earthquake Shook Kamchatka. The temblor that shook Russia's Kamchatka Peninsula last year was one of only six supersonic earthquakes ever identified [image] Jul 14, 2014 |By Becky Oskinand LiveScience • • A comparison between the 2013 Okhotsk earthquake and the 1994 Northridge earthquake. One of the world's deepest earthquakes was also a rare supersonic quake, upending ideas about where these unusual earthquakes strike. Only six supersonic (or supershear) earthquakeshave ever been identified, all in the last 15 years. Until now, they all showed similar features, occurring relatively near the Earth's surface and on the same kind of fault. But last year, a remarkably super-fast and super-deep earthquake hit below Russia's Kamchatka Peninsula, breaking the pattern. "This was very surprising," said Zhongwen Zhan, lead author of the study, published today (July 10) in the journal Science. "It's not only deep, it's supershear, and it's also quite small." The weird earthquake struck May 24, 2013, about 398 miles (642 kilometers) beneath the Sea of Okhotsk offshore of the Kamchatka Peninsula. The magnitude-6.7 quake was an aftershock to the largest deep earthquakeon record, a magnitude 8.3 that also hit May 24. [ Image Gallery: This Millennium's Destructive Earthquakes] The shaking provided the first sign that this was a strange quake. Earthquakes of similar size, such as the 1994 Northridge quake in Los Angeles, shimmy for seven to eight seconds. But this magnitude-6.7 temblor lasted for just two seconds. After dredging up all the available seismic recordings, Zhan and his co-authors realized the earthquake was extremely short because it was extremely fast. An earthquake occurs when two sides of a fault rip apart, opening up like a zipper. Faultscan slide side-by-side or up-and-down, or a combination of both directions. The event unleashes waves of seismic energy. Certain types of waves called shear waves usually travel faster than the rupture unzips, but in supershear earthquakes, the rupture catches the shear waves. When the rupturing fault moves faster than the shear waves, the waves of energy pile up like the Mach cone surrounding a jet flying faster than the speed of sound, creating a phenomenon akin to a seismic sonic boom. The Okhotsk quake's rupture speed clocked in at a zippy 5 miles per second (8 km/s), said Zhan, a seismologist at the Scripps Institution of Oceanography in La Jolla, California. Regular earthquakes, at shallower depths, break loose at about 2.2 miles per second (3.5 km/s), he said. 'U' is for unique Until now, seismologists had never documented a super-fast earthquake at such extreme depths. Nor have they seen supershear earthquakes on this kind of fault. Previously, the super-fast quakes were on strike-slip faults, where two slabs of the Earth slide past each other with no up-and-down motion. But the Okhotsk earthquake was in a subduction zone, where a fault thrusts one of Earth's tectonic plates down below another plate.