Archive for November, 2015

M4.7 – 25km WSW of Medford, Oklahoma – November 30, 2015

November 30, 2015

NTMA NEMAclick here for USGS Tectonic Summary

2015-11-30 09:49:13 UTC
Magnitude 4.7
25km (16mi) WSW of Medford, Oklahoma
Location  36.737°N   -98.006°W
Depth  5.0 km


M7.6 – 169km WNW of Iberia, Peru and M7.6 – 210km S of Tarauaca, Brazil – November 24, 2015

November 25, 2015

click here for USGS Tectonic Summary

2015-11-24 22:45:38 (UTC)
Magnitude 7.6169km (105mi) WNW of Iberia, Peru
Peru-Brazil border
-10.548°S   -70.904°W
depth=600.6 km (373.2 mi)

2015-11-24 22:50:53 (UTC)
Magnitude 7.6
210km (130mi) S of Tarauaca, Brazil
Peru-Brazil border
-10.047°S   -71.023°W
depth=611.7 km (380.1 mi)

From the USGS Tectonic Summary
“Both earthquakes occurred as the result of normal faulting at a depth of approximately 600 km, almost 1000 kilometers east of the Peru-Chile Trench within the subducted oceanic lithosphere of the Nazca plate. At the location of the earthquakes, the Nazca plate subducts to the east under the South America plate at a velocity of about 69 mm/yr.

The events were separated by approximately 50 km horizontally – slightly more than typical location uncertainties of global earthquakes – and just 6 km vertically.  The latter earthquake was almost certainly triggered by the earlier event. Seismologists sometimes refer to a pair of similarly sized earthquakes that occur at nearly the same time and location as an earthquake “doublet.”

Magnitude 4.7 – 13km SW of Cherokee, Oklahoma – November 19, 2015

November 20, 2015

Slide1 Slide2USGS Earthquake Summary
2015-11-19 07:42:12 (UTC)
Magnitude 4.7
13km (8mi) SW of Cherokee, Oklahoma
Location:  36.661°N   -98.461°W
depth=6.2 km

USGS Tectonic Report — Induced Seismicity
“As is the case elsewhere in the world, there is evidence that some central and eastern North America earthquakes have been triggered or caused by human activities that have altered the stress conditions in earth’s crust sufficiently to induce faulting. Activities that have induced felt earthquakes in some geologic environments have included impoundment of water behind dams, injection of fluid into the earth’s crust, extraction of fluid or gas, and removal of rock in mining or quarrying operations. In much of eastern and central North America, the number of earthquakes suspected of having been induced is much smaller than the number of natural earthquakes, but in some regions, such as the south-central states of the U.S., a significant majority of recent earthquakes are thought by many seismologists to have been human-induced. Even within areas with many human-induced earthquakes, however, the activity that seems to induce seismicity at one location may be taking place at many other locations without inducing felt earthquakes. In addition, regions with frequent induced earthquakes may also be subject to damaging earthquakes that would have occurred independently of human activity. Making a strong scientific case for a causative link between a particular human activity and a particular sequence of earthquakes typically involves special studies devoted specifically to the question. Such investigations usually address the process by which the suspected triggering activity might have significantly altered stresses in the bedrock at the earthquake source, and they commonly address the ways in which the characteristics of the suspected human-triggered earthquakes differ from the characteristics of natural earthquakes in the region.”

Further Reading:  For an excellent, more complete, analysis of the situation, see the USGS Science Feature article Man-Made Earthquakes Update, and follow the links in it.