Magnitude 6.9 – 93km NW of Coquimbo, Chile and Magnitude 6.9 – 98km NW of Coquimbo, Chile – November 11, 2015


Slide1 Slide2USGS Tectonic Summary
Time:  2015-11-11 01:54:37 UTC
Magnitude 6.9
93km (58mi) NW of Coquimbo, Chile
Location:  -29.439°S   -72.105°W
Depth:  10.0 km

Time:  2015-11-11 02:46:19 UTC
Magnitude 6.9
98km (61mi) NW of Coquimbo, Chile
Location:  -29.445°S  -72.173°W
Depth:  10.0 km

From, “Watching the Weather Using a Seismograph” by
Dr. John E. Ebel, Boston College, Weston Observatory:

“Seismologists have a name for natural background vibrations. We call them microseisms. Microseisms are observed at all frequencies on seismograms, although there are some frequencies at which they are especially strong. Microseisms at frequencies above about 1 Hz are generally associated with local weather conditions, while below 1 Hz they reflect regional weather and ocean conditions. For seismic stations near the ocean, a particularly strong microseismic signal at a frequency of about .17 Hz (6-second period) apparently is caused by movements of water offshore and against the shoreline.
Since microseisms are caused primarily by meteorological conditions, an analysis of microseismic patterns on seismograms can reveal information about current weather conditions or how weather patterns are changing. High-pressure systems generally bring fair skies and light winds, and these are often reflected on seismograms by low levels of background microseismic noise. Low-pressure systems are typically accompanied by stronger winds, clouds, and precipitation. Microseisms on seismograms often are greater at times when low-pressure systems are near a seismic station.
During the fall, winter, and spring, those of us who live along the northeastern North American coast are often visited by storms that New Englanders call “nor’easters.” These are low-pressure centers that travel from southwest to northeast along the coast, bringing cold, wind, rain, and higher-than-normal tides. Some can be quite strong, with wind gusts occasionally even exceeding hurricane force. During nor’easters, the microseisms on our seismograms in the New England region can be quite large. It is much more difficult to observe earthquake signals on our seismograms when such storms pass by.”


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