10 March 2014, 05:18 UTC (10:18 pm local time) a magnitude 6.8 earthquake struck the southern end of Cascadia subduction zone. The epicentre location of the earthquake was about 50 km west of the California coast. During the subsequent few hours many aftershocks followed in the same area (see map).
This map shows the location of the main shock and aftershocks at the Mendocino Triple Junction between the North America, Pacific and Gorda tectonic plates.
Ocean Networks Canada's NEPTUNE observatory is about 900 km to the north, and its seismometers in Cascadia Basin (NV.NC27), Barkley Canyon (NV.NCBC) and Clayoquot Slope (NV.NC89) recorded the powerful main shock as well as some of the larger (up to magnitude 4.6) aftershocks about 2 minutes after the event (see seismic recordings below).
NEPTUNE seismic recordings of the 10 March 2014 earthquake. Three components, North (HHN), East (HHE) and Vertical(HHZ) of 3 seismometers show the various earthquake signals arriving at different times. The top 3 traces were recorded by the ocean bottom seismometer in Cascadia Basin (NV.NC27); the second set of 3 traces are from the Barkley Canyon seismometer (NV.NCBC)and the lowest 3 traces are from the seismometer at Clayoquot Slope (NV.NC89) The green bar indicates the origin time of the main shock; about 2 minutes later the fast travelling P-waves (indicated by the red bars) arrive at the NEPTUNE stations, and almost 2 minutes later the S-waves arrive (indicated by the yellow bars), immediately followed by low-frequency surface wave shaking. About 10 minutes after the main shock, high frequency water sound arrives, as clearly visible in the top two panels. (Click to enlarge.)
The southern end of the Cascadia subduction zone terminates at the northern end of the San Andreas fault, meeting at the Mendocino triple junction between three tectonic plates, the North American plate in the east, the Pacific plate in the south, and the Gorda plate to the north. The San Andreas fault zone compensates mostly the lateral motion between the North American and the Pacific plate, whereas the Cascadia subduction thrust fault takes up the motion of the Juan de Fuca plate going down (subducting) below North America.
The 10 March earthquake occurred at a depth of around 16.6 km, and its fault mechanism was taking up mostly lateral motion from the interplay between the Pacific and the Gorda plate. It was not a subduction thrust earthquake and therefore did not generate any tsunami. However, as every earthquake redistributes tectonic stresses in the area (hence the many aftershocks), we will keep an extra eye out for any other seismicity in that region.
