By Jay Patton From Jay Patton online

Original web posting (see web link below) 2018/01/09 11:16 PM

NOTE: The following is an EXCERPT only from the original posting. Go to weblink below to read

Just a couple hours ago there was an earthquake along the Swan fault, which is the transform plate boundary between the North America and Caribbean plates. The Cayman trough (CT) is a region of oceanic crust, formed at the Mid-Cayman Rise (MCR) oceanic spreading center. To the west of the MCR the CT is bound by the left-lateral strike-slip Swan fault. To the east of the MCR, the CT is bound on the north by the Oriente fault.

Based upon our knowledge of the plate tectonics of this region, I can interpret the fault plane solution for this earthquake. The M 7.6 earthquake was most likely a left-lateral strike-slip earthquake associated with the Swan fault.

[Attached] is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 (and down to M ≥ 4.5 in a second poster).
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.3 earthquakes, in addition to some relevant historic earthquakes.There have been several M 6.7-M 7.5 earthquakes to the west of this fault in the last 4 decades or so.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
I include some inset figures.

In the upper left corner is a plate tectonic map showing the major plate boundary faults in the Caribbean region. Symithe et al. (2015) plot fault plane solutions for earthquakes M ≥ 6. I place a blue star in the general location of today’s M 7.6 earthquake.
In the upper right corner is a different plate tectonic map from García-Casco et al. (2011). I place a blue star in the general location of today’s M 7.6 earthquake.
In the lower right corner is a figure from Mann et al., (1991) that shows the magnetic anomalies in the oceanic crust of the Cayman trough. The short vertical subparallel black lines are magnetic anomalies, identified from magnetic surveys with ages constrained by rocks from the seafloor. As the crust spreads from the Mid Cayman Ridge, and Earth’s magnetic field polarity flips, the changes in magnetic polarity are recorded in the crust. The crust closest to the MCR is youngest. I place a blue star in the general location of today’s M 7.6 earthquake.
Above the Mann et al. (1991) map is a larger scale map from ten Brink et al. (2002). This map shows the quasi detailed bathymetry in the area of the MCR. They map that both the Swan and Oriente faults terminate at the MCR. Today’s M 7.6 earthquake is to the west of this map, so there is no little blue star. 🙁

Below are the USGS websites for the earthquakes plotted on the poster [attached].
M 7.6 – 44km E of Great Swan Island, Honduras
2018-01-10 02:51:31 UTC 17.469°N 83.520°W 10.0 km depth
https://earthquake.usgs.gov/earthquakes/eventpage/us1000c2zy#executive
M 7.5 – Guatemala
1976-02-04 09:01:43 UTC 15.324°N 89.101°W 5.0 km depth
https://earthquake.usgs.gov/earthquakes/eventpage/usp0000ex3#executive
M 6.8 – Cuba region
1992-05-25 16:55:04 UTC 19.613°N 77.872°W 23.1 km depth
https://earthquake.usgs.gov/earthquakes/eventpage/usp000582h#executive
M 6.7 – Guatemala
1999-07-11 14:14:16 UTC 15.782°N 88.330°W 10.0 km depth
https://earthquake.usgs.gov/earthquakes/eventpage/usp0009bed#executive
M 6.8 – Cayman Islands region
2004-12-14 23:20:13 UTC 18.958°N 81.409°W 10.0 km depth
https://earthquake.usgs.gov/earthquakes/eventpage/usp000daqt#executive
M 7.3 – offshore Honduras
2009-05-28 08:24:46 UTC 16.731°N 86.217°W 19.0 km depth
https://earthquake.usgs.gov/earthquakes/eventpage/usp000gxkj#executive
UPDATE: 2018.01.10 9 AM pacific time. There were two observations of a small amplitude (small wave height) tsunami recorded on tide gages in the region. Below are those observations.

[Attached] Here is the tectonic map from Symithe et al. (2015). I include their figure caption below in blockquote.

Seismotectonic setting of the Caribbean region. Black lines show the major active plate boundary faults. Colored circles are precisely relocated seismicity [1960–2008, Engdahl et al., 1998] color coded as a function of depth. Earthquake focal mechanism are from the Global CMT Catalog (1976–2014) [Ekstrom et al., 2012], thrust focal mechanisms are shown in blue, others in red. H = Haiti, DR = Dominican Republic, MCS = mid-Cayman spreading center, WP = Windward Passage, EPGF = Enriquillo Plaintain Garden fault.

For more on this story go to: http://earthjay.com/?p=6842