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| Picture 1. Earthquake generation is usually explainned through the Elastic Rebound Theory |
April 8 this year, 3 strong earthquakes hit Batangas: a magnitude 5.7 at around 3 pm, follwed by a 5.9. These were again followed by a magnitude 5.7 within 20 minutes.
We might be familiar of the fact that earthquakes, at least the tectonic ones, happen when stored and accumulated stress on rocks finally overcome its strength, and these are associated along faults, where two rock masses move past each other. However, for the area hit, no such faults were identified at the surface, as shown by the image below.
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| Picture 2. Seismicity Map of Region IV - A (Source: PHIVOLCS) |
Fortunately, PHIVOLCS - Seismic Observation and Earthquake Prediction Division (SOEPD) continually updates a publicly-accessible page recording data - coordinates, magnitude and depth - for every earthquake at least magnitude 1 that occurs in the Philippines. I've plotted the epicenters of all the earthquakes that have occurred around the Batangas area for the month of April, as shown below.
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| Picture 3. Earthquake epicenters of the area (April 2017). |
As we can see above, the earthquake epicenters already show a NNW trend, which would imply that the fault that has caused these quake swarm occurs this orientation.
Now, if we plot the earthquake foci, we could see the dip, and the planar feature of this previously unknown fault. I plotted the earthquake foci using QGIS 2.8.2, with the help of the Qgisthreejs plug-in. Here is an image, showing views parallel and perpendicular to the fault's approximated strike (around N 20 W).
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| Picture 4. Profile showing earthquake foci for the Batangas earthquakes and aftershocks. Magnitude increases as the bubble size increases, and as color changes from blue to red. 2x vertical exaggeration. Dip angle was measured to be 88 degrees, and by quick calculation (dip angle measurements in exaggerated scales must be corrected), the dip was found to be 86 degrees. |
While what I have shown lies more on an illustrative side, allow me to take a guess, and say that the attitude of the fault that caused the Batangas quakes is around N 20 W, 86 NE.
