EARTHQUAKES OF MEXICO
East Pacific Rise-(Baja spreading center-transforms segments), Figures 11 and 12
The Baja spreading center segment comprises the north- ern segment of the East Pacific Rise from Antarctic to the Southeastern Pacific Ocean. This spreading center is com- prised of many small individual spreading rises and the accompanying small transform faults. Figure 1 illustrates there were 11 earthquakes recorded in that area.
Figure 11. Dec. 5, 2010 M 4.9, D 24 km.
Discussion
Of the three questions posed earlier, two have been answered while the third remains unanswered. First, the earthquakes observed off Mexico’s west coast are from motion along the Rivera transform fault.
Second, apparently the Middle America subducting slab is somewhat unique. Instead of the slab descending into the man- tle at the trench, it remains horizontal for about 300 km and then descends beneath the volcanoes. This phenomenon has been named “flat slab subduction,” (Cal Tech Observatory). The Cal Tech report also suggests that because the “flat slab” is bordered by normal subduction zones, the volcanic arc bends inland in this region.
Third, while not answered here, I strongly suspect the waveforms have differing shapes due to the differing physi- cal and chemical changes within the crust. For example, the Baja spreading enter consists of thin, hot, young ocean crust while the continental areas contain thicker, cooler continental crust. The question of differing waveforms is worth pursuing.
Local Observations and Results
Figure 12. Sept. 10, 2011. M 4.6, D 10 km.
Rio Grande Rift, Figures 13 and 14
The Rio Grande Rift trends north-south, from the Colorado Plateau south to the Mexican state of Chihuahua, Mexico. Although there are earthquakes, high heat flow, and recent volcanism, it is not certain if the rift is active or failed.
The local earthquakes I have recorded demonstrate small seismic events continually near my home, including an earthquake on November 4, 2014 at 18:06:40 GMT (Figure 16). Normal earthquake waveforms decrease exponentially; however, the waveforms from that unknown seismic event (UKSE) do not. Using the equation (s-p) to plot travel times by implying that the first and strongest amplitude is the p-wave and second is the s-wave (Figure 17), a distance of 4.1° or 460 km is suggested. But other seismometers within the Texas seismometer network would have detected that amplitude and distance, which they did not. So I infer in the cases of these UKSEs, that the (s-p) method is not applicable because I don’t know the first arrival of the s-wave. There appear to be two strong pulses of equal amplitude. These seismic waves have a peak frequency of 0.533 Htz (1.88 sec period) with a range of 0.12 – 1.13 Htz (0.34 – 0.884 sec period), Figure 18. I wonder, what is the significance?
Figure 13. April 28, 2011. M 4.3, D 10.2 km.
Pacific Ocean, Figure 15
One earthquake was recorded within the Pacific Plate, west of Baja, California. I am not aware of its tectonic significance.
Figure 15. Dec. 14. 2012. M 6.3, D 11.3 km.
Figure 16 UKSE, Nov. 4, 2014.
Figure 14. May 2, 2011. M 4.3, D 5.8 km.
Although these events are not recorded on the seismometer network in Texas with one exception, the events appear to be real. They occur during the day, usually during the work week. This timing implies that the events may be induced by human activity. Are these events the result of quarrying, oilfield hydrfracturing, or some sort of construction activity? If only I had access to another seismometer or two in the area of Dallas, I might pinpoint the source and deduce the causes of these curious phenomena.
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