EARTHQUAKES OF MEXICO
AmaSeis, this allows discrimination among p-waves, s-waves and surface waves.
Texas is in a relatively aseismic area of the U.S. My seismometer usually records an earthquake once a week. Interestingly, several often occur within a small time period. One begins to wonder whether there is some type of seismic positive feedback suggesting that one earthquake may trigger another in a different location.
Figure 3A. Guatemala Earthquake, Full screen.
Figure 3B. Magnification of Figure 3A.
Once an earthquake is recorded on my computer, I use the AmaSeis program to interpret the seismic data. The data is recorded in a heliograph on the computer screen. Again, the units of both the X and Y axes of the heliograph are in time.
Mexican Tectonics Figure 3 C. S-P distance determination.
Figure 3D. Fourier analysis of earthquake.
Figures 3A-3D. Mexican Earthquakes. Earthquake recorded on December 7, 2014, 12:14 GMT. Magnitude 5.8, depth 50 km at the Middle America Trench, Guatemala, near Mexico western border. Figure 3A shows the full screen view of the earthquake. Figure 3B is a magnification of Figure 3A to observe more details of wave forms. Figure 3C illustrates (s-p) distance plot using AmaSeis software. The plot shows a distance of 12.9° about the corrected distance as calculated from the earthquakes epicenter and described on the USGS website. Figure 3D is a Fourier analysis of the first arrivals of the seis- mic waves using AmaSeis software. Maximum period varies from 1.21 to 2.56 seconds for the time period of 12:14:09 to 12:15:45 GMT. The location of this Guatemala earthquake is indicated by the blue star in Figure 1.
surface waves traveling at about 90% the velocity of s-waves. As these waves propagate outward from the focus, the waves spread out due to the velocity differences with the p-wave arriving first, s-waves, second, and trailing directly behind are the surface waves. Three parameters can be determined from the heliograph observations:
1. Distance to the earthquake epicenter (Figure 3C). This is accomplished by published travel time curves in AmaSeis and the USGS using the (s-p) wave travel time difference. Distances are usually in the form of degrees from the epicenter to the seismometer, and then are corrected for distance in either kilometers or miles. If the epicenter is far enough away to provide a wide separation of the surface waves, then the equation (surface waves-p-waves) may be employed.
2. Magnitude of earthquake. The magnitude values that are used from the USGS represent moment magnitudes and are labeled “M” on the USGS website. These values can be calculated either from the AmaSeis software and/ or a “magnitude calculator” from the USGS earthquake website. In general, the instrument sensitivity can record small earthquake magnitudes (~3.5) only within tens of kilometers from home, but larger earthquakes can be recorded from half-way around the world.
3. Frequency (period) content of the seismic waveforms (Figure 3D). Along with the frequency filter tool in
The earthquakes of Mexico observed and recorded on the EQ-1 are used as an example of the instrument’s ability as a learning tool. This example is presented because (1) many earthquakes are recorded and (2) the earthquakes reveal the presence all of the major elements of Plate Tectonics. Shown are divergent plate boundaries including the East Pacific Rise (Baja spreading center) and Rio Grande Rift, Transform Faults (San Andreas and Rivera), Subduction earthquakes (Volcanic Province), and Rivera plate triple junction. A brief summary is presented in Table 1. A tectonic sketch map of Mexico showing the major tectonic elements reveals areas of convergence, divergence, transform motion, and a triple junction (Figure 4).
Figure 4. Tectonic sketch map of Mexico illustrating three main tec- tonic plates, Pacific, North American, Cocos, and the Rivera Micro- plate (RP). Numbers are plate velocities in mm/year and arrows point to plate movement direction (DeMets and Traylan, 2000). Legend is MAT = Middle America Trench, VP = Volcanic Province, RT = Rivera Transform, EPR = East Pacific Rise (Baja Spreading Center), SAT = San Andreas transform, and RGR = Rio Grande Rift. A disturbed zone (DZ) is in purple at Rivera triple junction.
The triple junction is apparent in the area marked DZ (disturbed zone). This area is where the Rivera micro-plate is being subducted beneath the Middle America Trench. A glance at Figure 4 and comparison with Figure 1 shows the relationship between the tectonic elements and earthquakes.
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