Earthquakes of Mexico- As Observed from Home Seismometer in Palmer, Texas
Stephen E. Pierce, CPG-8726 Introduction
It is quite amazing and educational to observe the sensitiv- ity of a simple inexpensive seismometer. The EQ-1 seismom- eter is truly a remarkable instrument for not only education but is also suited for a modest home seismic observatory. Located in Palmer, Texas, I have recorded earthquakes in the U.S., Pacific Oceana, Indonesia, Japan, Western Russia, Mexico, and Central and South America.
Figure 2A & 2B. The EQ-1 seismometer in home at Palmer, Texas. Note 12” yellow ruler for scale. Image at right shows the instrument’s cover that reduces air currents.
ranging from less than 1 to 10s of seconds. This range allows the instrument to detect short period body waves including primary (p-waves), secondary (s-waves), and long period sur- face Raleigh waves. Because the seismometer is responsive to vertical motion only, lateral propagating surface waves such as Love waves remain undetected.
Figure 1. Location of Mexican earthquakes recorded on the EQ-1 seismometer, 2009-2014, Palmer, Texas. Numbered red diamonds relate to recorded seismic events. Seismometer location is shown as light blue tri- angle in East Texas. Base map from Google Earth.
The EQ-1 seismometer has recorded 46 earthquakes in Mexico from April 27, 2009, to July 27, 2014. These earth- quakes vary in magnitude from 4.6 to 7.2 and in focus depth from 5 to 190 km, with one at a depth of 525 km. The travel times have varied from 2 to 3 minutes.
Upon observing the epicenters of the earthquakes in Mexico, three questions immediately emerged. First, what is the significance of the earthquakes offshore? Second, one expects earthquakes associated with subduction zones to have epicenters at the trench, offshore. But in Mexico, why do the large earthquakes appear beneath the volcanoes? And, third, why do the seismic waveforms differ when they are from the same region?
Apparatus
The EQ-1 made by Next Generation Science is a small inexpensive seismometer that can be used at home, Figure 2A (left). The seismometer is rather simple but is very sensi- tive for detecting the vertical components of earthquakes and other seismic activities. The natural oscillation period is 1.6 seconds. This instrument can detect earthquakes of periods
The seismometer is located in a quiet area with very little foot traffic. The seismometer consists of an interface box and cables that are connected directly to a dedicated computer. The display of the seismic waves on the computer screen is called the “heliograph.” A plastic cover is placed over the instrument to prevent small air currents from disturbing it (Figure 2B).
Interpretation
The AmaSeis software that comes with the seismometer is developed by IRIS (Incorporated Research Institutions for Seismology). IRIS is a consortium dedicated to seismology research and is associated with many federal agencies, uni- versities, and private foundations. I have freely used their website, the USGS seismology website, and the seismology webpage of the University of Texas.
The Y axis of the heliograph is divided into 24-hour military time and the X axis is divided into 60 minutes (Figure 3A). In addition to 24-hour time intervals, the Y axis records seismic wave amplitude in internal units of the AmaSeis program. Magnification of the seismic event (Figure 3B) allows for detailed evaluation of the seismic wave-train, and (s-p) plots for determining distance to the focus (Figure 3C). A Fourier analysis of the waves also is available. When combined with filtering selected seismic waveforms, the Fourier application is important for a close study of the waveforms (Figure 3D).
It has been established that seismic waves in the crust travel between 1 and 6 km/sec depending upon the physical and chemical conditions (Plato Publications, 1983). Due to the elastic and bulk densities of rock, p-waves travel the fastest, with s-waves traveling about 60% of the velocity of p-waves and
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