Download Thomas Martin SIO 226 Paper Review Week 1 Isacks et al., 1968

Thomas Martin
SIO 226
Paper Review
Week 1
Isacks et al., 1968
The paper is largely observational paper of global seismicity and there locations
of epicenters and how they're located on the borders of plates. The authors are
hypothesising that seismology will be extremely important in “New Global Tectonics”.
Today, students drop the “new” as the ideas in the paper today are considered the
standard, and are taught in current undergraduate courses. The paper provides
seismological evidence that mid ocean ridges are sources of new crust, and island arcs
are areas where oceanic crust is destroyed. Tying these sections together, the authors
relate all plate motions on a global scale, and how earthquake epicenter locations tie it
all together that the mobile lithosphere hypothesis is true. The remaining parts of the
paper go over some of the some of the future questions that need to be addressed, and
some of the answers that seismology can possibly answer. The authors are not shy in
their belief of “New Global Tectonics” and the mobile lithosphere hypothesis, and state
in the paper they tried to stay as objective as possible, but the evidence is so
overwhelming in support.
The modeling assumptions are that the seismometers work, and the calculated
slip rate in figure 16 hold true from previous work by Xavier Le Pichon. This paper is
notable for the lack of math and calculations compared to modern papers on the
subject. One of the biggest assumptions though is based of off Le Pichon’s paper in
1968 that describes global plate tectonics as only 6 rigid plates. As this is a great
overview, today most would argue to do a more accurate representation of plate
motions, more plates would have to be added into the model. This was early in the
formation of plate tectonic theory. They realized within the paper that this is a much
simplified version of the actual earth, and that more detailed analysis was required a
later date.
The main type of data is largely observational earthquake epicenters. In figure 1,
they go over basic tectonic environments like subduction zones, mid ocean ridges and
transform faults. Figure two and three are basic tectonic boundary maps, with velocity
vectors. Most of the velocity data was taken from Le Pichon’s 1968 paper. Some of the
other figures are idealized oceanic-continental convergence zones (trenches) and a
diagram of transform faults on a mid ocean ridge. In one diagram, is just the bathymetry
of a trench system off of japan via seismic reflection. It is amazing to think of what is
standard today was truly revolutionary thinking back in the sixties. Isacks et al future
illustrate the complexities of the tectonics on a smaller scale with more localized
diagrams of the gulf of aden and the gulf of california. Both early stage oceanic
spreading centers, with similar tectonic regimes. On these diagrams they plotted
earthquake epicenters, which correlate to the boundary of the plates. It is stated in the
paper that the standardized seismograph network as described in Murphy 1966, vastly
improved the earthquake fault plane solutions.
The analysis is convincing, as this is a foundation paper in plate tectonics and
truly tells the story of the importance of earthquakes in relation to boundaries of plates.
Today this notion is pretty basic, but back in 1968, this was revolutionary. Another
aspect of this paper that is amazing, is the absence of math. It just simply relates
locations of epicenters along with “New Global Tectonics”. When I was a teaching
assistant for SIO 10 this fall, we called it “action at the edges”, SIO 10 is a lower division
earth science course, meant for non science majors. The ideas of “New Global
Tectonics” today are mainstream.
There case is made. In 1968 what was new, now today is the convention. It is
just an observational paper, so you don't have to get lost in the math. But with
improvements in technology and epicenter locations, the data can be improved on, but
the large scale ideas of seismology, and the theory of the mobile lithosphere hypothesis
relating to global tectonics still holds true.
I would be pleased with my name on this paper. Today, it seems pretty basic, as
the larger concepts get covered in an introductory undergraduate course, but in 1968
the field of plate tectonics was rapidly changing. At forty five pages it truly covers a lot of
ground in the field of tectonics, and is a great introduction to what the authors call “New
Global Tectonics”. As the data has improved in both quality and quantity, the basic
understanding from this paper and ideas hold true today, forty five years later.