The Effects of Age, Angle of Subduction, and Subduction Rate on Subduction Zones
Written by Master Geologist and Geophysicist Kevin Jensen
A special thanks to Kevin for providing this informative discussion on subduction. Subduction Zones Cause the Majority of Earthquakes
Subduction zones account for approximately 80 percent of all major earthquakes that occur throughout the world (World Book, 2009). Because of the amount of destruction that can be caused by damaging earthquakes, it is important to gain a better understanding of the processes and characteristics of subduction zones. In this study, six subduction zones are observed to determine effects of the age of the subducting oceanic lithosphere, and the angle and rate at which subducting oceanic lithosphere on subduction zone processes. These subduction zones are the South American, Middle American, Aleutian, Izu-Bonin, Kermadec-Tonga, and the Sumatra-Java subduction zones (Figure 1).
Methods to Understand Subduction
Earthquake data were taken from the United States Geological Survey’s (USGS) database for each of the areas contained within the rectangles in Figure 1, since January 1, 1990 to present. These data were then plotted and rotated to face perpendicular to the trench to look for characteristic differences within subduction zones, and then separated into sections according to variations of maximum depths (Figure 2). Once these data were plotted, the maximum and second maximum depths for each section of the subduction zones were plotted against thermal parameters calculated by using f = v*t*sin(q), where v is the rate of subduction, t is the age of the subducting lithosphere, and q is the angle of subduction (Table 1, Figure 3, Figure 4). Figure 5 shows the geothermal gradients calculated for oceanic lithosphere the age of that subducting in each of the trenches of the study, using T(z,t) = To + (Ta-To)*erf[z/(4*a*t)], where To is the initial temperature at the surface of the crust, Ta is the temperature of the lithosphere-asthenosphere boundary, z is the depth of the from 0 to 100 km, a is thermal diffusivity calculated from an average density of the lithosphere of 3.3 kg/m3, a thermal conductivity of 3 W/(m*K), a specific heat of 1.17 J/(kg*K), and then converted to km2/My. The t is for age of the lithosphere in my discussion.
Deep Earthquakes
Thermal parameter calculations, Figure 4, show that deeper earthquakes generally occur at trenches where the lithosphere is old, South America being the exception. These thermal parameters match fairly well with the thermal parameter calculations calculated by Kirby et al. in their 1996 study (Kirby et al., 1996). The thermal parameter shows that when it is greater than ~5000 km, then the earthquakes occur deep in the earth. When the parameter is less than 5000 km the earthquakes are generally shallow in comparison to deep earthquakes.
Earthquake Data Results
From thermal parameters, it can generally be seen that the older and faster the lithosphere is, when subducting, the deeper the earthquakes occur.
References
World Book Encyclopedia, “Interplate earthquakes,” 2009, May 6, 2009 <http://www.worldbook.com/wb/Students?content_spotlight/earthquakes/where_interplate >NOAA, (Age of oceanic crust. (See also Image:Earth seafloor crust age 1996.gif) Source: Excerpted from http://www.ngdc.noaa.gov/mgg/image/crustageposter.gif ; original upload in english wikipedia 31 March 2005 by en:User:SEWilco [[en:Image:{{subst:PAG)
Kirby, S. H., S. Stein, E. A. Okal, and D. C. Rubie, Metastable phase transformations and deep earthquakes in subducting oceanic lithosphere, Rev. Geophys., 34, 261-306, 1996.
See Fowler, C. M. R., The Solid Earth, Cambridge University Press, 2nd edition, 2004, p. 474
Fowler, C. M. R., The Solid Earth, Cambridge University Press, 2nd edition, 2004, p. 461
Arkansas Lecture, “Subduction zones – Trenches and Subducting Slabs Websites”
Earthquake Maps
Figure 1 shows the trenches where earthquake data were retrieved from the USGS. South America (red), Middle America (magenta), Aleutians (green), Izu-Bonin (black), Sumatra-Java (blue), and Kermadec-Tonga (cyan) trenches are examined in this study.
 
Figure 2 shows earthquakes retrieved from the USGS PDE catalog. South America is separated into four sections, the Aleutians are only one section, and the rest are two sections. These data are plotted in 3D, and rotated approximately perpendicular to the trench. Latitude and longitude are on the x-axis and depth is on the y-axis.
Figure 3 shows the ages of oceanic lithosphere, red being the youngest and blue the oldest (NOAA, 1996).
Trench |
Age (My) |
Angle (deg) |
Velocity (mm/yr) |
South America |
10, 25, 45, 5 |
30 |
90 |
Middle America |
10, 5 |
40 |
80 |
Aleutians |
55 |
50 |
60 |
Kermadec-Tonga |
120, 140 |
60 |
80 |
Java-Sumatra |
70,120 |
70 |
70 |
Izu-Bonin |
180, 160 |
80 |
100 |
Table 1 shows ages taken from Figure 3, and the angles and velocities taken from Fowler, p 461. The subduction angle of Izu-Bonin was altered from 60 degrees to 80 degrees based a lecture from University of Arkansas that said the subduction angle is near 90 degrees (Arkansas Lecture).
Figure 4 shows thermal parameters of each section plotted versus the maximum and second maximum depths each of trench section; South America (red), Middle America (magenta), Aleutians (green), Izu-Bonin (black), Sumatra-Java (blue), and Kermadec-Tonga (cyan)
 
Figure 5 shows the geothermal gradients calculated for each of the ages of oceanic lithosphere in this study.
 
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