5.2: Earthquakes and Plate Tectonics
- Page ID
- 33114
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The distribution of earthquakes across the globe is shown in the figure below. It is relatively easy to see the relationships between earthquakes and the plate boundaries. Along divergent boundaries like the mid-Atlantic ridge (middle of Atlantic Ocean) and the East Pacific rise (in ocean west of South America) earthquakes are common but restricted to a narrow zone close to the ridge, and consistently less than 30 km depth (red dots). Shallow earthquakes are also common along transform faults, such as the San Andreas Fault. Earthquakes are very common along subduction zones, and their depth below surface increases inshore from the subduction zone (green and blue dots).
Earthquakes also occur in a few intraplate locations, including the Rift Valley area of Africa (near the center of Africa), in the Tibet region of China and in the Lake Baikal area of Russia.
Earthquakes at Divergent and Transform Boundaries
Figure \(\PageIndex{2}\): provides a closer look at magnitude 4 and larger earthquakes in an area of divergent and transform boundaries in the mid-Atlantic region near to the equator. Here the segments of the mid-Atlantic ridge (divergent boundaries) are offset by some long transform faults, and there is side-by-side motion on those faults. Most of the earthquakes are located along the transform faults, rather than along the divergent boundaries, although there are clusters of earthquakes at some of the divergent-transform intersections. Some earthquakes do occur on divergent boundaries, but they tend to be small and infrequent because of the relatively high rock temperatures in the areas where spreading is taking place.
Earthquakes at Convergent Boundaries
The distribution and depths of earthquakes in the Caribbean and Central America area are shown on Figure \(\PageIndex{3}\). In this region the Cocos Plate is subducting beneath the North America and Caribbean Plates (an ocean-continent convergence), and the South and North America Plates are subducting beneath the Caribbean Plate (an ocean-ocean convergence). In both cases the earthquakes get deeper with distance from the trench.
All the very large earthquakes—M9 or higher—take place at subduction boundaries because there is the potential for greater rupture zone width on a gently dipping subduction boundary than on a steep transform boundary. The largest earthquakes on transform boundaries, like what is most of California marked by the San Andreas fault, are in the order of M8.
Media Attributions
- Figure \(\PageIndex{1}\): Dale Sawyer, Rice University, http://plateboundary.rice.edu Used with permission. All rights reserved.
- Figure \(\PageIndex{2}\): Dale Sawyer, Rice University. http://plateboundary.rice.edu. Used with permission. All rights reserved.
- Figure \(\PageIndex{3}\): Dale Sawyer, Rice University. http://plateboundary.rice.edu. Used with permission. All rights reserved.