Nature's Balanced Seismometers

Arizona has some of the most beautiful perched rocks in the Southwest. From the towering forests of rhyolitic spires in the Chiricahua Mountains to the roosting granitic boulders of the Granite Dells, our balanced rocks are fine examples of nature’s geomorphic and geologic products.

Perched rocks, also known as precariously balanced rocks (PBRs), are typically balanced on bedrock pedestals throughout upland drainage basins and pediments. They range from a few decimeters to several meters across and can weigh up to several tons. Granitic PBRs are formed in a two-stage process. The first stage occurs in the subsurface where chemically active waters infiltrate into joint and fracture surfaces. Chemical weathering is thus enhanced along these surfaces, resulting in the formation of spheroidally shaped corestones beneath the land’s surface. The second stage involves the mechanical stripping of the decayed rock material, leading to the exhumation of the spheroidal corestones. Vast fields of spheroidally weathered boulders and PBRs can be seen along many Arizona highways, such as State Route 87 (between Phoenix and Payson) and Interstate 10 (between Benson and Willcox).

The development of rounded boulders in precarious positions depends on several geologic and geomorphic factors. These include, but are not limited to, the slope of the ground, the spacing and orientation of bedrock joints, the internal lithologic properties of the bedrock, and climate. Current geomorphic research aims to quantify how these factors control the formation and preservation of PBRs in the landscape

Precariously balanced rocks are proving to be life-saving natural ornaments in seismically active regions. Over the past fifteen years, geoscientists recognized PBRs as negative indicators of extreme ground motions produced by historic earthquakes. In other words, the presence of PBRs is evidence that an area has not experienced large earthquake-induced ground motions since the formation of the precarious rocks. Otherwise, the PBRs would likely have toppled by the shaking. Thus, PBRs are used as natural seismometers to estimate paleo-earthquake magnitudes by physically constraining earthquake-generated maximum ground motions.

Precariously balanced rocks prove most useful in monitoring ground motions when constraints on their exposure times are established. The use of cosmogenic radionuclides (CRN) to determine exposure histories of rocks to cosmic rays make this possible. Incoming cosmic radiation bombard and react with nuclei in Earth’s atmosphere to produce atmospheric cosmogenic nuclides. Secondary subatomic particles (neutrons and muons) produced from these reactions interact with minerals in rocks to produce terrestrial cosmogenic nuclides. Careful measurement of nuclide production rates from rock surfaces provides estimates of exposure times used to constrain exhumation histories of PBRs. This, in turn, provides a first-order estimation of the elapsed time a PBR has been balanced since the occurrence of the most recent large earthquake.

Coupling surface exposure ages of PBRs with modeled PBR-toppling earthquake magnitudes yields estimates of recurrence intervals of large earthquakes. These are vital to probabilistic seismic hazard analyses because they are used to quantify the likely spatial extent of earthquake-induced damage to man-made structures (e.g., apartment buildings, schools, hospitals, businesses, roads, water and gas lines, etc.). PBRs are used as natural seismometers to evaluate the veracity of seismic hazard maps in Southern California and Nevada.

Whether we admire them for their photogenic properties or their apparent gravity-defying grace, PBRs will continue to safeguard our communities by silently monitoring our deserts. Meanwhile, interest in the formation of PBRs and how to more effectively use them as nature’s balanced seismometers will continue to grow.

suggested readings

Anooshehpoor, A., and Brune, J. N., 2002, Verification of precarious rock methodology using shake table tests of rock models: Soil Dynamics and Earthquake Engineering, v. 22, p. 917-922.

Bell, J. W., Brune, J. N., Liu, T., Zreda, M., and Yount, J. C., 1998, Dating precariously balanced rocks in seismically active parts of California and Nevada: Geology, v. 26, no. 6, p. 495-498.

Brune, J. N., 1996, Precariously balanced rocks and ground-motion maps for Southern California: Bulletin of the Seismological Society of America, v. 86, no. 1A, p. 43-54.

Haddad, D. E., and Arrowsmith, J. R., 2008a, A comparative assessment of the geomorphologic setting and geologic context of zones of precariously balanced rock in low-seismicity regions (proceedings and abstracts): SCEC Annual Meeting, Palm Springs, California, September 6-11, 2009, 18.

-, 2008b, Investigation of the geologic setting and geomorphic processes that control the formation and preservation of precarious rock zones: Eos Trans. AGU 89(52), Fall Meet. Suppl., Abstract H51D-0838.

Shi, B., Abdolrasool, A., Zeng, Y., and and Brune, J. N., 1996, Rocking and overturning of precariously balanced rocks by earthquakes: Bulletin of the Seismological Society of America, v. 86, no. 5, p. 1364-1371.


David E. Haddad
School of Earth & Space Exploration

Arizona State University
Tempe, AZ

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