Precarious Rocks Give Clues To Future Earthquake Movements


By Merry Morris

A new approach to predict the intensity and direction of earthquakes lies in an unlikely source — 10,000-yearold, improbably balanced rocks. You’ve probably seen pictures of these; they are astounding, seemingly impossible and a little scary. You certainly wouldn’t want to be standing downhill if they toppled over.

These geological features are called Precariously Balanced Rocks or “PBRs.” They are formed slowly when tectonic forces elevate granite rocks from below ground to the surface, and erosion whittles away the softer surrounding materials leaving the unlikely and amazing result.

Earthquake Predictions From A Strange Source

Of interest are certain PBRs in the San Bernardino Mountains located 90 miles east of Los Angeles near the active San Andreas and San Jacinto faults where the North American and Pacific tectonic plates meet. These PBRs should not be standing; they have been in place long enough that earlier ground-shaking earthquakes (1812 and 1857) should have knocked them out of balance. There must be some counteracting factors responsible for their long-lasting balancing act, reasoned researchers Julian Lozos of Stanford University and Lisa Grant Ludwig of University of California, Irvine, considering earthquakes occur every 200 to 300 years. Figuring out this puzzle would be of tremendous importance in predicting what future quakes might do and how to prepare in advance for the inevitable destruction.

The researchers propose that around the San Bernardino’s PBRs, earthquakes occur less frequently and with less energy. As a mechanism, they suggest that a quake initiating on the San Jacinto fault could “hop” into the San Andreas fault, giving up some of its energy and diverting the path of the seismic activity. This process would then alter the direction of quake energy and the location of most critical damage; in this case, sparing the PBRs. This likelihood is nice to know when developing plans for structures, water supplies and other support systems in quake-susceptible areas. Alternately, the “hop” might lead to ruptures along both faults simultaneously, multiplying the devastation. Also, very important for planning purposes.

Good news for quake-fearing Californians — the ability to make predictions by evaluating the PBRs in quake-prone areas may unlock some clues to dealing with the “big one.”

Extension Questions

  • Investigate the locations of other PBRs around the globe. Are these in earthquake-prone areas?
  • Research other methods of earthquake prediction. How do they compare with the evaluation of PBRs in susceptible areas?
  • Create a map showing the paths of the San Andreas and San Jacinto faults. What population centers are located within 50 miles of these faults?