| Line | In 1971 researchers hoping to predict earthquakes in |
| the short term by identifying precursory phenomena |
| (those that occur a few days before large quakes |
| but not otherwise) turned their attention to changes |
| (5) | in seismic waves that had been detected prior to |
| earthquakes. An explanation for such changes was |
| offered by “dilatancy theory,” based on a well-known |
| phenomenon observed in rocks in the laboratory: |
| as stress builds, microfractures in rock close, |
| (10) | decreasing the rock’s volume. But as stress |
| continues to increase, the rock begins to crack and |
| expand in volume, allowing groundwater to seep in, |
| weakening the rock. According to this theory, such |
| effects could lead to several precursory phenomena in |
| (15) | the field, including a change in the velocity of seismic |
| waves, and an increase in small, nearby tremors. |
| Researchers initially reported success in identifying |
| these possible precursors, but subsequent analyses |
| of their data proved disheartening. Seismic waves |
| (20) | with unusual velocities were recorded before some |
| earthquakes, but while the historical record confirms |
| that most large earthquakes are preceded by minor |
| tremors, these foreshocks indicate nothing about |
| the magnitude of an impending quake and are |
| (25) | indistinguishable from other minor tremors that occur |
| without large earthquakes. |
| In the 1980s, some researchers turned their |
| efforts from short-term to long-term prediction. |
| Noting that earthquakes tend to occur repeatedly in |
| (30) | certain regions, Lindh and Baker attempted to identify |
| patterns of recurrence, or earthquake cycles, on which |
| to base predictions. In a study of earthquake-prone |
| sites along the San Andreas Fault, they determined |
| that quakes occurred at intervals of approximately 22 |
| (35) | years near one site and concluded that there was a |
| 95 percent probability of an earthquake in that area |
| by 1992. The earthquake did not occur within the time |
| frame predicted, however. |
| Evidence against the kind of regular |
| (40) | earthquake cycles that Lindh and Baker tried |
| to establish has come from a relatively new |
| field, paleoseismology. Paleoseismologists |
| have unearthed and dated geological features |
| such as fault scarps that were caused by |
| (45) | earthquakes thousands of years ago. They have |
| determined that the average interval between ten |
| earthquakes that took place at one site along the |
| San Andreas Fault in the past two millennia was |
| 132 years, but individual intervals ranged greatly, |
| (50) | from 44 to 332 years. |