Dr. Peggy Hellweg
No, Dr. Peggy Hellweg of UC Berkeley’s Seismological Laboratory can’t predict the Bay Area’s next major earthquake. However, she did go over why scientists think the Hayward fault could get shaken up pretty soon. That was part of her talk during March’s enrichment lecture for Chabot volunteers and their guests.
Earthquakes occur when the edges of the major tectonic plates that make up the earth’s crust stick, rub or break off against each other. There are three different types of tectonic plate boundaries: transform, where the plates are moving past each other horizontally, convergent, where they move towards each other, and divergent, where the plates are moving apart. Most of California’s faults, including the large San Andreas fault, are transform faults. The North American plate, where our continent resides, is moving slowly westward, while the Pacific plate, adjacent to the west, is moving slowly eastward.
Until about 30 million years ago, the land that is now California was within a subduction zone, where part of one plate moves underneath another and becomes buried. The present-day San Andreas fault system formed from the tension created as the North American plate moved northwest.
Geologists figured out that continents moved over time by observing changes in the magnetic fields of rocks. When rocks heated above a certain temperature cool, they take on the magnetic field orientation present around them, which depends on where they are located in relation to the Earth’s magnetic poles. So when some rocks in a certain area have a different orientation from nearby rocky matter, we can infer that they have moved in from elsewhere. Researchers estimate that the earth’s tectonic plates move at about five centimeters per year.