The Complex Relationship Between Small And Large Earthquakes: Debunking Common Misconceptions.pdf

The article discusses the recent occurrence of multiple small earthquakes (magnitude 4s and a 3.7) in Northern California over a short period, leading to common questions about their significance and potential for triggering larger quakes. The author, Lori Dengler, an emeritus professor of geology, clarifies several misconceptions surrounding these events.

Key Points:

1. Misconceptions About Small Earthquakes:
- Small earthquakes do not necessarily relieve stress or prevent larger ones; they release energy, but not enough to significantly reduce the likelihood of a major quake.

2. Differentiating Between Earthquakes:
- The recent quakes had different magnitudes and occurred on distinct fault types (normal vs. strike slip), indicating they were independent events rather than aftershocks or foreshocks related to each other.

3. Stress Shadows and Statistics:
- Stress shadows, areas of stress change around an earthquake, did not extend far enough to affect the recent quakes.
- Statistical analysis of historical data shows that small earthquakes following a larger one (like the 1992 Landers quake) do not necessarily presage a "big one."

4. The "Big One" and Foreshocks:
- The odds of a major earthquake (e.g., on the Cascadia Subduction Zone) remain the same despite recent activity.
- USGS statistics provide probabilities for aftershocks and larger earthquakes, but these are based on historical patterns and may not be predictive in every case.

5. The Cascadia Subduction Zone:
- The CSZ has been unusually quiet seismically, making it unlikely that recent small quakes are foreshocks of a major event on this fault line.