Seismological research issues in the San Diego region

What is the nature of earthquake ground motions that can be expected in San Diego's foreseeable future? Although this is the most basic of questions underlying the adequate design of structures to resist earthquakes, answers to this question are disturbingly uncertain. A reasonable assumption is that future earthquake ground motions will be similar to those that have occurred in the past. When compared with San Francisco or Los Angeles, San Diego has historically experienced relatively mild earthquake shaking. Unfortunately, San Diego's written history is very short compared to the time scales of earthquake repetition. Are there sources of earthquakes that may cause damage in San Diego and what is their frequency? Mapping of geologic structures and the study of patterns of small earthquakes are the primary tools for recognizing potentially active faults. There are features in both the geologic structure and the seismicity that are suggestive of major active faults that could pose a serious hazard to San Diego. Furthermore, there is evidence that the rate of occurrence of small earthquakes has increased within the last 5 years when compared with the previous 50 years. However, these features are not well studied or understood. Even if the potential sources of earthquakes were well understood, the problem of anticipating the range of future ground motions is difficult. The nature of shaking from earthquakes is strongly affected by the nature of seismic wave propagation through complex geologic structures (path effects). Although path effects are likely to be of great importance in San Diego, relatively little specific information is available