Intensity of the 18 April 1906 earthquake in and near San Jose, California

Intensities of shaking for previously untapped, historical sources of damage data from the 1906 earthquake in and near San José, California, were developed with a refined and expanded version of the Modified Mercalli intensity (MMI) scale designed to bring out rich detail in the dense data. Sanborn fire insurance maps provide construction details and precise locations for 80 percent of the 607 intensity data points. Intensity values for 1906 range from MMI 6 to 9, with an average of 7.4. For damage data grouped by modern census blocks, the small variation in intensity within blocks indicates the 1906 intensity values estimated here reflected true ground motion. Within the 12-km by 12-km study area, different microzones from about 1.0 to 16.8 km 2 in size had characteristic 1906 intensities ranging from 6.6 to 8.2. Variations in 1906 intensity over small areas were correlated with site-specific geologic factors, primarily slowness of shear-wave velocity in the top 30 m (1/V s 30 ), thickness of unconsolidated sediment, proximity to buried basin edges, and likely resonance between shallow ground layers and buildings. Distance to the San Andreas fault and 1906 epicenter, depth to bedrock basement, surficial geologic map unit, and liquefaction did not control 1906 intensity. Agreement with modern ground-motion studies suggests that areas of high or low intensity have persisted through time