资源环境科技发展态势分析平台

Within the San Bernardino basin, some focal mechanisms show normal slip that is inconsistent with the expected interseismic strike-slip loading of the region. The discrepancy may owe to deep (>10-km depth), creep along the nearby northern San Jacinto fault. The enigmatic normal slip microseismicity occurs to the northeast of the fault and primarily below 10-km depth, consistent with off-fault deformation due to spatially nonuniform ongoing slip. Consequently, if these normal focal mechanisms are included in stress inversions from the seismic catalog, the results may provide inaccurate information about fault loading. Here we show that off-fault loading from models with deep interseismic creep on the northern San Jacinto fault match the first-order pattern of observed normal slip focal mechanisms in the basin and that this deep creep cannot be detected with GPS data due to the proximity of the San Andreas fault.

Plain Language Summary Over the past 36years, seismic stations have recorded the style of deformation from thousands of small earthquakes in the San Bernardino basin, California. Within this basin, many earthquakes below 10-km depth show deformation that does not match what we expect for this region during the current period between large damaging earthquakes along the San Jacinto and San Andreas faults. Rather than showing expected horizontal slip, many of these earthquakes show vertical movement. We use crustal deformation models to show that vertical movement can be produced in the basin if the northern portion of the San Jacinto fault creeps at depth; this portion of the fault is constantly moving rather than locked, like the San Andreas fault. Traditional GPS-based approaches to detect deep creep do not work here because the faults are too close to one another. The findings of this study demonstrate that small earthquakes that occur adjacent to and between faults can have very different style of deformation than the large ground rupturing earthquakes produced along active faults. This means that scientists should not use the information recorded by these small earthquakes in the San Bernardino basin to predict loading of the nearby San Andreas and San Jacinto faults.