TOPPLED COLUMNS AS STRONG MOTION SENSORS IN NORTHERN CALIFORNIA AND CENTRAL OREGON

Normal faulting characterizes regions of active extensional tectonics at many places on Earth, such as Iceland, the Basin and Range Province of North America, and the west flank of Kilahuea volcano on Hawaii. There is a common association between extensional faulting and basaltic volcanism so that many faults form scarps in jointed basalt. Characteristics of the jointed basalt, such as joint dimensions, scarp height, location along scarp segment, vesicle density, and free-face height, play a large role in the degree of degradation of the scarp. In particular, the weathering, erosion and failure of columns of different sizes dominates the degradation process. Under normal conditions, the basaltic columns with bases greater than 0.45 m in diameter have stable centers of gravity and will topple only with ground motion to induce rocking. In regions of low historical seismicity, precarious balanced rocks can be used to estimate strong ground motion. We propose that toppled columns can also be used as strong motion sensors. Toppled columns over 0.45 m in diameter were measured at three fault scarps in central Oregon and northern California. We calculated the ground acceleration necessary for toppling and determined that 0.39 to 0.87g is required to topple the columns. According to published seismic probability maps, the study regions will not feel the accelerations necessary to topple columns of this size in the next 50 years. We used the acceleration values to calculate Modified Mercali intensity necessary for toppling; these can be loosely correlated to minimum event magnitude. The minimum magnitude for local earthquakes to induce toppling of the columns is between 3.8-5.0. We compared these magnitudes to frequency-magnitude relationships for the region and determined these events reoccur in intervals between 10 and 100 years. Based on the lengths of fault segments, we estimate a regional maximum magnitude to be 6.7. The high frequency of toppling events suggest that regional ground shaking may play a large role in the degradation of basalt scarps by toppling stable columns of all sizes. With appropriate dating techniques to determine time since toppling, these columns might be used as paleoseismic indicators.