Uncertainties in Earthquake Stress Drop
Earthquake stress drop, the difference in stress on a fault before and after an earthquake, is an important earthquake parameter for seismic hazard analysis. An earthquake’s stress drop controls the level of high‐frequency ground motions that damage structures. However, the uncertainties in stress‐drop estimates are poorly understood. We compared stress drops for ∼900 earthquakes derived from two independent studies using teleseismic data and found practically zero correlation indicating significant stress-drop uncertainty. These large uncertainties in teleseismic estimates might lead to erroneous inferences about earthquake hazards. Our results suggest that the seismogram deconvolution process is a significant source of uncertainty in stress-drop estimation.
Earthquake Maximum Magnitudes
We often look to past earthquakes to inform us about the potential size of future ones. However, how do we know our records are long enough to capture the largest (and infrequent) earthquakes? We use earthquake magnitude distribution statistics to examine whether earthquake records are really long enough to capture the largest earthquakes. In places like eastern North America, it appears that the historical record is not long enough to safely assume it contains the largest possible earthquakes. In the case of global continental earthquake, the records do in fact appear long enough and demonstrate that maximum earthquake magnitude differs by fault geometry.
Modeling Earthquake Recurrence Patterns
Long records often show large earthquakes occurring in supercycles, sequences of temporal clusters of seismicity, cumulative displacement, and cumulative strain release separated by less active intervals. Traditional earthquake recurrence models do a poor job of describing this kind of behavior. We’ve developed a Long-Term Fault Memory model that replicates this earthquake supercycle behavior. This simple Markov model can be modified to match a range of earthquake behaviors. We are exploring what this model can tell us about the likelihood of future earthquakes.
Seismic Hazard Map Assessment
Seismic hazard maps are an important tool in crafting effective earthquake hazard mitigation policies. These maps show the likelihood of earthquake shaking exceeding a certain level over a given time period. Although these maps are widely used, little effort has been spent assessing their predictions. Using both historical shaking reports (which were not used to create the maps) and recent shaking reports from the USGS’s Did You Feel It? reporting tool, we assess seismic hazard maps in places like California and Oklahoma. For more info on the historical records used, check out the California Historical Intensity Mapping Project (CHIMP).
Developing Plate Boundaries
Studying how a new plate boundary develops is challenging since the process occurs on very long timescales (millions of years). Generally, we combine observations from multiple regions at different stages in the development process to construct the full picture. However, tectonic features called Subduction-Tranform Edge Propagator (STEP) faults provide a unique window into the plate boundary development process. STEP faults form from tearing subducting lithosphere and create a brand new transform boundary. Ongoing tearing means that younger parts of the boundary are closest to tear while the older sections are farther away. This age-distance relationship allows us to study millions of years of plate boundary development along a single tectonic feature. Observations along the San Cristobal Trough STEP fault near the Solomon Islands have shown that seismicity trends vary systematically with the plate boundary’s age.
