Seismological Indicators of Geologically Inferred Fault Maturity

Variations in fault zone maturity have intermittently been invoked to explain variations in some seismological observations for large earthquakes. However, the lack of a unified geological definition of fault maturity makes quantitative assessment of its importance difficult. We evaluate the degree of empirical correlation between geological and geometric measurements commonly invoked as indicative of fault zone maturity and remotely measured seismological source parameters of 34 MW & GE; 6.0 shallow strike-slip events. Metrics based on surface rupture segmentation, such as number of segments and surface rupture azimuth changes, correlate best with seismic source attributes while the correlations with cumulative fault slip are weaker. Average rupture velocity shows the strongest correlation with metrics of maturity, followed by relative aftershock productivity. Mature faults have relatively lower aftershock productivity and higher rupture velocity. A more complex relation is found with moment-scaled radiated energy. There appears to be distinct behavior of very immature events which radiate modest seismic energy, while intermediate mature faults have events with higher moment-scaled radiated energy and very mature faults with increasing cumulative slip tend to have events with reduced moment-scaled radiated energy. These empirical comparisons establish that there are relationships between remote seismological observations and fault system maturity that can help to understand variations in seismic hazard among different fault environments and to assess the relative maturity of inaccessible or blind fault systems for which direct observations of maturity are very limited. Earthquakes occur on faults, which are often complex structures. The relationship between the structure of faults and the seismological aspects of an earthquake rupturing that fault is an ongoing puzzle. Here we empirically compare geological and geometrical aspects of strike-slip fault zones and surface ruptures of major events with seismological attributes of the events. We consider factors such as the total offset on the fault that has accumulated over geological time and the number of segments in maps of earthquake rupture and investigate how they correlate with aspects of the resulting earthquake such as the number of aftershocks or the rupture speed. Several of these factors co-evolve as a fault accumulates slip and matures, thus the trends can be interpreted as indicative of the type of earthquakes observed on mature versus immature faults. We find that less mature faults tend to generate more aftershocks and have lower rupture velocity. Energy estimated from seismograms is relatively low for very immature faults, increases with fault evolution and then decreases as maturity further increases. These relationships help elucidate seismic hazard for fault systems of different maturity and delineate the important fault zone factors that have bearing on earthquake rupture and aftershock generation process. Strike-slip fault zone relative maturity is inferred from cumulative fault slip and from individual event surface rupture segment geometryMaturity measures are compared with relative aftershock productivity, rupture velocity, and moment-scaled radiated energyLess mature faults have higher relative aftershock productivity and lower rupture velocity than mature faults

Automated summary

There is a correlation between seismological observations and fault system maturity, which can help understand variations in seismic hazard and assess the relative maturity of inaccessible fault systems.