Rupture properties of clustered microearthquakes near intersecting intraplate faults of new Madrid seismic zone: Implications for fault weakening

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Journal of Geophysical Research-Solid Earth
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Apr 10
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Seismic waveforms from the portable array for numerical data acquisition network were used to identify sets of 21 similar microearthquakes near Ridgely, Tennessee, where two major segments of the New Madrid Seismic Zone (NMSZ) intersect. The events are relocated with a precision of about 30 m and are found to have occurred along four subfaults that are aligned to N50 degrees E, subparallel to the southern (Arkansas) segment of the NMSZ. The separations among the subfaults are a few hundred meters, while the span of hypocenters within each subfault is typically a few tens of meters. These subfaults are inferred from focal mechanism solutions to be oriented at high angles to N50 degrees E trend, striking roughly north-northwest and dipping moderately with reverse slip. This indicates that the subfaults belong to the central segment of the NMSZ and are en echelon. The similar events also cluster in time. The typical time span of events over one subfaults is about 3 weeks, while the intercluster times are a few months or more. An empirical Green's function analysis of the largest event placed a lower bound of its rupture dimension at 121 +/- 32 m, suggesting that this rupture overlapped with the smaller ruptures along the same subfault. On the basis of variations in fault zone properties previously inferred for the Ridgely junction, the rupture properties of the similar events and the result of a stress calculation, we infer that the similar events may be triggered by temporal weakening of the subfaults, possibly associated with a temporal elevation of pore fluid pressure. The nearly constant depths (similar to 5.7 km) and the reverse dip of the ruptures suggest that the en echelon subfaults may be intersected by subhorizontal veins, which could have been developed by episodic influx of fluid at superhydrostatic pressures.


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