Testing the Hypothesis that Detachment Faults are Active and Generate Microearthquakes

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Since low-angle normal, or detachment, faults and their role in accommodating large-scale extension were recognized in the 1970's, these enigmatic structures have captured the attention of a wide community of structural and tectonic geologists. Outstanding questions concerning detachment faulting include the putative mechanical impossibility of slip on gently dipping normal faults at and near the Earth's surface and if some detachment faults are active, why has not seismicity been observed? Are the faults slipping aseismically, or are recurrence intervals of large earthquakes longer than historic records? To address these questions, University of Washington researchers will install a ten-station, portable temporary network of seismometers in Death Valley for a period of six to nine months. Data recorded in the field will be analyzed to yield the locations of earthquake hypocenters. The seismometers will be situated on alluvial fans on the west side of Death Valley about 8 km above the projected positions of detachment faults at depth. West-dipping, geologically and possibly seismically active detachment faults are well exposed on the east side of Death Valley along the foot of the Black Mountains, as are range-front scarps of normal faults that formed due to slip during late Quaternary or Holocene earthquakes. The experiment will test the hypotheses that: (1) the gently dipping detachment faults are seismogenic, and (2) that rupture and slip nucleating at seismogenic depths produces microearthquakes. A positive result would challenge the hypothesis that slip on detachments is either mechanically impossible or aseismic.Low-angle normal, or detachment, faults are important tectonic elements in that they accommodate large-scale stretching in extensional regimes, such as the Basin and Range in western United States. However, these faults are paradoxical because mechanical analysis suggests movement should not occur at such low angles and earthquakes are rarely observed even though geological evidence suggests the detachments are active. This experiment, the first of its kind directed toward detachment faults in western North America, will explore these paradoxes. Observation of microearthquakes would challenge the widely-held idea that fault movement on detachments is either mechanically impossible or aseismic.