Relocations and Tectonic Implications of the Nine Mile Ranch Sequence From 2016-2018: 3 Mw 5.4-5.6 Near Hawthorne, Nevada

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

On Dec. 28^th, 2016 three moderate sized earthquakes occurred within the Walker Lane, southwest of Hawthorne, NV. The sequence began with a Mw 5.6 at 08:18 UTC and was followed 4 minutes later by an Mw 5.4. The third large event, a Mw 5.5, occurred at 09:13 UTC, 55 minutes after the initial earthquake. Our aim is to detect structures, characterize the spatio-temporal evolution, and place our observations within the context of the active tectonics in the region, which are poorly understood. These events give us the opportunity to investigate the area, in addition to compare to a 2011 sequence in the immediate vicinity. We update relocations, gather focal mechanisms and moment tensor solutions, and add a stress tensor inversion, GPS, and InSAR analysis to understand the aspects of the sequence. We hope to provide insight into triggering processes, stress release, and the initial stress environment. The Nevada Seismological Laboratory has located over 7000 events from the sequence, with depths ranging from 4-12 km. Moment tensor solutions show high-angle strike-slip faulting for the three main events, with focal mechanisms of smaller events showing predominantly strike-slip and normal ruptures. Relocations indicate several substantial unmapped structures. The largest structure in the sequence implies right-lateral strike slip faulting on a ~N45W striking fault plane. This fault plane is also dipping at a high angle to the NE. Two conjugate SW striking, vertical-dipping fault planes lie at both ends of the larger fault plane. The three main events are located very near one another at the intersection of the right lateral fault plane and the southern-most SW-striking conjugate structure. We compare the features of the Nine Mile Ranch Sequence to the 2011 Hawthorne sequence, which had over 50 earthquakes of M 3+, with the largest event at Mw 4.6. The 2011 and 2016-2018 Nine Mile Ranch sequences occur in the northwest corner of the Mina Deflection, a major structural right step within the Walker Lane, characterized by ENE-trending normal and sinistral faults.

Presenting Author: Rachel Hatch

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