Complex Fault Geometry of the 2020 Mww 6.5 Monte Cristo Range, Nevada, Earthquake Sequence

Abstract

On 15 May 2020 an Mww 6.5 earthquake occurred beneath the Monte Cristo Range in the Mina Deflection region of western Nevada. Rapid deployment of eight temporary seismic stations enabled detailed analysis of its productive and slowly decaying aftershock sequence (p=0.8), which included ∼18,000 autodetected events in 3.5 months. Double-difference, waveform-based relative relocation of 16,714 earthquakes reveals a complex network of faults, many of which cross the inferred 35-km-long east–northeast-striking, left-lateral mainshock rupture. Seismicity aligns with left-lateral, right-lateral, and normal mechanism moment tensors of 128 of the largest earthquakes. The mainshock occurred near the middle of the aftershock zone at the intersection of two distinct zones of seismicity. In the western section, numerous subparallel, shallow, north-northeast-striking faults form a broad flower-structure-like fault mesh that coalesces at depth into a near-vertical, left-lateral fault. We infer the near-vertical fault to be a region of significant slip in the mainshock and an eastward extension of the left-lateral Candelaria fault. Near the mainshock hypocenter, seismicity occurs on a northeast-striking, west-dipping structure that extends north from the eastern Columbus Salt Marsh normal fault. Together, these two intersecting structures bound the Columbus Salt Marsh tectonic basin. East of this intersection and the mainshock hypocenter, seismicity occurs in a narrow, near-vertical, east-northeast-striking fault zone through to its eastern terminus. At the eastern end, the aftershock zone broadens and extends northwest toward the southern extension of the northwest-striking, right-lateral Petrified Springs fault system. The eastern section hosts significantly fewer aftershocks than the western section, but has more moment release. We infer that shallow aftershocks throughout the system highlight fault-fracture meshes that connect mapped fault systems at depth. Comparing earthquake data with surface ruptures and a simple geodetic fault model sheds light on the complexity of this recent M 6.5 Walker Lane earthquake.

The 2020 Monte Cristo (Nevada) Earthquake Sequence: Stress Transfer in the Context of Conjugate Strike‐Slip Faults

“…Based on elastic dislocation theory (Okada, 1992), our preferred model constrained by GPS, InSAR and moment tensor solutions data consists of a combination of three conjugate segments with a mixed right‐lateral, left‐lateral and normal slip distributions (Table 1, Figure 2). The spatial variability of the geometry and the slip distributions from E to W derived from this model align with the spatial evolution of aftershocks and surface ruptures investigations (Koehler et al., 2021; Ruhl et al., 2021).…”

“…Using the empirical relations from Wells and Coppersmith (1994), the magnitude of the 2020 Monte Cristo event implies 0.6 m coseismic left‐lateral slip at depth and ∼20‐km complex‐wide rupture (Table 1, Figure 2) consistent with the GPS data and geological field investigations (Hammond et al., 2021; Koehler et al., 2021). The Monte Cristo slip model proposed in this study (Table 1) also mimics the change in orientation and sense of slip as observed in GPS data, seismicity analysis, and field investigations (Hammond et al., 2021; Koehler et al., 2021; Ruhl et al., 2021).…”

“…The Eastern zone of the rupture including the location of the 2020 Monte Cristo mainshock is quite different. The spatial evolution of seismicity and aftershocks moment tensor solutions shows a more complex distribution of faults with a cluster of events aligned N to NW associated with predominantly right‐lateral slip (Ruhl et al., 2021) comparable to the surface rupture mapping observations (Koehler et al., 2021). The seismicity near the hypocenter is concentrated on shallow oblique structures with ranging in depth between [3–10] km (Ruhl et al., 2021).…”

“…The 2020 Mw 6.5 Monte Cristo Range earthquake has been the site of numerous published studies as of the time of this writing with extensive documentation on earthquake rupture, aftershock distribution and surface faulting (Bormann et al., 2021; Hammond et al., 2021; Koehler et al., 2021; Ruhl et al., 2021). A precise relocation of ∼17,000 events, moment tensor solutions, and the rapid tectonic‐geomorphic reconnaissance demonstrate that the 2020 Monte Cristo earthquake occurred at the intersection of two distinct fault zones characterized by a ∼30 km E‐NE zone of complex fault‐ruptures with variable senses of slip that accommodate overall East‐West left‐lateral shear deformation (Bormann et al., 2021; Koehler et al., 2021; Ruhl et al., 2021). The Western zone is characterized by a wide area of damage characterized by a predominantly left‐lateral offsets with a right‐stepping en echelon fault‐rupture geometries (Koehler et al., 2021; Ruhl et al., 2021) comparable to those observed during the 2019 (Mw 6.4) Ridgecrest earthquake (Ponti et al., 2020).…”

“…Aftershock distribution and focal mechanism solutions (Ruhl et al., 2021; Figure 1), surface rupture mapping (Koehler et al., 2021), GPS measurement (Hammond et al., 2021), and surface deformation deduced from Synthetic Aperture Radar Interferogram (InSAR), indicates that the 2020 Monte Cristo event occurred within two distinct zone of seismicity from west to east characterized by a complex pattern of surface rupture between NE‐striking oblique left‐normal faults, with minor slip on north‐striking right‐lateral faults. Seismicity and geodesy show a predominantly E‐W rupture plane with hints of perpendicular intersecting structures.…”