Highly Heterogeneous Pore Fluid Pressure Enabled Rupture of Orthogonal Faults During the 2019 Ridgecrest Mw7.0 Earthquake

Abstract

Here, we show that the 2019 Mw7.0 Ridgecrest mainshock as well as its Mw6.5 foreshock ruptured orthogonal conjugate faults. We invert the waveforms recorded by the dense strong motion network at relatively high frequencies (up to 1 Hz for P; 0.25 Hz for S) to derive multiple-point source models for both events, aided by path calibrations from a Mw5.4 and a Mw5.5 earthquake. We demonstrate that the mainshock started from a shallow (3 km) depth with a Mw5.2 event and ruptured the main fault branches oriented in the NW-SE direction. At~11 s, two Mw6.2 subevents took place on the SW-NE oriented fault branches that conjugate to the main fault to the NE and SW. The SW branch rupture partially overlapped with the foreshock rupture. We suggest the coseismic rupture on nearly orthogonal faults was enabled by high pore fluid pressure, which greatly weakened the immature fault system in a heterogeneous way. Plain Language Summary Earthquakes, which are caused by shear dislocation processes on faults, often rupture single faults or multiple faults oriented at acute angles. However, rupture of orthogonal faults (i.e., faults oriented at 90°to each other) has until now been considered unfavorable based on the basic Mohr circle stress analysis. Here, we show that two large July 2019 earthquakes (Mw6.5 and Mw7.0) ruptured fault segments that are perpendicular to each other, with one NW trending and the other SW trending. We suggest that the complex fault slip is the result of a young fault system, aided by high heterogeneous pore fluid pressure.

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

“…Despite the similarities in fault interaction with others conjugate triggering mechanisms (Hudnut et al, 1989;Kariche et al, 2018), the static stress change modeling using an isotropic poroelastic model shows no correlation between the fluid redistribution caused by large right-lateral ruptures and the occurrence of moderate to strong earthquakes on conjugated en echelon left-lateral faults. Compared to several case studies of conjugate triggering mechanisms (Fialko, 2004;Kariche et al, 2018Kariche et al, , 2019Shi & Wei, 2020), the 'no apparent' fluid redistribution effect may also explain the large time delay between the Cedar Mountain and Monte Cristo earthquakes (∼88 years). Among the main historical and recent earthquakes, the static stress change modeling at half of the seismogenic depth shows that the 1932 Cedar Mountain earthquake (Mw 7.1) significantly influenced the stress field in Central Walker Lane.…”

Dynamics, interactions and delays of the 2019 Ridgecrest rupture sequence

“…Both earthquakes were highly complex (8), including likely fault reactivation (9,10), defying common, simplifying assumptions on earthquake physics and fault interaction. Peculiarly, the largest events were set apart in time by 34 hr (11,12) while driving aftershocks, shallow aseismic creep, and swarm activity (13,14).…”

High‐Resolution Imaging of Complex Shallow Fault Zones Along the July 2019 Ridgecrest Ruptures

“…The lack of significant off-fault deformation in this area may be due a relatively section of the fault with large surface displacement as indicated by kinematic source inversions (e.g., K. Wang et al, 2020), accommodating most of the slip on the primary fault traces. Finally, the strong heterogeneity of the shallow fault zone obtained from our surface wave dispersion suggests an incipient fault zone, with multiple fault segments not optimally oriented for failure (Crider & Peacock, 2004), as found in other areas (e.g., Goldberg et al, 2020;Lomax, 2020;Shelly, 2020;Shi & Wei, 2020). Such regions are less likely to host large earthquakes, as compared to mature faults, for example, the San Andreas fault.…”