ABSTRACT The July 2019 Ridgecrest, California, earthquake sequence involved two large events—the M 6.4 foreshock and the M 7.1 mainshock that ruptured a system of intersecting strike-slip faults. We present analysis of space geodetic observations including Synthetic Aperture Radar and Global Navigation Satellite System data, geological field mapping, and seismicity to constrain the subsurface rupture geometry and slip distribution. The data render a complex pattern of faulting with a number of subparallel as well as cross-cutting fault strands that exhibit variations in both strike and dip angles, including a “flower structure” formed by shallow splay faults. Slip inversions are performed using both homogeneous and layered elastic half-space models informed by the local seismic tomography data. The inferred slip distribution suggests a moderate amount of the shallow coseismic slip deficit. The peak moment release occurred in the depth interval of 3–4 km, consistent with results from previous studies of major strike-slip earthquakes, and the depth distribution of seismicity in California. We use the derived slip models to investigate stress transfer and possible triggering relationships between the M 7.1 mainshock and the M 6.4 foreshock, as well as other moderate events that occurred in the vicinity of the M 7.1 hypocenter. Triggering is discouraged for the average strikemore »
Rupture Process of the 2019 Ridgecrest, California Mw 6.4 Foreshock and Mw 7.1 Earthquake Constrained by Seismic and Geodetic Data
ABSTRACT The 2019 Ridgecrest earthquake sequence culminated in the largest seismic event in California since the 1999 Mw 7.1 Hector Mine earthquake. Here, we combine geodetic and seismic data to study the rupture process of both the 4 July Mw 6.4 foreshock and the 6 July Mw 7.1 mainshock. The results show that the Mw 6.4 foreshock rupture started on a northwest-striking right-lateral fault, and then continued on a southwest-striking fault with mainly left-lateral slip. Although most moment release during the Mw 6.4 foreshock was along the southwest-striking fault, slip on the northwest-striking fault seems to have played a more important role in triggering the Mw 7.1 mainshock that happened ∼34 hr later. Rupture of the Mw 7.1 mainshock was characterized by dominantly right-lateral slip on a series of overall northwest-striking fault strands, including the one that had already been activated during the nucleation of the Mw 6.4 foreshock. The maximum slip of the 2019 Ridgecrest earthquake was ∼5 m, located at a depth range of 3–8 km near the Mw 7.1 epicenter, corresponding to a shallow slip deficit of ∼20%–30%. Both the foreshock and mainshock had a relatively low-rupture velocity of ∼2 km/s, which is possibly related to the geometric complexity and immaturity of the eastern California shear zone faults. The 2019 more »
- Award ID(s):
- 1735448
- Publication Date:
- NSF-PAR ID:
- 10250849
- Journal Name:
- Bulletin of the Seismological Society of America
- Volume:
- 110
- Issue:
- 4
- Page Range or eLocation-ID:
- 1603 to 1626
- ISSN:
- 0037-1106
- Sponsoring Org:
- National Science Foundation
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Abstract Surface rupture from the 2019 Ridgecrest earthquake sequence, initially associated with the Mw 6.4 foreshock, occurred on 4 July on a ∼17 km long, northeast–southwest-oriented, left-lateral zone of faulting. Following the Mw 7.1 mainshock on 5 July (local time), extensive northwest–southeast-oriented, right-lateral faulting was then also mapped along a ∼50 km long zone of faults, including subparallel splays in several areas. The largest slip was observed in the epicentral area and crossing the dry lakebed of China Lake to the southeast. Surface fault rupture mapping by a large team, reported elsewhere, was used to guide the airborne data acquisition reported here. Rapid rupture mapping allowed for accurate and efficient flight line planning for the high-resolution light detection and ranging (lidar) and aerial photography. Flight line planning trade-offs were considered to allocate the medium (25 pulses per square meter [ppsm]) and high-resolution (80 ppsm) lidar data collection polygons. The National Center for Airborne Laser Mapping acquired the airborne imagery with a Titan multispectral lidar system and Digital Modular Aerial Camera (DiMAC) aerial digital camera, and U.S. Geological Survey acquired Global Positioning System ground control data. This effort required extensive coordination with the Navy as much of the airborne data acquisition occurred within their restricted airspacemore »
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ABSTRACT We investigate the deformation processes during the 2019 Ridgecrest earthquake sequence by combining Global Navigation Satellite Systems, strong-motion, and Interferometric Synthetic Aperture Radar datasets in a joint inversion. The spatial complementarity of slip between the Mw 6.4 foreshock, Mw 7.1 mainshock, and afterslip suggests the importance of static stress transfer as a triggering mechanism during the rupture sequence. The coseismic slip of the foreshock concentrates mainly on the east-northeast–west-southwest fault above the hypocenter at depths of 2–8 km. The slip distribution of the mainshock straddles the region above the hypocenter with two isolated patches located to the north-northwest and south-southeast, respectively. The geodetically determined moment magnitudes of the foreshock and mainshock are equivalent to moment magnitudes Mw 6.4 and 7.0, assuming a rigidity of 30 GPa. We find a significant shallow slip deficit (>60%) in the Ridgecrest ruptures, likely resulting from the immature fault system in which the sequence occurred. Rapid afterslip concentrates at depths of 2–6 km, surrounding the rupture areas of the foreshock and mainshock. The ruptures also accelerated viscoelastic flow at lower-crustal depths. The Garlock fault was loaded at several locations, begging the question of possible delayed triggering.
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The recent 2019 Ridgecrest earthquake sequence in Southern California jostled the seismological community by revealing a complex and cascading foreshock series that culminated in a M7.1 mainshock. But the central Garlock fault, despite being located immediately south of this sequence, did not coseismically fail. Instead, the Garlock fault underwent post-seismic creep and exhibited a sizeable earthquake swarm. The dynamic details of the rupture process during the mainshock is largely unknown, as is the amount of stress needed to bring the Garlock fault to failure. We present an integrated view of how stresses changed on the Garlock fault during and after the mainshock using a combination of tools including kinematic slip inversion, Coulomb stress change, and dynamic rupture modeling. We show that positive Coulomb stress changes cannot easily explain observed aftershock patterns on the Garlock fault, but are consistent with where creep was documented on the central Garlock fault section. Our dynamic model is able to reproduce the main slip asperities and kinematically estimated rupture speeds (≤ 2 km/s) during the mainshock, and suggests the temporal changes in normal and shear stress on the Garlock fault were greatest near the end of rupture. The largest static and dynamic stress changes onmore »
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- https://doi.org/10.1785/0120200108
