Whether the final properties of large earthquakes can be inferred from initial observations of rupture (deterministic rupture) is valuable for understanding earthquake source processes and is critical for operational earthquake and tsunami early warning. Initial (P-wave) characteristics of small to moderate earthquakes scale with magnitude, yet observations of large to great earthquakes saturate, resulting in magnitude underestimation. Whether saturation is inherent to earthquake dynamics or rather is due to unreliable observation of long-period signals with inertial seismic instrumentation is unclear. Seismogeodetic methods are better suited for broadband observation of large events in the near-field. In this study, we investigate the deterministic potential of seismogeodetically derived P-wave amplitude using a dataset of 14 medium-to-great earthquakes around Japan. Our results indicate that seismogeodetic P-wave amplitude is not a reliable predictor of magnitude, opposing the notion of strong determinism in the first few seconds of rupture.
Abstract The 4 July 2019 M w 6.4 and subsequent 6 July 2019 M w 7.1 Ridgecrest sequence earthquakes (CA, USA) ruptured orthogonal fault planes in a low slip rate (1 mm/year) dextral fault zone in the area linking the Eastern California Shear Zone and Walker Lane. This region accommodates nearly one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas Fault. We investigate the rupture process of these events using a novel simultaneous kinematic slip method with joint inversion of high‐rate GNSS, strong motion, GNSS static offset, and Interferometric Synthetic Aperture Radar data. We model the Coulomb stress change to evaluate how the M w 6.4 earthquake may have affected the subsequent M w 7.1 event. Our findings suggest complex interactions between several fault structures, including dynamic and static triggering, and provide important context for regional seismic source characterization and hazard models.
Abstract Traditional real-time (RT) seismology has relied on inertial sensors to characterize ground motions and earthquake sources, particularly for hazards applications such as warning systems. In the past decade, a revolution in high-rate, RT Global Navigation Satellite Systems (GNSS) displacement has provided a new source of data to augment traditional measurement devices. The Ridgecrest, California, earthquake sequence in 2019 provided one of the most complete recordings of RT-GNSS displacements to date, helping aid in an initial source characterization over the first few days. In this article, we analyze and make available the archived RT displacement streams and compare their performance to postprocessed results, which we also provide. We find good agreement for all stations showing a noticeable signal. This demonstrates that simple modeling in RT, such as peak ground displacement scaling, would be practically identical to postprocessed results. Similarly, we find good agreement across the full spectral range, from the coseismic offsets (∼0 Hz) to the Nyquist frequency. We also find low latency between the measurement acquisition at the field site and the position calculation at the data center. In aggregate, the performance during the Ridgecrest earthquakes is strong evidence of the viability and usefulness of RT-GNSS as a monitoring tool.
Abstract The U.S. Geological Survey (USGS) National Earthquake Information Center (NEIC) routinely produces finite-fault models following significant earthquakes. These models are spatiotemporal estimates of coseismic slip critical to constraining downstream response products such as ShakeMap ground motion estimates, Prompt Assessment of Global Earthquake for Response loss estimates, and ground failure assessments. Because large earthquakes can involve slip over tens to hundreds of kilometers, point-source approximations are insufficient, and it is vital to rapidly assess the amount, timing, and location of slip along the fault. Initially, the USGS finite-fault products were computed in the first several hours after a significant earthquake, using teleseismic body wave and surface wave observations. With only teleseismic waveforms, it is generally possible to obtain a reliable model for earthquakes of magnitude 7 and larger. Here, we detail newly implemented updates to NEIC’s modeling capabilities, specifically to allow joint modeling of local-to-regional strong-motion accelerometer, Global Navigation Satellite System (GNSS), and Interferometric Synthetic Aperture Radar (InSAR) observations in addition to teleseismic waveforms. We present joint inversion results for the 2015 Mw 8.3 Illapel, Chile, earthquake, to confirm the method’s reliability. Next, we provide examples from recent earthquakes: the 29 July 2021 Mw 8.2 Chignik, Alaska, United States, the 14 August 2021 Mw 7.2 Nippes, Haiti, and the 8 July 2021 Mw 6.0 Antelope Valley, California, United States, earthquakes. These examples confirm that jointly leveraging a variety of geophysical datasets improves the reliability of the slip model and demonstrate that such a combination can be leveraged for rapid response. The inclusion of these new datasets allows for more consistent finite-fault modeling of earthquakes as small as magnitude 6. As accelerometer, GNSS, and InSAR observations worldwide become more accessible, these joint models will become more routine, providing improved resolution and spatiotemporal constraints on rapid finite-fault models, and thereby improving the estimates of downstream earthquake response products.
Abstract Earthquake and local tsunami early warning is critical to mitigating adverse impacts of large‐magnitude earthquakes. An optimal system must rely on near‐source data to maximize warning time. To this end, we have developed a self‐contained seismogeodetic early warning system employing an optimal combination of high‐frequency information from strong‐motion accelerometers and low‐frequency information from collocated Global Navigation Satellite Systems (GNSS) instruments to estimate real‐time displacements and velocities. Like GNSS, and unlike broadband seismometers, seismogeodetic stations record the full waveform, including static offset, without clipping in the near‐field or saturating for large magnitude earthquakes. However, GNSS alone cannot provide a self‐contained system and requires an external seismic trigger. Seismogeodetic stations detect P wave arrivals with the same sensitivity as strong‐motion accelerometers and thus provide a stand‐alone system. We demonstrate the utility of near‐source seismogeodesy for event detection and location with analysis of the 2010 M w 7.2 El Mayor‐Cucapah, Baja, California and 2014 M w 6.0 Napa, California strike‐slip events, and the 2014 M w 8.2 Iquique, Chile subduction zone earthquake using observatory‐grade accelerometers and GPS data. We present lessons from the 2014 M w 4.0 Piedmont, California and 2016 M w 5.2 Borrego Springs, California earthquakes, recorded by our seismogeodetic system with Micro‐Electro Mechanical System (MEMS) accelerometers and GPS data and reanalyzed retrospectively. We conclude that our self‐contained seismogeodetic system is suitable for early warning for earthquakes of significance (> M 5) using either observatory‐grade or MEMS accelerometers. Finally, we discuss the effect of network design on hypocenter location and suggest the deployment of additional seismogeodetic stations for the western U.S.
Abstract A central question of earthquake science is how far ruptures can jump from one fault to another, because cascading ruptures can increase the shaking of a seismic event. Earthquake science relies on earthquake catalogs and therefore how complex ruptures get documented and cataloged has important implications. Recent investments in geophysical instrumentation allow us to resolve increasingly complex, multi-fault ruptures for even moderate-sized earthquakes. We combine dense seismic and geodetic measurements to reveal an enigmatic rupture in late 2021 at the Mendocino Triple Junction in northern California. We show that rupture was dynamically triggered, yet concurrent, on two distinct faults roughly 30 km apart. Thus, this rupture combines features of complex ruptures usually considered to be single earthquakes, and triggered ruptures considered as multiple earthquakes. This event illustrates that moderate-sized earthquakes can exhibit similar complexity to that more commonly documented for large earthquakes.
The July 4, 2019 Mw6.4 and subsequent July 6, 2019 Mw7.1 Ridgecrest Sequence earthquakes ruptured orthogonal fault planes in the Little Lake Fault Zone, a low slip rate (1 mm/yr) dextral fault zone in the area linking the Eastern California Shear Zone and Walker Lane. This region accommodates nearly one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas. We investigate the rupture process of these events using a novel simultaneous kinematic slip method with joint inversion of high-rate GNSS, strong motion, GNSS static offset, and InSAR data. We model the Coulomb stress change to evaluate how the first mainshock may have affected the second. Our findings suggest complex interactions between several fault structures, including dynamic and static triggering, and provide important context for regional seismic source characterization and hazard models.
