Weighted Template-Matching Algorithm (WTMA) for Improved Foreshock Detection of the 2019 Ridgecrest Earthquake Sequence

ABSTRACT The template-matching algorithm (TMA) has become an important tool for detecting small and/or unconventional earthquakes, and newly detected seismic events have improved our understanding of earthquake physics, regional tectonics, and geological hazards. Standard TMA-based detection methods do not take into account the fact that the template waveforms themselves are contaminated with noise. In this study, we propose a weighted template-matching algorithm (WTMA), in which the normalized cross-correlation time series are weighted according to the signal-to-noise ratios of the corresponding template waveforms before they are being stacked for event picking. We present results from an extensive collection of numerical experiments to demonstrate that WTMA is capable of improving the detection rate of small to tiny (e.g., ML<0.0) earthquakes while reducing the computational cost associated with the standard TMA-based detection methods. The application of our WTMA to the continuous waveform recordings before the Mw 6.4 earthquake in the 2019 Ridgecrest sequence allowed us to discover ∼200 foreshocks, a larger number than the ∼150 foreshocks detected using the standard TMA, and significantly larger than the 17 foreshocks documented in the Southern California Seismic Network (SCSN) catalog. Relocated hypocenters of the foreshocks suggest a complex faulting pattern that is consistent with the hierarchical interlocked orthogonal faulting that characterized the main Ridgecrest sequence. Magnitudes of the foreshocks obtained through careful waveform amplitude calibration provided a more robust estimate of the magnitude–frequency distribution, which reduced the minimum magnitude threshold and increased the b-value, as compared with those obtained from the SCSN catalog.

Constraints on Complex Faulting during the 1996 Ston–Slano (Croatia) Earthquake Inferred from the DInSAR, Seismological, and Geological Observations

This study, involving remote sensing, seismology, and geology, revealed complex faulting during the mainshock of the Ston–Slano earthquake sequence (5 September, 1996, Mw = 6.0). The observed DInSAR interferogram fringe patterns could not be explained by a single fault rupture. Geological investigations assigned most of the interferogram features either to previously known faults or to those newly determined by field studies. Relocation of hypocentres and reassessment of fault mechanisms provided additional constraints on the evolution of stress release during this sequence. Available data support the scenario that the mainshock started with a reverse rupture with a left-lateral component on the Slano fault 4.5 km ESE of Slano, at the depth of about 11 km. The rupture proceeded unilaterally to the NW with the velocity of about 1.5 km/s for about 11 km, where the maximum stress release occurred. DInSAR interferograms suggest that several faults were activated in the process. The rupture terminated about 20 km away from the epicentre, close to the town of Ston, where the maximum DInSAR ground displacement reached 38 cm. Such a complicated and multiple rupture has never before been documented in the Dinarides. If this proves to be a common occurrence, it can pose problems in defining realistic hazard scenarios, especially in deterministic hazard assessment.

Normal faulting above salt wedges in tilted continental margins: numerical modeling

<p>Most salt basins are highly deformed and consist of complex faulting systems that is difficult to reconstruct. In contrast, in the Levant basin, the deformation of the Pliocene-Quaternary overburden on top of the Messinian salt is relatively mild, providing a rare opportunity to explore a young salt basin in its early stages of evolution. In the Levant continental margin normal faulting occurs mainly above the wedge of the salt layer where it rapidly thins from a few hundred meters to less than 100m. Recently, chronology of faulting in the Levant continental margin improved. It was indicated that during the Pliocene (duration of 2.7 My) faulting activity was minor. In the Gelasian (duration of 0.8 My) faulting activity peaked alongside huge slumping. Then, in the past 1.8 My, faulting and slumping had both decreased, although they are still mildly active today.</p><p>These observations raise questions such as: why didn't faulting start immediately after salt deposition? Why had faulting peaked when it did, and then why did it decrease? In this work we wish to understand the mechanism of normal faulting in continental slopes bordering salt basins. What drives salt motion? How does this motion cause faulting in overriding rocks? Where exactly will faults initiate and how will they progress in space? What controls the rate of faulting and when will they shut down?</p><p>This study uses 2D numerical simulations to explore these questions. The model assumes that salt is viscous and its overriding rock is brittle and viscoelastic. The model uses a Stokes flow solver, specifically a finite difference/particle-in-cell numerical approach, that can simulate both viscous and elasto-plastic–brittle rheology.</p><p>Answering these questions will contribute to the understanding of halokinematics in young salt basins and will allow better assessment of seismic hazards related to salt related deformation.</p>

Pedro Miguel Fault Investigations: Borinquen Dam 1e Construction and the Panama Canal Expansion

Borinquen Dam 1E is part of the new Pacific Access Channel (PAC) of the Panama Canal Expansion. The 2.3-km-long zoned rockfill dam forms the navigational channel providing navigation access from the Gaillard Cut to the new Post-Panamax Pacific Locks. A key geologic objective during construction was to confirm locations and activity of faults mapped at the dam during design, namely the Pedro Miguel Fault (PMF) and its suspected newly mapped “main trace.” The design allowed for core and filter widening at the anticipated location of the PMF at the south abutment and at a west branch of the PMF (believed to be the main active trace of the fault) mapped along the dam axis about one-third of the way north from the south abutment. As-built geologic mapping revealed complex faulting associated with the PMF crossing the southeast half of the foundation, the PAC, and the nearby Dam 1W foundation along a north-south trend. Trenching and age dating of alluvium overlying the faults crossing the Dam 1E foundation and overlying the PMF at Dam 1W indicated the unfaulted alluvium was latest Pleistocene to early Holocene age. At Dam 1E, the core and filters were widened to accommodate potential fault rupture on the PMF and a previously unrecognized fault revealed across the width of the dam foundation. The west branch of the PMF (trenched and mapped during design investigations) was determined to not exist at Dam 1E based on mapping the dam foundation and other extensive excavations created for the PAC.

Produced-Water-Chemistry History Matching in the Janice Field

Summary Produced-water-chemistry (PWC) data are the main sources of information to monitor scale precipitation in oilfield operations. Chloride concentration is used to evaluate the seawater fraction of the total produced water per producing well and is included as an extra history-matching constraint to reevaluate a good conventionally history-matched (HM) reservoir model for the Janice field. Generally, PWC is not included in conventional history matching, and this approach shows the value of considering the nature of the seawater-injection front and the associated brine mixing between the distinctive formation water and injected seawater. Adding the extra constraint resulted in the reconceptualization of the reservoir geology between a key injector and two producers. The transmissibility of a shale layer is locally modified within a range of geologically consistent values. Also, a major lineament is identified which is interpreted as a northwest/southeast-trending fault, whereby the zero transmissibility of a secondary shale in the Middle Fulmar is locally adjusted to allow crossflow. Both uncertainties are consistent with the complex faulting known to exist in the region of the targeted wells. Other uncertainties that were carried forward to the assisted-history-matching phase included water allocation to the major seawater injectors; thermal fracture orientation of injectors; and the vertical and horizontal permeability ratio (Kv/Kh) of the Fulmar formation. Finally, a stochastic particle-swarm-optimization (PSO) algorithm is used to generate an ensemble of HM models with seawater fraction as an extra constraint in the misfit definition. Use of additional data in history matching has improved the original good HM solution. Field oil-production rate is interpreted as improved over a key period, and although no obvious improvement was observed in field water-production rate, seawater fraction in a number of wells was improved.