Homogenizing instrumental earthquake catalogs – a case study around the Dead Sea Transform Fault Zone

The creation of a homogenized earthquake catalog is a fundamental step in seismic hazard analysis. The homogenization procedure, however, is complex and requires a good understanding of the heterogeneities among the available bulletins. Common events within the bulletins have to be identified and assigned with the most suitable origin time and location solution, while all the events have to be harmonized into a single magnitude scale. This process entails several decision variables that are usually defined using qualitative measures or expert opinion, without a clear exploration of the associated uncertainties. To address this issue, we present an automated and data-driven workflow that defines spatio-temporal margins within which duplicate events fall and converts the various reported magnitudes into a common scale. Special attention has been paid to the fitted functional form and the validity range of the derived magnitude conversion relations. The proposed methodology has been successfully applied to a wide region around the Dead Sea Transform Fault Zone (27N-36N, 31E-39E), with input data from various sources such as the International Seismological Centre and the Geophysical Institute of Israel. The produced public catalog contains more than 5500 events, between 1900 and 2017, with moment magnitude Mw above 3. The MATLAB/Python scripts used in this study are also available

The spatial distribution of causal factors behind the seismicity in the Delaware basin

Our analysis hindcasts seismicity rates of magnitude (M) ≥ 1.5 in the Delaware Basin (West Texas) from 2017 through 2019 on a 5km grid using either HF or SWD data as input, and compares them against the null hypothesis of solely tectonic loading. Each block is assigned a p-value, indicating the statistical confidence of its causal link with either HF, shallow SWD or deep SWD injection operations. To do that we expanded the framework of Grigoratos et al. (2020) to daily hydraulic fracturing (HF) operations and pressure-driven modeling of wastewater disposal (SWD). Our results indicate that 60% of the earthquakes occurred in blocks with p ≤ 0.05 (95% confidence interval) for at least one examined causal factor, and are thus triggered by oil and gas activities. This percentage increases to 68% for p ≤ 0.10 (90% confidence interval). Overall, more events than previously identified are associated with HF, especially within Reeves county. That said, shallow SWD is linked to more earthquakes, likely reactivating parallel faults of significant length. The seismicity around Pecos city seems to be affected by more than one injection activity, with higher frequency operational data and better depth resolution needed to fully decouple causality. Finally, within Culberson county, the only area where the earthquakes occur predominantly within the basement, the identified triggering mechanism is far-field SWD from deep wells

Thousands of Induced Earthquakes per Month in West Texas Detected Using EQCCT

West Texas has been a seismically active region in the past decade due to the injection of industrial wastewater and hydrocarbon exploitation. The newly founded Texas seismological network has provided a catalog that characterizes the intense seismicity down to a magnitude of 1.5 Ml. However, there are numerous small-magnitude events (Ml < 1.0) occurring every day that are not analyzed and reported, due to the prohibitively high workload to manually verify the picks from automatic picking methods. We propose to apply an advanced deep learning method, the earthquake compact convolutional transformer (EQCCT), to unleash our power in analyzing hundreds of small earthquakes per day in West Texas. The EQCCT method is embedded in an integrated-detection-and-location framework to output a highly complete earthquake catalog, given a list of available seismic stations, in a seamless way. The EQCCT has enabled us to detect and locate 50-times more earthquakes (mostly smaller than magnitude 1) than we previously could. We applied the EQCCT-embedded detection and location workflow to the Culberson and Mentone earthquake zone (CMEZ) in West Texas and detected thousands of earthquakes per month for consecutively three months. Further relocation of the new catalog revealed an unprecedentedly high-resolution and precise depiction of shallow and deep basement-rooted faults. The highly complete catalog also offers significant insights into the seismo-tectonic status of the CMEZ. Association with nearby injection activities also revealed a strong correlation between the rate of injected fluid volume and the number of small earthquakes.ISSN:2076-326