High-precision Earthquake Locations Research Articles

Fault structure and earthquake clustering in Aswan region (Egypt) revealed by high-precision earthquake location from 35 years of recorded natural and induced seismicity

Starting from an initial catalogue of 7833 natural and reservoir-induced seismic events collected in Aswan region (south Egypt) from 1982 to 2016, we investigate the fault structure and triggering mechanisms by determining high-resolution hypocenters for 2562 earthquakes. Despite the complex network of intersecting fractures, high-precision earthquake locations reveal numerous discrete fault strands (some not mapped yet), whose kinematics have been determined from the focal mechanisms of Ml ≥ 2.5 earthquakes. Furthermore, the analysis of the space-time evolution of seismicity indicates an eastward migration of earthquakes and a progressive activation of different faults. This process could reflect fluid migration in the Wadi Kalabsha embayment likely controlled by permeability barriers at depth acting as seals, and the subsequent build-up of the pore pressure as the dominant driving mechanism of the observed seismicity in the region. A strong spatio-temporal earthquake clustering is observed in the region characterized by seismic sequences, repeated earthquakes, and long-lasting swarms. High variability of b-value in correspondence of the occurrence of mainshock-aftershock sequences also suggests the activation of faults and fractures within their respective fault zones under lower differential stress conditions due to fluids. The projection of earthquake hypocenters on an E-W vertical depth section of the study area shows a seismic gap with an approximate length of about 11 km along the Kalabsha fault, which suggests the presence of a locked fault patch that may generate a Mw 5.9 earthquake if this segment were to break in a single rupture episode.

Riedel Shear Structures Reactivation may Induce Earthquakes Through Long-Term Steam Injection: A Case Study of a Heavy Oil Production Field in Northwestern China

Conventional heavy oil exploitation methods involve steam stimulation and flooding. An oil field in northwest China has been producing heavy oil via steam injection for several decades. The production area was seismically quiet until an increase in seismicity occurred several years ago. An array of 40 seismographs was deployed between July and October 2021 to monitor seismicity and resolve the possible causes of the recent seismicity. Using an end-to-end machine learning-based high-precision earthquake location workflow, we analyzed a microseismic sequence comprising 178 events that occurred in the study area. Numerical simulations incorporating Coulomb failure stress suggest that prolonged steam injection can reactivate faults and induce seismic events. Similarly, fluid diffusion through conduits may achieve the same effect. Analysis of the focal mechanism solutions of 21 strike-slip and thrust events with ML≥2 in conjunction with the background stress regime (SHmax azimuth = N15°W) revealed that the stress distribution is compatible with a left-lateral Riedel Shear Structure (RSS) model. Therefore, we can speculate that steam injection may induce earthquakes by reactivating pre-existing RSS fault structures. To conclude, the recent seismic events could have been induced by two possible mechanisms: (i) long-term steam injection may cause the static stress level on the faults beneath the reservoir to build up to critical levels, following which a slight stress disturbance can trigger an earthquake; (ii) fluid conduits may transport condensed water to basement faults, weakening the faults through fluid diffusion.

Wasatch Fault Structure from Machine Learning Arrival Times and High-Precision Earthquake Locations

Abstract On 18 March 2020, a magnitude 5.7 earthquake hit the Salt Lake valley in the state of Utah, United States. Using a dense geophone deployment and machine learning (ML), an additional several thousand events were detected and located. Currently, both the mainshock and the majority of the aftershocks are suspected to have occurred on or near a deeper portion of the Salt Lake segment of the Wasatch fault—part of a large range-bounding fault system thought to be capable of generating an Mw 7.2 earthquake. However, a small subset of aftershocks may have occurred on a portion of the more steeply, eastward dipping, and poorly understood West Valley fault. Unfortunately, the catalog locations and lack of focal mechanisms for this subset of aftershocks provide only a crude constraint on the true fault structure. To better illuminate fault structure, we relocate the ML-generated catalog with a range of magnitudes from −2 to 4.6, using: (1) NonLinLoc, a nonlinear location algorithm, (2) source-specific station terms, and (3) waveform coherence. We further compute first-motion focal mechanisms for 68 events. Results of the relocation suggest a simpler, minimally listric Wasatch fault geometry, contrary to what has been previously proposed. We also find that analysis of the focal mechanisms and waveform similarity indicates minimal event similarity throughout the Magna sequence, suggesting a highly complex and heterogeneous rupture zone, as opposed to rupture on a single plane. These findings suggest an increased seismic hazard due to the overall shallowness of the earthquake sequence and highly varied rupture mechanisms.

LOC-FLOW: An End-to-End Machine Learning-Based High-Precision Earthquake Location Workflow

Abstract The ever-increasing networks and quantity of seismic data drive the need for seamless and automatic workflows for rapid and accurate earthquake detection and location. In recent years, machine learning (ML)-based pickers have achieved remarkable accuracy and efficiency with generalization, and thus can significantly improve the earthquake location accuracy of previously developed sequential location methods. However, the inconsistent input or output (I/O) formats between multiple packages often limit their cross application. To reduce format barriers, we incorporated a widely used ML phase picker—PhaseNet—with several popular earthquake location methods and developed a “hands-free” end-to-end ML-based location workflow (named LOC-FLOW), which can be applied directly to continuous waveforms and build high-precision earthquake catalogs at local and regional scales. The renovated open-source package assembles several sequential algorithms including seismic first-arrival picking (PhaseNet and STA/LTA), phase association (REAL), absolute location (VELEST and HYPOINVERSE), and double-difference relative location (hypoDD and GrowClust). We provided different location strategies and I/O interfaces for format conversion to form a seamless earthquake location workflow. Different algorithms can be flexibly selected and/or combined. As an example, we apply LOC-FLOW to the 28 September 2004 Mw 6.0 Parkfield, California, earthquake sequence. LOC-FLOW accomplished seismic phase picking, association, velocity model updating, station correction, absolute location, and double-difference relocation for 16-day continuous seismic data. We detected and located 3.7 times (i.e., 4357) as many as earthquakes with cross-correlation double-difference locations from the Northern California Earthquake Data Center. Our study demonstrates that LOC-FLOW is capable of building high-precision earthquake catalogs efficiently and seamlessly from continuous seismic data.

GIS applications in volcano monitoring: the study of seismic swarms at the Campi Flegrei volcanic complex, Italy

Abstract. Campi Flegrei caldera (Southern Italy) is one of the most hazardous volcanic complexes in the world since it is located inside the densely inhabited urban district of Naples-Pozzuoli. In the past, the caldera has produced devastating to moderate eruptions and periodically undergoes from strong to minor uplift episodes, named “bradyseism”, almost always accompanied by seismic swarms. Starting from 2005 Campi Flegrei has undergone an unrest crisis, characterized by ground uplift, localized gas emissions and seismicity, often occurring in seismic swarms. As a consequence, the monitoring activities have been progressively increasing, producing a huge amount of data, difficult to manage and match. GIS (Geographical Information System) represents a potent tool to manage great quantity of data, coming from different disciplines. In this study, we show two GIS technology applications to the seismic catalogue of Campi Flegrei. In the first one, a high-quality dataset is extracted from the GeoDatabase addressed to seismological studies that require high precision earthquake locations. In the second application, GIS are used to extract, visualize and analyse the typical seismic swarms of Campi Flegrei. Moreover, density and seismic moment distribution maps were generated for these swarms. In the last application, the GIS allow to highlight a clear variation in the temporal trend of the seismic swarms at Campi Flegrei.

Zooming into the Hindu Kush slab break-off: A rare glimpse on the terminal stage of subduction

The terminal stage of subduction sets in when the continental margin arrives at the trench and the opposite forces of the sinking slab and buoyant continent extend and ultimately sever the subducted lithosphere. This process, although common in geological history, is short-lived, and therefore rarely observed. The deep seismicity under the Hindu Kush (Central Asia), including the 2015 Mw7.5 event, is a rare case that testifies to this process. Here, we use new seismological data to create a high resolution picture of slab break-off and infer its dynamics. High precision earthquake locations and tomographic images show subduction of continental crust down to ∼180 km. A large dataset of source mechanisms indicates sub-vertical extension in the entire slab but a strain rate analysis showed that the deeper seismogenic portion of the slab, below the subducted crust, extends at higher rates (∼40 km/Ma). Most Mw>7 earthquakes between 1983–2015, relocated relative to our new well-constrained earthquake catalog, cluster in a small volume below 180 km, and indicate shearing on an overturned interface. A slip model for the latest 2015 Mw7.5 event suggests that it ruptured into a seismic gap on this interface. From this configuration we conclude that a horizontal slab tear develops along-strike of the Hindu Kush seismic zone at the base of the subducted continental crust. Below the subducted crust, the deepest and also largest earthquakes (180–265 km) are likely associated with deformation in the mantle lithosphere. From the seismicity distribution and the rupture mechanisms we further deduce that the dominant deformation mechanism in this deeper portion of the slab changes along-strike from simple to pure shear. The fastest detachment rates and largest earthquakes occur during the simple shear dominated stage. Earthquakes in the upper part (60–180 km), above the rapidly extending slab, might be triggered by processes related to the subduction of crustal rocks.

High-precision earthquake locations in Switzerland using regional secondary arrivals in a 3-D velocity model

We present a new approach to relocate earthquakes in the greater western Alpine region using main crustal phases (Pg, Pn, PmP) that takes advantage of recent developments in P-wave velocity models and modelling of the Moho topography in the region, as well as the ability to track reflected and refracted phases in three-dimensional (3-D) heterogeneous media. Our approach includes a new 3-D P-wave velocity model for Switzerland and surrounding regions that combines a first-order Moho discontinuity based on local earthquake tomography (LET) and controlled-source seismology (CSS) information and 3-D seismic velocity information based on LET. Traveltimes for the main crustal phases (Pg, Pn, PmP) are computed using a fast marching method. We use a non-linear, probabilistic approach to relocate earthquakes that has been extended to include the use of secondary phases. We validate our approach using synthetic data, which was computed for a real earthquake and different combinations of available phases (Pg, Pn, PmP). We also applied our approach to relocate four selected earthquakes, two shallow and two deep crustal events in the northern Alpine foreland, for which independent information (ground truth information) on their focal depths exist. Our results demonstrate that the precision and accuracy of focal depth estimates can be greatly improved if secondary phases are used. This gain is a combined effect of an improved range of take-off angles and the use of differential traveltimes between first and secondary arriving phases. Our results also show that reliable information on the Moho depth is crucial to obtain accurate focal depths, if Pn or PmP phases are used in the relocation process. Finally, our approach demonstrates that proper identification of the main crustal phases in combination with an appropriate model parametrization in the forward solver will significantly improve earthquake locations.

The Augustine magmatic system as revealed by seismic tomography and relocated earthquake hypocenters from 1994 through 2009

[1] We incorporate 14 years of earthquake data from the Alaska Volcano Observatory with data from a 1975 controlled-source seismic experiment to obtain the three-dimensional P and S wave velocity structure and the first high-precision earthquake locations at Augustine Volcano to be calculated in a fully three-dimensional velocity model. Velocity tomography shows two main features beneath Augustine: a narrow, high-velocity column beneath the summit, extending from ∼2 km depth to the surface, and elevated velocities on the south flank. Our relocation results allow a thorough analysis of the spatio-temoral patterns of seismicity and the relationship to the magmatic and eruptive activity. Background seismicity is centered beneath the summit at an average depth of 0.6 km above sea level. In the weeks leading to the January 2006 eruption of Augustine, seismicity focused on a NW-SE line along the trend of an inflating dike. A series of drumbeat earthquakes occurred in the early weeks of the eruption, indicating further magma transport through the same dike system. During the six months following the onset of the eruption, the otherwise quiescent region 1 to 5 km below sea level centered beneath the summit became seismically active with two groups of earthquakes, differentiated by frequency content. The deep longer-period earthquakes occurred during the eruption and are interpreted as resulting from the movement of magma toward the summit, and the post-eruptive shorter-period earthquakes may be due to the relaxation of an emptied magma tube. The seismicity subsequently returned to its normal background rates and patterns.

Optimized automatic pickers: application to the ANCORP data set

SUMMARY It has become increasingly important to develop fast and accurate automatic procedures to process and fully exploit increasing large seismic data sets. Traditionally these data sets are processed manually, which requires significant amounts of both manpower and time with sometimes-variable results. We have developed a cost minimization approach to train three automatic pickers: an Sta/Lta, Tpd and the PAI-K picker at each station within a dense temporary network located in northern Chile and southern Bolivia. The optimum picking parameters for each station show regional variability and need to be adjusted individually to achieve the best performance. We developed a weighting scheme that uses four independent predictors of weight calibrated using a handpicked data subset, which mimics the picking by an expert seismologist. We use the fact that each of the three pickers highlights different properties of the observed seismic trace to combine two pickers that work in tandem. The first makes an initial pick before the second picker refines and improves the accuracy of the automatic pick. We find the tandem pickers improve the accuracy of the automatic picks when compared to the single automatic pickers. We demonstrate that following the cost minimization procedure described here the automatic picks have sufficient accuracy that they would be suitable for high-precision earthquake location, focal mechanism determination or high-resolution seismic tomography.

Automatic seismic phase picking and consistent observation error assessment: application to the Italian seismicity

SUMMARY Accuracy of seismic phase observation and consistency of timing error assessment define the quality of seismic waves arrival times. High-quality and large data sets are prerequisites for seismic tomography to enhance the resolution of crustal and upper mantle structures. In this paper we present the application of an automated picking system to some 600 000 seismograms of local earthquakes routinely recorded and archived by the Italian national seismic network. The system defines an observation weighting scheme calibrated with a hand-picked data subset and mimics the picking by an expert seismologist. The strength of this automatic picking is that once it is tuned for observation quality assessment, consistency of arrival times is strongly improved and errors are independent of the amount of data to be picked. The application to the Italian local seismicity documents that it is possible to automatically compile a precise, homogeneous and large data set of local earthquake Pg and Pn arrivals with related polarities. We demonstrate that such a data set is suitable for high-precision earthquake location, focal mechanism determination and high-resolution seismic tomography.

High precision earthquake locations in Koyna‐Warna seismic zone reveal depth variation in brittle‐ductile transition zone

The Koyna‐Warna region located in the southwestern part of Deccan Volcanic Province is known for sustained seismicity during the last four decades. Catalog P and S phase data of local earthquakes recorded by the Koyna‐Warna digital network were used to relocate 609 earthquakes using double difference algorithm. The relocated seismicity patterns delineate the presence of three fault segments and variation in the brittle–ductile transition depths amongst the individual faults as the earthquake foci in the north are limited up to 11 km depth whilst in the south they are confined up to 6–8 km. The focal mechanism solutions also indicate distinct styles of faulting amongst the three inferred fault segments. The variation in brittle‐ductile transition zone is probably controlled due to the geometry of the fault system and the effective stress.

High precision earthquake locations reveal seismogenic structure beneath Mammoth Mountain, California

In 1989, an unusual earthquake swarm occurred beneath Mammoth Mountain that was probably associated with magmatic intrusion. To improve our understanding of this swarm, we relocated Mammoth Mountain earthquakes using a double difference algorithm. Relocated hypocenters reveal that most earthquakes occurred on two structures, a near‐vertical plane at 7–9 km depth that has been interpreted as an intruding dike, and a circular ring‐like structure at ∼5.5 km depth, above the northern end of the inferred dike. Earthquakes on this newly discovered ring structure form a conical section that dips outward away from the aseismic interior. Fault‐plane solutions indicate that in 1989 the seismicity ring was slipping as a ring‐normal fault as the center of the mountain rose with respect to the surrounding crust. Seismicity migrated around the ring, away from the underlying dike at a rate of ∼0.4 km/month, suggesting that fluid movement triggered seismicity on the ring fault.