Microearthquake Sequence Research Articles

Bayesian Focal Mechanism Estimation from P-, S-Wave Amplitudes, and Polarities for a Microearthquake Sequence in Irpinia, Italy

ABSTRACT The P-wave peak distribution in azimuth and takeoff angles, corrected for the distance through empirical attenuation laws, can reveal the radiation pattern amplitude of the source (Tarantino et al., 2019). This piece of information, jointly combined with the available polarities and S-wave peak amplitudes, can provide the full focal mechanism, constraining the solution also when only a few seismic station records are available, that is, in the case of microseismicity. We proposed a new technique, named P-, S-wave amplitude, and polarities (P-SAP), designed to compute the focal mechanism by jointly inverting the P-, S-wave amplitude ratios and P-wave polarities in a Bayesian framework and employing the octree strategy (Fang et al., 1996) to explore the space of possible solutions. The outputs are strike, dip, and rake angles of the most likely triplet (principal and auxiliary planes) with the related uncertainties, as well as other multiple solutions, if present. We tested the methodology to synthetic data, and we applied it to a microseismic sequence that occurred in Irpinia region, southern Italy. A background microseismicity occurs in a volume delimited by the faults activated during the 1980 M 6.9 Irpinia earthquake. This faults system is complex and composed of northwest–southeast-striking normal faults along the Apennines chain and an approximately east–west-oriented strike-slip fault, deep-seated in the Potenza area. A network of three-component accelerometers and velocimeters are currently deployed to monitor the area. The major topic of this work is validating the proposed P-SAP methodology for microseismicity studies. We also inferred the optimum stress tensor of the sequence, confirming that the microseismicity is controlled by the regional stress field and can reveal characteristics useful to highlight behaviors of larger-scale seismicity.

Hypocenter‐Based 3D Imaging of Active Faults: Method and Applications in the Southwestern Swiss Alps

AbstractDespite the fact that earthquake occurrence can be strongly influenced by the architecture of pre‐existing faults, it remains challenging to obtain information about the detailed subsurface geometries of active fault systems. Current geophysical methods for studying such systems often fail to resolve geometrical complexities at sufficiently high spatial resolutions. In this work, we present a novel method for imaging the detailed 3D architectures of seismically active faults based on high‐precision hypocenter catalogs, using nearest neighbor learning and principal component analysis. The proposed approach enables to assess variations in fault instabilities and kinematics. We apply the method to the relatively relocated St. Léonard (max.ML = 3.2) and Anzère (max.ML = 3.3) microearthquake sequences in the Southwestern Swiss Alps, revealing strike‐slip fault systems with interconnecting stepovers at depths of 3–7 km and lengths ranging from 0.5 to 2 km. In combination with additional information about fault instabilities and kinematics, we observe significantly reduced earthquake migration velocities and fault locking processes within the stepovers. Understanding such processes and their role in the propagation of strain across stepovers is of great relevance, as these structures can potentially limit earthquake ruptures but also represent possible locations for the nucleation of larger ruptures. Our proposed method is expected to be broadly useful for further applications such as monitoring hydraulic fracture stimulations or geothermal exploration of natural, fluid‐bearing faults. Conducting similar high‐resolution spatiotemporal analyses of microseismic sequences has the potential to greatly enhance our comprehension of how the 3D fault architecture impacts seismogenic fault reactivation.

Semi-automated template matching and machine-learning based analysis of the August 2020 Castelsaraceno microearthquake sequence (southern Italy)

The accurate characterization of microearthquake sequences allows seismologists to better understand the physical processes involved in earthquake nucleation and rupture propagation and to gain insights on fault geometry at depth. Standard procedures for seismic sequences analysis are based on manual detection and phase-picking, requiring a huge amount of work from expert seismologists, particularly in the case of microseismic events. Here we show how the investigation of a low-magnitude foreshock-mainshock-aftershock sequence, occurred in August 2020 close to Castelsaraceno village (southern Italy), greatly benefited from the application of a semi-automated template matching and machine-learning based workflow. The phase-picking was automatically performed through a deep-learning algorithm on 202 microearthquakes detected between July and October 2020, followed by an automatic multi-step absolute and relative earthquake location procedure. The 72 relocated events of the seismic sequence were clustered in time (7–12 August) and in a narrow range of depths (10–12 km). The Ml 2.1 foreshock doublet and the Ml 2.9 mainshock identified a persistent asperity. The joint analysis of aftershocks distribution, the mainshock focal mechanism, and the geology of the study area suggest the occurrence of the sequence along a NNE-SSW left-lateral, transtensional fault in the brittle portion of the crystalline basement.

An unusually productive microearthquake sequence brings new insights to the buried active thrust system of Montello (Southeastern Alps, Northern Italy)

In August 2021, an extremely productive seismic sequence took place in northeastern Italy. Within 1 month, we identified and localized 407 microearthquakes in the area monitored by the local Collalto Seismic Network. This is about 20 times the average monthly rate of events detected in the last decade; 92% of them are clustered near the village of Refrontolo at about 9 km depth (main event a ML2.5, MW2.4). None of the Refrontolo sequence earthquakes were felt by the population. The earthquakes, mostly identified by automatic procedures, were post-processed by manual picking of P, S-phases and polarities; they were localized by different techniques and define a small SE dipping volume that departs from the brittle surface previously enlightened by microseisms and assigned to the NNW dipping Montello Thrust. We interpret the sequence ruptured pre-stressed patches near to failure of sub-vertical, antithetically oriented faults. The unusual productivity rate and precursory foreshock activity support the hypothesis that the Montello system has a relevant aseismic component. We believe that this episode, of no importance from an energetic point of view, is of greatest interest for deciphering the seismic potential of this area and for strengthening automatic microearthquake detection and location procedures.

Seismological characteristics of microearthquake sequence near Suncheon, South Korea, from 2009 to 2020

Seismological characteristics of microearthquake sequence near Suncheon, South Korea, from 2009 to 2020

S/P Amplitude Ratios Derived from Single-Component Seismograms and Their Potential Use in Constraining Focal Mechanisms for Microearthquake Sequences

Abstract Focal mechanisms, which reflect the sense of slip in earthquakes, provide important constraints for understanding crustal tectonics and earthquake source physics, including the interactions among earthquakes during mainshock–aftershock sequences or seismic swarms. Focal mechanisms of small (magnitude ≲3.5) earthquakes are usually determined by first-motion P-wave polarities, sometimes supplemented by the ratio of S-wave to P-wave amplitudes (S/P). However, focal mechanisms of such events can be difficult to reliably constrain, particularly with sparse recording networks or very small magnitude events. Here, we describe a method for deriving S/P amplitude ratios from P/P and S/S amplitude ratios measured on single seismic components between pairs of nearby events, as is often performed during correlation-based earthquake detection and relocation. These measurements can be transformed into relative S/P amplitude ratios, or they can be combined with a smaller number of traditional S/P amplitude ratios to provide a single-channel estimation of full S/P ratios, even for low signal-to-noise-ratio events not routinely cataloged and not amenable to traditional S/P ratio processing. This approach has the potential to greatly expand the applicability of S/P amplitude ratios, providing additional constraints for focal mechanisms of small earthquakes, particularly for spatially concentrated seismicity sequences.

Possible Precursory Slow‐Slip to Two ML∼3 Mainevents of the Diemtigen Microearthquake Sequence, Switzerland

AbstractHow earthquakes initiate is still a largely debated question in earthquake science. On the lab scale, rupture initiation is well studied, and detailed models of how rupture nucleation evolves have been developed. Contrarily, for real earthquakes, only a few high‐resolution observations of this process are available today, mostly limited to mainshock magnitudes > M5. Consequently, there is still no consensus on whether and how laboratory results can be transferred to real earthquakes. Here we show that rupture nucleation phenomena observed on the lab scale can also be imaged on the microearthquake‐scale with little instrumental effort. Our results highlight the potential of the applied analysis workflow to significantly improve the observation quality of seismicity patterns and immediate foreshock phenomena in microearthquake sequences. Our approach can help to narrow the existing observational gap to the lab scale and may contribute to a better understanding of the mechanisms of earthquake initiation in the future.

Repeating microearthquake sequences interact predominantly through postseismic slip.

Studying small repeating earthquakes enables better understanding of fault physics and characterization of fault friction properties. Some of the nearby repeating sequences appear to interact, such as the ‘San Francisco' and ‘Los Angeles' repeaters on the creeping section of the San Andreas Fault. It is typically assumed that such interactions are induced by static stress changes due to coseismic slip. Here we present a study of the interaction of repeating earthquakes in the framework of rate-and-state fault models using state-of-the-art simulation methods that reproduce both realistic seismic events and long-term earthquake sequences. Our simulations enable comparison among several types of stress transfer that occur between the repeating events. Our major finding is that postseismic creep dominates the interaction, with earthquake triggering occurring at distances much larger than typically assumed. Our results open a possibility of using interaction of repeating sequences to constrain friction properties of creeping segments.

The importance of characterizing uncertainty in controversial geoscience applications: induced seismicity associated with hydraulic fracturing for shale gas in northwest England

Fracking of the Preese Hall-1 well in 2011 induced microseismicity that was strong enough to be felt. This occurrence of ‘nuisance’ microearthquakes, unexpected at the time, was a major factor resulting in adverse public opinion against shale gas in the UK and was thus of significant political importance. Despite this, and notwithstanding the technical importance of this instance of induced seismicity for informing future shale gas projects, it has received little integrated study; contradictory results have indeed been reported in analyses that lack integration. This instance therefore provides a case study to illustrate how a small but significant multi-disciplinary geoscience dataset may be put to best use, including how best to quantify uncertainties in key parameters, which may themselves be relatively poorly quantified but whose values may significantly affect the ability to understand the occurrence of induced seismicity. The best-recorded event in this induced microearthquake sequence (at 08:12 on 2 August 2011) is thus assigned an epicentre circa British National Grid reference SD 377358, south of the Preese Hall-1 wellhead, a focal depth of ∼2.5km, and a focal mechanism with strike 030°, dip 75°, and rake −20°, this NNE-striking nodal plane being the inferred fault plane. Like other parts of Britain, this locality exhibits high differential stress, with maximum and minimum principal stresses roughly north-south and east-west. This instance indeed fits an emerging trend of the occurrence of relatively large induced earthquakes in localities with high differential stress; such an association was predicted many years ago on the basis of experimental rock mechanics data, so observational confirmation might well have been anticipated and should thus not have been unexpected. Many steep faults, striking NNE-SSW or NE-SW, mostly Carboniferous-age normal faults, are present; the stress field is favourably oriented for their left-lateral reactivation, southward leakage of fracking fluid into one such fault having presumably caused the induced seismicity. Given the pervasive presence of similarly oriented faults, future occurrences of similar induced seismicity should be planned for; they pose a significant technical challenge to any future UK shale-gas industry.

Very detailed seismic pattern and migration inferred from the April 2010 Pietralunga (northern Italian Apennines) micro-earthquake sequence

We propose a very detailed picture of seismicity that occurred in the proximity of the Alto Tiberina Fault (ATF, northern Italian Apennines), a low angle normal fault, by presenting the pattern and evolution of a seismic sequence on the hanging wall of the ATF in the first months of 2010 that was characterized by about 1000 events with ML ranging from − 0.7 to 3.8. To capture the rupture kinematics of the investigated area, a cross-correlation technique was initially applied to calculate very accurate time shifts among the events of the sequence, and then to relocate them. The whole sequence was relocated with the double-difference method, which includes both absolute travel-time measurements and cross-correlation differential travel-times. The new locations confirm that the seismic activity was mainly arranged along a NW–SE-oriented structure that ranged in depth from 4 km to 6 km, with dipping towards NE at an angle of ca. 65°. In comparison with geological data, the position of the seismic sequence is compatible with the evaporite Triassic layer. The main nodal planes are consistent with the spatial evolution of the aftershocks and with the tensional state of stress in the area. An analysis of waveform similarity was performed at a reference station by merging the capability of the cross-correlation technique and the bridging algorithm. The detected mutiplets allow us to emphasize the anisotropic spatial and temporal migrations of the seismicity that occurred along a 307° N strike direction with an averaged propagation velocity of ca. 0.4 km/day. We explain the highlighted anisotropic behavior of the migration with the hypothesized presence of overpressured fluids and with physical properties of Triassic evaporites. This suggests the importance of such very detailed relocation of weak and micro-seismicity for improvement of our knowledge of fault system geometry and its evolution.

Strength statistics and the distribution of earthquake interevent times

The Weibull distribution is often used to model the earthquake interevent times distribution (ITD). We propose a link between the earthquake ITD on single faults with the Earth’s crustal shear strength distribution by means of a phenomenological stick–slip model. For single faults or fault systems with homogeneous strength statistics and power-law stress accumulation we obtain the Weibull ITD. We prove that the moduli of the interevent times and crustal shear strength are linearly related, while the time scale is an algebraic function of the scale of crustal shear strength. We also show that logarithmic stress accumulation leads to the log-Weibull ITD. We investigate deviations of the ITD tails from the Weibull model due to sampling bias, magnitude cutoff thresholds, and non-homogeneous strength parameters. Assuming the Gutenberg–Richter law and independence of the Weibull modulus on the magnitude threshold, we deduce that the interevent time scale drops exponentially with the magnitude threshold. We demonstrate that a microearthquake sequence from the island of Crete and a seismic sequence from Southern California conform reasonably well to the Weibull model.

Anatomy of a microearthquake sequence on an active normal fault

The analysis of similar earthquakes, such as events in a seismic sequence, is an effective tool with which to monitor and study source processes and to understand the mechanical and dynamic states of active fault systems. We are observing seismicity that is primarily concentrated in very limited regions along the 1980 Irpinia earthquake fault zone in Southern Italy, which is a complex system characterised by extensional stress regime. These zones of weakness produce repeated earthquakes and swarm-like microearthquake sequences, which are concentrated in a few specific zones of the fault system. In this study, we focused on a sequence that occurred along the main fault segment of the 1980 Irpinia earthquake to understand its characteristics and its relation to the loading-unloading mechanisms of the fault system.

Rupture directivity of microearthquake sequences near Parkfield, California

[1] The direction of propagation is an important factor that affects the pattern of ground motion generated by an earthquake. Characterizing factors favoring a potential rupture propagation direction is thus an important task. Here we analyze the earthquake directivity of repeating earthquake sequences located on the San Andreas fault near Parkfield, California. All earthquakes of a sequence have very similar waveforms and have overlapping surface ruptures. We show that subtle variations of the transfer function between earthquakes of a common sequence can be interpreted as a change of apparent rupture duration. Relative apparent rupture durations are computed for all pairs of events at all available stations and for each sequence. We invert these measurements to obtain an estimation of the apparent rupture duration for each individual event of the sequence relative to a reference event. Variation of apparent rupture duration with azimuth attests for the rupture directivity. We show that the majority of analyzed microearthquakes presents a rupture in the south-east direction. We also show that, on a given repeating sequence, most earthquakes tend to show the same rupture direction.

Structural interpretation of the coseismic faults of the Wenchuan earthquake: Three‐dimensional modeling of the Longmen Shan fold‐and‐thrust belt

The 2008 Mw 7.9 Wenchuan earthquake is a result of ongoing India‐Tibet collision and reflects the growth of the Longmen Shan fold‐and‐thrust belt. In this paper, we construct a 3‐D structural model of the geometry of the coseismic faults and related structures of the Wenchuan earthquake by integrating geological investigations, relocated aftershocks, and seismic reflection profiles. In the 3‐D structural model, the differences between the southern and northern segments of the rupture are highlighted. The structural transition zone between the two segments contains a major geometric segment boundary, reflecting differences in the structural configuration of the thrust ramp and the tectonic evolution of the fault system, which appears to have localized significant damage from Anxian to Beichuan. Within the northern segment, we identify a transverse structure across which the Beichuan fault plunges under the Tangwangzhai syncline. This boundary corresponds to a marked change in the nature of the surface rupture and is illuminated by a microearthquake sequence perpendicular to the Longmen Shan thrust belt. In the southern segment, our investigations confirm that uplift due to active faulting and folding is largely responsible for the areas of steepest topography. On the basis of this association, the southwestern segment of the Longmen Shan, south of the Wenchuan earthquake, is likely active and presents a significant earthquake hazard, despite the lack of historical earthquakes in this region. This study illustrates the importance of building 3‐D models to study active faulting and folding, as well as to assess earthquake hazard.

Intrinsic Qp at Mt. Etna from the inversion of rise times of 2002 microearthquake sequence

About three-hundred microearthquakes, preceeding and accompanying the 2002-2003 Mt. Etna flank eruption, were considered in this study. On the high-quality velocity seismograms, measurements of the first half cycle of the wave, the so-called rise time τ, were carried out. By using the rise time method, these data were inverted to infer an estimate of the intrinsic quality factor Qp of P waves and of the source rise time τ0 of the events, which represents an estimate of the duration of the rupture process. Two kind of inversions were carried out. In the first inversion τ0 was derived from the magnitude duration of the events, assuming a constant stress drop and Qp was inferred from the inversion of reduced rise times τ−τ0. In the second inversion both τ0 and Qp were inferred from the inversion of rise times. To determine the model parameters that realize the compromise between model sim- plicity and quality of the fit, the corrected Akaike information criterion was used. After this analysis we obtained Qp=57 ±42. The correlation among the inferred τ0 and Qp, which is caused by some events which concomitant- ly have high τ0 (>30 ms) and high Qp (>100) indicates that the technique used is able to model rise time versus travel time trend only for source dimensions less than about 80 m.

Evidence for Widespread Nonlinear Strong Ground Motion in the MW 6.9 Loma Prieta Earthquake

We exploit 55 repeating microearthquake sequences on the San Andreas Fault, just south of the rupture zone of the 1989 M W 6.9 Loma Prieta Earthquake, to search for time-dependent properties of the Earth's crust. Using moving window waveform cross correlation, we identify clear and systematic delays as large as 20 msec for the direct S wave and exceeding 50 msec in the early S -wave coda following the Loma Prieta mainshock. Others have also identified phase delays (velocity reductions) associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms. Here, we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake. A strong correlation between the spatial patterns of S delays and the intensity of strong ground motion in the Loma Prieta Earthquake suggests that physical damage, the formation or growth of cracks during strong ground motion, to the Earth's shallow crust is responsible for the observed velocity reductions. The strong spatial variability in S delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver. The effect is stronger at stations on young, soft rocks than at stations on old, hard rock. Disproportionately larger S coda delays than P coda delays suggest that the cracks formed by the strong shaking are fluid filled. In the 10 years after Loma Prieta, the initial increase in travel times reduces logarithmically with respect to time, often back to the premainshock levels. We attribute this behavior to the same “slow dynamic” healing observed in laboratory studies of the recovery of materials from transient nonlinear strain. Manuscript received 14 January 2004.

High-resolution Mechanics of a Microearthquake Sequence

Unique data from microseismic (MS) and acoustic emission (AE) systems monitoring a common rock volume have been analyzed from the Underground Research Laboratory in Canada. The two 16 sensor systems were installed to investigate stress-induced microcracking around clay and concrete bulkheads as part of the Tunnel Sealing Experiment. Single and double MS events are found to be spatially associated with clusters of between 19-86 higher frequency AE events. Each AE cluster is elongated with the longest axis between 15 and 50 cm. The AE events occurring before the associated MS event are termed foreshocks, and for two of the three analyzed AE clusters an acceleration in the AE frequency and cumulative magnitude occurs upto the time of the MS event, however with one AE cluster very few foreshocks are recorded, possibly indicating a more homogeneous failure plane. Time independent moment tensors were determined with the MS events showing significant deviatoric sources. The majority of the AE events have deviatoric mechanisms with a few crack opening and crack closure type events also determined. This study highlights the benefits of using small-scale seismic systems to investigate the temporal fracture mechanics of microcrack formation, and allows comparisons with larger more damaging seismicity.

Low-magnitude earthquakes in Rome: structural interpretation and implications for the local stress field

On 1995 June 12, a ML = 3.6 earthquake occurred in the Cecchignola area of southern Rome. A detailed structural investigation of this area and an analysis of macroseismic and instrumental data indicate a seismogenic area for the local low-magnitude seismicity that affects the city of Rome. A peripheral volcanic field in Cecchignola shows that the local seismicity could be related to the same volcanotectonics that affects the Colli Albani (Alban Hills) region. Fracture-field emission is proposed for the emplacement of about 2.5 km3 of lava, which developed a small plateau that was subsequently intruded by the feeder dyke of a large ignimbrite. The geometry of this dyke as well as the faults disrupting the lava field indicates NE–SW extension. Younger tectonic features show a NW-striking σ3 in the studied area, suggesting that dextral N–S strike-slip displacement generated structures that are not compatible with the local present-day stress field, characterized by a NE-striking σ3. An analysis of focal solutions and the spatial distribution of the 1995 June 12 microearthquake sequence suggests an E–W seismogenic structure whose trend is atypical of the structural setting of the area. The present study identifies surface faulting with the same trend as the focal mechanisms in the epicentral area and suggests that these faults represent an inactive surface expression of the deeper structures. It is also suggested that these structures originated during a different tectonic regime and are presently reactivated at depth by the NE–SW extension. The general NE orientation of the T axes from focal mechanisms for the Colli Albani earthquakes and for the 1995 June 12 Cecchignola event strongly supports a regional interpretation for the present-day stress field previously observed only in the Colli Albani area. It is likely that a combination of different tectonic styles during the last 600 kyr is necessary to explain the structural features of the Cecchignola area and of the Central Tyrrhenian Sea margin of Italy in general.

Characterisation of AP activation by normal B cells, raji cells and CR2-transfected cell lines

An experiment utilizing microearthquake and acoustic emission techniques has been designed to investigate the discrete failure and behavior of hard rock in response to storage of high level nuclear waste. Data are presently being collected from a 15-station, three-dimensional array of piezoelectric sensors surrounding 11 canisters of spent nuclear fuel and 6 electricaty heated simulator canisters. The canisters are arranged in a linear array 420 m beneath the surface in the Climax Stock at the Nevada Test Site. Because of the unique scale of the experiment (array dimension - 50 × 20 × 10 m), techniques employed are drawn from the fields of acoustic emission and earthquake analysis. The data are digitized and analyzed in-field via an Automated Seismic Processor (ASP) developed for earthquake studies. Although the discrete events are high frequency in nature (1–10 kHz) their behavior and appearance are typical of shear failure observed in microearthquake sequences. Sought are not only the spatial and temporal characteristics of the events with respect to induced thermal, mechanical, or radiation stresses, but also a complete understanding of the source characteristics relative to these induced stresses. The velocity and attenuation characteristics within the array imply that the rock is not homogenous at the observed frequencies. This paper reviews the basis for experiment's concept, the instrumentation emplaced, and preliminary results of the monitoring program.

Seimological studies at Parkfield V: Characteristic microearthquake sequences as fault-zone drilling targets

AbstractStudies at very high resolution of microearthquakes at Parkfield, California, since 1987 reveal a systematic organization in space and time, dominated by clustering of nearly identical, regularly occurring microearthquakes (characteristic events) on 10 to 20-m-wide patches within the fault zone. More than half of the 4000+ events in our 1987 to 1996 catalog exhibit this trait. In general, recurrence intervals (0.5 to 2 yr) scale with the magnitude of the repeating events for the on-scale range (Mw 0.2 to 1.3) in this study. The similar waveforms, superimposed locations, quasi-periodic recurrence, and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months. Clustered characteristic events occur at depths as shallow as about 3 km, and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate-boundary fault zone. At Parkfield, the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault-zone measurements.