To date, there are only few well documented examples of reverse polarity earthquakes worldwide, especially at intermediate depth. These are pairs or families of very closely located events, where one or more members show flipped polarities, indicating nearly reversed earthquake mechanisms. We identify four intermediate-depth reverse polarity earthquake families in a catalog of originally 3,153 repeating earthquake sequences for the North Chilean subduction zone from 2006 to 2024. A detailed analysis of the largest group, which consists of 38 events and contains five reverse events is presented. Relative event relocations, cluster properties, and inter event times suggest a very small active volume of the order of only a few hundred meters, partially with overlap of the rupture area, and a weak upward migration of seismicity over several years. As uncertainties remain, we discuss several possible models to explain the observations. We propose that they may reflect transient processes linked to dehydration embrittlement and fluid pulses.

Abstract Owing to the low N‐S convergence rates between Adria and Europe, crustal deformation rates in the Alps and its forelands are low. Active tectonics are, therefore, difficult to study, especially as non‐tectonic landscape forming processes can erase or modify the tectonic surface imprint. Large‐scale and dense seismological data recently offered insights into earthquakes and into the lithospheric structure beneath the Alps. Recent field studies added data on geological archives of active tectonics. In this review, we summarize the results from studies of seismic tomography, geodesy, seismology, historical seismology, archeoseismology, on‐fault and off‐fault paleoseismology, and fault gouge dating, focusing on the eastern part of the Alps during the last c. 1 Ma. We discuss the influence of the lithosphere on the localization of deformation, and we draw a generalized picture of active deformation. We show that deformation is primarily accommodated across the South Alpine Front (∼2 mm/yr out of max 3 mm/yr total shortening) and NW‐SE striking strike‐slip faults in western Slovenia (∼1.5 mm/yr shear). Large fault systems in the interior of the Eastern Alps are still actively accommodating extrusion of crustal material toward the east at very low rates (0.5–1.0 mm/yr). Diffuse deformation and clusters of seismicity are observed in the interior of the Alps. Minor shortening occurs at the North Alpine Front. Crustal strength controls the localization of deformation resulting in non‐deforming blocks. Neogene slab break‐off events do not control the present‐day deformation. Instead, present‐day uplift of 1.5–2 mm/yr correlates to areas glaciated during the Last Glacial Maximum.

Repeating earthquakes are nearly identical seismic events that are assumed to occur reiteratively on the same fault-patch with highly consistent focal mechanisms. Their magnitude-dependent recurrence intervals can be used to estimate local fault-slip and to infer spatial and temporal patterns of aseismically creeping zones at depth. We construct here the first long-term repeating earthquake catalog for Northern Chile. Using waveforms for 180,000 earthquakes from a recent regional seismicity catalog as templates, a GPU-based template matching is performed to search for repeating earthquakes in the continuous, multiannual seismological data of the permanent IPOC station network between 2006 and 2024. The resulting repeater catalog contains 10,684 events grouped into 3153 families. We observe a notable variability of size and behavior of the families, ranging from long-lasting, regular sequences to short-term, burst-type repeaters. Two megathrust earthquakes in 2007 and 2014 have a strong effect on the spatio-temporal distribution of repeaters. We compute the first time-dependent slip map for the interface between the subducting Nazca slab and the overriding South American plate. The catalog facilitates future detailed analysis of rupture processes, source structures and the spatio-temporal evolution of slow slip at depth. It also allows for comparative studies with other subduction zones, such as Japan.

We observe a time dependence of the median stress drop in spatiotemporal proximity of large earthquakes. The median stress drop of the early aftershock seismicity is elevated for only a few days after the mainshocks and then rapidly falls back to the long-term average. This short-term variation has remained largely unnoticed by previous studies, presumably due to their usually low temporal resolution. Our study uses a recent extensive stress drop catalog, which contains more than 51,000 events from northern Chile. It includes observations from three M-w > 7 megathrust earthquakes, namely the 2007 M-w 7.7 Tocopilla earthquake, the 2014 M-w 8.1 Iquique earthquake, and its largest M-w 7.6 aftershock, as well as another eleven M-w > 6 earthquakes. A detailed analysis reveals that the elevated stress drop is not primarily linked to an increase of seismic moment during the early aftershock phase but is rather attributable to higher measured corner frequencies in the corresponding time interval. We propose two possible explanations: (1) The high stress changes induced by the mainshock allow failure of strong structures at or adjacent to the main rupture area, which produces higher stress drop events. After a few days, afterslip activity has reduced the in situ stresses and thereby the failure potential of the stronger regions. (2) The mainshock disturbs the fluid-sealing plate interface, allowing overpressured fluids to escape, exhibiting the so-called fault-valve behavior. The effect appears to persist only for several days until the sealing effect is restored and average stress drop levels are recovered.

The installation of the temporary, large-N Swath-D seismic network in the years 2017-2019 (Heit et al., 2021) provided the basis for the recent compilation of a high-resolution, consistently processed seismicity catalogue for the eastern and southern Alps (Hofman et al., 2023). The catalogue contains more than 6,000 earthquakes with magnitudes down to −1.7 ML. In the present study, we analyse in more detail several of the newly detected microseismic clusters in the study area, which includes the most active parts of the Alps as well as particularly quiet regions with very little previously reported seismicity. We combine inter-event waveform similarity clustering, catalogue statistics and rupture mechanisms to characterise the clustered seismicity swarms and mainshock-aftershock sequences. We apply a relative location technique based on differential Ts-Tp arrival times to better resolve the seismogenenic structures. For subgroups of microseismic events with magnitudes Mw 1.2-3.0, we obtain moment tensor solutions using the flexible probabilistic inversion framework Grond, which allows to combine different fitting targets and frequency bands, while providing meaningful estimates of uncertainties (Heimann et al., 2018, Petersen et al., 2021). This adds to resolve subtle, but systematic variations of the inner-cluster seismicity.Thanks to the outstanding network density, we can report a variability of seismic sequences and microseismic event mechanisms across the study area and interpret them with in terms of long-term tectonic and intermediate triggering processes.

The southern and eastern Alps are a fascinating target region for a seismological study because they include the deformation front of Adria-Europe convergence with historically significant events (e.g., M 6.0 Friuli 1976) as well as areas where seismicity seems more or less absent despite geologically mapped large fault systems and past deformation fronts. The large-N installations of the Swath-D (2017-2019) and AlpArray (2016-20219) seismic networks provide unmatched opportunities to study the microseismicity in the Eastern Alps in unprecedented detail. For the first time in the study area, the homogeneous station spacing allows a consistent analysis of seismicity across the entire area. These detailed seismological analyses provide the opportunity to characterize deformation in the upper 15 km of the crust.We show how a combined workflow, including clustering, relocations, and MT inversions, sheds light on the seismicity and the ongoing active deformation. We observe strong zonations of seismic activity rates, sequence characteristics, and rupture mechanisms, coinciding with dominant tectonic deformation styles and subsurface properties such as Qp attenuation. We identify and characterize multiple likely unknown fault systems that experience local stresses deviating from the regionally dominant Adria-Europe convergence. Our findings agree well with the occurrence of large historical earthquakes while simultaneously shedding light on much smaller seismogenic features.

Uniformly spaced large-N seismic networks like the Swath-D (2017-2019), a densified part of the AlpArray initiative, provide unmatched opportunities to study microseismic activity and fault structures. Here, we show how the combined analysis of spatially and temporally clustered seismicity, precise relocations, waveform-based clustering, and moment tensor solutions for 67 earthquakes (1.1< M-w<3.3) help to characterize the heterogeneous study region in the south-eastern Alps. We observe a strong zonation, with clustered microseismicity predominantly in the SE and NW parts of the study area. The identified sequences indicate a dominance of swarms in the NW compared to more mainshock-aftershock sequences in the SE, while both sequence types can occur in both regions. We identify multiple short faults in the NW with lengths of a few hundred meters, and distinguish two close fault structures activated in one sequence based on waveform similarity and focal mechanisms. Compared to predominant thrust faulting in the SE, normal and strike-slip faulting in the NW points to high regional complexity, with local stresses deviating from simple expectations of thrust faulting resulting from the Europe-Adria convergence. We find that zones of increased microseismic activity match zones of high P wave attenuation from a recent Qp model developed in the AlpArray initiative (Jozi Najafabadi et al., 2023, https://doi.org/10.1186/s40623-023-01942-0), supporting our interpretations of spatiotemporal patterns concerning crustal properties and tectonic activity. Our findings agree well with the occurrence of large historical earthquakes while simultaneously shedding light on much smaller seismogenic features.

At the northern Chilean subduction zone the IPOC network monitors seismicity since 2007. During the observation time period two very large earthquakes occurred, the 2007 MW 7.7 Tocopilla earthquake and the 2014 MW 8.1 Iquique earthquake and until today the subduction zone shows a vast amount of seismic activity. A large catalog was compiled and published including over 100000 events by Sippl et al. 2018. Therein, seismicity ranges from close to the trench till deep into the mantle to about 300km depth. Consequently, events occur under a broad variability of physical conditions. We extend the aforementioned catalog by applying a template matching technique to identify additional events, that are colocated with catalog events. Based on these events we apply an empirical Green’s function method called spectral ratio approach to estimate stress drops. The results cover different nucleation provinces i.e. the data set includes stress drops obtained at the interface, within the subducting plate, from crustal events, intermediate depth events, and from deep to very deep seismicity. The study therefore bears a great potential to better understand the stress drop distribution within an entire subduction zone. First results show no depth dependency in the shallowest 100 km but spatial variability with high stress drops focused to particular regions on the interface. We also find increased stress drop values in the crust when compared to events close or at the interface.

Tectonic (or non-volcanic) tremors have been extensively documented at subduction zones and are considered as the signature of transport processes of dehydration-related fluids in subduction zones, often recorded in close association with geodetically observed shear induced slow-slip events. However, to the best of our knowledge, they have not yet been reproduced in the laboratory at subduction zones P–T conditions in such way that their first-order controlling mechanisms remain enigmatic. This work investigates the mechanism of these seismic events by performing dehydration-deformation experiments combined with detailed investigations of mineral reactions and acoustic emissions. Experiments were carried out on chlorite-peridotite powders (Balmuccia peridotite with synthetically added chlorite, a mineral that is typically found in subduction zone lithologies), following a subduction zone geothermal gradient using a high-pressure apparatus (Griggs-type). The experiments were conducted from ambient conditions to maximum pressures of 1.5-3.0 GPa and temperatures of 750-800 °C. Experiments were executed under hydrostatic conditions and an additional one with deformation. An ultrasonic transducer (0.5-10MHz dynamic range) was employed to monitor and detect the micro-seismic events. High-resolution electron beam techniques (EMPA, SEM and TEM) have been applied for analyzing the sample material.Dehydration of ~15 vol.% of the initial chlorite suffices to trigger acoustic emissions, which display waveforms reminiscent of those of tectonic tremors. The moment distribution statistics of these laboratory tremor-like signals follows the Gutenberg-Richter relationship and a scaling between moment vs. event duration. Finally, we observe a match between the ratios of size and typical frequency of natural over laboratory tremors. Microstructural observations document metamorphic olivine and pyroxene growth in the decomposing chlorite and demonstrate that an almost isochemical dehydration of the chlorite took place. Accordingly, the appearance of the tremor-like acoustic emissions after crossing a temperature of 600 °C can be linked to a dehydration process related to the chlorite breakdown in the sample. Thermodynamic calculations show that a small amount of released fluids (breakdown of ~1.5 vol.% of a hydrous phase) is enough to trigger seismic signals analogues to tremors. The experiment with additional deformation produced no tremor-like acoustic emission suggesting that the large macroscopic shear stress suppressed the development of the processes that lead to acoustic emissions. We conclude that fluid release during dehydration is the cause of tectonic tremors, whereas shear-stress seems to counteract their development with no occurrence of tremors at high rates of deformation. According to the results from this study, the triggering mechanism can be tentatively interpreted as a fluid propagation front resulting in the vibration of grain boundaries.

We produced a comprehensive stress drop catalog for northern Chile. To improve reliability, we applied a combination of two different stress drop estimation approaches. The result is a mapped stress drop distribution for more than 30,000 events covering the subduction zone from the trench to a depth of about 150 km. The stress drops were computed on the basis of a recently updated version of the IPOC seismic catalog, now spanning the years 2007 to 2021, using the spectral stacking technique as well as the spectral ratio technique. The resulting distribution reveals a segmentation of median stress drop values for different seismogenic parts of the subduction zone: We find the lowest stress drops for interface events and slightly increased values for the two parallel bands of seismicity below, which lie inside the subducting plate. The upper plate events, show higher stress drops and the intermediate depth events bear the highest median stress drop. The variation of the median stress drops between classes is small: from 1.3 MPa for interface events to about 3.2 MPa for intermediate depth events. This being the values of the spectral ratio results. Using spectral ratios we find the exact same order of median stress drops between the classes with a range of 2.0 MPa to 5.8 MPa for interface and intermediate depth events, respectively. Interestingly, there is no stress drop increase with dept in the uppermost ~80 km, i.e. within each of the classes except for the intermediate depth events. Additionally, we observe spatial stress drop variability, a noticeable increase with distance from the plate interface, and temporal variability connected with the two megathrust events in the study region, the Mw7.6 2007 Tocopilla event and the Mw 8.1 Iquique event.

&lt;p&gt;We exploit a new template matching based catalogue to study the spatial&lt;br /&gt;and temporal patterns of seismicity in the Eastern and Southern Alps. Data&lt;br /&gt;from the AlpArray Swath-D network from late 2017 to late 2019 were used to&lt;br /&gt;enhance the resolution of the seismic catalogues provided by local&lt;br /&gt;agencies. The template matching method was implemented using our own&lt;br /&gt;GPU-accelerated algorithm to deal with the large data volume. All events&lt;br /&gt;are relocated using waveform-based picking methods.&lt;/p&gt; &lt;p&gt;Based on this result, we study statistical characteristics of the extended&lt;br /&gt;seismicity catalogue, which now has a magnitude of completeness of Mc=0.6&lt;br /&gt;and contains about 7,500 seismic events. We analyse the main spatial and&lt;br /&gt;temporal features of seismicity revealed by this novel dataset. Finally,&lt;br /&gt;we analyse specific event clusters that originated from the template&lt;br /&gt;matching method, and seek to link event interconnectivity to geometrical&lt;br /&gt;properties of the clusters.&lt;/p&gt;

At the northern Chilean subduction zone, where the IPOC network has been monitoring seismicity since 2007, we have identified multiple families of repeating earthquakes. High data quality and long observation time allow analyzing these sequences in detail. Often repeaters are searched to be used as creep proxies and their spatio-temporal cumulative displacement is compared with the tectonic plate convergence rate or GPS based slip rate estimates for smaller fault patches. Repeaters can be classified into periodic, pseudo-periodic or aperiodic types. Put into relation with large earthquakes such as the 2014 MW8.1 Iquique earthquake, repeaters may be described as continuous or burst type families. A precondition for such an analysis is that events are collocated and show highly similar mechanism. This is usually ensured via high cross correlation values between waveforms or by catalog location, or both. Errors in grouping would heavily bias the analysis for individual groups. Therefore, we not only use cross correlation values, but we analyze the intra-family relations in detail. Events are relocated relative to each other based on phase based cross correlation refined s-p travel time differences. Rupture sizes are estimated and intra-family rupture histories are resolved. Having confirmed the characteristics of true repeating earthquake families in this way, we make the classifications and compute the slip rates mentioned above. This study shows that the concept of repeating earthquakes holds beautifully in the case of the northern Chilean subduction zone. Repeater families repeatedly rupture the same patches, and they are observed to respond different to the 2014 Iquique with a strong dependence on their location. Particularly, the time around the Iquique megathrust event shows very interesting patterns in several families. We observe clear precursor patterns, burst reactions and unresponsive families simultaneously.

Despite recent tectonic activity, the Alpine mountain range in central Europe is mostly characterized by weak to moderate seismicity. Low earthquake magnitudes and the heterogeneous crust comprising of different tectonic units challenge centroid moment tensor inversions in this region. Thanks to the dense AlpArray seismic network, comprising more than 600 stations across the Alps, as well as the adoption of a flexible, bootstrap-based inversion tool, we were able to reduce the magnitude threshold for moment tensor inversion to Mw 3.0. The inversion set-up was implemented after systematic tests of different frequency bands, distance ranges, input data types and azimuthal gaps. We quantified the uncertainties of centroid locations and moment tensors, and assessed the reliability of potential non double couple components. Here, we present ~80 deviatoric moment tensor solutions and compare our results to strain rates, historic and recent seismic activity as well as to other published focal mechanisms. We identify three main seismically active subregions, namely the Western Alps, the Lake Garda region and the SE Alps, and two clusters further away from the study region, in the Dinarides and the Apennines. Seismicity is particularly low in the NE Alps and in parts of the central Alps. Additionally, we apply a focal mechanism clustering algorithm to the joint catalog, including our moment tensor solutions and those from existing catalogs. While typical E-W to ENE-WSW striking thrust faulting is observed in the Friuli area in the SE Alps, strike-slip faulting with a similarly oriented pressure axis is observed along the central Alps and in the Dinarides. NW-SE striking normal faulting is observed in the NW Alps with a similar strike direction as the dominant normal faulting events in the Apennines. In the W Alps as well as in the SE Alps, rotations of mechanisms are observed. Both, our centroid depths as well as hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow, with 80 % of the studied events being shallower than 10 km.

The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating different tectonic units and blocks in close proximity to sedimentary foreland basins. Centroid moment tensor inversion provides insight into the faulting mechanisms of earthquakes and related tectonic processes but is significantly aggravated in such an environment. Thanks to the dense AlpArray seismic network and our flexible bootstrap-based inversion tool Grond, we are able to test different setups with respect to the uncertainties of the obtained moment tensors and centroid locations. We evaluate the influence of frequency bands, azimuthal gaps, input data types, and distance ranges and study the occurrence and reliability of non-double-couple (DC) components. We infer that for most earthquakes (Mw≥3.3) a combination of time domain full waveforms and frequency domain amplitude spectra in a frequency band of 0.02–0.07 Hz is suitable. Relying on the results of our methodological tests, we perform deviatoric moment tensor (MT) inversions for events with Mw>3.0. Here, we present 75 solutions for earthquakes between January 2016 and December 2019 and analyze our results in the seismotectonic context of historical earthquakes, seismic activity of the last 3 decades, and GNSS deformation data. We study regions of comparably high seismic activity during the last decades, namely the Western Alps, the region around Lake Garda, and the eastern Southern Alps, as well as clusters further from the study region, i.e., in the northern Dinarides and the Apennines. Seismicity is particularly low in the Eastern Alps and in parts of the Central Alps. We apply a clustering algorithm to focal mechanisms, considering additional mechanisms from existing catalogs. Related to the N–S compressional regime, E–W-to-ENE–WSW-striking thrust faulting is mainly observed in the Friuli area in the eastern Southern Alps. Strike-slip faulting with a similarly oriented pressure axis is observed along the northern margin of the Central Alps and in the northern Dinarides. NW–SE-striking normal faulting is observed in the NW Alps, showing a similar strike direction to normal faulting earthquakes in the Apennines. Both our centroid depths and hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow; about 80 % of the studied events have depths shallower than 10 km.

Asperities are patches where the fault surfaces stick until they break in earthquakes. Locating asperities and understanding their causes in subduction zones is challenging because they are generally located offshore. We use seismicity, interseismic and coseismic slip, and the residual gravity field to map the asperity responsible for the 2014 M 8.1 Iquique, Chile, earthquake. For several years prior to the mainshock, seismicity occurred exclusively downdip of the asperity. Two weeks before the mainshock, a series of foreshocks first broke the upper plate then the updip rim of the asperity. This seismicity formed a ring around the slip patch (asperity) that later ruptured in the mainshock. The asperity correlated both with high interseismic locking and a circular gravity low, suggesting that it is controlled by geologic structure. Most features of the spatiotemporal seismicity pattern can be explained by a mechanical model in which a single asperity is stressed by relative plate motion.

Abstract. Despite our general knowledge of earthquake processes, it is still not fully understood how earthquake ruptures nucleate and propagate and why they stop. Also, the controlling factors of the frequency and of the size of earthquakes are subject of ongoing research. We aim to address these questions with a comprehensive study of seismicity in deep South African gold mines. We find here the unique situation that the seismicity consists of both induced earthquakes and aftershocks triggered by the M5.5 Orkney earthquake which occurred in August 2014. We separate the cataloged seismicity and group the events into three classes: the aftershock sequence, seismicity induced by fluids, and seismicity induced by mining activities. We examine statistical properties of earthquakes in each of the three classes. We conclude that the magnitude statistics of both aftershocks and induced earthquakes are influenced by the finite size and geometry of the rock volume of stress perturbation resulting in an absence of larger magnitude events. The magnitude frequency distributions obey the Lower Bound model of magnitude probability. The statistics of dynamic stress drop of aftershocks and induced earthquakes satisfy log-normal distributions but the value range is different, it means that aftershocks are generally characterized by higher stress drops. Another key aspect in our study is the imaging of the propagating rupture of the M5.5 earthquake. We apply the back projection imaging approach using seismological data from two different local networks and retrieve similar results. We conclude that the rupture of the M5.5 earthquake propagated predominantly unilaterally, nearly from North to South over a distance of about 6km. The hypocenters of the aftershock sequence are situated unilaterally in respect to the hypocenter of the main shock and are aligned to the South which confirms the obtained rupture propagation image and the directivity of the main shock.

We used data from >100 permanent and temporary seismic stations to investigate seismicity patterns related to the 1 April 2014 M8.1 Iquique earthquake in northern Chile. Applying a multistage automatic event location procedure to the seismic data, we detected and located ~19,000 foreshocks, aftershocks, and background seismicity for 1 month preceding and 9 months following the mainshock. Foreshocks skirt around the updip limit of the mainshock asperity; aftershocks occur mainly in two belts updip and downdip of it. The updip seismicity primarily locates in a zone of transitional friction on the megathrust and can be explained by preseismic stress loading due to slow‐slip processes and afterslip driven by increased Coulomb failure stress due to the mainshock and its largest aftershock. Afterslip further south also triggered aftershocks and repeating earthquakes in several EW striking streaks. We interpret the streaks as markers of surrounding creep that could indicate a change in fault mechanics and may have structural origin, caused by fluid‐induced failure along presumed megathrust corrugations. Megathrust aftershocks terminate updip below the seaward frontal prism in the outer continental wedge that probably behaves aseismically under velocity‐strengthening conditions. The inner wedge locates further landward overlying the megathrust's seismogenic zone. Further downdip, aftershocks anticorrelate with the two major afterslip patches resolved geodetically and partially correlate with increased Coulomb failure stress, overall indicating heterogeneous frictional behavior. A region of sparse seismicity at ~40‐ to 50‐km depth is followed by the deepest plate interface aftershocks at ~55‐ to 65‐km depth, which occur in two clusters of significantly different dip.