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The article highlights a study on seismic attenuation beneath the Lesser Antilles arc, an end-member system that slowly subducts old, tectonized lithosphere to understand the view of fundamental mantle wedge processes. It focuses on the end-member Lesser Antilles arc (LAA) system due to its slow consumption of old (80 to 120 million years), slow-spread lithosphere and also shows the l variations in the hydration state of the oceanic lithosphere before its subduction into the Antilles trench.

• Low velocity and high Vp/Vs anomalies to 150 km depth imply significant serpentinisation at the top of the Antilles slab. • Strong slab Vp/Vs anomalies indicate enhanced volatile concentrations correlating with inherited oceanic plate structure. • Fluids released into the cold, entrained part of the wedge give rise to moderate Vp and high Vp/Vs anomalies. Volatiles play a pivotal role in subduction zone evolution, yet their pathways remain poorly constrained. Studying the Lesser Antilles subduction zone can yield new constraints, where old oceanic lithosphere formed by slow-spreading subducts slowly. Here we use local earthquakes recorded by the temporary VoiLA (Volatile recycling in the Lesser Antilles) deployment of ocean-bottom seismometers in the fore- and back-arc to characterize the 3-D seismic structure of the north-central Lesser Antilles subduction zone. Along the slab top, mapped based on seismicity, we find low Vp extending to 130–150 km depth, deeper than expected for magmatic oceanic crust. The slab's most prominent, elevated Vp/Vs anomalies are beneath the fore- and back-arc offshore Guadeloupe and Dominica, where two subducted fracture zones lie with the obliquely subducting boundary between Proto-Caribbean and Equatorial Atlantic lithosphere. These structures, therefore, enhance hydration of the oceanic lithosphere as it forms and evolves and the subsequent dehydration of mantle serpentinite when subducted. Above the slab, we image the asthenosphere wedge as a high Vp/Vs and moderate Vp feature, indicating slab-dehydrated fluids rising through the overlying cold boundary layer that might induce melting further to the west. Our results provide new evidence for the impact of spatially-variable oceanic plate formation processes on slab dehydration and mantle wedge volatile transfer that ultimately impact volcanic processes at the surface, such as the relatively high magmatic output observed on the north-central islands in the Lesser Antilles. [ABSTRACT FROM AUTHOR]

Monitoring small magnitude induced seismicity requires a dense network of seismic stations and high-quality recordings in order to precisely determine events' hypocentral parameters and mechanisms. However, microseismicity (e.g. swarm activity) can also occur in an area where a dense network is unavailable and recordings are limited to a few seismic stations at the surface. In this case, using advanced event detection techniques such as template matching can help to detect small magnitude shallow seismic events and give insights about the ongoing process at the subsurface giving rise to microseismicity. In this paper, we study shallow microseismic events caused by hydrofracking of the PNR-2 well near Blackpool, UK, in 2019 using recordings of a seismic network which was not designed to detect and locate such small events. By utilizing a sparse network of surface stations, small seismic events are detected using template matching technique. In addition, we apply a full-waveform moment tensor inversion to study the focal mechanisms of larger events (ML > 1) and used the double-difference location technique for events with high-quality and similar waveforms to obtain accurate relative locations. During the stimulation period, temporal changes in event detection rate were in agreement with injection times. Focal mechanisms of the events with high-quality recordings at multiple stations indicate a strike-slip mechanism, while a cross-section of 34 relocated events matches the dip angle of the active fault. [ABSTRACT FROM AUTHOR]

Following the Mw 8.8 Maule megathrust earthquake that occurred on February 27, 2010, in central Chile, a sequence of normal faulting crustal earthquakes occurred close to the city of Pichilemu. This activity lasted several months and included two large events (Mw = 6.9 and Mw = 7.0) on March 11, 2010. An initial investigation of this activity analyzed a data set of about 630 earthquakes recorded locally by 8 short-period seismic stations and attributed this seismicity to the activation of the "Pichilemu fault system." While there is no visible surface faulting associated with this system, it is inferred from discontinuities in lithofacies in the metamorphic complex of Pichilemu and from morphological breaks attributed to post-Pliocene neotectonic activity. There are no recorded shallow crustal earthquakes in the Pichilemu area prior to the 2010 events. In this study, we combine locally recorded earthquake data from the 2010 seismic deployment and data from 20 seismic stations (short period, three components, continuous recording) deployed in 2017 around Pichilemu, Chile, to create a more detailed characterization of the Pichilemu fault system through local earthquake tomography (LET). The combined data set composed of P- and S-wave arrival times from 3691 events was inverted to generate a 3D elastic wave speed model from the surface to about 50-km depth. One hundred twenty-two focal mechanisms for relocated earthquakes with M ≥ 1 were also generated. Relocated hypocenters show that most of the recorded seismicity is associated with the Pichilemu fault system; its main structure is oriented N145°E, and it is seismically active along about 50 km long. Normal faulting mechanisms predominate for events with M≥2, being similar to the mechanisms of the Mw 7.0 March 11, 2010, Pichilemu earthquakes. Low-velocity anomalies correlate with fracture zones associated with the Pichilemu fault, and a high contrast in Vp/Vs coincides with known structures of Paleozoic to Mesozoic age. A high Vp/Vs ratio is observed where a projection of the fault reaches the interplate contact, suggesting that this zone of the forearc crust is likely weakened by the presence of fluids from the slab. A high Vp anomaly is contiguous with the fault system and appears to be related to the presence of granitic rocks which belong to the Coastal Batholith within the Cordillera de la Costa. We suggest that the location of the Pichilemu fault system is governed by rheological contrasts inherited from the evolution of the subduction complex represented by the current Cordillera de la Costa, and we infer that the orientations of those structures, the possible hydration of them from the interplate contact and associated crustal blocks, play a key role in fault activation following the stressing by a great subduction earthquake. [ABSTRACT FROM AUTHOR]

Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three‐dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (<1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (>1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period. Plain Language Summary: Using seismic data recorded by the permanent Ecuadorian network and the large emergency installation after the 2016 Pedernales earthquake, we obtained the seismic velocity structure together with precise earthquake locations for the coastal Ecuadorian margin. Our images highlight the heterogeneities of the subduction zone affected by seamounts and ridges comprising the oceanic crust. These features play an important role in controlling the seismic behavior of the margin. While seamounts can contribute to the occurrence of intermediate (M ∼ 7–7.5) megathrust earthquakes in the north, the Carnegie Ridge seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region. Key Points: 3D Vp and Vp/Vs models were calculated using local earthquake tomography in the region affected by the 2016 Pedernales, Ecuador earthquakeTomographic images highlight the heterogeneities of the margin affected by seamounts and ridges comprising the oceanic crustCarnegie Ridge seems the main feature controlling the seismic activity and the offshore extent of large megathrust earthquakes in the region [ABSTRACT FROM AUTHOR]

The Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a M w 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of ∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We also observe significant north–south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a M w >7.4 has ruptured south of ∼0.5°S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influance the occurrence and spatial distribution of high magnitude seismicity in the region. [ABSTRACT FROM AUTHOR]

The heterogeneous seafloor topography of the Nazca Plate as it enters the Ecuador subduction zone provides an opportunity to document the influence of seafloor roughness on slip behavior and megathrust rupture. The 2016 Mw 7.8 Pedernales Ecuador earthquake was followed by a rich and active postseismic sequence. An internationally coordinated rapid response effort installed a temporary seismic network to densify coastal stations of the permanent Ecuadorian national seismic network. A combination of 82 onshore short and intermediate period and broadband seismic stations and six ocean bottom seismometers recorded the postseismic Pedernales sequence for over a year after the mainshock. A robust earthquake catalog combined with calibrated relocations for a subset of magnitude ≥4 earthquakes shows pronounced spatial and temporal clustering. A range of slip behavior accommodates postseismic deformation including earthquakes, slow slip events, and earthquake swarms. Models of plate coupling and the consistency of earthquake clustering and slip behavior through multiple seismic cycles reveal a segmented subduction zone primarily controlled by subducted seafloor topography, accreted terranes, and inherited structure. The 2016 Pedernales mainshock triggered moderate to strong earthquakes (5 ≤ M ≤ 7) and earthquake swarms north of the mainshock rupture close to the epicenter of the 1906 Mw 8.8 earthquake and in the segment of the subduction zone that ruptured in 1958 in a Mw 7.7 earthquake.Key Points: A dense temporary seismic network provides a detailed view of the evolution of a megathrust rupture postseismic sequenceThe postseismic sequence is characterized by clusters of seismicity containing earthquake swarms and moderate to large aftershocksSeafloor topography and upper plate structure segment the subduction zone and control slip behavior across the seismic cycle [ABSTRACT FROM AUTHOR]

On April 16th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Instituto Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with M L > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P- and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap <200°, are simultaneously inverted for, obtaining the minimum 1D velocity model for the rupture region, along with hypocentral locations and station corrections. We observe an average Vp/Vs of 1.82 throughout the study region, with relatively higher Vp/Vs values of 1.95 and 2.18 observed for the shallowest layers down to 7.5 km. The high relative Vp/Vs ratio (1.93) of the deeper section, between 30 km and 40 km, is attributed to dehydration and serpentinization processes. For the relocated seismicity distribution, clusters of events align perpendicular to the trench, and crustal seismicity is also evidenced, along with earthquakes located close to the trench axis. We also compute Regional Moment Tensors to analyze the different sources of seismicity after the mainshock. Aside from thrust events related to the subduction process, normal and strike-slip mechanisms are detected. We suggest that the presence of subducting seamounts coming from the Carnegie Ridge act as erosional agents, helping to create a scenario which promotes locking and allows seismicity to extend up to the trench, along zones of weakness activated after large earthquakes. Unlabelled Image • 1D velocity model, station corrections and hypocentral locations were obtained based on the aftershocks of the 2016 Pedernales earthquake. • Our minimum 1D velocity model allows us to observe and discuss depth ranges and to first order the velocity structure with depth. • Station correction terms are consistent with areas affected by site effects during the earthquake. • A portion of the relocated seismicity occurs very close to the trench (~5 – 10 km). • Regional moment tensors suggest the presence of a strike-slip fault in the marine forearc. [ABSTRACT FROM AUTHOR]

Gas‐and‐ash explosions at the Santiaguito dome complex, Guatemala, commonly occur through arcuate fractures, following a 5‐ to 6‐min period of inflation observed in long‐period seismic signals. Observation of active faults across the dome suggests a strong shear component, but as fault propagation generally proceeds through the coalescence of tensile fractures, we surmise that explosive eruptions require tensile rupture. Here, we assess the effects of temperature and strain rate on fracture propagation and the tensile strength of Santiaguito dome lavas. Indirect tensile tests were conducted on samples with a porosity range of 3–30% and over diametral displacement rates of 0.04, 0.004, and 0.0004 mm/s. At room temperature, the tensile strength of dome rock is rate independent (within the range tested) and inversely proportional to the porosity of the material. At eruptive temperatures we observe an increasingly ductile response at either higher temperature or lower displacement rate, where ductile deformation is manifest by a reduction in loading rate during constant deformation rate tests, resulting in slow tearing, viscous flow, and pervasive damage. We propose a method to conduct indirect tensile tests under volcanic conditions using a modification of the Brazilian disc testing protocol and use brittleness indices to classify deformation modes across the brittle‐ductile transition. We show that a degree of ductile damage is inevitable in the lava core during explosions at the Santiaguito dome complex and discuss how strain leading to rupture controls fracture geometry, which would impact gas pressure release or buildup and regulate explosive activity. Plain Language Summary: Using instruments we installed at Santiaguito, an active lava dome complex in Guatemala, we detected repeating cycles of inflation and deflation. The inflation took less time leading up to explosions compared to weak gas puffing, which led us to suspect that lava breaking and flowing might be responsible. To investigate this, we made laboratory tests where we put lava samples under tension, which is the most common way they break. We ran tests where we squeezed lavas at faster and slower rates in a press, and we also heated the lavas to their eruption temperatures—about 800 °C—for some tests. Since the lavas contain volcanic glass, in some high temperature tests the glass partially flowed. In other tests the lavas were completely brittle, which means they stored up stress and then broke without flowing. The lava's behavior depends on the temperature and how fast they are squeezed. Finally, we considered how fast dome lavas at Santiaguito would have to be deformed to either break or stay intact during inflation of the dome. This study gives us a better idea of how dome lavas deform and how that affects hazardous activity during eruptions. Key Points: Low‐to‐high temperature indirect tensile tests simulate tensile failure during monitored inflation phases at Santiaguito dome complexWe recommend a protocol to conduct indirect tensile tests at high temperatures and describe novel Brittleness Indices for this regimeAbove the glass transition temperature greater ductile deformation and strain are observed, and deformation mode varies with strain rate [ABSTRACT FROM AUTHOR]

The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel-time data of local earthquakes for vp and vp=vs velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean-bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, Mw D 8:7) and to the south in 2007 (Bengkulu earthquake, Mw D 8:5). The 2-D and 3-D vp velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3-D velocity model shows prevailing trench-parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at ~180 km distance to the trench. vp velocities beneath the magmatic arc and the Sumatran fault zone (SFZ) are around 5 km s-1 at 10 km depth and the vp=vs ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated vp=vs values of 1.85 at ~150 km distance to the trench in the region of the Mentawai Fault. vp=vs ratios suggest an absence of large-scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced vp velocities beneath the forearc basin covering the region between the Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust. [ABSTRACT FROM AUTHOR]

Seismograms of about 12,000 aftershocks of the M w 6.7 Coquimbo-La Serena earthquake of 20 January 2019 that were recorded during six months by an array of 33 short-period seismic stations are used to generate wavespeed images of the subduction wedge beneath the Chilean forearc between 28.0°-30.6°S. This part of the margin is near the northern terminus of the 2015 M w 8.4 Illapel earthquake and the southern terminus of the 1922 M w 8.5 Copiapo earthquake. The distribution of hypocenters and wavespeeds at the slab interface and within the subduction wedge, are like those determined by Comte et al. (2019) in the Illapel aftershock region, and we infer that the same processes of subduction erosion and re-accretion inferred by them are active here. Moreover, anomalous wavespeeds are found in the immediate vicinity of a dense cluster of aftershocks located in a well-defined double seismic zone, within the subducting Nazca plate. Vp/Vs ratios in the upper seismic zone are uniformly high (∼1.9), those in most of the lower seismic zone are average to low, and those in the intraslab region of the aftershock cluster are exceptionally low (∼1.6). These values suggest dehydration of the slab interior and hydration of the mantle near the slab interface. While the origin of this feature is enigmatic, its limited spatial extent suggests that it most likely is due to an inherited structure, such a seamount or petit-spot volcano, in the Nazca plate, rather than a feature that developed in situ. The high rate of activity coupled with the low wavespeeds suggests a region of local weakness that could eventually evolve into a tear or hole in the slab, and its location at the bounds of two megathrust earthquakes suggests that it plays a significant role in delimiting the rupture of such events. • Most Coquimbo-La Serena earthquake aftershocks are confined to a dense cluster. • Wavespeeds show dehydration of the slab interior and hydration of the overlying mantle. • Remnants of a seamount or petit-spot volcano are found in the subducted Nazca plate. • The aftershock region may arrest the rupture of Andean margin megathrusts. • The region is subject to the same subduction erosion processes as Illapel to the south. [ABSTRACT FROM AUTHOR]

We analyze records of ambient seismicity from Central Chile to investigate mechanisms of tectonic erosion and accretion in an Andean margin. The seismograms were recorded by a temporary network of 39 stations deployed between 32.5°S and 34.0°S from April to November 2017. We employ an automatic earthquake catalogue generator to estimate 8463 P-wave and 5482 S-wave arrival times from 924 hypocenters, which were then used to jointly determine hypocenters and 3D models of Vp and Vp/Vs. Previous investigations of the nearby Illapel region suggest that the Coastal Cordillera acts as a subduction wedge, at the bottom of which are accreted crustal slices detached from the overriding plate by tectonic erosion. These slices may be subducted beyond the downdip frictional limit in a way that differs from a traditional orogenic wedge (type A-subduction). Our results reveal a wedge geometry with: (1) body wavespeed anomalies near the interplate contact below 30 km depth, which we interpret as accretionary complexes formed by basal accretion of crustal slices, and (2) latitudinal differences of those anomalies, which we attribute to eroded material from both plates that changes the basalt-eclogite transition depth in the subducted Nazca plate. Based on these, we infer that the development of this basal accretionary complex by subduction erosion could be responsible for the uplift of Coastal Cordillera in Central Chile, and that the retroshear zone of the subduction wedge is controlled by local conditions inside the mantle wedge. Moreover, we suggest that the accretionary complex beneath the Central Chile subduction wedge behaves as seismic barrier that can explain the 1985 Mw 8.0 Valparaíso earthquake slip distribution and the southeastward migration of aftershocks of 2017 Mw 6.9 Valparaíso earthquake. Finally, we suggest that the subduction wedge influences continental structures on a scale of millions of years. • There is a subduction wedge in Central Chile with a basal accretionary complex. • Central Chile subduction wedge has along-strike variations. • Basal accretionary complex could generate Coastal Cordillera uplift. • Subduction wedge retroshear zone could be controlled by mantle wedge conditions. • Central Chile basal accretionary complex could be a seismic barrier. [ABSTRACT FROM AUTHOR]

Long-term instrumental monitoring of open-vent volcanoes provides the necessary datasets to characterize volcanic activity and unravel its temporal changes. This is particularly important for active lava domes, which can undergo rapid transitions in behavior over the course of their eruption. Here, we analyzed seismic, acoustic infrasound and thermographic data collected between January 2018 and September 2020 to resolve volcanic processes taking place at the Santiaguito lava dome complex in Guatemala. During this period lava effusion filled the crater of the active Caliente lava dome. The extrusive activity was accompanied by small-to-moderate explosions, prolonged episodes of gas emissions, and occasional rockfalls. Automated algorithms were applied to identify seismic signals associated with different processes and to characterize the temporal evolution of activity. We identified ~70–250 tectonic events per week and detected signals associated with gas-and ash explosions occurring at a rate of ~70–100 events/week. Lava dome growth activity was accompanied by the emplacement of a lava flow along the eastern upper flank of Caliente and seismicity possibly due to the occurrence of rockfalls. We observed episodes of harmonic tremor in seismic and acoustic data associated with sustained gas emissions, estimated to originate at shallow depths of about 500–750 m below the crater. Data indicated that both the recurrence rate of tremor (~10–50 events/week) and its duration (~40–130 min/week) were slightly lower and shorter between January 2019 and March 2020 than in rest of the study period, despite minor variations in explosive activity. Finally, within a period of 11 weeks, between 18 January and 4 April 2018, we found 129 volcano tectonic earthquakes; we were able to locate 10 of them at depths between 1.3 and 2.3 km, ~1.5 km southwest of Caliente. This multi-parametric study provides valuable insights into geophysical signals and associated processes at Santiaguito, helping to resolve temporal occurrence of each event type during protracted effusive-explosive activity. • Analysis of seismic and infrasound data at Santiaguito revealed a variety of signals. • The data were classified using automated search algorithms. • Signals associated with gas-and-ash explosions showed an impulsive onset and a rapid amplitude decay. • Gliding harmonic tremor signals were observed in the seismic and acoustic data. [ABSTRACT FROM AUTHOR]

The region around the Mejillones Peninsula is characterized by major onshore and offshore fault systems. The existence of major splay faults in these fault systems has been debated. To improve our undestanding of seismotectonic processes affecting the MP, we built a new catalog of aftershocks of the 1995 Antofagasta earthquake (Mw 8.1) recorded by the CINCA seismic network. In addition, we build a three-dimensional tomographic model to determine the physical properties of the upper plate using arrival times from our new catalog combined with other earthquakes catalogs derived from several networks deployments across the MP. We detected seismicity in the outer wedge of the upper plate. The seismicity is partially defined by a clear lineament between 22 and 5 km depth that we defined as a splay fault. The focal mechanisms indicate a rupture plane with NE orientation and SE dip. The seismicity in the upper plate and splay fault are located in the areas in which the Vp and Vs velocities are decreased and the Vp/Vs ratio is increased, suggesting a fractured fault zone within the outer wedge of the upper plate. We propose the presence of fluids in the fracture fault zone within the outer wedge because the decrease in Vs larger than that of Vp and repeating earthquakes clusters were detected. Our observations present clear evidence of the activation of the splay fault in the outer wedge of the North Chilean offshore forearc. • The seismicity in the outer wedge of the forearc of northern Chile is associated with a splay fault. • Increased Vp/Vs ratios, decreased Vp and Vs values, and repeating earthquakes indicate fractures and the presence of fluids. • The results reveal the structure of the offshore forearc of the MP and are beneficial for the mitigation of seismic and tsunamigenic risks. [ABSTRACT FROM AUTHOR]

On September 26, 2019, a significant, Mw 6.5, earthquake shook Ambon region, Indonesia, causing severe damage on the Island(s) of Ambon. Due to the complexity of the fault network and in-situ stress field it was, up to now, not possible to define the fault plane using data from the BMKG regional seismic network. In this study, we analyze the fault plane of the 2019 Ambon earthquake and the reactivation potential of the surrounding faults using local networks. Eleven stations were deployed to monitor the aftershocks from October 18th to December 15th, 2019 augmented with data of four regional stations. During the monitoring period, 1,778 events were identified comprised of 10,938 P- and 10,315 and S- wave arrival times. The locations of aftershock were determined in a stepwise approach, i.e. (i) initial location determination using a non-linear approach, (ii) updating the velocity model, and (iii) relative double-difference relocation. Slip inversion using teleseismic data was performed to infer of high strain relief of the mainshock and to compute its associated static stress transfer (ΔCFF). Based on aftershock distribution and finite fault modeling, we conclude that the Mw 6.5 Ambon earthquake occurred on a N-S oriented fault plane. Two clusters consisting of ~60% of total events are located at both tips of the plane. Another cluster ~30% was sharply aligned in a NE-SW trend, 10 km westward, starting by an Mw 5.2 event on November 2nd, 2019. The b-value of the NE-SW events is ~0.25 lower than the other clusters with a b-value of 0.85±0.14. ΔCFF imparted by the mainshock caused ~0.5 Bar stress increase on the NE-SW fault. We concluded that the NE-SW trend was the reactivation of a preexisting fault crossing Ambon Island. The triggered large aftershock caused further significant damages to already weakened infrastructure and, thus, had the largest mapped damage area. • Dense local network for assessing source mechanism and triggered large aftershocks. • 1,778 aftershocks with 10,938 P- and 10,315 and S- wave arrival times were identified. • Fault plane was identified using seismic cluster, slip inversion and stress transfer. • Reactivation of the vicinity fault network (Mw 5.2) due to an Mw 6.5 strike slip event. • Existing NE-SW fault in the Ambon and Seram Island might be critically stress. [ABSTRACT FROM AUTHOR]

Megathrust ruptures and the ensuing postseismic deformation cause stress changes that may induce seismicity on upper plate crustal faults far from the coseismic rupture area. In this study, we analyze seismic swarms that occurred in the north Ecuador area of Esmeraldas, beginning two months after the 2016 M w 7.8 Pedernales, Ecuador megathrust earthquake. The Esmeraldas region is 70 km from the Pedernales rupture area in a separate segment of the subduction zone. We characterize the Esmeraldas sequence, relocating the events using manual arrival time picks and a local a-priori 3D velocity model. The earthquake locations from the Esmeraldas sequence outline an upper plate fault or shear zone. The sequence contains one major swarm and several smaller swarms. Moment tensor solutions of several events include normal and strike-slip motion and non-double-couple components. During the main swarm, earthquake hypocenters increase in distance from the first event over time, at a rate of a few hundred meters per day, consistent with fluid diffusion. Events with similar waveforms occur within the sequence, and a transient is seen in time series of nearby GPS stations concurrent with the seismicity. The events with similar waveforms and the transient in GPS time series suggest that slow aseismic slip took place along a crustal normal fault during the sequence. Coulomb stress calculations show a positive Coulomb stress change in the Esmeraldas region, consistent with seismicity being triggered by the Pedernales mainshock and large aftershocks. The characteristics of the seismicity indicate that postseismic deformation involving fluid flow and slow slip activated upper plate faults in the Esmeraldas area. These findings suggest the need for further investigation into the seismic hazard potential of shallow upper plate faults and the potential for megathrust earthquakes to trigger slow-slip and shallow seismicity across separate segments of subduction zones. • Fluid flow and likely slow slip caused damaging earthquake swarm on crustal fault. • Postseismic deformation triggered fluid flow and slow slip in adjacent segment. • Hazard potential of crustal faults in Esmeraldas after megathrust is demonstrated. [ABSTRACT FROM AUTHOR]

• Eruptive activity shifted to larger, irregular, ash-rich explosions in 2015–16. • The eruptive transition was assessed using a multi-parametric approach. • Activity was triggered by the injection of higher temperature magma from depth. • Microlite textures of eruptive products indicate contrasting magma ascent styles. • Explosions switched from low energy shear-driven to high energy overpressure-driven. Protracted volcanic eruptions may exhibit unanticipated intensifications in explosive behaviour and attendant hazards. Santiaguito dome complex, Guatemala, has been characterised by century-long effusion interspersed with frequent, small-to-moderate (<2 km high plumes) gas-and-ash explosions. During 2015–2016, explosions intensified generating hazardous ash-rich plumes (up to 7 km high) and pyroclastic flows. Here, we integrate petrological, geochemical and geophysical evidence to evaluate the causes of explosion intensification. Seismic and infrasound signals reveal progressively longer repose intervals between explosions and deeper fragmentation levels as the seismic energy of these events increased by up to four orders of magnitude. Evidence from geothermobarometry, bulk geochemistry and groundmass microlite textures reveal that the onset of large explosions was concordant with a relatively fast ascent of a deeper-sourced (∼17–24 km), higher temperature (∼960–1020 °C) and relatively volatile-rich magma compared to the previous erupted lavas, which stalled at ∼2 km depth and mingled with the left-over mush that resided beneath the pre-2015 lava dome. We interpret that purging driven by the injection of this deep-sourced magma disrupted the long-term activity, driving a transition from low energy shallow shear-driven fragmentation, to high energy deeper overpressure-driven fragmentation that excavated significant portions of the conduit and intensified local volcanic hazards. Our findings demonstrate the value of multi-parametric approaches for understanding volcanic processes and the triggers for enigmatic shifts in eruption style, with the detection of vicissitudes in both monitoring signals and petrological signatures of the eruptive products proving paramount. [ABSTRACT FROM AUTHOR]

Subduction zones generate the largest earthquakes on Earth, yet their detailed structure, and its influence on seismic and aseismic slip, remains poorly understood. Geological studies of fossil subduction zones characterize the seismogenic interface as a 100 m-1 km thick zone 1-3 in which deformation occurs mostly on metres-thick faults 1,3-6 . Conversely, seismological studies, with their larger spatial coverage and temporal resolution but lower spatial resolution, often image the seismogenic interface as a kilometres-wide band of seismicity 7 . Thus, how and when these metre-scale structures are active at the seismic-cycle timescale, and what influence they have on deformation is not known. Here we detect these metres-thick faults with seismicity and show their influence on afterslip propagation. Using a local three-dimensional velocity model and dense observations of more than 1,500 double-difference relocated earthquakes in Ecuador, we obtain an exceptionally detailed image of seismicity, showing that earthquakes occur sometimes on a single plane and sometimes on several metres-thick simultaneously active subparallel planes within the plate interface zone. This geometrical complexity affects afterslip propagation, demonstrating the influence of fault continuity and structure on slip at the seismogenic interface. Our findings can therefore help to create more realistic models of earthquake rupture, aseismic slip and earthquake hazard in subduction zones., (© 2024. The Author(s).)

The margins of the Caribbean and associated hazards and resources have been shaped by a poorly understood history of subduction. Using new data, we improve teleseismic P-wave imaging of the eastern Caribbean upper mantle and compare identified subducted-plate fragments with trench locations predicted from plate reconstruction. This shows that material at 700-1200 km depth below South America derives from 90-115 Myr old westward subduction, initiated prior to Caribbean Large-Igneous-Province volcanism. At shallower depths, an accumulation of subducted material is attributed to Great Arc of the Caribbean subduction as it evolved over the past 70 Ma. We interpret gaps in these subducted-plate anomalies as: a plate window and tear along the subducted Proto-Caribbean ridge; tearing along subducted fracture zones, and subduction of a volatile-rich boundary between Proto-Caribbean and Atlantic domains. Phases of back-arc spreading and arc jumps correlate with changes in age, and hence buoyancy, of the subducting plate., (© 2021. The Author(s).)

Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging 1 . Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform 2 . Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc 3 , we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine-that is, hydrated mantle rather than crust or sediments-is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards.