Crustal Architecture of Puerto Rico Using Body-Wave Seismic Tomography and High-Resolution Earthquake Relocation

Puerto Rico is a highly seismically active island, where several damaging historical earthquakes have occurred and frequent small events persist. It situates at the boundary between the Caribbean and North American plates, featuring a complex fault system. Here, we investigate the seismotectonic crustal structure of the island by interpreting the 3D compressional-wave velocity VP and compressional- to shear-wave velocity ratio VP/VS models and by analyzing the distribution of the relocated earthquakes. The 3D velocity models are obtained by applying the simul2000 tomographic inversion algorithm based on the phase arrivals recorded by the Puerto Rico seismic network. We find high-VP and low-VP/VS anomalies in the eastern and central province between the Great Northern Puerto Rico fault zone and the Great Southern Puerto Rico fault zone, correlating with the Utuado pluton. Further, there are low-VP anomalies beneath both the Great Southern Puerto Rico fault zone and the South Lajas fault, indicating northerly dipping structures from the southwest to the northwest of the island. We relocate 19,095 earthquakes from May 2017 to April 2021 using the new 3D velocity model and waveform cross-correlation data. The relocated seismicity shows trends along the Investigator fault, the Ponce faults, the Guayanilla rift, and the Punta Montalva fault. The majority of the 2019–2021 Southwestern Puerto Rico earthquakes are associated with the Punta Montalva fault. Earthquakes forming 17° northward-dipping structures at various depths possibly manifest continuation of the Muertos trough, along which the Caribbean plate is being subducted beneath the Puerto Rico microplate. Our results show complex fault geometries of a diffuse fault network, suggesting possible subduction process accommodated by faults within a low-velocity zone.

Teleseismic P waves at the AlpArray seismic network: wave fronts, absolute travel times and travel-time residuals

We present an extensive dataset of highly accurate absolute travel times and travel-time residuals of teleseismic P waves recorded by the AlpArray Seismic Network and complementary field experiments in the years from 2015 to 2019. The dataset is intended to serve as the basis for teleseismic travel-time tomography of the upper mantle below the greater Alpine region. In addition, the data may be used as constraints in full-waveform inversion of AlpArray recordings. The dataset comprises about 170 000 onsets derived from records filtered to an upper-corner frequency of 0.5 Hz and 214 000 onsets from records filtered to an upper-corner frequency of 0.1 Hz. The high accuracy of absolute and residual travel times was obtained by applying a specially designed combination of automatic picking, waveform cross-correlation and beamforming. Taking travel-time data for individual events, we are able to visualise in detail the wave fronts of teleseismic P waves as they propagate across AlpArray. Variations of distances between isochrons indicate structural perturbations in the mantle below. Travel-time residuals for individual events exhibit spatially coherent patterns that prove to be stable if events of similar epicentral distance and azimuth are considered. When residuals for all available events are stacked, conspicuous areas of negative residuals emerge that indicate the lateral location of subducting slabs beneath the Apennines and the western, central and eastern Alps. Stacking residuals for events from 90∘ wide azimuthal sectors results in lateral distributions of negative and positive residuals that are generally consistent but differ in detail due to the differing direction of illumination of mantle structures by the incident P waves. Uncertainties of travel-time residuals are estimated from the peak width of the cross-correlation function and its maximum value. The median uncertainty is 0.15 s at 0.5 Hz and 0.18 s at 0.1 Hz, which is more than 10 times lower than the typical travel-time residuals of up to ±2 s. Uncertainties display a regional dependence caused by quality differences between temporary and permanent stations as well as site-specific noise conditions.

The 2003 Ms6.1 Minle Earthquake: An Earthquake in the Minle-Yongchang Reverse Fault-Related Fold Belt in the Hexi Corridor, NW China

The Minle-Yongchang fault is an active reverse fault-related fold structural belt developed in the Hexi Corridor Basin on the northeastern edge of the Tibetan Plateau. An earthquake of Ms6.1 occurred near the Minle-Yongchang fault zone in 2003. The deformation pattern of the Minle-Yongchang fault and its relationship with this strong earthquake, however, are still not well known. In this study, we used the methods of HYPOINVERSE absolute location and double-difference location with waveform cross-correlation technology to relocate the 2003 Minle earthquake sequence. In total, 383 earthquakes are precisely relocated. Based on the results of precise seismic relocation, using the method of determining fault planes by small earthquakes, the seismogenic fault is found to be a low-angle thrust with a strike of 311°, a dip of NE, and a dip angle of 14°. It does not rupture the surface, extends to 19–20 km depth, and is hidden beneath the Yonggu Anticline. We also employed the cut-and-paste (CAP) method with a broadband waveform to determine the focal mechanism of the mainshock in 2003: the strike is 311°; the dip is 34°; and the rake is 90°. The fault plane parameters obtained in these two ways are roughly consistent. We also used a digital elevation model (DEM) derived from the SPOT 6 stereo image pair and high-precision differential Global Positioning System (GPS) to measure the displacement of terraces. Topographic profiles along the terraces across the Minle-Yongchang fault show that high alluvial terrain exhibits fold deformation. The vertical offsets of the T2 and T3 terraces along the Tongziba River are approximately 2.3 and 22 m, respectively. Optically stimulated luminescence (OSL) dating indicates that the ages of T2 and T3 are 11.3 and 106 ka, respectively. We calculated an average uplift rate of 0.21 ± 0.05 mm/a by dividing the vertical offset by age. According to the spatial distribution of the relocated earthquake sequence and terrace deformation in the study area, the Ms6.1 Minle earthquake in 2003 was caused by the latest activity of a blind reverse fault-related fold in the Hexi Corridor Basin.

Slowly migrating tectonic microearthquake swarms in the Icelandic Rift Zone: driven by pore-pressure or aseismic slip transients?

<p>Intense swarms of microearthquakes have been detected in the rift zone of Central Iceland since the 1970s, but the cause of their clear swarm-like nature remains enigmatic. We use the QuakeMigrate earthquake detection and location software<sup>1</sup> to produce a highly complete catalogue of microseismicity from 2007-2020, using data from a dense local seismic network. Automatic hypocentre locations have been refined using waveform cross-correlation and double-difference relocation, and tightly constrained focal mechanisms have been obtained by manual analysis of a subset of events.</p><p>The resulting high-resolution earthquake catalogue reveals a network of conjugate strike-slip faults, oriented to accommodate plate-boundary extension. Sharply defined fault planes imaged by the microearthquake hypocentres range from 1-10 km in length, and are found between 1 and 8 km b.s.l., with their orientations closely matching the fault plane geometry inferred from the fault plane solutions. Seismicity within individual swarms displays a systematic migration of hypocentres at velocities of ~ 1 km/day. In the majority of swarms we also observe clusters of identical repeating events, providing evidence for re-loading of brittle asperities.</p><p>For a selection of swarms our high resolution seismic observations are complemented by GPS and InSAR measurements, allowing us to place constraints on the amount of fault slip. Comparing this, and the area of the fault plane activated in the swarm, to the seismic moment release reveals a significant contribution of aseismic slip, or very low effective stress drop. Analysis of swarms within this fault network triggered by the 2014 Bárðarbunga-Holuhraun dike intrusion provides further constraint on the amplitude of the stress cycle.</p><p>We combine our observations with comparisons to numerical & laboratory modelling studies, observed swarm scaling properties and knowledge of the geological and permeability structure of the Icelandic crust to determine the nature of the transient forcing driving these exceptionally well-recorded tectonic earthquake swarms.</p><p> </p><p>1: https://github.com/QuakeMigrate/QuakeMigrate Tom Winder, Conor Bacon, Jonathan D. Smith, Thomas S. Hudson, Julian Drew, & Robert S. White. (2021, January 15). QuakeMigrate v1.0.0 (Version v1.0.0). Zenodo. http://doi.org/10.5281/zenodo.4442749</p>

Discriminating glacial and volcanic seismicity at Llaima and Villarrica volcanoes, Chile

<p>The monitoring of seismic activity at active glacier-hosting volcanoes is challenging as volcanic and glacial earthquakes (i.e. icequakes) can have overlapping characteristics (i.e. frequencies, waveform shape and magnitude). Here we present results from the first study to target glacial activity at active ice-covered volcanoes in the Southern Chile. The primary focus so far has been on Llaima volcano, one of the largest and most active volcanoes in the region while hosting >14 km<sup>2</sup> of glacial ice on the flanks. We use a combination of automatic multi-station event detection and waveform cross-correlation to find candidate repeating icequakes in seismic data from the permanent volcano monitoring network recorded in early 2019. We identified dozens of low magnitude families of repeating seismic events across two months, the largest of which included over 200 events. These findings are comparable to results from analysis of seismic data recorded at Llaima volcano during the same time period in 2015. The persistent, repetitive nature of these events combined with their waveform characteristics and source locations suggest they originated from multiple sub-glacial stick-slip sources around the upper flanks of the volcano. We also deployed a network of seismo-acoustic sensors at Villarrica volcano in early 2020 to record glacial activity in concurrence with the lava lake and strombolian activity at the summit. We conclude that icequakes at Llaima volcano may be more common than previously thought and has implications for how seismic data at ice-covered volcanoes may be used for assessing future volcanic and glacial hazard potential.</p>

Repeating earthquakes follow afterslip gradient in the aftermath of the 16th April 2016 M7.8 Pedernales earthquake in Ecuador

<p>Repeating earthquakes are earthquakes that repeatedly break a single, time-invariant fault patch. They are generally associated with aseismic slip, which is thought to load asperities, leading to repeated rupture. Repeating earthquakes are therefore useful tools to study aseismic slip and fault mechanics, with possible applications to earthquake triggering, loading rates and earthquake forecasting.</p><p>In this study, we analyze one year of aftershocks following the 16<sup>th</sup> April 2016 Mw 7.8 Pedernales earthquake in Ecuador to find repeating families, using data recorded by permanent and temporary seismological stations. In our area, seismicity during both the inter-seismic and post-seismic periods has been previously linked to aseismic slip. We calculate waveform cross-correlation coefficients (CC) on all available catalogue events, which we use to sort events into preliminary families, using a minimum CC of 0.95. These events were then stacked and used to perform template-matching on the continuous data. In total, 376 earthquakes were classified into 62 families of 4 to 15 earthquakes, including 8 from the one-year period before the mainshock. We later relocated these earthquakes using a double-difference method, which confirmed that most of them did have overlapping sources.</p><p>Repeating earthquakes seem to concentrate largely around the areas of largest afterslip release, where afterslip gradient is the highest. We also find an increase in the recurrence time of repeating events with time after the mainshock, over the first year of the postseismic period, which highlights a possible timeframe for the afterslip’s deceleration. Our results suggest that while most repeating aftershocks are linked to afterslip release, the afterslip gradient may play a bigger role in determining their location than previously thought.</p>

Dominant strike-slip faulting and near-constant stress drop of induced earthquakes in the Kiskatinaw area, northeastern British Columbia, Canada

<p><span>An increasing number of hydraulic fracturing (HF) operations in­ low-permeable tight shales in the Kiskatinaw area, northeastern British Columbia, have been associated with M3+ earthquakes in the last decade, including a M<sub>L</sub> 4.5 on 11/30/2018 near Dawson Creek. Here, we use a catalog of 8285 events ranging from magnitude ML -0.5 to 4.5 between July 2017 and July 2020 to investigate their source parameters. We identify event families using waveform cross-correlation and event temporal correlation, and estimate the focal mechanism solutions (FMS) of the highest-magnitude event within each family using the probabilistic earthquake source inversion framework </span><span><em>Grond</em></span><span>. We also estimate FMS for events with a magnitude larger than M</span><sub><span>L</span></sub><span> 2.5 that do not belong to a family (independent events). We compile a FMS catalog using the robustly constrained solutions for the largest events, and associate all smaller earthquakes with a cross-correlation coefficient (CCC) > 0.8 with the corresponding FMS. In addition, we estimate seismic moment and static stress drop values using spectral fitting methods on both single spectra and spectral ratios, and investigate their scaling relations.</span></p><p> </p><p><span>In total, we constrain 65 FMS, of which 53 are clustered events, and the remaining 12 are independent events. An additional 4255 events have a CCC > 0.8 with one of the constrained FMS and are listed accordingly in the catalog. Of the total 4320 FMS, 93% are strike-slip events with one nodal plane at low angles to S</span><sub><span>H</span></sub><span>, 3% are dominantly strike-slip with thrust-faulting components, and the remaining 4% have a dominantly thrust-faulting mechanisms perpendicular to S</span><sub><span>H. </span></sub><span> The thrust-style events comprise the relatively larger magnitudes contained in the catalog, and may indicate slip on pre-existing faults. Most strike-slip events are part of an event family with multiple matching waveforms, while most thrust-faulting events are isolated with a low number of matching waveforms. </span></p><p> </p><p><span>We fit the spectral corner frequency of 2360 P-phases and 1981 S-phases using single spectra estimates, and 1031 P-phases and 919 S-phases using the spectral ratios. While results from spectral ratios suggest a roughly constant stress drop of ~1 MPa for all magnitudes, the constant stress drop trend from single spectrum fitting breaks down at magnitudes smaller ~ M</span><sub><span>L</span></sub><span> 2.0, as has commonly been observed for events recorded by surface stations. We do not observe significant dependence of stress-drop values with the faulting style, nor with event depth. </span></p>

An upper crust shear-wave splitting study for the period 2013-2014 in the Western Gulf of Corinth (Greece)

<p>Seismic anisotropy is investigated by performing an upper crust shear-wave splitting study in the Western Gulf of Corinth (WGoC). The study area, which is a tectonic rift located in Central Greece, is one of the most seismically active regions in Europe, characterized by a 10 to 15 mm/year extension rate in a NNW-SSE direction and E-W normal faulting. Intense seismic activity has been recorded in the WGoC during 2013-2014, including the 2013 Helike swarm, at the southern coast, and the offshore 2014 seismic sequence between Nafpaktos and Psathopyrgos, including an Mw 4.9 event on 21 September 2014. The largest event of the study period was an Mw 5.0 earthquake that occurred in November 2014, offshore Aigion, followed by an aftershock sequence. Seismicity was relocated using the double-difference method, including waveform cross-correlation differential travel-time data, yielding a high-resolution earthquake catalogue of approximately 9000 local events. This dataset was utilized in order to determine the shear-wave splitting parameters in seven stations installed at the WGoC, using a fully automatic technique based on the eigenvalue method and cluster analysis. A smaller subset was analyzed with the visual inspection method (polarigrams and hodograms) for verification of the automatic measurements. All selected station-event pairs were within the shear-wave window and had adequately high signal-to-noise ratio. The orientation of the seismometers of all stations used in the present study has been measured and verified in order to ensure the validity of the obtained fast shear-wave polarization directions and to apply corrections for borehole instruments. Mean anisotropy directions are in general agreement with the horizontal component of the dominant stress field, with some deviations, likely related to mapped faults and local stress anomalies. Temporal variations of time-delays between the two split shear-waves are examined in order to investigate their connection to possible stress field variations, related either to the occurrence of moderate to strong events or to fluid migration.</p><p>Acknowledgements</p><p>We would like to thank the personnel of the Hellenic Unified Seismological Network (http://eida.gein.noa.gr/) and the Corinth Rift Laboratory Network (https://doi.org/10.15778/RESIF.CL) for the installation and operation of the stations used in the current article. The present research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning 2014-2020» in the context of the project “The role of fluids in the seismicity of the Western Gulf of Corinth (Greece)” (MIS 5048127).</p>

Discrimination of earthquakes and quarry blasts in Mecsek Mountain region (Hungary) and its vicinity by using a linear discrimination function

<p>Contamination of earthquake catalogues with anthropogenic events largely complicates seismotectonic interpretation. It is especially true for relatively low seismicity areas, such as Hungary. In the present study, we analyze the characteristics of earthquakes and blasts of quarries occurred between 2015 and 2020 in the Mecsek Mountains in southern Hungary within 120 km to MORH and KOVH stations.</p><p>The objective of this study was to determine the linear discrimination line between the two classes earthquakes and explosions. We investigated the effectiveness of P/S amplitude ratios using filtered waveforms at different ranges of frequencies. We applied waveform cross-correlation to build correlation matrices at both stations and performed hierarchical cluster analysis to identify event clusters. Because most of the quarry blasts were carried out by ripple-fire technology, we computed spectrograms and examined the spectral ratio between low and high frequencies and the steepness of spectra.</p><p>Classes of earthquakes and quarry blasts have separated well from each other by combining the amplitude ratio, waveform similarity and the different spectral methods. We compare the discrimination parameters and capability of both stations to identify the explosions in analyzed quarries that were misclassified as earthquakes in the Hungarian National Bulletins.</p>

Variations in Seismogenic Thickness Along the Central Alpine Fault, New Zealand, Revealed by a Decade's Relocated Microseismicity

©2018. American Geophysical Union. All Rights Reserved. The Alpine Fault is an oblique strike-slip fault that is known to fail in large magnitude (M7–8) earthquakes, yet it is currently seismically quiescent. We examine the low-magnitude earthquake activity occurring along the central portion of the Alpine Fault using seismic data from five temporary seismic networks deployed for various lengths of time between late 2008 and early 2017. Starting from continuous seismic data, we detect earthquake arrivals and construct the longest and most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes. This enables us to study the distribution and characteristics of the seismicity in unprecedented detail. Earthquake locations are constrained by high-quality automatic and manual picks, and we perform relocations using waveform cross-correlation to better constrain hypocenters. We have derived a new local magnitude scale calibrated by M w values. Magnitudes range between M L −1.2 and 4.6, and our catalog is complete above M L 1.1. Earthquakes mainly occur southeast of the Alpine Fault (in the hanging wall) and exhibit low magnitudes. We observe a lack of seismicity beneath Aoraki/Mount Cook, which we associate with high uplift rates and high heat flow. Seismogenic cutoff depths vary along the strike of the Alpine Fault from 8 km, beneath the highest topography, to 20 km in the adjacent areas.