Microearthquake Swarms Research Articles

Development of a Deep Neural Network Model for the Relocation of Mining-Induced Seismic Event

The precise relocation of seismic events is critical for many engineering projects. Swarms of minor or micro earthquakes typically reveal stress concentration and spots of greater seismic hazards. Particularly in the context of deep underground mining, advanced techniques that can accurately relocate microseismicities are urgently in demand. Here, we developed a neural network-based modeling training method that can precisely relocate seismicities and invert for velocities at the same time, with preconfigured receiver network locations. Our model can be iteratively improved with field recorded data. We showed that, with roughly eight iterations, we can reasonably resolve for the earthquake locations for both clusters of events, namely spatially distributed with linear pattern or randomly scattered. Our initially trained model, which only focused on events that had a linear distribution pattern, was used as the base for the training of the subsequent models which could better resolve for randomly scattered event locations. Although we stopped at the eighth iteration, the process reported here can be continued, as the model will have a better performance with more iterations.

Magmatic System and Seismicity of the Arxan Volcanic Group in Northeast China

AbstractThe Arxan volcanic group is one of the largest active volcanic fields in China. Its last eruption occurred ∼2,000 years ago, but it still has a potential of eruption in the future. The spatial distribution and migration channel of its magmatic reservoirs are still unclear. Here we precisely locate microearthquakes and study the detailed 3‐D velocity structure of the volcanic area using a large number of high‐quality P and S wave arrival‐time data recorded at 227 portable seismometers. A swarm of microearthquakes (M < 1.5) related to magmatic activity were recorded, which lasted about 4 days during 13–16 September 2020. Our tomographic results show that 12 Arxan volcanoes share the same magmatic system. The melt fraction of a magma chamber below the Tianchi volcano at ∼0–8 km depths is estimated to be ∼6.1%–8.6% from the seismic velocity reductions. Our results indicate that the Arxan magmatic system is still active at present.

Cyclic tidalites and seismites at a submarine hydrothermal system for a 2450 Ma banded iron formation, Hamersley Basin, Western Australia

The Hamersley Group in the Hamersley Basin, Western Australia, contains major deposits of banded iron formation of Neoarchean–Paleoproterozoic age (2629–2449 Ma). The 2450 Ma Weeli Wolli Formation near the top of the group includes banded iron formation with microbands (≤0.05–0.5 mm to rarely 1 mm thick) of chert and iron oxide lamina couplets that display distinctive cycles. Some adjacent, thin microbands have merged with compaction and the diagenetic removal of chert laminae, and maximum counts of cycle period by different studies is preferred, suggesting a period of ∼28 ± 2 microbands. Associated cyclic iron formation shows soft-sediment plastic deformation and brittle fracturing. Rare earth element geochemistry indicates that much of the iron in late Archean–early Paleoproterozoic banded iron formations was derived from submarine hydrothermal systems. Monitoring of hydrothermal vents on mid-ocean ridges and seamounts has identified tidal modulation of the discharge, temperature and dispersal of deep-sea hydrothermal plumes principally by semidiurnal variations in tidal loading and earth tides. Microearthquake swarms are associated with deep-sea hydrothermal systems at the crests of mid-ocean ridges, and tidal triggering of microearthquake activity is indicated for a hydrothermal system on the East Pacific Rise. The microband cycles in the Weeli Wolli Formation are interpreted as tidalites, comprising ∼28 ± 2 semidiurnal microbands per lunar (synodic) fortnightly cycle, related to the tidally modulated activity of a submarine hydrothermal system. The associated plastically deformed and brittle-fractured sediments are viewed as seismites resulting from microearthquakes also related to the hydrothermal system. The Weeli Wolli microband cycles and seismites may represent a rarely identified instance of iron-formation deposition near a submarine hydrothermal system. KEY POINTS Submarine hydrothermal venting was a major source of iron in late Archean–early Paleoproterozoic banded iron formations. Semidiurnal variations in tidal loading and earth tides commonly modulate present-day deep-sea hydrothermal activity, and microearthquake swarms are associated with deep-sea hydrothermal systems. Early Paleoproterozoic banded iron formation from the Hamersley Basin displays cycles with an estimated period of ∼28 ± 2 microbands and associated plastic deformation and brittle fracturing. The microband cycles are interpreted as tidalites comprising ∼28 ± 2 semidiurnal increments grouped in fortnightly cycles at a tidally modulated submarine hydrothermal system, and the deformed sediments as seismites caused by related microearthquake activity.

New insights into the High Agri Valley deep structure revealed by magnetotelluric imaging and seismic tomography (Southern Apennine, Italy)

We present an electrical resistivity model obtained from a 2D Magnetotelluric survey across a large sector of the Southern Apennine in the High Agri Valley (HAV), a NW-SE trending intra-mountain basin, with a high seismogenic potential. The intensive hydrocarbon exploitation (Val d'Agri oilfield) makes this area also affected by induced seismicity. In this HAV sector, the injection of salt-water in an unproductive disposal well (Costa Molina 2) causes localized swarms of microearthquakes; a second cluster of continuous induced seismicity is also observed SW of the Pertusillo Lake and it is associated to the seasonal fluctuations of the reservoir's water level.The major insight inferred from this study concerns a better understanding of the geological and tectonic framework in the HAV. The electrical resistivity model images the subsurface as conductive sedimentary sequences (Allochthonous Units) upon the carbonate Apulian Platform Unit characterized by higher resistivity values. Both these units appear composed of thrust-and-fold system deepening with larger wavelength anticlines N-E toward. Most of the structures identified in the magnetotelluric model are rather superficial and confined within the Allochthonous Units. A sudden break of the Apulian platform under the central part of the MT profile defines a conductive zone possibly associated to a major SW-dipping reverse fault or to several branches, as closely spaced thrust-sheets cutting eastern flanks of the Agri Valley.Additional information on the HAV deep structures comes from the joint interpretation of the resistivity model and a 3D seismic tomographic model obtained from the inversion of passive seismic data collected in the period 2002–2018. The availability of this elastic representation of the subsurface allowed us to perform a cluster analysis on the electrical resistivity and seismic P-wave velocity distribution within the subsoil. This joint quantitative interpretation unveiled new insights, otherwise hidden by individual models, on the subsurface structure distinguishing some rheological zones in terms of barriers and asperities.

Monsoonal rainfall induced shallow earthquake Swarm in the Amravati district of the central India

Sadrabadi and Zilphi villages of Dharni Taluka in Amravati district, Maharashtra, experienced small magnitude earthquakes between September 1, 2018 and December 9, 2018. We deployed a four station temporary network of three component broadband seismographs to understand and characterize the earthquake sequence. We find that the short lived (~2 months) micro-earthquake (M ≤ 2.4) swarm, accompanying rumbling sound, was highly clustered (3×3 km2) and occurred at extremely shallow depth (<0.4 km). Analysis of the data reveal that the earthquake swarm might have been induced by the percolation of monsoonal rainwaters (reaching ~100 mm/day) through the cracked volume of soil/rock, associated with the fault system of the Narmada Son failed rift region. Besides revealing the science of earthquake swarm and characterizing it, the monitoring of earthquake swarm and its analysis contributed significantly in reducing the public panic.

A tectono-geomorphological perspective of micro-earthquake swarm activity prone areas around Bhavnagar and Jamnagar in Saurashtra, western India

Areas around Bhavnagar and Jamnagar in eastern and northern parts of Saurashtra respectively, are identified previously as prone to recurrent earthquake swarm activity. Sparse seismotectonic studies so far have not revealed any linkage with known faults in Bhavnagar area while no structures are known to occur in the area around Jamnagar. We carried out detailed tectono-geomorphic studies with contrasting geomorphic settings with a view to provide an insight into the swarm activity from a geomorphological perspective. The study is based on extensive field studies and quantitative geomorphic analysis of landscape and drainage. The area around Bhavnagar is divisible into two tectono-geomorphic domains – the E-W trending Sihor Fault zone located to the west of Bhavnagar (domain-I) and the terrain to its south which shows undulating landscape controlled by N–S trending structural trends (domain-II). The quadrant in the SW between these two domains consists of southward titled trappean hills which form source area for drainages flowing northward through Sihor Fault zone and those flowing eastward through domain-II. Late Quaternary, possibly Holocene tectonic activity along the Sihor Fault is indicated by the northward tilt of alluvial plain surface, knickpoint in rivers and small scale minor faults along the fault zone, prominent decrease in depth of fluvial incision away from the fault zone and youthful landscape indicated by drainage basin analysis. Domain-II comprises the largely erosional pediplain developed over trappean and Tertiary rocks to the south of Bhavnagar. The drainages show an early stage of landscape development due to ongoing tectonic activity. The area around Jamnagar is dominantly a peneplain formed over subhorizontal trappean lava flows. The peneplain is a manifestation of the prolonged period of non-erosion and deposition during the Cenozoic. The rivers show youthful landscape in the central part of the peneplain which includes, large knickpoints, rapids, anomalous river bends, steepening of channel gradient and large potholes. This ENE-WSW trending zone of anomalous fluvial geomorphic anomalies points to the existence of a fault or fold in the subsurface. The structure appears to be active as indicated by the denser cluster of earthquake swarm along this zone. We infer that the generation of earthquake swarm activity comprising low to moderate magnitude shocks is caused by the sheared rocks along the Sihor Fault near Bhavnagar and the ENE-WSW trending subsurface structure near Lalpur which provide avenues for downward percolation of rainwater to deeper levels causing loss of frictional resistance along weak planes.

SH-waveform modeling of small local seismic events in Ladoga lake

A lot of seismic phenomena with small magnitude has been taking place in the Ladoga lake region (northwestern Russia). One of them is the earthquake which was recorded on the 31st of July, 2010 followed by the swarm of microearthquakes near Nikonovsky cape. There is only one station in the region that is why it is difficult to process the data. In this work, we propose a new approach to construct synthetic seismograms of the main event with minimum data. Also this approach applies us to calculate parameters of the source. The approximate magnitude of the main event is $$M_L=-0.8$$ . For swarm events, we have magnitude values from $$-\,1.0$$ to $$-\,2.6$$ . The data of swarm events (microearthquakes) is used to test the technique of constructing synthetic seismograms. The approach we propose is based on Green’s function method with using two temporal $$\delta$$ -sequences (the Lorentzian function and the continuous Dirichlet kernel). A comparison of synthetic and real seismograms shows that the Lorentzian function is better to describe small local events, and the continuous Dirichlet kernel is more appropriate for earthquakes. This approach can be generalized to various problems of different nature.

Transition from passive to pre-extrusion degassing of ascending andesitic magma before the lava dome-building eruption as derived from the seismic signals and tilt changes: Volcán de Colima, México, August–September 2004

Volcanoes emit volcanic gas during various types of activity. In the case of andesitic and dacitic volcanoes the high viscosity of silicic magma inhibits the release of volatiles. This increases the possibility that gas escape will occur explosively. The explosive emission of volatile constituents, presented in magmas, generates the seismic signals which may give information about the process of degassing at different stages of eruption. Transition between the eruptive episodes of lava dome-building at andesitic volcanoes represents the transition from passive degassing of the final stage of one episode to pre-extrusion degassing of the next episode. In this article we use the seismic parameters of volcanic explosions, volcanic tremor and micro-explosions, recorded at Volcán de Colima during August–September 2004, to describe the process of transition from passive to pre-extrusion degassing from episode 2 (May 2001–September 2004) to episode 3 (30 September 2004–January 2007). Variations in the tilt changes allowed us to identify three periods in development of this transition: period of passive degassing at the final stage of episode 2 (until 6 September) and two periods of pre-extrusive degassing at the initial stage of episode 3 defined by processes of inflation and deflation of volcanic structure from 6 to 30 September. Intensity of degassing of magma, expressed in the energy of volcanic explosions, decreased during transition from passive to pre-extrusive stages. During pre-extrusion periods of inflation and deflation of volcanic structure the depths of explosive events became shallower. Period of deflation was characterized also by a dominance of degassing processes occurring within the permeable cap of ascending magma and generating volcanic tremors and swarms of micro-earthquakes. Comparison of degassing of magma during pre-extrusive period and co-extrusive and post-extrusive periods showed the similarity in intensity of degassing of magma during the pre-extrusive and co-extrusive periods but a sharp increase in degassing intensity during the post-extrusive period of lava dome destruction with large explosions. It was shown also minor intensity of degassing processes just preceding the beginning of the lava dome building comparing with intensity of degassing processes just preceding the beginning of the lava dome destruction with great explosions.

The July 2019 Ridgecrest, California, Earthquake Sequence Recorded by Creepmeters: Negligible Epicentral Afterslip and Prolonged Triggered Slip at Teleseismic Distances

AbstractWe report sequential triggered slip at 271–384 km distances on the San Andreas, Superstition Hills, and Imperial faults with an apparent travel-time speed of 2.2 ± 0.1 km/s, following the passage of surface waves from the 4 July 2019 (17:33:49 UTC) Mw 6.4 and 6 July 2019 (03:19:53 UTC) Mw 7.1 Ridgecrest earthquakes. Slip on remote faults was not triggered instantaneously but developed over several minutes, increasing in duration with distance. Maximum slip amplitudes varied from 10 μm to 5 mm within minutes of slip nucleation, but on the southernmost San Andreas fault slip continued for two months and was followed on 16 September 2019 by a swarm of microearthquakes (Mw≤3.8) near Bombay Beach. These observations add to a growing body of evidence that fault creep may result in delayed triggered seismicity. Displacements across surface faults in the southern epicentral region and on the Garlock fault in the months following the Ridgecrest earthquakes were negligible (&amp;lt;1.1 mm), and they are interpreted to characterize surface strain adjustments in the epicentral region, rather than to result from discrete slip on surface faults.

Temporal variations of repeating low frequency volcanic earthquakes at Ngauruhoe Volcano, New Zealand

A persistent swarm of micro-earthquakes was observed at Ngauruhoe volcano, New Zealand from 2005 to 2010, marking the first unrest episode at the volcano since 1975. Detailed waveform and spectral analysis of the continuous seismic record revealed eight earthquake families (F1–F8) within a sequence of 8561 low-frequency earthquakes. An analysis of stacked waveforms of each family allowed the families to be linked systematically into four super-families that may indicate four different mechanisms, and/or source locations or geometries.The first super-family mostly occurred in 2005 and showed stable waveforms and no significant evolution. The second super-family was dominant over a two-year period starting in mid-2006 and displayed a slow evolution with progressively higher frequency. The third super-family emerged from early 2008 with intermittent occurrence and high seismicity, and progressed to higher peak frequency in 2010. The fourth super-family was mainly observed from late 2008 to mid-2009 and had the highest peak frequency in 2009.The observed evolutionary pattern may be driven by a modest multi-year input of gas and heat from a deeper relict magmatic zone into the shallow volcano hydrothermal system. The interplay of gas from below and seasonal meltwater from above may have modulated the seismic activity and may relate to repeated over-pressurization and failure of constricted evolving pathways within a pervasive crack system.

Characteristics of micro-earthquake swarms preceding eruptions at Showa crater of Sakurajima volcano, Japan

The Showa crater of Sakurajima, an andesitic volcano in Japan, resumed eruptive activity in June 2006 for the first time in 58 years. Explosive eruptions at the Showa crater have been active since 2009. Micro-earthquake swarms have sometimes been observed in the 1 or 2 h preceding these eruptions since 2012 and have frequently occurred during periods of low eruptive activity. These precursory micro-earthquakes are dominated by high-frequency (5–6 Hz) components and no clear S-wave and are thus classified as BH-type earthquake swarms. The hypocenters of the earthquakes are located in a shallow region (depth of approximately 1 km) near the Showa crater. Prior to the eruptions, the onset of inflation has been observed by extensometers, followed by precursory earthquake swarms 30 min to 1 h later. In the case of insufficient ground contraction, the precursory earthquakes reoccur when the strain reaches the same level it showed just before the eruptions occurred. From the occurrence period and pattern of the precursory earthquakes, it appears they may be generated within a narrow conduit leading to the Showa crater.

Detecting Real Earthquakes Using Artificial Earthquakes: On the Use of Synthetic Waveforms in Matched‐Filter Earthquake Detection

AbstractMatched‐filters are an increasingly popular tool for earthquake detection, but their reliance on a priori knowledge of the targets of interest limits their application to regions with previously documented seismicity. We explore an extension to the matched‐filter method to detect earthquakes and low‐frequency earthquakes on local to regional scales. We show that it is possible to increase the number of detections compared with standard energy‐based methods, with low false‐detection rates, using suites of synthetic waveforms as templates. We apply this to a microearthquake swarm and an aftershock sequence, and to detect low‐frequency earthquakes. We also explore the sensitivity of detections to the synthetic source's location and focal mechanism. Source‐receiver geometry has a first‐order control on how sensitive matched‐filter detectors are to variations in source location and focal mechanism, and this likely applies to detections made using both synthetic and real templates.

Spatial and Temporal Variations in Earthquake Stress Drop on Gofar Transform Fault, East Pacific Rise: Implications for Fault Strength

AbstractOn Gofar Transform Fault on the East Pacific Rise, the largest earthquakes (6.0 ≤ MW ≤ 6.2) have repeatedly ruptured the same portion of the fault, while intervening fault segments host swarms of microearthquakes. These long‐term patterns in earthquake occurrence suggest that heterogeneous fault zone properties control earthquake behavior. Using waveforms from ocean bottom seismometers that recorded seismicity before and after an anticipated 2008 MW 6.0 mainshock, we investigate the role that differences in material properties have on earthquake rupture at Gofar. We determine stress drop for 138 earthquakes (2.3 ≤ MW ≤ 4.0) that occurred within and between the rupture areas of large earthquakes. Stress drops are calculated from corner frequencies derived using an empirical Green's function spectral ratio method, and seismic moments are obtained by fitting the omega‐square source model to the low frequency amplitude of the displacement spectrum. Our analysis yields stress drops from 0.04 to 3.2 MPa with statistically significant spatial variation, including ~2 times higher average stress drop in fault segments where large earthquakes also occur compared to fault segments that host earthquake swarms. We find an inverse correlation between stress drop and P wave velocity reduction, which we interpret as the effect of fault zone damage on the ability of the fault to store strain energy that leads to our spatial variations in stress drop. Additionally, we observe lower stress drops following the MW 6.0 mainshock, consistent with increased damage and decreased fault strength after a large earthquake.

Focal mechanisms and size distribution of earthquakes beneath the Krafla central volcano, NE Iceland

AbstractSeismicity was monitored beneath the Krafla central volcano, NE Iceland, between 2009 and 2012 during a period of volcanic quiescence, when most earthquakes occurred within the shallow geothermal field. The highest concentration of earthquakes is located close to the rock‐melt transition zone as the Iceland Deep Drilling Project‐1 (IDDP‐1) wellbore suggests and decays quickly at greater depths. We recorded multiple swarms of microearthquakes, which coincide often with periods of changes in geothermal field operations, and found that about one third of the total number of earthquakes are repeating events. The event size distribution, evaluated within the central caldera, indicates average crustal values with b = 0.79 ± 0.04. No significant spatial b value contrasts are resolved within the geothermal field nor in the vicinity of the drilled melt. Besides the seismicity analysis, focal mechanisms are calculated for 342 events. Most of these short‐period events have source radiation patterns consistent with double‐couple (DC) mechanisms. A few events are attributed to non‐shear‐faulting mechanisms with geothermal fluids likely playing an important role in their source processes. Diverse faulting styles are inferred from DC events, but normal faulting prevails in the central caldera. The best fitting compressional and tensional axes of DC mechanisms are interpreted in terms of the principal stress or deformation rate orientations across the plate boundary rift. Maximum compressive stress directions are near‐vertically aligned in different study volumes, as expected in an extensional tectonic setting. Beneath the natural geothermal fields, the least compressive stress axis is found to align with the regional spreading direction. In the main geothermal field both horizontal stresses appear to have similar magnitudes causing a diversity of focal mechanisms.

Inferring fault mechanical conditions from the source parameters of a complex microseismic multiplet in the Corinth rift, Greece

AbstractWe develop a mechanical model of tight clusters of coplanar seismic asperities, to investigate a particular microearthquake swarm located at 8 km depth in the Corinth rift in Greece, which was active between 2001 and 2007. Although it is classified as a multiplet based on waveform similarity, this seismic sequence is much more complex than a repeating earthquake sequence and cannot be interpreted as the regular failure of a single asperity forced by surrounding aseismic creep. Here we suggest that such complex sequences could be generated by the failure of a set of coplanar asperities interacting in a small region of a fault segment. We show that in order to reproduce the dynamics of the observed sequence and the characteristics of the events, the cluster of asperities has to be located very close to an aseismically slipping fault segment, which could be an updip extension of the deep detachment zone in the rift, creeping at 1.5 cm/yr. For more general cases of coplanar clustered asperities, we show that the shape of the cumulative coseismic displacement pattern associated with the repeated failures of the asperities is strongly controlled by the behavior of the fault area surrounding the asperity cluster. In particular, if the cluster is part of a locked fault area, the resulting long‐term cumulative displacement is maximum at the center of the cluster. In contrast, an asperity cluster surrounded by aseismic creep leads to a uniform cumulative coseismic slip pattern. The ratio between cumulative slip at the center of the seismogenic patch and cumulative slip at its periphery could therefore be an indicator of the mechanical conditions prevailing on the fault. A systematic study of the source parameters of complex microseismic sequences could therefore provide insights into the mechanical state of active faults continuously generating microseismicity.

Analysis of the 2009 earthquake swarm near Sunset Crater volcano, Arizona

A swarm of microearthquakes occurred on October 31, 2009 within 5km of the Sunset Crater, Arizona, volcano. A detailed study of the swarm was warranted because of its location near a young volcanic construct and its proximity to the population center of Flagstaff, Arizona. The question posed in this study was whether the swarm was the result of tectonic stress release during fault slip, or due to stresses driven by magmatic processes. This question was addressed by analyzing and comparing the physical and seismic characteristics of the swarm to the regional tectonic environment and to the characteristics of tectonic swarms in Arizona and magmatic/volcanic swarms elsewhere. This analysis included swarm duration, frequency of events, b-value, focal depths and epicentral pattern of the swarm. The comparison of the salient features of the 2009 Sunset Crater swarm to both magmatic and tectonic swarms indicates that the Sunset Crater swarm has features similar to magmatic swarms and is a potential magmatic swarm candidate.

Seismic velocity constraints on the material properties that control earthquake behavior at the Quebrada‐Discovery‐Gofar transform faults, East Pacific Rise

Mid‐ocean ridge transform faults (RTFs) vary strongly along strike in their ability to generate large earthquakes. This general observation suggests that local variations in material properties along RTFs exert a primary control on earthquake rupture dynamics. We explore these relationships by examining the seismic structure of two RTFs that have distinctly different seismic coupling. We determine the seismic velocity structure at the Gofar and Quebrada faults on the East Pacific Rise (EPR) using P wave traveltime tomography with data from two active‐source wide‐angle refraction lines crossing the faults. We image low‐velocity zones (LVZs) at both faults, where P wave velocities are reduced by as much as 0.5–1.0 km/s (∼10–20%) within a several kilometer wide region. At the Gofar fault, the LVZ extends through the entire crust, into the seismogenic zone. We rule out widespread serpentinization as an explanation for the low velocities, owing to the lack of a corresponding signal in the locally measured gravity field. The reduced velocities can be explained if the plate boundary region is composed of fault material with enhanced fluid‐filled porosity (1.5–8%). Local seismic observations indicate that the high‐porosity region lies within a ∼10 km long portion of the fault that fails in large swarms of microearthquakes and acts as a barrier to the propagation of large (M ∼ 6.0) earthquakes. Tomographic images of fault structure combined with observed earthquake behavior suggest that EPR transform segments capable of generating large earthquakes have relatively intact gabbro within the seismogenic zone, whereas segments that slip aseismically or via earthquake swarms are composed of highly fractured, ≥2 km wide damage zones that extend throughout the crust.

Cascadia tremor spectra: Low corner frequencies and earthquake-like high-frequency falloff

The discovery of non-volcanic tremor (NVT) has opened a new window to observe major Earth plate boundaries. However, the spectral characteristics of NVT have not been well studied due to poor signal-to-noise ratio (SNR) on individual seismograms. We estimate the spectral content of Cascadia tremor between 2.5 and 20 Hz by suppressing noise using array analysis, and compute empirical path corrections using nearby small earthquakes. We demonstrate that the displacement spectra of the Cascadia tremor have corner frequencies around 3–8 Hz and fall off at f−2 to f−3 at higher frequencies. Our results have the following implications. (1) The high-frequency falloff of tremor agrees with the observations of regular earthquakes, suggesting that tremor can be analyzed using standard spectral models. Prior analyses that have shown a tremor spectral falloff proportional to f−1 may reflect only the spectral behavior over a limited frequency band. (2) Tremor may be no different from a swarm of microearthquakes with abnormally small stress drops on the order of kPa, likely due to the presence of fluids. Alternatively the low corner frequencies of tremor may reflect abnormally slow ruptures. (3) Fitting a standard Brune (1970) spectral model implies a moment release rate of Cascadia tremor of 3.8 × 1010 N·m/s assuming the tremor signals are P waves (or 1.4 × 1010 N·m/s assuming S-waves). This implies that a typical 20-day long tremor episode releases moment equivalent to Mw 5.1 (P-wave) or Mw 4.9 (S-wave), although these may be underestimates if the spectra deviate substantially from the Brune model at very low frequencies.

Dynamics of dyke intrusion in the mid-crust of Iceland

We have captured a remarkable sequence of microearthquakes showing progressive melt intrusion of a dyke moving upward from a sill at 18 km depth in the mid-crust of the northern volcanic rift zone in Iceland. Two-thirds of the earth's crust is created at mid-ocean rifts. Two-thirds of that crust is formed by intrusion and freezing before it erupts of molten rock generated within the underlying mantle. Here we show seismicity accompanying melt intrusion from 17.5 to 13.5 km depth along a dyke dipping at 50° in the mid-crust of the Icelandic rift zone. Although the crust at these depths is normally aseismic, high strain rates as melt intrudes generate microearthquakes up to magnitude 2.2. Moment tensor solutions show dominantly double-couple failure, with fault mechanisms sometimes flipping between normal and reverse faulting within minutes in the same location, but breaking along fault planes with the same orientations. We suggest several possible reasons for the flipping fault mechanisms: the breakage of solidified plugs of basalt within the dyke itself as more melt intrudes; intrusion along sub-parallel fractures or dykelet fingers into the local stress field created near the tip of a propagating dyke; or movement on small jogs or offsets between adjacent en echelon dykes. Although the faulting is caused ultimately by melt movement, there is no resolvable volumetric component in the moment tensor solutions. The inferred fault planes from microearthquakes align precisely with the overall plane of the dyke delineated by hypocentres. Melt injection occurs in bursts propagating at 2–3 m/min along channels c. 0.2 m thick, producing swarms of microearthquakes lasting several hours. Intervening quiescent periods last tens to hundreds of hours.

Relocation and characterization of the August 2009 microearthquake swarm above the Socorro magma body in the central Rio Grande Rift

Earthquake swarms, often attributed to magma or fluid movement in the uppermost crust, have differing statistics depending on tectonic setting, with volcanic swarms producing b‐values of up to 2.5 and continental rift swarms having b‐values ∼0.8‐1. Along the Rio Grande continental rift (RGR) in the western US, earthquake swarms have occurred in the area of the Socorro Magma Body (SMB), including the 2009 swarm of 431 events (magnitudes −1.0 to 2.5) over 26 days. We characterize the 2009 swarm to define spatial and temporal patterns to help understand the role of the SMB on the swarm. Our relocations suggest a small seismogenic volume of 34.5 km3 and focal mechanisms of the largest events are consistent with regional stress patterns. The b‐value of 0.9 is more characteristic of continental rift zones than volcanic swarms, suggesting more emphasis on the preexisting highly fractured crust rather than direct influence of magmatic movement.