Low-frequency Earthquakes Activity Research Articles

Deep learning detects uncataloged low-frequency earthquakes across regions

Documenting the interplay between slow deformation and seismic ruptures is essential to understand the physics of earthquakes nucleation. However, slow deformation is often difficult to detect and characterize. The most pervasive seismic markers of slow slip are low-frequency earthquakes (LFEs) that allow resolving deformation at minute-scale. Detecting LFEs is hard, due to their emergent onsets and low signal-to-noise ratios, usually requiring region-specific template matching approaches. These approaches suffer from low flexibility and might miss LFEs as they are constrained to sources identified a priori. Here, we develop a deep learning-based workflow for LFE detection and location, modeled after classical earthquake detection with phase picking, phase association, and location. Across three regions with known LFE activity, we detect LFEs from both previously cataloged sources and newly identified sources. Furthermore, the approach is transferable across regions, enabling systematic studies of LFEs in regions without known LFE activity.

Comparison of statistical low-frequency earthquake activity models

Slow earthquakes are slow fault slip events. Quantifying and monitoring slow earthquake activity characteristics are important, because they may change before large earthquakes occur. Statistical seismicity models are useful for quantifying seismicity characteristics. However, no standard statistical model exists for slow earthquake activity. This study used a high-quality catalog of low-frequency earthquakes (LFEs), a type of slow earthquake, in the Nankai subduction zone from April 2004 to August 2015 and conducted the first comparison of existing statistical LFE activity models to determine which model better describes LFE activity. Based on this comparison, this study proposes a new hybrid model that incorporates existing model features. The new model considers the LFE activity history in a manner similar to the epidemic-type aftershock sequence (ETAS) model and represents the LFE aftershock rate (subsequent LFE occurrence rate) with a small number of model parameters, as in the Omori–Utsu aftershock law for regular earthquakes. The results show that the proposed model outperforms other existing models. However, the new model cannot reproduce a feature of LFE activity: the sudden cessation of intense LFE bursts. This is because the new model superimposes multiple aftershock activities and predicts extremely high seismicity rates during and after the LFE bursts. I suggest that reproducing and successfully predicting the sudden cessation of intense LFE bursts is critical for the further improvement of statistical LFE activity models. In addition, the empirical equations formulated in this study for the LFE aftershock rates may be useful for future statistical and physical modeling of LFE activity.Graphical

Hidden Markov Models for Low-Frequency Earthquake Recurrence

Low-frequency earthquakes (LFEs) are small magnitude earthquakes with frequencies of 1–10 Hertz which often occur in overlapping sequence forming persistent seismic tremors. They provide insights into large earthquake processes along plate boundaries. LFEs occur stochastically in time, often forming temporally recurring clusters. The occurrence times are typically modeled using point processes and their intensity functions. We demonstrate how to use hidden Markov models coupled with visualization techniques to model inter-arrival times directly, classify LFE occurrence patterns along the San Andreas Fault, and perform model selection. We highlight two subsystems of LFE activity corresponding to periods of alternating episodic and quiescent behavior.

Complexity of Deep Low‐Frequency Earthquake Activity in Shikoku (Japan) Imaged From the Analysis of Continuous Seismic Data

AbstractWe derive an extensive catalog of low‐frequency earthquakes (LFEs) in western and central Shikoku applying an automatic, coherency‐based detection and location method to 4‐year long continuous data from selected seismic stations. Our catalog is the first LFE catalog in this area that does not rely on a priory templates and results from a homogeneous data processing scheme. It allows investigating in detail main characteristics of LFE activity in space and time. We observe clear heterogeneity of the LFE space distribution in the along‐strike direction of the subducting Philippine Sea plate corresponding to segments with different sizes, some of which are highly productive in LFE generation. More detailed statistical and correlation‐based analysis of LFE occurrence patterns allows quantification of the along‐strike segmentation and examination of the migration and event interaction during the LFE sequences and inter‐sequence periods. The analysis indicates that a strong interaction exists among LFE sources during slow slip events, but it otherwise varies significantly among the along‐strike segments. We suggest that the observed segmentation of LFE activity is related to static heterogeneity, such as structural property variations along the subduction interface, or dynamic heterogeneity, corresponding to memory‐dependent stress variations or possible fluid transients. We also confirm that regions with the highest LFE productivity correspond to spots of tectonic tremor triggered by teleseismic earthquakes' surface waves supporting a localized fluid‐rich environment with possible fluid transients. Our results illustrate how high‐resolution LFE catalog can contribute to the characterization and quantification of slow earthquake processes through detailed statistical analysis of their activity.

The Role of Fluid‐Related Heterogeneous Structures in Controlling the Fault Slip Behavior in the Slow‐Earthquake Source Region Along the Nankai Subduction Zone, Southwest Japan

AbstractA seismic array observation across the slow‐earthquake source region in the eastern Kii Peninsula, southwest Japan, reveal the detailed structure and seismicity along the upper boundary of the subducted Philippine Sea Plate. Tomography analysis and earthquake reflection imaging clarify the geometry of the upper boundary of the Philippine Sea Plate with a subduction angle of ∼15° at a depth range of 22–30 km. We observe intraslab low‐frequency earthquakes (LFEs) in and around the low‐Vp and high‐Vp/Vs zones, which are located above the intraslab earthquakes that are controlled by the dehydration embrittlement of serpentine. The intraslab LFE activity may be related to fluid movement. Fluids, which are derived from both dehydration of the serpentinized oceanic mantle and crustal eclogitization, may control the fault slip behavior. Therefore, the fluid behavior around the subducting plate boundary has an important role in controlling the downdip limit of the seismogenic zone.

Daily measurement of slow slip from low-frequency earthquakes is consistent with ordinary earthquake scaling.

Slow slip transients on faults can last from seconds to months and stitch together the earthquake cycle. However, no single geophysical instrument is able to observe the full range of slow slip because of bandwidth limitations. Here, we connect seismic and geodetic data from the Mexican subduction zone to explore an instrumental blind spot. We establish a calibration of the daily median amplitude of the seismically recorded low-frequency earthquakes to the daily geodetically recorded moment rate of previously established slow slip events. This calibration allows us to use the precise evolution of low-frequency earthquake activity to quantitatively measure the moment of smaller, subdaily slip events that are unresolvable by geodesy alone. The resulting inferred slow slip moments scale with duration and inter-event time like ordinary earthquakes. These new quantifications help connect slow and fast events in a broad spectrum of transient slip and suggest that slow slip events behave much like ordinary earthquakes.

Focal Mechanisms of Deep Low‐Frequency Earthquakes Beneath Zao Volcano, Northeast Japan, and Relationship to the 2011 Tohoku Earthquake

AbstractSmall, deep low‐frequency earthquakes (LFEs) with dominant frequencies of 2–8 Hz occur at depths of 20–40 km and are thought to be related to movement of magma or crustal fluid, but their physical source processes remain largely unknown. Therefore, we determine the focal mechanisms of LFEs beneath Zao volcano, Northeast Japan, using the S/P amplitude ratio. Most focal mechanisms are classified into five groups: three types of double couples, a compensated linear vector dipole group, and a single force group. Double couples in 2007–2012 are consistent with those expected for the regional stress field, but no such events have been observed since 2013. This transition in focal mechanisms was simultaneous with a rapid increase in LFE activity beneath Zao. Our results suggest that LFEs beneath Zao were controlled mainly by the local stress field, but the stress field changed about two years after the 2011 Tohoku earthquake.

Spatial and temporal distributions of b-values related to long-term slow-slip and low-frequency earthquakes in the Bungo Channel and Hyuga-nada regions, Japan

Long-term slow-slip events (SSEs) and low-frequency earthquakes (LFEs) repeatedly occur in the area encompassing the Bungo Channel and Hyuga-nada region in Japan. SSEs are slow earthquake events with a duration of months to years, characterized by a type of slip that is considered transitional between the fast rupture of regular earthquakes and stable sliding along a plate interface, and have characteristics sensitive to stress states near the plate interface. In addition, since LFEs are also thought to occur at the plate interface, it would be useful to infer the stress state in this region. This study focuses on the stress-dependent characteristics of the b-values of regular earthquakes, SSEs, and LFEs, as well as the spatio-temporal relationships between b-values and these events in this area, using the Japan Unified high-resolution relocated Catalog for Earthquakes (JUICE). The b-value, which represents the relative proportion of large to small earthquakes, is inversely related to differential stress or effective stress. The b-values are moderate to low (b = 0.6–1.1) in regions with a high incidence of long-term SSEs. This spatial b-value anomaly is attributed to the partial release of accumulated stress in regions with a relatively high differential stress due to SSE activity. Furthermore, the b-value distributions estimated in this study provide constraints on the mechanisms that generate LFEs. On the other hand, the b-values increase and decrease before and during periods of enhanced LFE activity in the Bungo Channel, respectively. The increase and following decrease in b-values are thought to reflect episodes of low and high shear strength along the plate interface, which were caused by the presence of abundant slab-derived fluids under undrained conditions above the plate interface, and decreased pore pressure related to fluid depletion via LFE generation, respectively.

Variations in Cascadia Low‐Frequency Earthquake Behavior With Downdip Distance

AbstractWe identify and characterize four different low‐frequency earthquake (LFE) families (LFEs 1–4, ordered updip to downdip) that span the width of the transition zone in the Cascadia Subduction Zone beneath western Washington State. We find that LFE swarm duration, recurrence interval, and event size decrease systematically with increasing depth. LFE moments are observed to follow an exponential distribution rather than a power law distribution, allowing us to determine a characteristic event size for each family. Absolute locations of these LFE families place them very near the inferred Juan de Fuca‐North America plate interface. Relative relocation of individual LFEs within each family reveal elongation parallel to the plate convergence direction and narrow depth distributions with dips corresponding to plate interface models. We interpret the behavior of our LFE families as indicative of slow slip at those locations on the plate interface. The tidal response of LFE activity at LFEs 1, 2, and 3 during a large episodic tremor and slip (ETS) event follows a previously observed evolution, in which tidal response during the initial 1 to 2 days of slip at an LFE patch is much weaker than that during the next few days of slip. The brief bursts of activity at LFE4, although little affected by large episodic tremor and slip events, respond strongly to tidal stresses. Our observed along‐dip contrasts in recurrence behavior and tidal response provide evidence for stress transfer and increased frictional strength with position updip through the tremor zone.

Imbricated slip rate processes during slow slip transients imaged by low-frequency earthquakes

Low Frequency Earthquakes (LFEs) often occur in conjunction with transient strain episodes, or Slow Slip Events (SSEs), in subduction zones. Their focal mechanism and location consistent with shear failure on the plate interface argue for a model where LFEs are discrete dynamic ruptures in an otherwise slowly slipping interface. SSEs are mostly observed by surface geodetic instruments with limited resolution and it is likely that only the largest ones are detected. The time synchronization of LFEs and SSEs suggests that we could use the recorded LFEs to constrain the evolution of SSEs, and notably of the geodetically-undetected small ones. However, inferring slow slip rate from the temporal evolution of LFE activity is complicated by the strong temporal clustering of LFEs. Here we apply dedicated statistical tools to retrieve the temporal evolution of SSE slip rates from the time history of LFE occurrences in two subduction zones, Mexico and Cascadia, and in the deep portion of the San Andreas fault at Parkfield. We find temporal characteristics of LFEs that are similar across these three different regions. The longer term episodic slip transients present in these datasets show a slip rate decay with time after the passage of the SSE front possibly as t−1/4. They are composed of multiple short term transients with steeper slip rate decay as t−α with α between 1.4 and 2. We also find that the maximum slip rate of SSEs has a continuous distribution. Our results indicate that creeping faults host intermittent deformation at various scales resulting from the imbricated occurrence of numerous slow slip events of various amplitudes.

Insights into the causal relationship between slow slip and tectonic tremor in Guerrero, Mexico

AbstractSimilar to other subduction zones, tectonic tremors (TTs) and slow‐slip events (SSEs) take place in the deep segment of the plate interface in Guerrero, Mexico. However, their spatial correlation in this region is not as clear as the episodic tremor and slip observed in Cascadia and Japan. In this study we provide insights into the causal relationship between TTs and SSEs in Guerrero by analyzing the evolution of the deformation fields induced by the long‐term 2006 SSE together with new locations of TTs and low‐frequency earthquakes (LFEs). Unlike previous studies we find that the SSE slip rate modulates the TT and LFE activity in the whole tremor region. This means that the causal relationship between the SSE and the TT activity directly depends on the stressing rate history of the tremor asperities that is modulated by the surrounding slip rate. We estimated that the frictional strength of the asperities producing tremor downdip in the sweet spot is around 3.2 kPa, which is ~2.3 times smaller than the corresponding value updip in the transient zone, partly explaining the overwhelming tremor activity of the sweet spot despite that the slow slip there is smaller. Based on the LFE occurrence‐rate history during the interlong‐term SSE period, we determined that the short‐term SSEs in Guerrero take place further downdip (about 35 km) than previously estimated, with maximum slip of about 8 mm in the sweet spot. This new model features a continuum of slow slip extending across the entire tremor region of Guerrero.

Slow slip hidden in the noise: The intermittence of tectonic release

AbstractReferred to as slow slip events, the transient aseismic slip that occurs along plate boundaries can be indirectly characterized through colocated seismicity, such as tectonic tremor and low‐frequency earthquakes (LFEs). Using the timing of cataloged LFE and tremor activity in Guerrero, Mexico, and northern Cascadia, I decompose the interaseismic GPS displacement, defined as the surface deformation between previously detected slow slip events, into separate regimes of tectonic loading and release. In such a way, previously undetected slow slip events that produce less than a millimeter of surface deformation are extracted from the geodetic noise. These new observations demonstrate that the interaseismic period is not quiescent and that slow slip occurs much more often than previously thought. This suggests that the plate interface where slow slip and tremor occur is in fact strongly coupled and that slow aseismic release occurs over a wide spectrum of time scales.

Magnitudes and moment‐duration scaling of low‐frequency earthquakes beneath southern Vancouver Island

AbstractWe employ 130 low‐frequency earthquake (LFE) templates representing tremor sources on the plate boundary below southern Vancouver Island to examine LFE magnitudes. Each template is assembled from hundreds to thousands of individual LFEs, representing over 269,000 independent detections from major episodic‐tremor‐and‐slip (ETS) events between 2003 and 2013. Template displacement waveforms for direct P and S waves at near epicentral distances are remarkably simple at many stations, approaching the zero‐phase, single pulse expected for a point dislocation source in a homogeneous medium. High spatiotemporal precision of template match‐filtered detections facilitates precise alignment of individual LFE detections and analysis of waveforms. Upon correction for 1‐D geometrical spreading, attenuation, free surface magnification and radiation pattern, we solve a large, sparse linear system for 3‐D path corrections and LFE magnitudes for all detections corresponding to a single‐ETS template. The spatiotemporal distribution of magnitudes indicates that typically half the total moment release occurs within the first 12–24 h of LFE activity during an ETS episode when tidal sensitivity is low. The remainder is released in bursts over several days, particularly as spatially extensive rapid tremor reversals (RTRs), during which tidal sensitivity is high. RTRs are characterized by large‐magnitude LFEs and are most strongly expressed in the updip portions of the ETS transition zone and less organized at downdip levels. LFE magnitude‐frequency relations are better described by power law than exponential distributions although they exhibit very high b values ≥∼5. We examine LFE moment‐duration scaling by generating templates using detections for limiting magnitude ranges (MW<1.5, MW≥2.0). LFE duration displays a weaker dependence upon moment than expected for self‐similarity, suggesting that LFE asperities are limited in fault dimension and that moment variation is dominated by slip. This behavior implies that LFEs exhibit a scaling distinct from both large‐scale slow earthquakes and regular seismicity.

Using systematically characterized low‐frequency earthquakes as a fault probe in Guerrero, Mexico

AbstractStudies of low‐frequency earthquakes (LFEs) have focused on detecting events within previously identified tectonic tremor. However, the principal LFE detection tools of matched‐filter searches are intrinsically incapable of detecting events that have not already been characterized previously as a template event. In this study, we therefore focus on generating the largest number possible of LFE templates by uniformly applying a recently developed LFE template detection method to a 2.5 yearlong data set in Guerrero, Mexico. Using each of the detected templates in a matched‐filter search, we then form event families that each represents a single source. We finally develop simple, empirical statistics to select the event families that represent LFEs. Our resulting catalog contains 1120 unique LFE sources and a total of 1,849,486 detected LFEs over the 2.5 yearlong data set. The locations of the LFE sources are then divided into subcatalogs based on their distance from the subduction trench. Considering each LFE as a small unit of slip along the subduction interface, we observe discrete episodes of LFE activity in the region associated with large slow‐slip events; this is in direct contrast to the near‐continuous activity observed 35 km farther downdip within the previously identified LFE/tremor sweet spot.

Low‐frequency earthquakes reveal punctuated slow slip on the deep extent of the Alpine Fault, New Zealand

AbstractWe present the first evidence of low‐frequency earthquakes (LFEs) associated with the deep extension of the transpressional Alpine Fault beneath the central Southern Alps of New Zealand. Our database comprises a temporally continuous 36 month‐long catalog of 8760 LFEs within 14 families. To generate this catalog, we first identify 14 primary template LFEs within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross‐correlation routine. The hypocentres of 12 of the 14 LFE families lie within 10 km of the inferred location of the Alpine Fault at depths of approximately 20–30 km, in a zone of high P‐wave attenuation, low P‐wave speeds, and high seismic reflectivity. The LFE catalog consists of persistent, discrete events punctuated by swarm‐like bursts of activity associated with previously and newly identified tremor periods. The magnitudes of the LFEs range between ML – 0.8 and ML 1.8, with an average of ML 0.5. We find that the frequency‐magnitude distribution of the LFE catalog both as a whole and within individual families is not consistent with a power law, but that individual families' frequency‐amplitude distributions approximate an exponential relationship, suggestive of a characteristic length‐scale of failure. We interpret this LFE activity to represent quasi‐continuous slip on the deep extent of the Alpine Fault, with LFEs highlighting asperities within an otherwise steadily creeping region of the fault.

Focal mechanisms of deep low‐frequency earthquakes in Eastern Shimane in Western Japan

AbstractDeep low‐frequency earthquakes (LFEs) are small earthquakes (M < 2) that occur at depths of ~10–45 km with seismic wave radiation mainly in the frequency range of 2–8 Hz. LFEs in western Japan are categorized into tectonic LFEs in the Nankai subduction zone, volcanic LFEs beneath active volcanoes, and semi‐volcanic LFEs that occur far from active volcanoes but are otherwise similar to volcanic LFEs. While tectonic LFEs are considered to reflect shear slip on a plate boundary, the mechanisms of volcanic and semi‐volcanic LFEs remain unclear. We have determined the mechanisms of 38 semi‐volcanic LFEs in eastern Shimane, which is the site of the most frequent semi‐volcanic LFE activity in Japan. For each event, velocity seismograms at five stations were inverted into a focal mechanism and moment rate function by a combined grid search and linear inversion analysis. Synthetic waveforms were calculated for a one‐dimensional structure and the local site amplification effects were corrected using body waves from deep earthquakes. The estimated moment rate function was found to often oscillate between positive and negative values, which are unlike those of regular earthquakes. The focal mechanisms for many LFEs are dominated by a compensated linear vector dipole with symmetry axes parallel to the lineation formed by the hypocenter distribution and the direction of the minimum principal axis of regional stress.

Evidence for episodic aseismic slip across the subduction seismogenic zone off Costa Rica: CORK borehole pressure observations at the subduction prism toe

Slow slip events, or “silent” earthquakes, may relieve a significant amount of stress at many subduction plate boundaries, both downdip of the limit of seismogenesis, and within the seismogenic zone itself in cases where seismic energy release accounts for only a fraction of the plate tectonic displacement rate (Schwartz and Rokosky, 2007). Slow slip has been identified in several instances downdip of the landward limit of the seismogenic zone and is often accompanied by seismic tremor or low-frequency earthquake activity along and above the plate interface (referred to as “episodic tremor and slip”, or ETS). Little is known, however, about the spatial distribution and history of slip between great earthquakes along the seismogenic thrust interface itself which lies mostly offshore. In this article we present formation pressure transients observed in two deep-sea boreholes near the toe of the subduction prism off Nicoya Peninsula, Costa Rica, which followed ETS events observed on shore by 1–2 weeks. The signatures of the transients are consistent with local slip on the shallow part of the thrust interface, with the underthrusting plate experiencing relaxation and the outer prism experiencing contraction. The delay between the tremor activity and the pressure transients observed c. 100 km seaward at the prism toe suggests either slow propagation across the seismogenic zone or delayed deformation at the outer part of the prism triggered by the slip beneath Nicoya. Such slip may serve generally to relieve stress at subduction zones, but also to increase stress in parts of the plate boundary where interseismic slip does not occur.

Observations of volcanic tremor during the January–February 2005 eruption of Mt. Veniaminof, Alaska

Mt. Veniaminof, Alaska Peninsula, is a stratovolcano with a summit ice-filled caldera containing a small intracaldera cone and active vent. From January 2 to February 21, 2005, Mt. Veniaminof erupted. The eruption was characterized by numerous small ash emissions (VEI 0 to 1) and accompanied by low-frequency earthquake activity and volcanic tremor. We have performed spectral analyses of the seismic signals in order to characterize them and to constrain their source. Continuous tremor has durations of minutes to hours with dominant energy in the band 0.5–4.0 Hz, and spectra characterized by narrow peaks either irregularly (non-harmonic tremor) or regularly spaced (harmonic tremor). The spectra of non-harmonic tremor resemble those of low-frequency events recorded simultaneously with surface ash explosions, suggesting that the source mechanisms might be similar or related. We propose that non-harmonic tremor at Mt. Veniaminof results from the coalescence of gas bubbles while low-frequency events are related to the disruption of large gas pockets within the conduit. Harmonic tremor, characterized by regular and quasi-sinusoidal waveforms, has duration of hours. Spectra containing up to five harmonics suggest the presence of a resonating source volume that vibrates in a longitudinal acoustic mode. An interesting feature of harmonic tremor is that frequency is observed to change over time; spectral lines move towards higher or lower values while the harmonic nature of the spectra is maintained. Factors controlling the variable characteristics of harmonic tremor include changes in acoustic velocity at the source and variations of the effective size of the resonator.