Subduction Zone Events Research Articles

Debris Avalanches in the Northern California Coast Range Triggered by Plate-Boundary Earthquakes

ABSTRACT Determining the timing and cause for ancient hillslope failures is challenging in the western United States, yet it is critical because it can inform ground-motion estimates for large-to-great earthquakes on plate-boundary faults. We identify two preinstumental debris avalanches (3×106 and 4.28×106 m3) in the Coast Ranges of northern California. These debris avalanches are well suited for studying the timing (to the exact year) and trigger, as the densely forested landscape enables effective use of dendrochronology and high-resolution radiocarbon to pinpoint the time of failure, and the legacy of landslide studies in the region provides context for evaluating a seismic trigger as the most likely triggering mechanism. Neither of the debris avalanches was triggered by the largest precipitation event of the instrumental record in 1964 C.E., and the two debris avalanche sites are physiographically suited to experience topographic amplification of seismic shaking. Through a suite of geologic and dendrochronologic evidence, we establish the likely time of failure of the two debris avalanches as 1906 C.E. and 933 C.E. The failure of the younger landslide coincides with the 1906 C.E. San Francisco earthquake on the Pacific/North American plate boundary, whereas the older landslide dates to 933 C.E., within the broad age range (850–966 C.E.) of a likely megathrust earthquake on the Cascadia subduction zone as recorded in coastal marshes in Humboldt Bay, California. If the older landslide was triggered by a subduction earthquake, then the precise age of the 933 C.E. debris avalanche would trim the uncertainty on a Cascadia subduction zone event from 116 yr to 1 yr. Utilizing debris avalanche records from sites suitable to record seismic shaking improves understanding of plate-boundary earthquake timing and extent of shaking.

Unraveling Earthquake Clusters Composing the 2014 Alto Tiberina Earthquake Swarm via Unsupervised Learning

AbstractEarthquake swarms represent a particular mode of seismicity, not directly related to the occurrence of large earthquakes (e.g., aftershocks) but rather driven by external forcing such as aseismic deformation or fluid migration in fault systems. Sometimes their occurrence overlaps with observable geodetic signals in space and time, indicating a direct link. However, the low resolution of geodetic observations tends to obscure the small scale spatial and temporal dynamics of swarms. In this work, we automatically extract clusters of seismicity related to the 2014 Alto Tiberina swarm sequence (Italy) using an unsupervised clustering approach that exploits space and time information of the seismicity. The quantitative characterization of each cluster indicates that the overall swarm is composed of spatially and temporally confined (sub) swarms each of which could potentially be driven by small‐scale aseismic deformation process. This observation aligns with similar findings during slow slip events in subduction zones.

Characteristics of ring-shaped seismicity at depths up to 110 km prior to large and great earthquakes in subduction zones of the Pacific

We have been considering some seismicity characteristics at depths up to 110 km prior to 38 large and great earthquakes (Mw=7.0-9.0) in subduction zones of the Pacific. We revealed ring-shaped seismicity structures in three depth ranges: 0-33, 34-70 and 71-110 km. These structures are being formed during a few decades prior to large and great events with hypocenters at depths of 0-40 and 42-110 km. We call these events conditionally as shallow and deep ones correspondingly. The structures are characterized by threshold magnitude values: Mt1, Mt2 and Mt3 respectively. We analyzed differences of Mt1-Mt2, Mt2-Mt3 and Mt1-Mt3 values. It was established that parameters Mt2-Mt3 and Mt1-Mt3 are higher considerably for large shallow earthquakes in comparison to deep ones. Besides that, we found differences of mean Mt1-Mt2 values at the west and east of Pacific. We discuss the reasons of ring-shaped structures formation which most likely are connected with dehydration of the subducting plate material and deep-seated fluid migration. We estimate роssibilities of depths prognosis for preparing large earthquakes using characteristics of ring-shaped seismicity structures. The data obtained are essential for shaking intensity forecast and also for tsunami danger estimate prior to large and great events in subduction zones.

Probabilistic seismic damage and loss assessment methodology for wastewater network incorporating modeling uncertainty and damage correlations

Maintaining the functionality of wastewater networks is critical to individual well-being, business continuity, public health, and safety. However, seismic damage and loss assessments of wastewater networks traditionally use fragility functions based on median repair rates without considering relevant sources of uncertainty and correlations of damage when estimating potential damage states and pipe repairs. This study presents a probabilistic methodology to incorporate modeling uncertainty (e.g. model parameter and model class uncertainty) and spatial correlations (e.g. spatial auto- and cross-correlation) of pipe repairs. The methodology was applied to a case study backbone system of a wastewater network in Portland, OR, using the expected hazard intensity maps for multiple deterministic earthquake scenarios, including a moment magnitude M6.8 Portland Hills Fault and M8.1, M8.4, M8.7, and M9.0 Cascadia Subduction Zone (CSZ) events. As spatial-correlation models of pipeline damage were non-existent in the literature and local information on costs to repair the pipes was limited at the time of this study, correlation methods and repair costs were proposed to estimate lower and upper bounds of pipe damage and loss. The results show how the consideration of different levels of uncertainty and spatial correlation for pipe repair rate could lead to different probabilistic estimates of damage and loss at the system level of the wastewater network, even though the point estimates, such as the mean and median, remain essentially unaltered.

Geological records of transient fluid drainage into the shallow mantle wedge.

Pore fluid pressure on subduction zone megathrusts is lowered by fluid drainage into the overlying plate, affecting subduction zone seismicity. However, the spatial and temporal scales of fluid flow through suprasubduction zones are poorly understood. We constrain the duration and velocity of fluid flow through a shallow mantle wedge based on the analyses of vein networks consisting of high-temperature serpentine in hydrated ultramafic rocks from the Oman ophiolite. On the basis of a diffusion model and the time-integrated fluid flux, we show that the channelized fluid flow was short-lived (2.1×10-1 to 1.1×101years) and had a high fluid velocity (2.7×10-3 to 4.9×10-2meters second-1), which is close to the propagation velocities of seismic events in present-day subduction zones. Our results suggest that the drainage of fluid into the overlying plate occurs as episodic pulses, which may influence the recurrence of megathrust earthquakes.

Influence of Creep Compaction and Dilatancy on Earthquake Sequences and Slow Slip

AbstractFluids influence fault zone strength and the occurrence of earthquakes, slow slip events, and aseismic slip. We introduce an earthquake sequence model with fault zone fluid transport, accounting for elastic, viscous, and plastic porosity evolution, with permeability having a power‐law dependence on porosity. Fluids, sourced at a constant rate below the seismogenic zone, ascend along the fault. While the modeling is done for a vertical strike‐slip fault with 2D antiplane shear deformation, the general behavior and processes are anticipated to apply also to subduction zones. The model produces large earthquakes in the seismogenic zone, whose recurrence interval is controlled in part by compaction‐driven pressurization and weakening. The model also produces a complex sequence of slow slip events (SSEs) beneath the seismogenic zone. The SSEs are initiated by compaction‐driven pressurization and weakening and stalled by dilatant suctions. Modeled SSE sequences include long‐term events lasting from a few months to years and very rapid short‐term events lasting for only a few days; slip is ∼1–10 cm. Despite ∼1–10 MPa pore pressure changes, porosity and permeability changes are small and hence fluid flux is relatively constant except in the immediate vicinity of slip fronts. This contrasts with alternative fault valving models that feature much larger changes in permeability from the evolution of pore connectivity. Our model demonstrates the important role that compaction and dilatancy have on fluid pressure and fault slip, with possible relevance to slow slip events in subduction zones and elsewhere.

Do upper-plate material properties or fault frictional properties play more important roles in tsunami earthquake characteristics?

Tsunami earthquakes are a type of shallow subduction zone events that rupture slowly (<1.5 km/s) with exceptionally long duration and depleted high frequency radiation, resulting in a large discrepancy of Mw and Ms magnitudes and abnormally large tsunami along coastal areas. Heterogeneous fault frictional properties at shallow depth have been thought to dominate tsunami earthquake generation. Some recent studies propose heterogeneous upper-plate material properties determine rupture behavior of megathrust earthquakes, including characteristics of tsunami earthquakes. In this study, we use a recently developed 3D dynamic earthquake simulator to explore tsunami earthquake generation and systematically examine roles of upper-plate material properties and fault frictional properties in tsunami earthquake characteristics in a physics-based framework. For heterogeneous fault friction, we consider isolated asperities with strongly velocity-weakening properties embedded in a conditionally stable zone with weakly velocity-weakening properties. For heterogeneous upper-plate properties, we consider a generic depth profile of seismic velocity and rigidity constrained from seismic surveys. We design a set of models to explore their effects on tsunami earthquake generation and characteristics. We find that the conditionally stable zone can significantly slow down rupture speeds of earthquakes that nucleate on asperities to be <1.5 km/s over a large depth range (1–20 km), while heterogeneous upper-plate properties can only reduce rupture speeds to be ∼1.5–2.0 km/s over a narrow depth range (1-3 km). Nevertheless, heterogeneous upper-plate properties promote cascading rupture over multiple isolated asperities on the shallow subduction plane, contributing to large tsunami earthquake generation. We also find that fault frictional properties play much more important roles than upper-plate material properties in long normalized duration, high-frequency depletion and low moment-scaled radiated energy in tsunami earthquakes. In addition, the effective normal stress on the subduction plane, which affects fault frictional strength, also influences the characteristics of tsunami earthquakes, including duration, stress drop and moment-scaled radiated energy.

Dilatancy Toughening of Shear Cracks and Implications for Slow Rupture Propagation

AbstractDilatancy associated with fault slip produces a transient pore pressure drop which increases frictional strength. This effect is analyzed in a steadily propagating rupture model that includes frictional weakening, slip‐dependent fault dilation and fluid flow. Dilatancy is shown to increase the stress intensity factor required to propagate the rupture tip. With increasing rupture speed, an undrained (strengthened) region develops near the tip and extends beyond the frictionally weakened zone. Away from the undrained region, pore fluid diffusion gradually recharges the fault and strength returns to the drained, weakened value. For sufficiently large rupture dimensions, the dilation‐induced strength increase near the tip is equivalent to an increase in toughness that is proportional to the square root of the rupture speed. In general, dilation has the effect of increasing the stress required for rupture growth by decreasing the stress drop along the crack. Thermal pressurization has the potential to compensate for the dilatant strengthening effect, at the expense of an increased heating rate, which might lead to premature frictional melting. Using reasonable laboratory parameters, the dilatancy‐toughening effect leads to rupture dynamics that is quantitatively consistent with the dynamics of observed slow slip events in subduction zones.

Migration-Based Local Event-Location Workflow for Ocean-Bottom Seismometer (OBS) Records in Subduction Zones: A Practical Approach for Addressing a Large Number of Events

ABSTRACT An efficient event-location workflow is highly desired to analyze large numbers of local earthquakes recorded by ocean-bottom seismometers (OBSs) in subduction zones. The present study proposes a migration-based event-location approach for evaluating OBS records to examine local subduction-zone earthquakes. This approach can significantly reduce the amount of manual time picks compared with conventional methods. The event-location workflow was designed to detect arrival onsets of both P and S phases. Synthetic tests have shown that the proposed migration-based event-location method is robust against different types of noise, such as environmental noise and local spike noise. This workflow was then applied to real OBS data in the off-Ibaraki region at the southern end of the Japan trench. The results show that this approach is applicable to real data from subduction-zone events: It gives reasonable agreement with manual time picks for both P and S waves and reasonable error bars, and it demonstrates a clear down-dip trend of seismicity. The results also show fair agreement with event distributions from previous studies of the off-Ibaraki region. This proposed workflow can be used to examine the seismicity of local earthquakes around the subduction zone using OBSs. This approach is especially effective when the seismicity is high and/or in cases in which long-term OBS monitoring has recorded a large number of events.

Ring-shaped seismicity structures, being formed in the Alaska region: Justi-fied prediction of the place and magnitude of the Simeonof earthquake of July 22, 2020 (Mw 7.8)

We have been studying some seismicity characteristics connected with Simeonof earthquake of July 22, 2020 (Mw 7.8) in the area of Shumagin seismic gap, located in the region of South Alaska. 8 years before this event, the authors picked up here ring-shaped seismicity structure. Using characteristics of this structure we made a conclusion on preparation for large earth-quake (Mw~8), similarly to many events in subduction zones. We used data on threshold magnitude and size of the ring structure, and earlier obtained dependences of these parame-ters on magnitudes of main events for the subduction zones in the Eastern Pacific. Accumula-tion of new seismicity data prior to 2020/07/22 allowed us to specify the source location and magnitude of the Simeonof earthquake: Mw=7.9±0.3. It was shown that after this event new ring structures continued to form, which can correspond to preparation for larger earthquake: Mw=8.2±0.2. Geodynamical processes, which lead to formation of the ring structures, are discussing.

Ocean-bottom and surface seismometers reveal continuous glacial tremor and slip

Shearing along subduction zones, laboratory experiments on analogue faults, and sliding along glacier beds are all associated with aseismic and co-seismic slip. In this study, an ocean-bottom seismometer is deployed near the terminus of a Greenlandic tidewater glacier, effectively insulating the signal from the extremely noisy surface seismic wavefield. Continuous, tide-modulated tremor related to ice speed is recorded at the bed of the glacier. When noise interference (for example, due to strong winds) is low, the tremor is also confirmed via analysis of seismic waveforms from surface stations. The signal resembles the tectonic tremor commonly observed during slow-earthquake events in subduction zones. We propose that the glacier sliding velocity can be retrieved from the observed seismic noise. Our approach may open new opportunities for monitoring calving-front processes in one of the most difficult-to-access cryospheric environments.

Seismic Velocity Heterogeneity of the Hikurangi Subduction Margin, New Zealand: Elevated Pore Pressures in a Region With Repeating Slow Slip Events

AbstractWe investigated the seismic velocity structure of the Hikurangi margin in New Zealand to uncover the physical features of the subduction zone and explore the relationships between microearthquake seismicity, seismic velocity structure, and slow slip events. Using local earthquake tomography with data collected from both temporary ocean bottom seismometers and on‐land permanent seismic stations, we used the tomography code TomoFD to iteratively perform a damped least squared inversion of absolute P and S arrival times to obtain relocated hypocenters and generate 3D velocity models for Vp and Vp/Vs. The seismic tomography images show two high Vp/Vs anomalies, one offshore and adjacent to a subducted seamount and the other beneath the North Island of New Zealand. The ∼50‐km wide offshore anomaly extends ∼10 km beneath the plate interface and lies directly beneath the area that slipped at least 50 mm during the 2 week‐long 2014 slow slip event. High Vp/Vs values may be related to high pore fluid pressures from subducted sediments, and such increases in pore fluid pressures have been suggested to trigger the occurrence of slow slip events in active subduction zones. The second onshore high Vp/Vs anomaly is located in the overlying plate and subducting slab and correlates with areas suggested by other geophysical techniques to be rich in fluids. Our seismic imaging supports interpretations that subduction processes in the Hikurangi margin are highly dependent on physical features such as subducted seamounts and fluid‐rich sediments.

Friction properties of hornblende and implications for slow-slip events in subduction zones

In order to understand the friction properties of amphibole as a primary constituent mineral in subduction zones, we used hornblende as the simulated gouge material to conduct two series of shearing experiments at temperatures from 101 to 607 °C under confining pressures of 136 MPa and 83 MPa, a pore pressure of 30 MPa and loading rates of 0.0488–1.22 μm/s. Steady-state friction coefficients in the high confining pressure series exhibited values between 0.70 and 0.72 with an average of ~0.71, without showing a systematic temperature dependence. The counterpart in the low confining pressure series ranged from 0.67 to 0.73 and exhibited a temperature-dependent decline for temperatures above 400 °C. Velocity-weakening behaviors were observed at 303, 505 and 607 °C in the high confining pressure series, and at 303 and 403 °C in the low confining pressure series. The |a-b| parameter associated with velocity weakening of hornblende in our high and low confining pressure experiments are 0.0005–0.0014 and 0.0014–0.0015, respectively. The b values, which represent the evolution effect in friction, were found to increase with increasing temperature, suggesting a thermally-activated process that produced creep at asperity. Ubiquitously precipitated particles in the deformed gouge suggest that dissolution-precipitation creep may be the most likely mechanism associated with the evolution effect and therefore velocity weakening. The very low velocity weakening observed for hornblende (|a-b| ≤ 0.0015) provides a favorable condition for nucleation of slow-slip events to occur in subduction zones over a wider range of effective normal stress.

Investigating the spatial variability of ground motions during the 2017 Mw 7.1 Puebla-Mexico City earthquake via idealized simulations of basin effects

We use strong motion records of the Mw 7.1 19 September 2017 mainshock and one- and two-dimensional (1D and 2D) simulations at six stations along a linear array across Mexico City, to shed light to the site amplification and ground motion variability observed during the mainshock. The 2017 Puebla-Mexico City earthquake occurred at the edge of the flat slab segment of the subducted Cocos plate beneath central Mexico, rupturing the top half of the plate. Compared to the 1985 subduction zone event, the intraslab Mw 7.1 mainshock was characterized by a much richer high-frequency content. This characteristic transpired into site amplification of the incoming shaking by the sediments of the transition zone, contrary to the 1985 Mw 8.1 Michoácan earthquake that was amplified by the deeper lake sediments. By means of idealized 1D site and 2D basin models, this paper seeks to disentangle site response from basin resonance and basin edge diffraction effects manifesting in the ground motion records. We specifically compare ground surface observations from the 2017 Mw 7.1 event to 1D and 2D analyses in the time and frequency domain; and empirical transfer functions from several earthquakes to numerical transfer functions computed using 1D and 2D models. Comparison of empirical and simulated amplification spectra shows that, although our 2D model does not capture all the peaks and troughs of the frequency response, theory and observations are nonetheless in good agreement across the entire frequency spectrum. And while frequency peaks of the 1D and 2D models are aligned in the range f>0.2Hz, the long period (f<0.01Hz) match between simulations and observations demonstrates that the 2D model appropriately captures the effects of the largest characteristic lengths of the basin, namely the basin width. In the intermediate frequency range (0.01 Hz <f<0.2Hz), our 2D model captures the scattering of the energy by the concave shape of the basin-rock interface via spectral deamplification, albeit not as strongly as is evidenced by the observations. One-dimensional site response on the other hand cannot capture the larger scale features below 0.2 Hz, which were likely relevant to the performance of lifelines and other infrastructure networks.

Empirical relocation of large subduction-zone earthquakes via the teleseismic network correlation coefficient method

The network correlation coefficient method is a powerful tool for relocating events under low signal-to-noise ratio conditions, as well as small (M_{text{W}} < 5.0) near-field earthquakes, where the relocation is performed by maximizing the sum of the waveform correlation coefficients between pairs of events in the network. Here we extend the method to relocating large (6.0 le M_{text{W}} le 7.3) teleseismic subduction-zone events. We apply a source duration correction to better correlate different-magnitude events. Four case studies are presented where we precisely locate interplate events near the rupture areas of giant megathrust earthquakes, namely the 2011 M_{text{W}} ;9.1 , Tohoku, 2010 M_{text{W}} ;8.8 Maule, 2005 M_{text{W}} ; 8.6 Nias, and 2007 M_{text{W}} ;8.5 Bengkulu earthquakes. We demonstrate the extensive links between these interplate events, with a clear alignment of earthquakes observed along each subduction interface.

Subducted seamount diverts shallow slow slip to the forearc of the northern Hikurangi subduction zone, New Zealand

Fluid migration and pore fluid pressure have been implicated in generating the transitional fault zone properties thought to be important for slow slip events (SSEs) in subduction zones. At the northern Hikurangi margin, New Zealand, overpressurized sediments spatially correlated with shallow SSEs are imaged downdip of subducted seamounts, providing an excellent environment to study the relationship between subducted oceanic relief, fluids, shallow SSEs, and related microseismicity. Using data from ocean-bottom seismometers, we detect “burst-type” repeating earthquakes coincident with tremor on an upper-plate fracture network above a subducted seamount. This activity occurred at the edge of a large, shallow SSE recorded offshore of Gisborne, New Zealand, in September–October 2014, but began days to weeks after the SSE and continued for nearly two months. We propose that during the large plate-boundary SSE, fluids migrated from the downdip overpressurized sediments into the fracture network, diverting aseismic slip to multiple faults in the upper plate. Thus, seamount subduction appears to play a key role in controlling the mechanics of shallow slow slip and microseismicity at the northern Hikurangi margin.

Low‐Frequency Earthquakes and Pore Pressure Transients in Subduction Zones

AbstractLow‐frequency earthquakes (LFEs) have been observed in subduction zones and some major tectonic faults and may well be the most important constituents of tectonic tremors. In subduction zones, they were initially attributed to fluids released by dehydration reactions in downgoing slabs. Their seismic radiation pattern, however, is consistent with shear slip on the subduction interface, and this rapidly became the favored model. Recent studies indicate that the source duration of LFEs does not scale with magnitude, which can hardly be explained by shear rupture. We revisit the characteristics of LFE events in subduction zones as retrieved from local seismic arrays. We demonstrate that they can be explained equally well by forces acting in the direction of fluid motion. Such forces may be generated by a fast local pressure variations associated with unsteady fluid motion. The amount of fluid required for LFE activity is consistent with dehydration reaction rates.

Effect of Tsunami Damage on Passenger and Forestry Transportation in Pacific County, Washington

The outer coast of Washington State is exposed to significant seismic and tsunami hazards. A Cascadia subduction zone (CSZ) event is expected to cause high earthquake intensities and tsunami inundation, resulting in considerable infrastructure loss, inundation of developed land, and degraded functioning of coastal communities. One area of particular concern is Pacific County, in southwest Washington, where more than 85% of the population is expected to experience severe shaking intensities. This paper establishes the predisaster passenger and freight transportation patterns and the predicted damaged postdisaster road network in Pacific County. The hazard used in the analysis was a CSZ magnitude 9.1 earthquake with Washington asperity and resulting tsunami. The anticipated change in trip distance and the percentage of trips that could no longer be completed were used to compare passenger travel and forestry freight travel. Because passenger and freight travel have different purposes and patterns, understanding how they are affected differently can serve as a foundation for community-based disaster recovery planning to increase community resilience in earthquakes and tsunamis.

Background seismicity in Boso Peninsula, Japan: Long‐term acceleration, and relationship with slow slip events

AbstractSlow slip events (SSEs) in subduction zones can trigger earthquake swarms, especially at shallow depth. The monitoring of seismicity rates has therefore the potential to help detect and characterize SSEs, and transient changes in coupling. However, the relationship between seismicity rate and slow slip rate during a SSE is unknown and made complicated by aftershock triggering within the swarm. Here we propose to complement geodetic methods with an objective measure of the seismicity rate that is directly associated with changes in slip rate. We show that this measure, applied to known occurrences of SSEs in the Boso area, Japan, yields an estimate, albeit indirect, of their seismic moment, hence their slip rate. We finally prove that the background rate in Boso has been accelerating since 1990; this explains previous observations of the shortening of the recurrence time between SSEs in Boso, that clearly predate the 2011 Mw9.0 Tohoku‐Oki earthquake.

Diagnosing the precatastrophic state of the environment in a subduction zone prior to very strong mega-seismic events

The barrier-free seismic process in the subduction zone is considered. The Kurils–Kamchatka subduction zones are characterized by two major environments of oscillatory processes expressed in seismic events distributed long-term throughout the zone and concentrated short-term in a local regional area. The first environment is related to the background seismic process. The fast localization of processes confirms the idea of synchronous movement of regional environment components and is indicative of precatastrophic conditions in the zone. Diagnosis of precatastrophic conditions in the zone makes it possible to control the transition from the background seismic process to the short-term regime of the most likely very strong seismic events. A short-term forecast of the epicentral area of the strongest mega-seismic events in subduction zones is implemented.