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Tectonic setting of south central Alaska, where the Pacific plate subducts beneath the North American plate. At the eastern end of the Aleutian‐Alaska subduction zone, the subducting Yakutat microplate is shown in green. Slab depth contours (Hayes et al., 2012) are indicated by the black thin lines. The right‐lateral strike‐slip Denali fault is shown by the thicker black line. The 1,000‐km‐long rupture area of the Mw 9.2 1964 earthquake is indicated by the yellow area (Christensen & Beck, 1994) and approximate contours for the geodetically observed years‐long slow slip events by the red areas. The black dots indicate the tectonic tremor locations (Wech, 2016). Red triangles show major volcano locations. Note the gap associated with the width of the Yakutat microplate. The red contour is the close‐up corresponding to Figure 2.
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We jointly investigate aseismic slip transients and tremor activity in south central Alaska. Near the eastern downdip edge of the Mw 9.2 1964 Prince Williams earthquake rupture, kinematic modeling of the Mw=7.6 2009–2013 slow slip event suggests cumulative transient slip of up to 55 cm. During this 5‐year transient event, tectonic tremors were co‐l...
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Citations
... (Evans et al., 1981;King et al., 1973;Mortensen et al., 1977). The velocities on the SHF are also remarkably similar to the propagation speeds of subduction zone ETS events, which appear to be ∼10 km/day (Bartlow et al., 2011;Hirose & Obara, 2010;Rousset et al., 2019). ...
Shallow creep events provide opportunities to understand the mechanical properties and behavior of faults. However, due to physical limitations observing creep events, the precise spatio‐temporal evolution of slip during creep events is not well understood. In 2023, the Superstition Hills and Imperial faults in California each experienced centimeter‐scale slip events that were captured in unprecedented detail by satellite radar, sub‐daily Global Navigation Satellite Systems, and creepmeters. In both cases, the slip propagated along the fault over 2–3 weeks. The Superstition Hills event propagated bilaterally away from its initiation point at average velocities of ∼9 km/day, but propagation velocities were locally much higher. The ruptures were consistent with slip from tens of meters to ∼2 km depths. These slowly propagating events reveal that the shallow crust of the Imperial Valley does not obey purely velocity‐strengthening or velocity‐weakening rate‐and‐state friction, but instead requires the consideration of fault heterogeneity or fault‐frictional behaviors such as dilatant strengthening.
... This amount of strain is relatively low and if the brittle yield strength is exceeded, this strain can be accumulated through common brittle deformation events, such as megathrust earthquakes, intra-slab earthquakes at the slab's upper boundary, or low-frequency earthquakes in the tremor-producing zone. Given the characteristic displacements associated with these events, 395 ranging from centimeters for LFEs to meters for larger earthquakes (Schmidt and Gao, 2010;Nishimura et al., 2013;Michel et al., 2019;Radiguet et al., 2012;Ozawa, 2017;Takagi et al., 2019;Rousset et al., 2019;Bostock et al., 2015; ical earthquake or slow slip and tremor cycles for km-scale oceanic blocks or lenses. Although exact lengthscales are unknown, this implies that brittle deformation in coarse-grained oceanic crust at blueschist facies conditions 400 can persist over multiple earthquake cycles but is otherwise expected to be short/transient on long-term geologic timescales. ...
We constrained the rheology of glaucophane aggregates deforming near its brittle-ductile transition with general shear deformation experiments conducted both at constant rate and using strain rate steps. In the experiments, glaucophane first underwent work hardening and strain weakening associated with brittle grain size reduction and incipient dislocation processes, then evolved at strains of ~1 gamma to steady-state dislocation creep with bulge nucleation recrystallization and limited climb. We developed a flow law from these experiment that can be used to approximate the rheological behavior of mafic oceanic crust at blueschist facies conditions. Extrapolations of the flow law to natural conditions indicate blueschists are stronger than quartz- or mica-dominated rocks such as metasediments but weaker than eclogites over the blueschist stability field.
... We present tremor observations during an example slow slip event in Cascadia in Figure 5. Despite a general trend of tremors migrating southward in that specific event, we also observe intermittent occurrence of tremor, punctuated by pauses with no radiation interpreted as temporary stalling of tremor-initiating slow slip (Frank & Brodsky, 2019;Frank et al., 2018;Jolivet & Frank, 2020;Mouchon et al., 2023;Rousset et al., 2019), and large variability in centroid locations of the tremors occurring in 2-hr intervals. Aspects of these observations mirror our lab-based findings, albeit with greater complexity, and may suggest the potential reopening of sealed or healed fault valves (e.g., backward ruptures). ...
... However, recent geological investigations (Condit & French, 2022;Mindaleva et al., 2020;Schmidt & Platt, 2022;Ujiie et al., 2018) and numerical simulations (Cruz-Atienza et al., 2018;Farge et al., 2021) reveal that fluid propagation can potentially generate seismic behaviors similar to tectonic tremor via a combined mechanism of shear and tensile deformations. While we focus only on the tensile mechanisms due to our experimental limitations, our results suggest previous studies of tremor intermittency that attribute it exclusively to sporadic fault locking and stalled tremor-initiating slow shear slip (Frank & Brodsky, 2019;Frank et al., 2018;Jolivet & Frank, 2020;Mouchon et al., 2023;Rousset et al., 2019) should also consider a role for variable pore pressures that may cause tensile fracturing and sealing. Additionally, in the model we propose in Figure 4, the cracking and sealing of fractures actually modify the potential for shear slip and thus tremor cannot be considered strictly as a result of the overall driving slow slip event, but instead as a facilitator of shear slip. ...
The fracture of Earth materials occurs over a wide range of time and length scales. Physical conditions, particularly the stress field and Earth material properties, may condition rupture in a specific fracture regime. In nature, fast and slow fractures occur concurrently: tectonic tremor events are fast enough to emit seismic waves and frequently accompany slow earthquakes, which are too slow to emit seismic waves and are referred to as aseismic slip events. In this study, we generate simultaneous seismic and aseismic processes in a laboratory setting by driving a penny-shaped crack in a transparent sample with pressurized fluid. We leverage synchronized high-speed imaging and high-frequency acoustic emission (AE) sensing to visualize and listen to the various sequences of propagation (breaks) and arrest (sticks) of a fracture undergoing stick-break instabilities. Slow radial crack propagation is facilitated by fast tangential fractures. Fluid viscosity and pressure regulate the fracture dynamics of slow and fast events, and control the inter-event time and the energy released during individual fast events. These AE signals share behaviors with observations of episodic tremors in Cascadia, United States; these include: (a) bursty or intermittent slow propagation, and (b) nearly linear scaling of radiated energy with area. Our laboratory experiments provide a plausible model of tectonic tremor as an indicative of hydraulic fracturing facilitating shear slip during slow earthquakes.
... Channel in Japan and Costa Rica (Brown et al., 2009;Hirose et al., 2010). Bursts of tremor co-located with or near the downdip limit of long-term SSEs have also been detected in Mexico, Alaska, and Japan, with a higher frequency of tremors during long-term SSEs (Frank et al., 2018;Hirose et al., 2010;Rousset et al., 2019). Frank et al. (2018) and Rousset et al. (2019) suggested that long-term SSEs were actually composed of a cluster of short ETS-like events while Rousset et al. (2019) also proposed the long-term SSE may have occurred updip from a cluster of short ETS-like events. ...
... Bursts of tremor co-located with or near the downdip limit of long-term SSEs have also been detected in Mexico, Alaska, and Japan, with a higher frequency of tremors during long-term SSEs (Frank et al., 2018;Hirose et al., 2010;Rousset et al., 2019). Frank et al. (2018) and Rousset et al. (2019) suggested that long-term SSEs were actually composed of a cluster of short ETS-like events while Rousset et al. (2019) also proposed the long-term SSE may have occurred updip from a cluster of short ETS-like events. The difference in the variability in tremor behavior within and between subduction zones is not well understood. ...
... Bursts of tremor co-located with or near the downdip limit of long-term SSEs have also been detected in Mexico, Alaska, and Japan, with a higher frequency of tremors during long-term SSEs (Frank et al., 2018;Hirose et al., 2010;Rousset et al., 2019). Frank et al. (2018) and Rousset et al. (2019) suggested that long-term SSEs were actually composed of a cluster of short ETS-like events while Rousset et al. (2019) also proposed the long-term SSE may have occurred updip from a cluster of short ETS-like events. The difference in the variability in tremor behavior within and between subduction zones is not well understood. ...
Plain Language Summary
In between the Earth's tectonic plates, energy builds over time and can be released along faults suddenly (seconds‐minutes; i.e., earthquakes) or slowly (weeks‐years; i.e., slow slip events). Slow slip often happens with low‐frequency earthquakes (i.e., tectonic tremor). The North Island, New Zealand features two colliding tectonic plates with the potential to generate large earthquakes. The interface between these plates has both deep tectonic tremor and large, long‐lasting slow slip, but the tectonic tremor is deeper on the fault than the large slow slip. Studies have suggested small, short‐lasting slow slip, usually not able to be detected, occur where tectonic tremor is found. In this study we tried a different approach to find the small slow slip. While small slow slip are not detected by themselves, we were able to detect their cumulative effect in the tectonic tremor area. We modeled small slow slip during tectonic tremor to find the mean sliding rate on the fault that is between the tectonic plates. The large long‐lasting slow slip may drive this smaller slow slip by making them slip faster. The question remains as to the cause of the many types of slow slip in New Zealand.
... Their source area corresponds to the subducting Yakutat microplate and overlaps with that of UCI L-SSEs (e.g., Wech, 2016) ( Figure 1). Rousset, Fu et al. (2019) discovered an M w 6.9 S-SSE lasting 27 days in southcentral Alaska for the first time. This S-SSE began in early September 2010, after the onset of the 2009-2013 UCI L-SSE, and the slip propagated from east to west, corresponding to the migration of tremors. ...
... We show fault models with observed and calculated displacements for all detected events ( Figures S9-S59 in Supporting Information S1) and picked up notable events in Figure 3. The 2010 September S-SSE was detected by both Rousset, Fu et al. (2019) and our study (Figure 3a). The fault location is not significantly different between the two studies, whereas the estimated magnitude and duration is less in our study than in those of Rousset, Fu et al. (2019). ...
... The 2010 September S-SSE was detected by both Rousset, Fu et al. (2019) and our study (Figure 3a). The fault location is not significantly different between the two studies, whereas the estimated magnitude and duration is less in our study than in those of Rousset, Fu et al. (2019). Although the differences between the assumed plate interface model and slip inversion strategy possibly affected the estimation, a wider station distribution compared with that of Rousset, Fu et al. (2019) is the main reason for the smaller moment than the previous one. ...
Plain Language Summary
Slow and transient fault slips, called slow slip events (SSEs), are important phenomena accommodating plate motion during interseismic periods. However, detecting SSEs, especially short‐term SSEs (S‐SSEs) that last from days to weeks, is sometimes difficult because of their weak signals. In southcentral Alaska, previous studies have detected S‐SSEs as several discrete events synchronizing with tectonic tremors, but their spatiotemporal distribution and the features of their magnitude and duration are still unclear. We applied a systematic detection method to 14 years of daily Global Navigation Satellite System position data and successfully detected 31 S‐SSEs. We found two major groups of S‐SSEs (S‐SSE clusters) at a depth from 35 to 45 km, which corresponds to a deeper extension of the source of the 1964 Alaska earthquake. These clusters are located in the region where the Yakutat microplate subducts. Maximum cumulative slip reaches 0.27 and 0.43 m in the western and eastern clusters, respectively, and it suggests that S‐SSEs contribute to the reduction of the large amount of interplate slip in their source areas.
... B. Frank & Brodsky, 2019). This empirical relationship implicitly redefines the rupture duration of slow slip to periods of low-frequency earthquake activity, ignoring the intermittent periods of loading hidden within the long-term geodetic signature of slow slip (Fujita et al., 2019;Rousset et al., 2019), to establish that slow slip moment scales with its duration cubed similar to ordinary earthquakes. This result demonstrates that observational constraints at the shortest time scales are needed to capture the physical mechanisms governing slow fault slip, the dominant faulting process just downdip of the seismogenic source region. ...
Plain Language Summary
Slow slip events can be observed in many subduction zones where they play an important role in the earthquake cycle. Decades after their discovery, slow slip events are now captured routinely in geodetic datasets with slip dynamics occurring over a broad range of time scales. Using high‐time resolution seismological observations together with the geodetic record allows us to go beyond the coarse daily GNSS sampling rate to image slow slip dynamics at short time scales. Here we use the temporal evolution of seismic slip produced by low‐frequency earthquakes to study the subdaily dynamics of a slow slip event cycle, reproducing the geodetic record of slow slip using only seismological observations. We develop a simple model where long‐term loading is in competition with the intermittent release of stress tied to the seismic slip of low‐frequency earthquakes. We show the full slow slip cycle is driven by bursts of slip at subdaily time scales that low‐frequency earthquake events witness only in their immediate source region. This result implies that the low‐frequency earthquake rupture process is incidental to slow fault slip and does not play a major role in the slow slip cycle.
... Recently, in July and October of 2020, the gap was partially ruptured by two M w > 7.5 earthquakes (Crowell & Melgar, 2020), and in July of 2021, an M w 8.2 earthquake occurred just east of the gap, 10.1029/2022JB024767 4 of 24 likely triggered by the previous year's events (Elliott et al., 2022). Although this subduction zone has hosted several documented SSEs and tremor episodes down dip of the seismogenic zone (Brown et al., 2013;Li & Freymueller, 2018;Rousset et al., 2019); to date, there is only limited evidence for SSEs within or updip of the seismogenic zone . ...
In subduction zones worldwide, seafloor pressure data are used to observe tectonic deformation, particularly from megathrust earthquakes and slow slip events (SSEs). However, such measurements are also sensitive to oceanographic circulation‐generated pressures over a range of frequencies that conflate with tectonic signals of interest. Using seafloor pressure and temperature data from the Alaska Amphibious Community Seismic Experiment, and sea surface height data from satellite altimetry, we evaluate the efficacy of various seasonal and oceanographic pressure signal proxy corrections and conduct synthetic tests to determine their impact on the timing and amplitude prediction of ramp‐like signals typical of SSEs. We find that subtracting out the first mode of the complex empirical orthogonal functions of the pressure records on either the shelf or slope yields signal root‐mean‐square error (RMS) reductions up to 73% or 80%, respectively. Additional correction with proxies that exploit the depth‐dependent spatial coherence of pressure records provides cumulative variance reductions up to 83% and 93%, respectively. Our detectability tests show that the timing and amplitude of synthetic SSE‐like ramps can be well constrained for ramp amplitudes ≥4 cm on the shelf and ≥2 cm on the slope, using a fully automated detector. The principal limits on detectability are residual abrupt changes in pressure that occur as part of the transition to and from summer to winter conditions but are not adequately characterized by our seasonal corrections, as well as the inability to properly account for instrumental drift, which is not readily separated from the seasonal signal.
... In addition to the Cascadia subduction zone and Nankai Trough, clear ETSs have been observed in the subduction zones of Costa Rica (Walter et al. 2013), Mexico (Rousset et al. 2017), and Alaska (Rousset et al. 2019). However, note that short-term SSEs and tremor bursts do not always occur at exactly the same time. ...
Slow earthquakes are episodic slow fault slips. They form a fundamental component of interplate deformation processes, along with fast, regular earthquakes. Recent seismological and geodetic observations have revealed detailed slow earthquake activity along the Japan Trench—the subduction zone where the March 11, 2011, moment magnitude (Mw) 9.0 Tohoku-Oki earthquake occurred. In this paper, we review observational, experimental, and simulation studies on slow earthquakes along the Japan Trench and their research history. By compiling the observations of slow earthquakes (e.g., tectonic tremors, very-low-frequency earthquakes, and slow slip events) and related fault slip phenomena (e.g., small repeating earthquakes, earthquake swarms, and foreshocks of large interplate earthquakes), we present an integrated slow earthquake distribution along the Japan Trench. Slow and megathrust earthquakes are spatially complementary in distribution, and slow earthquakes sometimes trigger fast earthquakes in their vicinities. An approximately 200-km-long along-strike gap of seismic slow earthquakes (i.e., tectonic tremors and very-low-frequency earthquakes) corresponds with the huge interplate locked zone of the central Japan Trench. The Mw 9.0 Tohoku-Oki earthquake ruptured this locked zone, but the rupture terminated without propagating deep into the slow-earthquake-genic regions in the northern and southern Japan Trench. Slow earthquakes are involved in both the rupture initiation and termination processes of megathrust earthquakes in the Japan Trench. We then compared the integrated slow earthquake distribution with the crustal structure of the Japan Trench (e.g., interplate sedimentary units, subducting seamounts, petit-spot volcanoes, horst and graben structures, residual gravity, seismic velocity structure, and plate boundary reflection intensity) and described the geological environment of the slow-earthquake-genic regions (e.g., water sources, pressure–temperature conditions, and metamorphism). The integrated slow earthquake distribution enabled us to comprehensively discuss the role of slow earthquakes in the occurrence process of the Tohoku-Oki earthquake. The correspondences of the slow earthquake distribution with the crustal structure and geological environment provide insights into the slow-earthquake-genesis in the Japan Trench and imply that highly overpressured fluids are key to understanding the complex slow earthquake distribution. Furthermore, we propose that detailed monitoring of slow earthquake activity can improve the forecasts of interplate seismicity along the Japan Trench.
... The segmentation is probably related to the along-strike variations in the effective normal stress on the fault plane due to fluid release from the subducting Yakutat Plateau [21]. Slip pulses that have been identified from GPS site velocities in south-central Alaska are generally associated with the tremor bursts during the 5-year transient events from 2009 to 2013 [28]. ...
Fast and slow earthquakes are predominantly generated along faults constituting active plate boundaries. Characterized by repeated devastating earthquakes and frequent slow slip events and tremors, the Alaska megathrust presents a chance to understand the complicated dynamics of a subduction system changing from steep to shallow dips associated with enigmatically abundant fast and slow seismic events. Based on three-dimensional thermal modeling, we find that the downgoing metamorphosed oceanic crust containing bound water releases a large amount of fluid and causes the recurrence of fast and slow earthquakes by elevated pore fluid pressure and hydrofracturing. The seismogenic interface and the slow slip events (SSEs) identified beneath the Upper Cook Inlet coincide well with the slab metamorphic dehydration regions. The observed slow earthquakes with quasi-stable fault slips preferentially occur, accompanied by high dehydration and temperature downdip along the transition zone.
... While slow-slip phenomena show much lower slip rates, for example, slow slip events (SSEs), fault creep, or slip related to fluid injection. For example, in the case of SSEs in subduction zones, the peak slip rates vary around 0.1 ∼ 3 cm/day (Bletery & Nocquet, 2020;Ozawa et al., 2019;Radiguet et al., 2011;Rousset et al., 2019). In the case of the episodic creep event, the slip rates in continental faults are 0.5 ∼ 3 cm/year (Hussain et al., 2016;Jolivet et al., 2012;Schmidt et al., 2005;Scott et al., 2020). ...
... Such patterns have been observed during regular earthquakes but are also associated with slow-slip phenomena: with slow slip transients migrating further away from where they started along strike (or dip) or remain stationary through time. Observations of some SSEs and "Episodic Tremor and Slip" (ETS) show pulse-like rupture characteristics with elongated slipping areas, for example, the Cascadia subduction zone (Michel et al., 2019), and with along strike migration speeds of ∼10 km/ day (Rousset et al., 2019;Wech et al., 2009). In contrast, slip propagation of meter-scale fluid injection experiments indicates stationary patterns: Bhattacharya and Viesca (2019) proposed a model in which the slip grows as an expanding ellipse, with the injection point as the slipping center. ...
... Furthermore, by taking advantage of the improved method for estimating slip rates during temporally overlapping InSAR time frames, one can image the fault behavior over a long period in a relatively high temporal resolution. This new method is expected to be applied to investigate the temporal evolution of slow fault slip, for example, transient slow slip (Khoshmanesh et al., 2015;Klein et al., 2018;Kyriakopoulos et al., 2013), afterslip (Thomas et al., 2014) and SSEs in subduction zones (Bletery & Nocquet, 2020;Ozawa et al., 2019;Rousset et al., 2019). ...
Improved imaging of the spatio‐temporal growth of fault slip is crucial for understanding the driving mechanisms of earthquakes and faulting. This is especially critical to properly evaluate the evolution of seismic swarms and earthquake precursory phenomena. Fault slip inversion is an ill‐posed problem and hence regularization is required to obtain stable and interpretable solutions. An analysis of compiled finite fault slip models shows that slip distributions can be approximated with a generic elliptical shape, particularly well for M ≤ 7.5 events. Therefore, we introduce a new physically informed regularization to constrain the spatial pattern of slip distribution. Our approach adapts a crack model derived from mechanical laboratory experiments and allows for complex slipping patterns by stacking multiple cracks. The new inversion method successfully recovered different simulated time‐dependent patterns of slip propagation, that is, crack‐like and pulse‐like ruptures, directly using wrapped satellite radar interferometry (InSAR) phase observations. We find that the new method reduces model parameter space, and favors simpler interpretable spatio‐temporal fault slip distributions. We apply the proposed method to the 2011 March–September normal‐faulting seismic swarm at Hawthorne (Nevada, USA), by computing ENVISAT and RADARSAT‐2 interferograms to estimate the spatio‐temporal evolution of fault slip distribution. The results show that (a) aseismic slip might play a significant role during the initial stage and (b) this shallow seismic swarm had slip rates consistent with those of slow earthquake processes. The proposed method will be useful in retrieving time‐dependent fault slip evolution and is expected to be widely applicable to studying fault mechanics, particularly in slow earthquakes.
