PBO BOREHOLE STRAINMETERS OBSERVE OVER A DECADE OF EPISODIC TREMOR AND SLIP ALONG THE CASCADIA SUBDUCTION ZONE: SOUTHERN VANCOUVER ISLAND, BC, CANADA THROUGH OLYMPIA, WA, USA
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We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.
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Abstract The Sumatra subduction zones have been known to produce large destructive earthquakes and tsunami, such as the 26 December 2004 M9 earthquake at the western offshore of Aceh. This large destructive earthquake usually occurred at the shallow portion of the subduction zone between 10 – 35 km depth, while the intermediate-to-deep earthquake usually occurred at the greater depth, deeper than 65 km along the subduction, which produces small to moderate earthquake with a magnitude between M3-5. Thus, these deep events along the subduction zone usually being neglected. However, the recent study has shown that an anomalous large ground movement associated with a deep earthquake at the subduction zone has been observed along the forearc region while in the backarc or at its epicenter, it remains low. Hence, the characteristics of the body-wave at these regions are different, which could be utilized for hazard mitigation. For this reason, this study aims to identify the body-wave characteristics between the forearc and backarc regions at the Sumatra subduction zone. Several selected deep regional earthquake records with a depth greater than 100 km provided by the Agency for Meteorology, Climatology, and Geophysics of Indonesia (BMKG) were analyzed. The finding indicates that the body-waves recorded in the forearc region are significantly different, with the main characteristics are; 1. Amplitude: preserve high amplitude, 2. Arrival time: shows a delayed signal of P- and S-waves, and 3. Spectral: conserve energy at low to high frequency and shows as a dispersion. These findings suggest that the signal recorded at the forearc region travel within the subducting slab, and the late arrival of the signal indicates that the seismic waves record at forearc travel at the top of the subducting slab, i.e., the oceanic crust, which is having lower velocity to the surrounding mantle.
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Abstract In recent years, it has been discovered that sections of the subduction interface slip aseismically in slow slip events, during which stress is intermittently transferred to the section of the subduction zone that generates large or great earthquakes. Within the Cascadia subduction zone, the magnitude and frequency of SSEs and accompanying tectonic tremor exhibit complex patterns that vary systematically with depth. However, the loading mechanisms and interactions that precede great subduction earthquakes are poorly understood. Here we present results from physics‐based simulations that reproduce the continuum of SSE characteristics reported for the Cascadia subduction zone. The simulations provide a basis for understanding the interactions that control both the observed complex patterns of SSEs and stress transfer to the seismogenic section that produce great earthquakes.
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Continuously operating global positioning system sites in the North Island of New Zealand have revealed a diverse range of slow motion earthquakes on the Hikurangi subduction zone. These slow slip events (SSEs) exhibit diverse characteristics, from shallow (<15 km), short (<1 month), frequent (every 1–2 years) events in the northern part of the subduction zone to deep (>30 km), long (>1 year), less frequent (approximately every 5 years) SSEs in the southern part of the subduction zone. Hikurangi SSEs show intriguing relationships to interseismic coupling, seismicity, and tectonic tremor, and they exhibit a diversity of interactions with large, regional earthquakes. Due to the marked along-strike variations in Hikurangi SSE characteristics, which coincide with changes in physical characteristics of the subduction margin, the Hikurangi subduction zone presents a globally unique natural laboratory to resolve outstanding questions regarding the origin of episodic, slow fault slip behavior. ▪ New Zealand's Hikurangi subduction zone hosts slow slip events with a diverse range of depth, size, duration, and recurrence characteristics. ▪ Hikurangi slow slip events show intriguing relationships with seismicity ranging from small earthquakes and tremor to larger earthquakes. ▪ Slow slip events play a major role in the accommodation of plate motion at the Hikurangi subduction zone. ▪ Many aspects of the Hikurangi subduction zone make it an ideal natural laboratory to resolve the physical processes controlling slow slip.
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Abstract Slow slip events (SSEs) detected on the Cascadia Subduction Zone interface at 30–50 km depth imply a release of accumulated strain. However, studies of interseismic deformation in Cascadia typically find coupling on the upper 30 km of the interface, which is generally accepted as defining the seismogenic zone. Estimates of coupling using net interseismic velocities (including SSE effects) and restricting coupling to the shallow interface may underestimate slip deficit accumulation at depths >30 km. Here, we detect reversals in GPS motion as indications of SSEs, then use SSE displacements to estimate cumulative slow slip from 2007 to 2021. We calculate pure interseismic velocities, correcting for SSE displacements, and use them to constrain an elastic block model, estimating slip deficit on the subduction interface down to 50 km. By evaluating slip deficit and slow slip independently, we examine SSEs’ effect on interseismic strain accumulation, and the effect of inter‐SSE slip deficit and slow slip on vertical deformation of the forearc. We find that moderate to high coupling extends to 40 km depth, and while shallow coupling is consistent with previous estimates of the seismogenic zone, a deeper region of slip deficit beneath the Olympic Peninsula may be partially (61%) relieved aseismically by SSEs. Patterns of surface uplift suggest that complete relief of deep coupling over multiple decades may be accomplished by time‐varying rates of aseismic slip.
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Geodetic and seismic signatures of episodic tremor and slip in the northern Cascadia subduction zone
Slip events with an average duration of about 10 days and effective total slip displacements of severalc entimetres have been detected on the deeper (25 to 45 km) part of the northern Cascadia subduction zone interface by observing transient surface deformation on a network of continuously recording Global Positioning System (GPS) sites. The slip events occur down-dip from the currently locked, seismogenic portion of the subduction zone, and, for the geographic region around Victoria, British Columbia, repeat at 13 to 16 month intervals. These episodes of slip are accompanied by distinct, low-frequency tremors, similar to those reported in the forearc region of southern Japan. Although the processes which generate this phenomenon of episodic tremor and slip (ETS) are not well understood, it is possible that the ETS zone may constrain the landward extent of megathrust rupture, and conceivable that an ETS event could precede the next great thrust earthquake.
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