Large Slow Slip Events Research Articles

Intermittence of transient slow slip in the Mexican subduction zone

In the Mexican subduction zone, slow slip events of various duration, from a few days to several months have previously been identified in the regions of Guerrero and Oaxaca. They occur along with microseismicity: low-frequency earthquakes and tectonic tremors. In this study, we analyzed a 10-year long tremor catalog covering multiple SSE cycles, to identify periods of high tremor activity. We then used the temporal information of these tremor bursts to decompose GNSS time series. Surface velocities estimated for tremor and non-tremor periods are then inverted to locate slip rates on the plate interface. Combining the detected microseismicity information and GNSS time series, we show that large slow slip events are made of clusters of short-lasting slip events occurring at tremor times and located updip of the tremor area. This analysis also allowed us to unveil low amplitude transients in between large slow slip events, that last 8−9 days, occurring at tremor times and locations, in both Guerrero and Oaxaca regions. Finally, we show that the plate locking amplitude in between short and large slow slip events increases by a factor up to 0.5 at the location of tremors.

Using Seafloor Geodesy to Detect Vertical Deformation at the Hikurangi Subduction Zone: Insights From Self‐Calibrating Pressure Sensors and Ocean General Circulation Models

AbstractSeafloor pressure sensor data is emerging as a promising approach to resolve vertical displacement of the seafloor in the offshore reaches of subduction zones, particularly in response to slow slip events (SSEs), although such signals are challenging to resolve due to sensor drift and oceanographic signals. Constraining offshore SSE slip distribution is of key importance to understanding earthquake and tsunami hazards posed by subduction zones. We processed seafloor pressure data from January to October 2019 acquired at the Hikurangi subduction zone, offshore New Zealand, to estimate vertical displacement associated with a large SSE that occurred beneath the seafloor array. The experiment included three self‐calibrating sensors designed to remove sensor drift, which, together with ocean general circulation models, were essential to the identification and correction of long‐period ocean variability remaining in the data after applying traditional processing techniques. We estimate that long‐period oceanographic signals that were not synchronous between pressure sensors and reference sites influenced our inferred displacements by 0.3–2.6 cm, suggesting that regionally deployed reference sites alone may not provide sufficient ocean noise correction. After incorporating long‐period ocean variability corrections into the processing, we calculate 1.0–3.3 cm of uplift during the SSE offshore Gisborne at northern Hikurangi, and 1.1–2.7 cm of uplift offshore the Hawke's Bay area at central Hikurangi. Some Hawke Bay displacements detected by pressure sensors near the trench were delayed by 6 weeks compared to the timing of slip onset detected by onshore Global Navigation Satellite System sites, suggesting updip migration of the SSE.

Along-strike distribution of seismicity and large slow slip correlated with gravity at the Nankai Trough

Various earthquakes and slow slip phenomena occur in different locations in the down-dip direction at subduction zones, for example, at the Nankai Trough, Japan. Many earthquakes, tectonic tremors, and slow slip events have occurred on the deeper side of the source areas of the past megathrust earthquakes. We show that three regions with high background seismicity correspond to the source areas of the large (> equivalent moment magnitude 6.5) slow slip events. These regions do not correspond to active areas of tectonic tremors. We note that considering broad-scale variations along the strike of the subduction zone, the three regions also correspond to areas with low free-air gravity anomalies. This fact implies that widespread stress perturbations in the along-strike direction, possibly coupled with pore fluid effects, may control the occurrence of seismic and aseismic slip on subduction faults regardless of whether at the plate interface or within the plate.

Slow slip events and flank instability at Mt. Etna volcano (Italy)

We analyzed a set of 11 slow slip events occurred during the 2006–2016 period and affecting the GNSS (Global Navigation Satellite System) stations of the unstable flank of Mt. Etna volcano. Observed surface deformation for most of the detected slow slip events, concentrates on the south-eastern edge of the unstable flank while the slow slip events involving the north-eastern edge are less frequent. Such a pattern highlights the existence of two distinct families of events, involving two contiguous sectors of the unstable flank, which occasionally slip together in large slow slip events. The modelled slips also highlight that both contiguous sectors extend ~10–12 km offshore, on areas where active tectonic lineaments such as the ESE (northward of Catania Canyon) and the N102° (along the southern slope of the Riposto Ridge) ones have been recently discovered. Equivalent seismic moments of slow slip events occurred in the last ten years (corresponding to magnitudes in the range 5.4–5.9) are larger than those associated to seismic events observed in the last 200 years, suggesting that most of the deformation affecting the eastern flank occurs aseismically.

Continuous Tremor Activity With Stable Polarization Direction Following the 2014 Large Slow Slip Event in the Hikurangi Subduction Margin Offshore New Zealand

AbstractMany types of slow earthquakes have been discovered at subduction zones around the world. However, the physical process of these slow earthquakes is not well understood. To monitor offshore slow earthquakes, a marine seismic and geodetic experiment was conducted at the Hikurangi subduction margin from May 2014 to June 2015. During this experiment, a large slow slip event (Mw 6.8) occurred directly beneath the ocean bottom seismometer (OBS) network. In this study, S‐wave splitting and polarization analysis methods, which have been previously used on onshore data to investigate tremor and anisotropy, are applied to continuous OBS waveform data to identify tremors that are too small to detect by the envelope cross correlation method. Continuous tremor activity with stable polarization directions is detected at the end of the 2014 slow slip event and continued for about 2 weeks. The tremors are generated around a southwest bend in the slow slip contours and at the landward edge of a subducted seamount. Our findings corroborate a previous interpretation, based on burst‐type repeating earthquakes and intermittent tremor, that localized slow slip and tremor around the seamount was triggered by fluid migration following the large plate boundary slow slip event and indicate tremor occurred continuously rather than as isolated and sporadic individual events.

Slow slip events precursory to the 2014 Iquique Earthquake, revisited with long-base tilt and GPS records

SUMMARY The M= 8.1, 1 April 2014 Iquique earthquake, which broke part of the northern Chile seismic gap, was preceded by a strong foreshock sequence starting early January 2014. The reported analysis of the continuous records of the nearby GPS stations from the Integrated Plate Observatory Chile, North Chile array lead to contradictory results concerning the existence and location of slow slip events (SSEs) on the interplate contact. Resolving this controversy is an important issue, as although many SSEs are reported in subduction zones, only a few were found to be precursory to large earthquakes. Here we show that the records of a long base tiltmeter installed near Iquique, when corrected for coseismic steps, long-term drift, tidal signals and oceanic and atmospheric loading, show significant residual signals. These can be modelled with a sequence of four SSEs located close to Iquique. Their signature was already reported on some GPS stations, but their source was then characterized with a very low resolution in time and space, leading to contradicting models. With the tilt records, we can rule out the previously proposed models with a single large SSE closer to the main shock. Combining tilt with GPS records greatly improves the resolution of GPS alone, and one could locate their sources 100–180 km south–southeast to the main shock epicentre, with moment magnitudes between 5.8 and 6.2, at the edge of the main aftershock asperities. These moderate SSEs thus did not directly trigger the main shock, but contributed to trigger the main foreshock and the main aftershock. Only the sensitivity and resolution of the tiltmeter, added to the GPS records, allowed us to describe with unprecedented accuracy this precursory process as a cascade of cross-triggered, short-term aseismic slip events and earthquakes on the interplate contact. This three months of precursory activation appears to be the final acceleration burst of a weaker, longer term SSE which started mid-2013, already reported, with a moment release history which we could quantify. From the methodological point of view, our study takes advantage of an interesting complementarity of tilt and GPS measurements, due to their different dependence in distance to the source of strain, which turns out to be very efficient for resolving location and moment of strain sources, even when both instruments are close to each other. It finally demonstrates the efficient removal of sequences of small or even undetected coseismic steps from high resolution tilt record signal in order to retrieve the purely aseismic signal, a presently impossible task for high time resolution GPS records due to low signal to noise.

Seismicity and velocity structure in the vicinity of repeating slow slip earthquakes, northern Hikurangi subduction zone, New Zealand

Using an 11-month (April 2012 to March 2013) deployment of Ocean Bottom Seismographs along the northern Hikurangi subduction zone, New Zealand, we located 2005 earthquakes (M 0.7-4.5) and obtained detailed 3-D Vp and Vs models for a region where a spectrum of subduction fault slip behavior is observed. Our analysis over a 120 × 110 km2 onshore-offshore area reveals high Vp/Vs (>1.8) in the subducting 10 km thick Hikurangi Plateau oceanic crust below an area of repeating slow slip events (SSEs), that are assumed to occur on the plate megathrust. Low Vp/Vs is mapped in a 15 km wide domain along the coastline, offshore Gisborne, marked by low Qs/Qp. Seismicity is absent from the core of high Vp/Vs anomalies (∼2.0) and within the band with lower Vp/Vs, but earthquakes are distributed vertically and are widespread in adjacent high Vp/Vs (∼1.85-1.95) regions along the trend of the coastline. Down-dip and along-strike variation of the Vp/Vs structure of the subducting Hikurangi Plateau could be the result of dehydration process caused by reactivated bending-related faults. Zones of vertical extensional fault–fracture in the subducting crust may act as conduits for the discharge of overpressured fluid and episodic hydrofracturing. During our deployment a large SSE (Mw6.8) occurred in February 2013. In response, seismicity became active in central Hawke Bay and at the down-dip edge of a previously mapped high-amplitude reflectivity zone. Microseismicity appeared synchronous with the slip propagation of the SSEs and may have been initiated by the slip. Our results hold implications for understanding of the physical environments conducive to slow slip generation and represent an important baseline for future seismicity work in the region.

Time to seismic failure induced by repeating SSEs in a single-degree-of-freedom spring-slider model

SUMMARYSome large earthquakes may have been triggered by large slow slip events (SSEs). This paper studies the time it takes from the most recent SSE to seismic failure in a situation where earthquakes are influenced by periodic SSEs, unlike in earlier studies that investigated the impact of a single SSE imposed at an arbitrary timing during a seismic cycle. A single-degree-of-freedom (SDOF) spring-slider model obeying a rate- and state-dependent friction (RSF) law is used to study the time to failure. The results are compared with those in Ohtani et al., which simulates megathrust earthquakes triggered by large, spontaneous SSEs along the deeper extension of a seismogenic zone in a 2-D elastic medium. Suppose a model fault under steady-rate tectonic loading is also impacted by stress steps induced by periodic SSEs. In the absence of SSEs, there is only a unique value of the characteristic slip distance L of RSF for a given seismic return period T. In the presence of SSEs, by contrast, synchronization occurs, and there exists a finite range of L values that corresponds to the same T. The timing (phase difference) of earthquakes relative to the SSEs varies continuously with L within that range. This study focuses on the case where T is triple the SSE return period, TSSE, (T = 3TSSE) to allow comparison with Ohtani et al. For each value of L in that range, disturbance tests that assign random values to T0, the timing of the third SSE within the seismic cycle, are conducted to obtain a probability distribution for tf, the time from that SSE to seismic failure. The distribution is converted into P(t; L), the cumulative probability that seismic failure occurs within time t of the SSE. The P(t; L) is averaged over the different L values to produce $\bar{P}$(t), which takes account of variability in the strength excess on the seismic fault at the time of the SSE, a parameter that is generally unknown. These numerical disturbance tests are conducted for three different values of an SSE size parameter. It is found that the larger the SSEs, the more intensely seismic failure is concentrated within short time intervals following an SSE. For the largest SSE, whereby a single SSE accounts for 3/10 of the total stress accumulated during a seismic cycle, there is a 50 per cent probability that seismic failure occurs within 132 d. These results contrast with those of Ohtani et al., in which tf was found to be concentrated more intensely within even shorter time intervals following an SSE (80 per cent probability that seismic failure occurs within 2.78 d of the SSE). It is suggested the timing of SSE-triggered seismic failure is concentrated more strongly on a fault embedded in a continuum because of a factor that cannot be taken into account by an SDOF model—a spatial structure of stress concentration that is already there on the fault even before the triggering event, the SSE.

Migration of shallow and deep slow earthquakes toward the locked segment of the Nankai megathrust

The Nankai megathrust is located offshore Shikoku and Kyushu, Japan and is characterized by various kinds of slow earthquakes whose relative motions across the plate boundary faults are slower than regular earthquakes. In the area, the interplate locking is stronger in the northern area (offshore Shikoku) than in the southern area (offshore Kyushu) and Mw ∼8 earthquakes (Nankai earthquakes) have occurred repeatedly in the northern area. In this paper, the spatio-temporal distributions of slow earthquakes (very low frequency earthquakes, tremors and slow-slip events) are examined based on the analyses of repeating earthquakes and slow earthquakes with special focus on the interaction between different activities. A comprehensive analysis of the seismic and geodetic data from 2003 to 2016 indicates complementary distribution of various types of slow earthquakes down to 35–50 km depth outside the Nankai main locking area. We also found interactions between different kinds of activities. The interactions between the repeating earthquakes and slow earthquakes suggest that the area of the repeating earthquakes activity can be divided into deeper (depth ≥ 20 km) and shallower (depth < 20 km) areas. The analyses of deep repeating earthquakes and the inland Global Navigation Satellite System (GNSS) data suggests slow northward migrations of long-term slow slip events (SSEs) in 20–50 km (offshore Kyushu) and 20–35 km (under Shikoku) depths along the plate boundary. These migrations occurred during a period of 2–3 years that includes the 2003 and 2010 large slow-slip events in the Bungo channel located in between Kyushu and Shikoku. The analysis has also shown interaction between shallow repeating earthquakes and shallow very low frequency earthquakes which indicates faster northward migrations of short-term SSEs from the shallow plate boundary offshore Kyushu to the deeper area under Shikoku over the duration of a month during the 2010 long-term slow-slip episode. The deep slow migration and the shallow to deep fast migration of SSEs in a ∼300 km area towards and around the source area of the recurrent Nankai earthquake (Mw 8.0–8.6) indicates the occurrence of a widespread non-steady stress build-up process around the source area of the Nankai megathrust earthquake.

Seismicity at the Northern Hikurangi Margin, New Zealand, and Investigation of the Potential Spatial and Temporal Relationships With a Shallow Slow Slip Event

AbstractIn 2014–2015, the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip experiment deployed seafloor absolute pressure gauges and ocean bottom seismometers directly above a large slow slip event, allowing examination of the relationship between slow slip and earthquakes in detail. Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip data were combined with nearby existing land stations to create a catalog of microseismicity consisting of 2,300 earthquakes ranging in magnitude between 0.5 and 4.7 that is complete to magnitude 1.5, yielding almost twice as many events as detected by the onshore networks alone. This greatly improves the seismicity catalog for this active subduction zone margin, especially in the offshore portion that was difficult to study using only the inland permanent seismic network. The new locations for the events within the footprint of the offshore network show that earthquakes near the trench are systematically shallower than and NW (landward) of their locations using only land‐based stations. Our results indicate that Hikurangi seismicity is concentrated in two NE‐SW bands, one offshore beneath the outer forearc wedge, one onshore beneath the eastern Raukumara Peninsula, and the majority of earthquakes are within the subducting Pacific plate with a smaller percent at the plate interface. We find a 20‐km wide northeast trending gap in microseismicity between the two bands and beneath the inner forearc wedge and this gap in seismicity borders the downdip edge of a slow slip patch.

Slow Slip Event Detection in Cascadia Using Vertical Derivatives of Horizontal Stress Rates

AbstractWe present a new approach assuming elastic behavior at the Earth's surface to invert continuous GPS time series for Vertical Derivatives of Horizontal Stress rates. This approach enables detection of transient slow slip events at the Cascadia subduction zone. Vertical Derivatives of Horizontal Stress rates are the most spatially compact expression of surface deformation due to subsurface deformation sources (compared to GPS displacement/velocity fields, and strain rates), and enable identification of transient slip events and temporal evolution of locking on the interface throughout the slow slip cycle. Slow slip events (SSEs) identified by our method coincide spatially and temporally with tremor episodes, and agree well with SSEs identified by previous geodetic studies. We present daily slip models that fit the Vertical Derivatives of Horizontal Stress rates, showing phases of locking, unlocking, and SSEs on the subduction interface for the 2009–2015 period. Similar to previous studies, our results highlight along‐strike variations in SSEs and locking behavior from northern Cascadia to southern Cascadia. Between events, the SSE source zone in northern Cascadia appears to be completely locked. In contrast, we suggest that some fraction of plate motion in the SSE zone in central and southern Cascadia is accommodated by steady creep (or very small, frequent SSEs) between large SSE events. We observe rapid relocking of the interface (within weeks) following the end of most SSEs, with implications for the time scales of fault healing following deformation events. Our results present a promising approach to transient deformation detection, with a fine temporal and spatial resolution of the surface expression of deformation.

Revealing the cluster of slow transients behind a large slow slip event.

Capable of reaching similar magnitudes to large megathrust earthquakes [Mw (moment magnitude) > 7], slow slip events play a major role in accommodating tectonic motion on plate boundaries through predominantly aseismic rupture. We demonstrate here that large slow slip events are a cluster of short-duration slow transients. Using a dense catalog of low-frequency earthquakes as a guide, we investigate the Mw 7.5 slow slip event that occurred in 2006 along the subduction interface 40 km beneath Guerrero, Mexico. We show that while the long-period surface displacement, as recorded by Global Positioning System, suggests a 6-month duration, the motion in the direction of tectonic release only sporadically occurs over 55 days, and its surface signature is attenuated by rapid relocking of the plate interface. Our proposed description of slow slip as a cluster of slow transients forces us to re-evaluate our understanding of the physics and scaling of slow earthquakes.

Observing and Modeling the Spectrum of a Slow Slip Event

AbstractWe estimate and model the normalized moment rate power spectrum of large slow slip events in Cascadia. We estimate the spectrum using data from GPS‐derived slip inversions, borehole strain records, and beamforming‐based tremor amplitudes. The normalized power spectrum initially decreases with frequency but then may flatten at periods of 1 to 10 days before decaying as frequency at higher frequencies, where nm is between 1.1 and 1.4 when estimated from tremor and between 0.4 and 1.5 when estimated from strain. We explore one way to understand the observed spectrum: by modeling a month‐long slow slip event as the sum of a steady background moment rate and a population of subevents. The subevents represent the wide variety of observed slow earthquakes, ranging from 0.5‐s‐long tremor to 3‐hr‐long rapid tremor reversals. We parameterize the subevents' magnitude distribution and moment‐duration scaling, and we examine how the subevent population determines the slow slip spectrum. There are not enough data to rigorously test the subevent model, but we show that the data are consistent with a single continuum of slow earthquakes whose moments scale linearly with their duration, as has been proposed previously.

Characteristics of a Sensitive Well Showing Pre-Earthquake Water-Level Changes

Water-level data recorded at a sensitive well next to a fault in central Japan between 1989 and 1998 showed many coseismic water-level drops and a large (60 cm) and long (6-month) pre-earthquake drop before a rare local earthquake of magnitude 5.8 on 17 March 1997, as well as 5 smaller pre-earthquake drops during a 7-year period prior to this earthquake. The pre-earthquake changes were previously attributed to leakage through the fault-gouge zone caused by small but broad-scaled crustal-stress increments. These increments now seem to be induced by some large slow-slip events. The coseismic changes are attributed to seismic shaking-induced fissures in the adjacent aquitards, in addition to leakage through the fault. The well’s high-sensitivity is attributed to its tapping a highly permeable aquifer, which is connected to the fractured side of the fault, and its near-critical condition for leakage, especially during the 7 years before the magnitude 5.8 earthquake.

Relating the long-term and short-term vertical deformation across a transect of the forearc in the central Mexican subduction zone

Author(s): Ramirez-Herrera, MT; Gaidzik, K; Forman, S; Kostoglodov, V; Burgmann, R; Johnson, CW | Abstract: Earthquake-cycle deformation, which includes earthquake ruptures, interseismic strain, and transient slow slip events, spans spatial scales ranging from fractions of a meter to thousands of kilometers. Similarly, temporal scales range from seconds during an earthquake rupture to thousands of years of strain accumulation between earthquakes. We discuss results regarding the vertical crustal deformation associated with both slow and rapid crustal deformation across a transect of the central Mexican subduction forearc in the Guerrero seismic gap, where the Cocos plate underthrusts the North America plate. This sector of the subduction zone is characterized by a flat-slab geometry with zones of sharp bending-unbending of the slab, irregularly distributed seismicity, and exceptionally large slow slip events. We used the river network, topography, geomorphic features, and morphometry on a transect across the forearc to assess Quaternary crustal deformation. The Papagayo drainage network shows that the forearc has been uplifted since the late Cenozoic (~25 Ma), and that rates of uplift increased since the beginning of the Holocene. Uplift is not homogeneous but shows a trend of increase away from the coast. This vertical deformation is strongly influenced by subduction processes. Thus, the Papagayo River network is strongly controlled by Holocene earthquake cycle processes. This is particularly true for the southern section of the drainage basin, where E-W-striking left-lateral strike-slip faults with a vertical component offset the course of the main river. These faults are accommodating part of the oblique plate convergence at the Mexican subduction zone. We measured the height of a series of terraces and dated quartz extracts by optically stimulated luminescence, and we calculated long-term rates of uplift ranging from 0.5 to 4.9 mm/yr. We discuss associations of forearc topography, faults, and long-term crustal deformation with the Cocos slab geometry, distribution of slow slip events, and earthquake-cycle deformation.

Variations in slow slip moment rate associated with rapid tremor reversals in Cascadia

AbstractDuring large slow slip events, tremor sometimes propagates in the reverse along‐strike direction for a few hours, at speeds 10 to 40 times faster than the forward propagation. We examine the aseismic slip that underlies this rapidly propagating tremor. We use PBO (Plate Boundary Observatory) borehole strainmeter data to search for variations in the slow slip moment rate during 35 rapid tremor reversals (RTRs) that occurred beneath Vancouver Island. The strain records reveal that, on average, the strain rate increases by about 100% ( ) during RTRs. Given the Green's functions expected for slip in the RTR locations, these strain rate increases imply 50 to 130% increases in the aseismic moment rate. The median moment released per RTR is between 8 and 21% of the daily slow slip moment, equivalent to that of a MW 5.0 to 5.1 earthquake. By combining the RTR moments with the spatial extents suggested by tremor, we estimate that a typical RTR has peak slip of roughly one‐sixth of the peak slip in the main slow slip event, near‐front slip rate of a few to ten times the main front slip rate, stress drop around half the main event stress drop, and strain energy release rate around one‐tenth that of the main front. Our observations support a picture of RTRs as aseismic subevents with high slip rates but modest strain energy release. RTRs appear to contribute to but not dominate the overall slow slip moment, though they may accommodate most of the slip in certain locations.

Repeating seismicity in the shallow crust modulated by transient stress perturbations

Recent studies have reported seismic phenomena that are modulated by small stress perturbations (∼10kPa), revealing their critically stressed nature. Such observations have been principally limited to plate interfaces with their occurrence linked to high fluid pore-pressure. In this study, we report observations of nine repeating seismic sources in the shallow crust in Guerrero, Mexico that emit events at rates comparable to other seismic phenomena in low stress environments. Testing their susceptibility to small stress perturbations, we find that all nine sources appear to be modulated by mining activity, tides, and a large slow slip event (Mw7.5). Our results suggest that the fault conditions necessary for low effective stress seismicity can occur away from plate interfaces.

Transient crustal movement in the northern Izu–Bonin arc starting in 2004: A large slow slip event or a slow back-arc rifting event?

The Izu–Bonin arc lies along the convergent boundary where the Pacific Plate subducts beneath the Philippine Sea Plate. Horizontal velocities of continuous Global Navigation Satellite System stations on the Izu Islands move eastward by up to ~1cm/year relative to the stable part of the Philippine Sea Plate suggesting active back-arc rifting behind the northern part of the arc. Here, we report that such eastward movements transiently accelerated in the middle of 2004 resulting in ~3cm extra movements in 3years. We compare three different mechanisms possibly responsible for this transient movement, i.e. (1) postseismic movement of the 2004 September earthquake sequence off the Kii Peninsula far to the west, (2) a temporary activation of the back-arc rifting to the west dynamically triggered by seismic waves from a nearby earthquake, and (3) a large slow slip event in the Izu–Bonin Trench to the east. By comparing crustal movements in different regions, the first possibility can be shown unlikely. It is difficult to rule out the second possibility, but current evidence support the third possibility, i.e. a large slow slip event with moment magnitude of ~7.5 may have occurred there.

Along-fault pore-pressure evolution during a slow-slip event in Guerrero, Mexico

Slow earthquakes are important constituents of the seismic cycle and are involved in the stress transfer between the viscously slipping portion of the plate interface and the seismogenic zone. Their occurrence is likely associated with the near-lithostatic pore pressure in the slow earthquake source region, where fluids might modify fault friction and whose presence is indicated by high ratios of compressional (P)–wave velocity to shear (S)–wave velocity observed at the interface between the subducting plate and the overlying crust. Here we compare two slow earthquake phenomena observed in the Guerrero region of the Mexican subduction zone: low-frequency earthquakes (LFEs) and a slow-slip event (SSE) recorded by GPS. We observe variations of the LFE occurrence rates over month-long time scales during a large SSE that we interpret as a manifestation of transient changes in the fault shear strength. We argue that these transient changes are caused by a pore pressure fluctuation that migrates updip along the subduction interface. This mechanism suggests that fluids do not only passively weaken the plate interface but also play an active role in slow earthquake source regions.

Tidal modulation and triggering of low‐frequency earthquakes in northern Cascadia

AbstractWe analyze the influence of Earth and ocean tides on the triggering of low‐frequency earthquakes (LFEs) in northern Cascadia using three LFE catalogs for southern Vancouver Island and Washington state from episodic tremor and slip events between 2003 and 2013. Sensitivities of LFE families to tidally induced fault normal stress, updip shear stress (UDSS), and corresponding time derivatives are computed and their geographic variability is mapped. We find localized areas showing higher sensitivity to UDSS than their surroundings, suggesting that tidal sensitivity depends on laterally heterogeneous physical properties such as variable pore fluid pressures and frictional properties along the plate interface. We observe that sensitivity of LFEs to UDSS rises dramatically from near zero on the first day of strong activity to a maximum ∼4 days later. In addition, the peak LFE rate transitions from a correlation with peak tidal shear stress rate to a correlation with peak tidal shear stress through large slow slip events. We identify 64 Rapid‐Tremor‐Reversals (RTRs) that start a few days after the main slip front. The RTRs have an average stress drop of ∼0.8 kPa and a majority (72%) occurs during periods of large positive UDSS. The combined observations imply that RTRs play an important role in slow slip processes and that modulation of creep rate due to tidal stress and tidal triggering of secondary events are jointly responsible for the observed tidal sensitivity.