Improved Characterization of Slow

United States Geological Survey Earthquake Hazards Program

Final Technical Report

USGS Award Numbers G17AP00038 and G17AP00039 Improved Characterization of Slow Slip in Cascadia by Stacking GPS on Tremor Times: Collaborative Research with University of Alaska-Fairbanks and Miami University Authors:

Stephen Holtkamp Geophysical Institute University of Alaska Fairbanks 903 Koyukuk Dr., PO Box 757320 Fairbanks, AK, 99775 Office: (907)474-5751 [email protected]

Michael Brudzinski Department of Geology and Environmental Earth Science Miami University 250 S. Patterson Ave. Miami University Oxford, OH 45056 Office: (513) 280-0660 [email protected]

Other Personnel:

Nealey Sims, UAF graduate student Shannon Fasola, Miami University graduate student

Award Term:

February 1 2017 – January 31 2018

Acknowledgement of Support and Disclaimer: This material is based upon work supported by the U.S. Geological Survey under Grant No. G16AP00140. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the opinions or policies of the U.S. Geological Survey. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Geological Survey.

Abstract The origins of slow slip and how transient slip processes factor into the nature of the subduction zone seismic cycle have been the focus of research since the discovery of slow slip in subduction zones. Such efforts have included, but are not limited to, (1) understanding the along-dip extent of slow slip events, with one goal of this being to determine how slow slip events may relieve or increase stress on the locked portions of the megathrust (Beroza & Ide, 2011); and (2) probing the underlying fault physics for this new class of slow earthquakes, for example by determining various scaling relations (Aguiar et al., 2009). Despite these efforts, many key observations could be improved upon or are poorly resolved, such as the updip extent of slip (the vertical component of geodetic observations is often ignored because of high noise levels), or the nature of slip during inter-ETS events. In this study, we have investigated the viability of improving our characterization of slow slip to answer the questions posed above. We focused our efforts on two techniques to stack GPS time series (for each station individually, not cross-station stacking) using times of tremor episodes to produce geodetic observations of slow slip which have significantly higher signal to noise ratios. First, we utilized catalogs of individual tremor events to determine the timing of GPS stacks. Second, we utilized single seismic station mean amplitude processing to determine the timing of when to stack GPS signals. Both methods produced observations with increased signal to noise ratios that improve our ability to resolve slow slip. We have found that by stacking even a modest number of events (~10), we are able to resolve slips of ~0.5 mm. For ETS events, GPS stacking produces significantly better vertical observations and observations from stations up to ~200km away from the slipping patch. For the smaller inter-ETS events, which previously have only been observed geodetically in strainmeter time series, GPS stacking produces observations robust enough to invert for slip on the interface. Preliminary inversions of surface displacements to slip on the plate interface reveal that slow slip during inter-ETS times are deeper on the plate interface, consistent with the independent evidence from that inter-ETS tremor locations occur deeper as well. However, the location of inter-ETS slow slip is still shifted slightly updip from the zone of inter-ETS tremor, similar to what has been observed for ETS. Inversions of enhanced signals at the southern end of the Cascadia margin also indicate that some slow slip is occurring at the southern edge of the subducted slab, about 150 km south of the surface location of the Mendocino triple junction.

1. Introduction

Fault displacement during large earthquakes is a consequence of interseismic locking, the degree and spatial variability of coupling between the two plates. The discovery (Dragert et al., 2001; Ozawa et al., 2001) and subsequently recognized ubiquity (Peng & Gomberg, 2010) of slow slip phenomena at subduction margins worldwide has obscured the nature of interseismic coupling along subduction margins, but at the same time has led to a proliferation of theories regarding potential interaction between slow slip and large megathrust earthquakes (Bouchon et al., 2011; Kato et al., 2016; Mavrommatis et al., 2015). Aside from any potential relation to seismogenic locking, observations of slow slip phenomena have greatly informed theoretical efforts to understand strain accumulation and release along the transition zone from seismogenic locking and extending into the stable sliding zone (Wech et al., 2009; Wech & Creager, 2011). A variety of seismic and geodetic signals comprise the continuum of slow slip phenomena. Seismic examples recorded at subduction margins include tectonic tremor (Obara, 2002; Rogers & Dragert, 2003), low frequency earthquakes (LFE’s) (Shelly et al., 2006), very low frequency earthquakes (VLFE’s) (Ghosh et al., 2015; Ito et al., 2007), and earthquake swarms (though not directly – aseismic slip is often inferred) (Holtkamp & Brudzinski, 2011; Lohman & McGuire, 2007). Geodetic manifestations of slow slip include fault afterslip (Heki et al., 1997), short- (Obara & Hirose, 2006), and long- term (Larson, 2004; Ohta et al., 2006) slow slip events (SSE’s), which are readily observed in GPS and/or strainmeter data. In Cascadia, slow slip events and tremor occur together and are called Episodic Tremor and Slip (ETS). They were coined “episodic” because (1) they can occur with a high degree of regularity, and (2) they represent repeated failure of the same source region along the subduction megathrust. While slow slip events have been observed updip from the locked zone at other convergent margins worldwide (i.e., in the forearc wedge), ETS events in Cascadia are observed down-dip from the locked zone. Geodetic studies of interseismic locking in Cascadia generally agree that the locked zone is separated from the tremor zone by a resolvable, sometimes significant gap (Schmalzle et al., 2014), with several researchers proposing a potential zone of persistent fault creep separating the locked and tremor regions (Hyndman, 2013). Alternatively, slip during ETS events could be propagating up-dip from the tremor source region, a hypothesis supported by some geodetic modeling (Wech et al., 2009). In any case, the region between the locked zone and the tremor region is crucial to understanding any potential relation between ETS and seismic hazard. Aside from hazard implications, the nature of ETS is now considered to be crucial to understanding fault physics and mechanics, particularly with respect to depth on the megathrust. Wech et al. (2011) show, using tremor observations, that slip size and periodicity vary considerably with depth. They find that tremor on the deepest portion of the tremor zone occurs as smaller, more frequent bursts, and that tremor on the shallowest portion of the tremor zone occurs as larger, less frequent bursts, with a continuum of behaviors in between. This implies that the fault is weakening with depth. ETS events begin with deep tremor before transitioning into shallower regions with geodetically observable slip, at which point it is considered to be an ETS event. The similarity between the sources of tremor and slip during ETS events (Bartlow et al., 2011) has led to the development of a scaling law (Aguiar et al., 2009) relating the moment magnitude of the slow slip to the total duration of tremor (e.g., number of hours tremor is present). Unfortunately, GPS displacements expected from inter-ETS events are below the noise level of modern GPS receivers (<1mm maximum expected displacement). Wech et al. (2011) has shown that inter-ETS and ETS tremor behave in a self-similar manner, but a lack of GPS observations from inter-ETS events has prevented comparisons to ETS slip, and has prevented the scaling law hypothesized by Aguiar et al. (2009) from being extended to small events. Our objective with this project was to produce high quality stacked GPS observations of both ETS and inter-ETS events in Cascadia, and invert those surface displacements to slip on the plate interface. Current geodetic transient detection methods operate on individual slow slip events in the geodetic time series (e.g., Schmalzle et al., 2014). These detection methods require that the signal from a slow slip event is greater than the noise level of the geodetic network. In the horizontal component, this threshold is typically greater than 1 mm, and in the vertical component can be several mm. In Cascadia, this means that slow slip events with cumulative magnitude less than about Mw = 6 do not generate displacements at the surface which are robust. Also, most studies of slow slip events in Cascadia have not utilized vertical observations of slip, even though this component is most sensitive to the depth distribution of slip. Through GPS stacking (Frank et al., 2015), we first show that we can achieve significantly greater signal to noise ratio observations of ETS events along the Cascadia margin. We focus on two regions: Puget Sound and the Mendocino Triple Junction. We chose Puget Sound as our first focus area because this region has been extensively benchmarked by prior geodetic studies (Bartlow et al., 2011; Dragert et al., 2001; Holtkamp & Brudzinski, 2010). We show that we are able to resolve the horizontal deformation field out to hundreds of km away from the source region. We are also able to resolve the vertical deformation field throughout the region of interest, a key advancement from prior efforts. We next focus on the Mendocino region because very few studies have been able to extract robust estimates of slow slip in this tectonically complex region. We show that we are able to resolve observations of slow slip well south of the Mendocino Triple Junction, in contrast with prior studies (Schmalzle et al., 2014; Szeliga, et al., 2004). We next turn our attention to inter-ETS observations. Again, we focus on northern Cascadia (Puget Sound region), because this region has been extensively benchmarked. We show that we are able to resolve inter-ETS slow slip events with surface displacements of about ¼ mm. We are not able to resolve the vertical deformation field for inter-ETS events. Lastly, we attempted to push the limits of this geodetic stacking procedure by stacking borehole strainmeter data on an isolated family of low frequency earthquakes which displays recurrent swarm- like behavior, but we were not able to resolve this family of events geodetically.

2. Results

2.1 Tremor event catalogs

In this section, we utilize tremor event catalogs to identify the timing of ETS and inter-ETS events. We found the greatest success utilizing the Cascadia WECC catalog (Waveform Envelope Cross Correlation), which is automatically produced using the method of Wech et al (2008). We also attempted to utilize LFE catalogs from northern Cascadia to attempt to geodetically resolve slip from a small down-dip family of inter-ETS LFE’s, but the results of both GPS stacking and borehole strainmeter stacking were inconclusive.

2.1.1 Inter-ETS events Our objective with GPS stacking of inter-ETS events is to constrain the deformation field associated with inter-ETS events and invert those displacements for slip on the megathrust. First, we separate ETS tremor from inter-ETS tremor by selecting a large window, 1.0 degrees, and examining all

Figure 1: (top) Tremor histograms for the Puget Sound region. (top left) Weekly histogram of tremor activity shown in the bottom panel. Red symbols indicate ETS tremors and black/blue symbols indicate inter-ETS tremors. (top right) Daily histogram of tremors shown in the bottom panel. (bottom panel) Mapview showing tremor locations (red – ETS tremor; black/blue – inter-ETS tremor; blue – inter-ETS tremor subset for GPS stacking. Here, the separation in space between the red and black/blue tremor locations shows that our procedure is effectively isolating the more frequent down- dip inter-ETS tremor from ETS tremor. WECC locations within that latitude range. ETS events are characterized by high, sustained tremor rates, so we empirically set the ETS threshold here to 1000 events per week, or within two weeks of a 600 event week. After removing these events from the catalog, we next look at daily tremor counts, since inter-ETS event durations are on the order of hours to days. Here, we select 30 counts per day to represent inter-ETS events. Figure 1 shows the spatial distribution of ETS and inter-ETS events, as well as the timing of ETS and interETS events. The along-dip separation of ETS and inter-ETS events here, with inter-ETS events down dip from ETS events, shows that this method is effectively separating the two here. We isolate 26 interETS events for GPS stacking with these thresholds.

Next, we extract GPS position time series, ± 25 days from each inter-ETS event, for all stations within 0.5 degrees latitude of our region of interest, and remove any linear trend. We only select time periods for which there is at least 90% of the daily positions at each station, and only select stations with at least 10 events to stack. We then stack these observations and attempt to fit a hyperbolic tangent curve of the form: ( ) = + tanh 1 (1) 0 where x(t) are the GPS positions, V is the 𝑈𝑈velocity before𝑡𝑡−𝑇𝑇 and after the slow slip event, U is the 𝑥𝑥 𝑡𝑡 𝑉𝑉𝑉𝑉 2 � � 𝜏𝜏 � − � displacement during the ETS event, T0 is the median time of the transient, and is the duration of the

transient displacement. Here, we fix T0=0 (the timing of the inter-ETS events) and =1 day (the assumed duration of inter-ETS events) and fit Equation 1 to the resulting stack for each station𝜏𝜏 to solve for U and V. We tried several stacking techniques, and found that taking the median of each𝜏𝜏 daily (relative to the inter-ETS timing) set of positions was least sensitive to sporadic daily GPS solutions. Select stations from this inter-ETS stacking procedure are shown in Figure 2.

Figure 2: Illustration of the GPS stacking procedure and results for inter-ETS events. (left) Illustration of the GPS stacking procedure for station P440 in northern Washington. (left top) individual GPS position time series for each of the 26 inter-ETS events identified in Figure 1, plotted on the same axes. (left bottom) median stack of each relative daily position (blue line, also visible on left top). Best fit result from the hyperbolic tangent curve fitting is shown as a red line. U is the displacement of the inter-ETS stack. (B) Results from stations BELI and ARLI.

2.1.2 ETS events In this section, we apply the GPS stacking method using the WECC tremor solutions to attempt to get better geodetic observations of ETS slow slip during these repeating events. We focus on the Puget Sound region, which has been thoroughly benchmarked geodetically. Then, we focus on the Mendocino region, which has proven difficult to observe geodetically due to the small nature of slow slip and increased tectonic complexity (and common shallow earthquakes) in the southern Cascadia margin. The results of the Mendocino study will be presented in the Mean Amplitude Processing section.

For Puget Sound, we focus on the region centered at 48.5 degrees North latitude, corresponding to segment 5 in Holtkamp and Brudzinski (2010). We extract all WECC tremor solutions within 0.5

Figure 3: Illustration of the GPS stacking procedure and results for ETS events and typical results. (A) Illustration of the GPS stacking procedure for station COUP in northern Washington. (A top) individual GPS position time series for each of the 11 ETS events, plotted on the same axes. (A bottom) median stack of each relative daily position (blue line, also visible on left top). Best fit result from the hyperbolic tangent curve fitting is shown as a red line. U is the displacement of the inter-ETS stack. (B,C) Vertical component results from stations SC02 and SC03. (D) Contour map of the vertical deformation field.

degrees latitude of the region center, and bin those solutions into weekly counts. We classify WECC solutions as “ETS” tremors if there are more than 600 solutions per week, or within 2 weeks of a 600 solution week, taking the peak of ETS tremors as the center of the slow slip event (11 events total). We then extract all GPS time series, centered on the determined ETS events, with a duration of 80 days. We take the median of the daily positions at each day relative to the ETS timing determined above, and stack all solutions that have at least 90% data availability for these times. We do this for all components of the GPS time series – vertical observations are difficult to make for individual time series and are often ignored when producing solutions of slow slip displacements. Figure 3 shows examples of this stacking procedure and the quality of results we achieved with this method, along with a map of the vertical deformation field.

2.1.3 Inversions of ETS and inter-ETS solutions in northern Cascadia

Figure 4 shows our inversion for the stacked inter-ETS and ETS events in Northern Washington. All solutions go into the model. Uncertainties are taken from the standard error of the displacement

Figure 4: Fault slip inversions for ETS events (top) inter-ETS events (bottom). For the ETS inversion, we were able to incorporate vertical observations, as well as horizontal observations up to ~150km arcward from the slipping patch, a marked improvement on current observations of single episodes.

coefficient from the hyperbolic tangent curve fitting. We invert the resulting surface displacements for slip on the plate boundary interface using an elastic half-space. We use a recent formulation of Meade (Meade, 2007) for calculation of displacements through an elastic half-space from triangular dislocation elements, which has the advantage of providing a representation of the curved fault surface that is free of gaps. Solutions are smoothed (L2-norm regularization), and the best fit solution is picked off of an L- curve of misfit vs. model roughness.

Figure 5 shows a comparison between the ETS and inter-ETS slip models and tremor activity for the Puget Sound region. The ETS displacements used in the ETS inversion, like the displacements calculated from stacking of inter-ETS events, are calculated from stacks of the GPS displacements of ETS times derived from the WECC catalog (Figure 1a). The best fitting solution appears to suggest that a self- similar process occurs between ETS and inter-ETS slip magnitudes as well as tremor.

Figure 5: Comparison of ETS and inter-ETS slip and tremor. Red lines represent ETS events, and blue lines represent inter-ETS events. Solid lines represent slip on the interface, and dashed lines represent tremor epicenters mapped to the plate interface. Both the ETS and inter-ETS slip extend up-dip of the tremor locations. The inter-ETS slip is more heavily weighted towards the down-dip edge, consistent with the separation in tremor locations.

2.2 Mean Amplitude Processing In this section, we utilize single station mean amplitude processing to identify the timing of ETS events. Brudzinski and Allen (2007) processed continuous seismic data by removing instrument response, bandpass filtering from 2-6 Hz, and apply a Hilbert transform to calculate the envelope. Then, they calculated a rolling mean of the envelope, but exclude times with high max/mean ratio, which typically indicate seismic events. After high pass filtering to remove longer term seasonal trends and averaging over a rolling 4-day window, peaks in the resulting time series indicate periods where tremor dominates. It is important to note that this technique is empirically adjusted (e.g., 4-day averaging) to favor ETS events, but inter-ETS events are sometimes still visible. This technique is applicable to any station which records in the 2-6 Hz passband. The advantage of this technique is that we can identify ETS events for any region which has had even a single seismic station operating continuously, which has pushed back the time that we can extract GPS solutions to the onset of good GPS positioning (1997).

2.2.1 ETS events

Here, we focus on the Mendocino region at the southern Cascadia margin, centered on 40 degrees North latitude (±1 degree latitude). We take all stations with mean amplitude processing results from 1997 to 2007, which we will use to augment the ETS timing results from the WECC method discussed above (2007 and beyond). Since most stations were not operating continuously over this time frame, we interpolate these results onto a common timing, setting a value of 0 when the station was not recording. Then, in order to aggregate the results from different stations into one detection stream, we utilize the singular value decomposition. We first arrange each station time series (normalized on [0,1]) as a row in a matrix, then take the SVD of that matrix. We truncate the decomposition in order to capture the common signal at all stations, and take the peaks of that function to identify the timing of ETS events. With this procedure, we identify 32 ETS events from 1997 – 2017 (Figure 6).

Figure 6: Detected ETS events in the Mendocino Triple Junction region. (top) An aggregate of the SVD detection method on mean amplitude processing (light blue time series) and WECC detections (dark blue histogram). Red dots on both panels represent the timing of detected events. (bottom) GPS station TRND, the longest continuously operating station near the MTJ.

Figure 7 (top) shows representative time series from GPS stacking near the Mendocino Triple Junction. With the GPS stacking technique, we were able to resolve trenchward surface displacements down to 39 degrees South, well south of the Mendocino triple junction. Figure 7 (bottom) shows the displacement field and interface slip inversion for the Mendocino region. Interestingly, the inversion for this ensemble event requires some slip to occur at the southern edge of the slab, about 150 km south of the Mendocino triple junction. While the surface displacements near 39 degrees South are small (on the order of 0.5 mm), this suggests that trenchward slow slip is still occurring well south of the Mendocino Triple Junction.

Figure 7: GPS stacking and fault slip inversion in the vicinity of the Mendocino Triple Junction (top time series) GPS stacking results for four stations in the region: two just south of the MTJ, and two about 150 km south of the MTJ (bottom) Geodetic inversion of the GPS stack near MTJ.

2.3 Margin-wide stacking on individual tremor events

Lastly, we seek to investigate the relative efficiency of the ETS process in producing tremor (i.e., how much slip occurs for a given tremor detection. Although the WECC technique is perhaps not uniform in how tremors are detected along the margin, we start with a simplistic approach of treating each individual detection in the WECC catalog as an “event” to stack on. First, we segment the entire margin into 0.4 degree latitude bins. Within the bin, we extract GPS time series surrounding each event. For example, if there are 50 tremors in one day, that day gets stacked on 50 times. In this way, more emphasis is placed on days which have more tremor activity. A significant advantage of this technique is that it is completely automated – for example, no thresholding values are considered or separation of ETS and inter-ETS events. Figure 8 shows the results of this technique, normalized along strike by the number of tremors in each latitude bin. From this we can see that central Cascadia has the most slip per tremor, followed by northern Cascadia and then southern Cascadia. This pattern correlates with the recurrence intervals of ETS reported by Brudzinski and Allen (2007). One possible interpretation is that the region with long recurrence intervals produce larger magnitude slip for any given episode while the amount of tremor does not scale up with the larger amount of slip.

Figure 8: GPS stacking on every individual tremor event in the WECC catalog. This is a fully automated technique. Almost all stations display trenchward motions (including southern Cascadia, though they are difficult to see on this map).

3. Products of the Funded Research

Holtkamp, S.G., Brudzinski, M.R., Frank, W., Fasola, S. (2017) Stacking GPS observations on ETS and inter-ETS tremor times to improve geodetic observations of slip in Cascadia , EOS, Transactions AGU, 98 (52), Fall Meeting Supplement, Abstract S41C-0817.

Holtkamp, S.G., Brudzinski, M.R. (in preparation) Stacking GPS observations on ETS and inter-ETS tremor times to improve geodetic observations of slip in Cascadia, in preparation for submission to EPSL

We are still in the process of exploring the intricacies of the stacking and inversion procedure and results, particularly with respect to vertical ETS observations. As of now, we have significant systematic errors in the inversion of our ETS events with vertical observations (trenchward displacements are overpredicted and arcward displacements are underpredicted), which we suspect may be due to the inhomogenous structure of the crust overlying the subducting plate (Figure 9). A complete discussion and final results will be presented in a peer reviewed article.

Figure 9: Systematic errors in the vertical component of geodetic inversions. Here, you can see that the uplift signal is systematically overpredicted, while the subsidence signal is systematically underpredicted. This is the model presented in Figure 4 (top). We have the same issue with both the Audet et al. (2010) and McCaffrey (2009) plate interface models.

References Aguiar, A. C., Melbourne, T. I., & Scrivner, C. W. (2009). Moment release rate of Cascadia tremor constrained by GPS. Journal of Geophysical Research, 114(July 2008), 1–11. http://doi.org/10.1029/2008JB005909 Audet, P., Bostock, M. G., Boyarko, D. C., Brudzinski, M. R., & Allen, R. M. (2010). Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip. Journal of Geophysical Research: Solid Earth, 115(B4). Bartlow, N. M., Miyazaki, S., Bradley, A. M., & Segall, P. (2011). Space-time correlation of slip and tremor during the 2009 Cascadia slow slip event. Geophysical Research Letters, 38(18), 1–6. http://doi.org/10.1029/2011GL048714 Beroza, G. C., & Ide, S. (2011). Slow Earthquakes and Nonvolcanic Tremor. Annual Review of Earth and Planetary Sciences, 39(1), 271–296. http://doi.org/10.1146/annurev-earth-040809-152531 Bouchon, M., Karabulut, H., Aktar, M., Ozalaybey, S., Schmittbuhl, J., & Bouin, M.-P. (2011). Extended nucleation of the 1999 Mw 7.6 Izmit earthquake. Science, 331(6019), 877–880. doi:10.1126/science.1197341 Brudzinski, M. R., & Allen, R. M. (2007). Segmentation in episodic tremor and slip all along Cascadia. Geology, 35(10), 907. http://doi.org/10.1130/G23740A.1 Dragert, G., Wang, K., & James, T. S. (2001). A silent slip event on the deeper Cascadia subduction interface. Science, 292(5521), 1525–8. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11313500 Frank, W. B., Radiguet, M., Rousset, B., Shapiro, N. M., Husker, A. L., Kostoglodov, V., … Campillo, M. (2015). Uncovering the geodetic signature of silent slip through repeating earthquakes. Geophysical Research Letters, 42(8), 2774–2779. http://doi.org/10.1002/2015GL063685 Ghosh, A., Huesca-Pérez, E., Brodsky, E., & Ito, Y. (2015). Very low frequency earthquakes in Cascadia migrate with tremor. Geophysical Research …, 3228–3232. doi:10.1002/2015GL063286. Heki, K., Miyazaki, S., & Tsuji, H. (1997). Silent fault slip following an interplate thrust earthquake at the Japan Trench. Nature, 386, 595–598. Retrieved from http://www.nature.com/nature/journal/v386/n6625/abs/386595a0.html Holtkamp, S., & Brudzinski, M. R. (2010). Determination of slow slip episodes and strain accumulation along the Cascadia margin. Journal of Geophysical Research, 115, 1–21. http://doi.org/10.1029/2008JB006058 Ito, Y., Obara, K., Shiomi, K., Sekine, S., & Hirose, H. (2007). Slow Earthquakes Coincident with Episodic Tremors and Slow Slip Events. Science, (315), 503–506. doi:10.1007/s13398-014-0173-7.2 Kato, A., Fukuda, J., Kumazawa, T., & Nakagawa, S. (2016). Accelerated nucleation of the 2014 Iquique, Chile Mw 8.2 Earthquake. Scientific Reports, 6(April), 24792. doi:10.1038/srep24792 Larson, K. M. (2004). Crustal deformation measurements in Guerrero, Mexico. Journal of Geophysical Research, 109(B4), 1–19. doi:10.1029/2003JB002843 Lohman, R. B., & McGuire, J. J. (2007). Earthquake swarms driven by aseismic creep in the Salton Trough, California. Journal of Geophysical Research-Solid Earth, 112(B4), B04405. Mavrommatis, A. P., Segall, P., Uchida, N., & Johnson, K. M. (2015). Long-term acceleration of aseismic slip preceding the Mw 9 Tohoku-oki earthquake: Constraints from repeating earthquakes. Geophysical Research Letters, 1–9. doi:10.1002/2015GL066069 McCaffrey, R. (2009). Time‐dependent inversion of three‐component continuous GPS for steady and transient sources in northern Cascadia. Geophysical Research Letters, 36(7). Meade, B. J. (2007). Algorithms for the calculation of exact displacements, strains, and stresses for Computers & Geosciences 33 triangular dislocation elements in a uniform elastic half space☆. , (8), 1064–1075. http://doi.org/10.1016/j.cageo.2006.12.003 Obara, K. (2002). Nonvolcanic deep tremor associated with subduction in southwest Japan. Science (New York, N.Y.), 296(5573), 1679–81. doi:10.1126/science.1070378 Obara, K., & Hirose, H. (2006). Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone. Tectonophysics, 417(1-2), 33–51. doi:10.1016/j.tecto.2005.04.013 Ohta, Y., Freymueller, J., Hreinsdottir, S., & Suito, H. (2006). A large slow slip event and the depth of the seismogenic zone in the south central Alaska subduction zone. Earth and Planetary Science Letters, 247(1-2), 108–116. doi:10.1016/j.epsl.2006.05.013 Ozawa, S., Murakami, M., & Tada, T. (2001). Time-dependent inversion study of the slow thrust event in the Nankai trough subduction zone, southwestern Japan. Journal of Geophysical Research, 106(B1), 787–802. Retrieved from http://www.agu.org/journals/ABS/2001/2000JB900317.shtml Peng, Z., & Gomberg, J. (2010). An integrated perspective of the continuum between earthquakes and slow-slip phenomena. Nature Geoscience, 3(9), 599–607. doi:10.1038/ngeo940 Rogers, G., & Dragert, H. (2003). Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip. Science (New York, N.Y.), 300(5627), 1942–3. doi:10.1126/science.1084783 Schmalzle, G. M., McCaffrey, R., & Creager, K. C. (2014). Central Cascadia subduction zone creep. Geochemistry, Geophysics, Geosystems, 15(4), 1515–1532. http://doi.org/10.1002/2013GC005172 Shelly, D. R., Beroza, G. C., Ide, S., & Nakamula, S. (2006). Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip. Nature, 442(7099), 188–91. doi:10.1038/nature04931 Szeliga, W., Melbourne, T. I., Miller, M. M., & Santillan, V. M. (2004). Southern Cascadia episodic slow earthquakes. Geophysical Research Letters, 31(16). http://doi.org/10.1029/2004GL020824 Wech, A. G., & Creager, K. C. (2008). Automated detection and location of Cascadia tremor. Geophysical Research Letters, 35(20), 1–5. http://doi.org/10.1029/2008GL035458 Wech, A. G., & Creager, K. C. (2011). A continuum of stress, strength and slip in the Cascadia subduction zone. Nature Geoscience, 4(9), 624–628. doi:10.1038/ngeo1215 Wech, A. G., Creager, K. C., & Melbourne, T. I. (2009). Seismic and geodetic constraints on Cascadia slow slip. Journal of Geophysical Research, 114(B10), 1–9. doi:10.1029/2008JB006090