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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Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip

The Electronic Seismologist (ES) has been known to actually do some research in the field of seismology from time to time. As an operator of a seismic monitoring network the research done often is related to the seismicity of the monitored region. Detecting changes or trends in seismicity is relevant to earthquake and volcano hazards, but are the trends detected real or only an artifact of changes in the network operating parameters? Because all seismic networks evolve, change staff, change software and hardware, there is always the nagging feeling, if not outright knowledge, that interesting patterns in the catalog reflect network changes rather than changes in the Earth. How can one tell the difference?

The ES is happy to report that there is a handy-dandy software package ideally suited to answering exactly this question (and many others). ZMAP , developed by Stefan Wiemer, allows the user to examine an earthquake catalog from many different angles. Not only does it include the traditional map, cross-section, and time sequence parameters, but also several others, such as event size and mechanism. These can be combined in interesting ways to present the user with different “views” into the data. Considerable seismological acumen lies behind the use and presentation of these parameters, which helps the user get the most out of the analyzed catalog. ZMAP is fairly intuitive to use and produces attractive output. In fact, the ES actually has fun “playing” with it and gets useful results besides. Perhaps one of the best ways to get a sense of how ZMAP might be used is to take a tour of case studies. The following includes many examples, and if they're not enough there are a slew of references where one can find more. In his traditional groveling way the ES has prevailed on Stefan …

A Software Package to Analyze Seismicity: ZMAP

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion No seismicity was associated with this event The sudden displacements are best explained by ∼2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 67 This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses

https://science.sciencemag.org/content/sci/292/5521/1525.full.pdf

A silent slip event on the deeper Cascadia subduction interface.

We test the concept that seismicity prior to a large earthquake can be understood in terms of the statistical physics of a critical phase transition. In this model, the cumulative seismic strain release increases as a power law time to failure before the final event. Furthermore, the region of correlated seismicity predicted by this model is much greater than would be predicted from simple elastodynamic interactions. We present a systematic procedure to test for the accelerating seismicity predicted by the critical point model and to identify the region approaching criticality, based on a comparison between the observed cumulative energy (Benioff strain) release and the power law behavior predicted by theory. This method is used to find the critical region before all earthquakes along the San Andreas system since 1950 with M≥6.5. The statistical significance of our results is assessed by performing the same procedure on a large number of randomly generated synthetic catalogs. The null hypothesis, that the observed acceleration in all these earthquakes could result from spurious patterns generated by our procedure in purely random catalogs, is rejected with 99.5% confidence. An empirical relation between the logarithm of the critical region radius (R) and the magnitude of the final event (M) is found, such that log R∝0.5M, suggesting that the largest probable event in a given region scales with the size of the regional fault network.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JB00792

An observational test of the critical earthquake concept

This study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes. Data on past great earthquake seismic limits have been compiled for four continental subduction zones, Cascadia, SW Japan, south Alaska, and Chile. These limits have then been compared to the predictions of several models for what constrains great earthquake rupture. Temperatures on the subduction thrusts have been estimated by finite element numerical models. The landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100°C, that is, depths of about 2 to 10 km for the subduction zones studied. This temperature agrees with the dehydration of stable sliding smectite clays to illite-chlorite. The temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow. The downdip limits correspond to the depth on the thrust where either (1) the temperature reaches about 350°C, which corresponds to thermally activated stable-sliding behavior for crustal rocks (with a transition to 450°C), or (2) about 40 km depth if 350°C is reached at greater depth. Depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc Moho, and this downdip limit may be a consequence of stable-sliding serpentinite or talc and other hydrated forearc mantle rocks. The primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile. Secondary control comes from the thickness of incoming sediment, the convergence rate, and the radioactive heat generation in the overlying forearc. The 100°C updip limit occurs near the trench for young subducting plates with a thick sediment section such as Cascadia (6–8 Ma), and up to 80 km landward for older oceanic crust such as south Alaska (∼50 Ma). The 350°C downdip thermal limit is applicable for young oceanic plates (e.g., Cascadia and Nankai), whereas the forearc mantle limit applies for older plates (e.g., south Alaska and Chile except near the Chile Rise). For the margins studied that have experienced great earthquakes, there is generally good agreement between the postulated thermal and Moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform, tsunami and dislocation modeling. The downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900060

The updip and downdip limits to great subduction earthquakes: Thermal and structural models of Cascadia, south Alaska, SW Japan, and Chile

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for ...

Summary of Coastal Geologic Evidence for past Great Earthquakes at the Cascadia Subduction Zone

The high likelihood of a gap-filling thrust earthquake in the Alaska subduction zone within this decade is indicated by two independent methods: analysis of historic earthquake recurrence data and time-to-failure analysis applied to recent decades of instrumental data. Recent (May 1993) earthquake activity in the Shumagin Islands gap is consistent with previous projections of increases in seismic release, indicating that this segment, along with the Alaska Peninsula segment, is approaching failure. Based on this pattern of accelerating seismic release, we project the occurrence of one or more M ≥ 7.3 earthquakes in the Shumagin-Alaska Peninsula region during 1994–1996. Different segments of the Alaska-Aleutian seismic zone behave differently in the decade or two preceding great earthquakes, some showing acceleration of seismic release (type “A” zones), while others show deceleration (type “D” zones). The largest Alaska-Aleutian earthquakes—in 1957, 1964, and 1965—originated in zones that exhibit type D behavior. Type A zones currently showing accelerating release are the Shumagin, Alaska Peninsula, Delarof, and Kommandorski segments. Time-to-failure analysis suggests that the large earthquakes could occur in these latter zones within the next few years.

Seismicity trends and potential for large earthquakes in the Alaska-Aleutian region

The volume-frequency distribution of rock falls and rock slides in the Yosemite Valley are well described by a simple power-law relationship, where log N(Vol) = 3.48 0.57(log Vol). This relationship, based on 214 documented rock-fall and rock-slide events that occurred from 1900 to 1992, allows determination of estimated return periods and probabilities for rock-fall events of different sizes. Based on this relationship, the largest prehistoric rock fall in the Yosemite Valley at Mirror Lake has an estimated return period of 325 years. On 10 March 1987 two massive rock falls from Middle Brother, with a combined volume of 600,000 m, spread across the talus cone and covered Northside Drive, blocking the primary exit from the Yosemite Valley. Brittle fracture indicated by rock popping noises and suggesting release of horizontal residual stress accompanied small ( <50 m) rock falls that preceded these two massive rock falls from Middle Brother. Removal of "key blocks" by the smaller rock falls may have released the interlocked geometry of closely jointed and fissured rock of the face of Middle Brother and permitted the failure of the much larger rock mass. During the subsequent months the number of continuing small rock falls at Middle Brother exhibited an inverse power law decay with time.

Stuart P. Nishenko

Papers

Summary of Coastal Geologic Evidence for past Great Earthquakes at the Cascadia Subduction Zone

Seismicity trends and potential for large earthquakes in the Alaska-Aleutian region

Analysis of rock falls in the Yosemite Valley, California