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We present seafloor pressure records from the Cascadia Subduction Zone, alongside oceanographic and geophysical models, to evaluate the spatial uniformity of bottom pressure and optimize the geometry of sensor networks for resolving offshore slow‐slip transients. Seafloor pressure records from 2011 to 2015 show that signal amplitudes are depth‐dependent, with tidally filtered and detrended root‐mean‐squares of <2 cm on the abyssal plain and >6 cm on the continental shelf. This is consistent with bottom pressure predictions from circulation models and comparable to deformation amplitudes from offshore slow slip observed in other subduction zones. We show that the oceanographic component of seafloor pressure can be reduced to ≤1‐cm root‐mean‐square by differencing against a reference record from a similar depth, under restrictions that vary with depth. Instruments at 100–250 m require depths matched within 10 m at separations of <100 km, while locations deeper than 1,400 m are broadly comparable over separations of at least 300 km. Despite the significant noise reduction from this method, no slow slip was identified in the dataset, possibly due to poor spatiotemporal instrument coverage, nonideal deployment geometry, and limited depth‐matched instruments. We use forward predictions of deformation from elastic half‐space models and hindcast pressure from circulation models to generate synthetic slow‐slip observational records and show that a range of slip scenarios produce resolvable signals under depth‐matched differencing. For future detection of offshore slow slip in Cascadia, we recommend a geometry in which instruments are deployed along isobaths to optimize corrections for oceanographic signals. Slow‐slip earthquakes, a special class of earthquake in which slip along a fault occurs over periods of days to years without producing shaking, have increasingly been found to occur in the subsea segments of subduction zones worldwide. There is evidence that these offshore slow‐slip earthquakes can occur within the same region where large, damaging earthquakes are generated and that they may precede and potentially trigger these events. Offshore slow‐slip earthquakes cause the seafloor to move vertically by up to a few centimeters, which can be detected by measuring the pressure on the seafloor if the effects of ocean tides and circulation can be corrected for. In this study, we search for evidence of slow‐slip earthquakes in seafloor pressure data from the Cascadia Subduction Zone, off the U.S. and Canadian west coast, and assess how to best reduce oceanographic signals in these data in order to reliably observe slow‐slip deformation. No evidence for these events is seen in Cascadia pressure data from 2011 to 2015. Using a combination of observational data and model simulations, we show that oceanographic signals can be largely eliminated from seafloor pressure measurements if instruments are placed in rows of constant depth. A network of 22 instruments would be sufficient to monitor for large offshore slow‐slip earthquakes offshore central Oregon, but smaller events would be difficult to detect with this method. By increasing our ability to detect these slow‐slip earthquakes, we can better understand subduction zone processes and their relation to large, damaging megathrust earthquakes. In Cascadia, differencing tidally filtered, depth‐matched seafloor pressure records can reduce oceanographic noise to less than 1‐cm RMSModels of offshore SSEs in Cascadia suggest that a network of depth‐matched sensors at 30‐km spacing can detect Mw6.4 and larger eventsNo offshore slow‐slip earthquakes are detected in the available Cascadia seafloor pressure data from 2011 to 2015

Graphical Abstract:

The boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up‐dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self‐calibrating pressure recorder. A piston‐gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey‐style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long‐term, sensor‐independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co‐located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation. The Cascadia subduction zone has produced large earthquakes and tsunamis whose potential size and interval is affected by the amount and distribution of locking between the tectonic plates. A large portion of the subduction zone is offshore, where typical land‐ and satellite‐based methods are ineffective at measuring crustal changes. Seafloor water pressure observations can be used to measure height changes, but pressure gauges inherently drift at rates typically greater than the expected vertical seafloor deformation rates. The absolute self‐calibrating pressure recorder (ASCPR) measures the true, absolute, sensor‐independent seafloor pressure by addressing and correcting sources of error caused by the internal piston gauge calibrator and recording pressure gauges. The accuracy of our measurements is about 25 ppm of the water depth, or about 2.5 cm of height per 1,000 m of water. Campaign survey‐style measurements using the ASCPR at seven benchmarks off the coast of Newport, Oregon from 2014 to 2017 establish a long‐term record of absolute measurements that can be referenced by studies decades or more in the future or can estimate and correct drift of nearby continuous pressure gauges. Continued measurements can provide insights on seafloor deformation and thus locking in Cascadia. Campaign‐style surveys of absolute calibrated seafloor pressure measurements were made in the Cascadia subduction zone from 2014 to 2017These sensor‐independent measurements act as long‐term, absolute reference values that can be used for future vertical deformation studiesWe document and quantify the sources of error in the technique with a total uncertainty of ∼25 ppm, or ∼0.25 kPa per 1,000 m water depth Campaign‐style surveys of absolute calibrated seafloor pressure measurements were made in the Cascadia subduction zone from 2014 to 2017 These sensor‐independent measurements act as long‐term, absolute reference values that can be used for future vertical deformation studies We document and quantify the sources of error in the technique with a total uncertainty of ∼25 ppm, or ∼0.25 kPa per 1,000 m water depth

Land‐surface subsidence is a major contributor to land loss in many river deltas. New approaches yielding high‐resolution data are needed to parse the relevant driving forces. In 2016, we established a novel “subsidence superstation” ∼2 km from the Mississippi River in coastal Louisiana (USA) to measure compaction in a global reference frame as a function of depth in Holocene sediments and deeper subsidence. The site features three borehole optical fiber strainmeters to obtain continuous records of displacement between ∼1.3 m below the surface and depths of ∼11, 25, and 38 m. These data are complemented by an adjacent station providing hydrologic data and near‐surface compaction. We also installed three GPS antennas, one of which is mounted to a rod cemented into the Pleistocene basement. A core from one of the boreholes provides insight into the sediment properties of the entire Holocene succession. Five years of records reveal the compaction rate in the material between 1.3 and 38 m is likely less than 0.25 mm/yr. The GPS records yield a subsidence rate of 2.5 mm/yr regardless of the depth of the anchor, thus corroborating the low compaction rates observed by the strainmeters. The new instrumental records show that current subsidence at this location is governed mostly by deformation of the Pleistocene or underlying strata rather than compaction of Holocene material, with the exception of the uppermost meter. The methodology represents an important new approach to mapping subsidence rates at varying depths, providing insight into the mechanisms governing delta subsidence. It is important to learn about the rate and causes of land subsidence along low‐lying coastlines and deltas where a large fraction of the world's population lives. In the Mississippi Delta, sediment deposited during the last ∼11,000 years (Holocene) has gradually compacted under its own weight. Constraining the path of the Mississippi River with levees over the past century has prevented deposition of new sediment over most of the delta, causing the land surface to drop with respect to sea level, increasing the threat from inundation by seawater. We measured the compaction of a nearly 40 m thick sediment layer at one site in the Mississippi Delta using a combination of optical fiber strainmeters in boreholes to several depths and GPS antennas mounted near the land surface and to deeper points. We found that the subsidence at our site, which occurs at a rate of several mm per year, is caused by very shallow (of order 1 m) and very deep (below 40 m) processes and is not driven by compaction of the bulk of the 38 m thick Holocene layer. First integration of multiple coastal subsidence monitoring techniques at one siteHigh‐resolution records from co‐located optical fiber strainmeters and GPS instruments corroborate one anotherThe site exhibits isostatic subsidence with a potential role of fluid extraction and negligible compaction First integration of multiple coastal subsidence monitoring techniques at one site High‐resolution records from co‐located optical fiber strainmeters and GPS instruments corroborate one another The site exhibits isostatic subsidence with a potential role of fluid extraction and negligible compaction

In many interferometers, two fringe signals can be generated in quadrature. The relative phase of the two fringe signals depends on whether the optical path length is increasing or decreasing. A system is developed in which two quadrature fringe signals are digitized and analyzed in real time with a digital signal processor to yield a linear, high-resolution, wide-dynamic-range displacement transducer. The resolution in a simple Michelson interferometer with inexpensive components is 5 × 10^-13 m Hz^-1/2 at 2 Hz.

Conventional tide gauges are usually housed along the coast. Satellite altimetry works well in the open ocean but poorly near the coast due to signal contamination by land returns. These limitations lead to an observational gap in the transition zone between the coast and open ocean. Using data collected by a GPS installed on top of an anchored spar‐buoy in Tampa Bay, we retrieved water levels through a combination of precise positioning and interferometric reflectometry. Individual water level retrievals agree with a nearby acoustic tide gauge (19.5 km distance) at ∼15 cm level. Amplitude and phase of the major tidal constituents are well recovered by the GPS spar‐buoy measurements. Over a 2.9‐year period, agreement of de‐tided daily mean sea levels measured by the GPS spar‐buoy and the nearby acoustic tide gauge is 4.4 cm. When sea level data measured by the GPS spar‐buoy are included in the local coastal ocean circulation model, low‐frequency error propagated from the open boundary is significantly reduced. GPS receivers record direct signals from satellites as well as reflected signals from local objects. The reflected signal can interfere with the direct signal, enhancing or reducing overall signal strength. This characteristic can be used to measure the height difference between the GPS antenna and the reflecting surface. We used GPS data collected by a spar‐buoy anchored in Tampa Bay to calculate water levels at different times. The calculated water levels can be used to study sea level change, ocean circulation, and tidal height predictions. An anchored spar‐buoy seafloor geodetic system is used to measure offshore sea levels based on Global Positioning System interferometric reflectometry (GPS‐IR)Agreement of de‐tided daily mean sea levels measured by the GPS spar‐buoy and the closest acoustic tide gauge (19.5 km away) is 4.4 cmSea levels measured with the GPS spar‐buoy can help improve coastal ocean circulation models An anchored spar‐buoy seafloor geodetic system is used to measure offshore sea levels based on Global Positioning System interferometric reflectometry (GPS‐IR) Agreement of de‐tided daily mean sea levels measured by the GPS spar‐buoy and the closest acoustic tide gauge (19.5 km away) is 4.4 cm Sea levels measured with the GPS spar‐buoy can help improve coastal ocean circulation models

In an absolute gravity meter, a laser interferometer measures the position of a test mass that is falling in a vacuum. The calculated value of gravity is the average acceleration of the mass during a set of drops. Since systematic accelerations of the optical system will bias the measured value of gravity, various interferometer geometries have been implemented in the past to isolate the optical system from ground motion. We have developed and tested a low-finesse fiber-optic extrinsic Fabry–Perot interferometer that is fixed to the mass of a critically damped seismometer in which the effects of systematic ground motion and acoustic vibrations are reduced.

Successful development of a few parts in 10 9portable gapparatus (which corresponds to a height sensitivity of about 1 cm) would have an impact on large areas of geodynamics as well as having possible application to earthquake prediction. Furthermore, the use of such an instrument in combination with classical leveling or extraterrestrially determined height data would yield information on internal mass motions. The plans for the development of such an instrument at JILA using the method of free fall will be given. The proposed interferometric method uses one element of an optical interferometer as the dropped object. Recent work has resulted in substantial progress towards the development of a new type of long-period ( T> 60 sec) suspension for isolating the reference mirror (corner cube) in the interferometer. Improvements here over the isolation methods previously available, together with state-of-the-art timing and interferometric techniques, are expected to make it possible to achieve a few parts in 10 9accuracy with a field-type instrument.

We report on laboratory experiments on single-mode optical fibers for use in measuring earth strain. We have monitored the long-term stability of 25-m long tensioned fibers and found their rates of fractional change in optical path lengths to be no more than 2 × 10^−6/yr. The optical temperature coefficients for several fibers whose physical lengths were held constant were found to be within 4% of 1.17 × 10^−5 apparent strain/°C. The strain sensitivity (the ratio of observed optical path change to physical path change) was determined to be within 1% of 1.16 for all the fibers tested. Initial field tests indicate that fibers are suitable for earth strain measurements of moderate precision.

The frequency of a commercial hard-sealed He–Ne laser has been stabilized to the center of the discharge gain profile using the mode splitting caused by an axial magnetic field. The absolute frequency has been measured repeatedly during intermittent operation over 20 months. Its drift over most of this period has been 2.6 ± 0.7 MHz/yr. The lock point has been found to depend slightly on the laser tube's temperature.

A five-year record of the lockpoint frequency of a Zeeman stabilized laser shows an observed drift rate of 0.3 ± 0.5 MHz/yr following an initial drift of 5.7 ± 2.2 MHz/yr in the first eighteen months of intermittent operation. A second Zeeman laser drifted at a rate of −0.8 ± 1.0 MHz/yr over the last 2.5 yr; the frequency drift was −0.2 ± 0.6 MHz/yr over the last 3.3 yr. Empirical temperature corrections to laser frequency measurements produce a slight variance reduction in the data but no effective bias in the drift estimates.