G21B-0688
Completion of the PBO Borehole Strainmeter Network: Network Results and Review of Processing Techniques.
By October 2008 the NSF funded geodetic component of Earthscope, the Plate Boundary Observatory will be
complete. As of September 2008 the strainmeter network within PBO consists of 70 co-located borehole
strainmeters and seismometers, with borehole tiltmeters included at volcanic sites. The instruments are
installed in arrays from Vancouver Island, Canada, to Anza in southern California. The network provides an
improved ability to record the evolution of strain transients such as those associated with Episodic Tremor
and Slip (ETS) events and slow fault slip. Already, the strainmeters in the Pacific North West have recorded
three Cascadia ETS events, providing unique temporal and spatial resolution of the signals. Raw data are
available to the community from the IRIS DMC and the NCEDC in SEED format within minutes of being
downloaded from the loggers. In addition to the raw data, UNAVCO's PBO-Borehole Strainmeter Analysis
Center provides a processed (Level 2) data set with a barometric pressure, tide and long-term trend
correction that is updated every 14 days.
The main purpose of the strainmeter network is to record strain transients that fall between the detection
levels of seismology and GPS. To identify such transients, strain changes induced by non-tectonic signals
must be removed from the data. Two main sources of noise are barometric pressure and borehole relaxation.
The current Level 2 barometric pressure corrections are produced assuming pressure and strain changes
are linearly related. However, this simple model does not completely remove the pressure signal and, in some
cases, can even introduce noise. We shall compare the current linear correction with a frequency-
dependant correction to seek an improved pressure correction technique. We shall also compare strain
residuals from trend models generated using a combination of linear and exponential terms with residuals
calculated using high pass filtering to remove the long-term borehole trends. If the improved pressure and
trend corrections lead to a cleaner processed data set then these new methods should be included in routine
Level 2 data strainmeter data products.
http://pboweb.unavco.org/
G21B-0689
A new method to process borehole strainmeter data; least squares with correlated data
The newly installed Plate Boundary Observatory (PBO) strainmeters record signals from tectonic activity,
Earth tides, and atmospheric pressure. Some of the tectonic signals have amplitudes close to those of tides
and pressure loading. If incorrect assumptions are made regarding the background noise in the data, then
adjusting these strain data will produce incorrect results that can obscure or contaminate any underlying
tectonic signal. The use of simplifying assumptions that data are uncorrelated can lead to such incorrect
results and, for example, pressure loading will not be completely removed from the raw data. Instead, any
algorithm used to process strainmeter data must incorporate the strong temporal correlations that are
inherent with these data. For instance, techniques based on auto-regressive methods or Kalman filters can
successfully remove the pressure load and the Earth tides. The technique described here is adapted from
error analysis of geodetic time-series of ground displacements. The technique uses least squares but
employs data covariance that describes the temporal correlation of strainmeter data. There are several
advantages to this method since many parameters are estimated simultaneously. These parameters include:
1) functional terms that describe the underlying error model, 2) the tidal terms, 3) the pressure loading
term(s) 4) amplitudes of offsets, either those from earthquakes or from the instrument, 5) rate and changes
in rate, and 6) the amplitudes and time constants of either logarithmic or exponential curves that can
characterize postseismic deformation or diffusion of fluids near the strainmeter. With the proper error model,
realistic estimates of the standard errors of the various parameters are obtained; this is especially critical in
determining the statistical significance of a postulated, tectonic strain signal. Because the algorithm uses a
maximum likelihood method, it is cpu-intensive. However, obtaining the error model by fitting a power-law
relation to the power spectrum of the adjusted data greatly reduces the computations. The algorithm
described here also provides a method of tracking the various adjustments required to process strainmeter
data. In addition, the algorithm provides several plots to assist with identifying either tectonic signals or other
signals that may need to be removed before any geophysical signal can be identified.
http://quake.wr.usgs.gov/research/deformation/twocolor/CLEANSTR/
G21B-0690
Tidal Calibration of Multicomponent Borehole Strainmeters: The Roles of Vertical and Shear Coupling
To measure tectonic strain, a borehole strainmeter is designed to deform more than the surrounding formation, but the precise coupling parameters relating strainmeter output to formation strain depend on local rock moduli and therefore must be estimated after installation. Usually this "calibration" procedure entails estimating the amplitudes and phases of the M2 and O1 earth tide variations in the strainmeter output, and choosing coupling parameters to reconcile them with model-calculated tides. For many of the Plate Boundary Observatory Gladwin Tensor Strainmeters (PBO GTSMs), two modifications to previously published coupling models can greatly improve agreement with model tides. The PBO GTSMs each consist of four horizontal extensometers. The first important modification is that each extensometer is coupled to vertical strain, a feature motivated by the PBO GTSMs' large responses to atmospheric pressure changes. Some PBO GTSMs respond strongly enough to vertical coupling that the free-surface extension induced by areal contraction results in a negative apparent areal-strain coupling parameter. Rotating the tidal strains to x,y coordinates with the x-axis parallel to each extensometer illuminates the second modification. The extensometer is expected to be positively coupled to εxx, negatively and less strongly coupled to εyy, and uncoupled to εxy. However, allowing some extensometers of some PBO GTSMs to have a small coupling to εxy can bring the model and observed tides into good agreement, and is not unreasonable given that the strainmeter host rock is not perfectly uniform or isotropic. Graphical inspection of the extensometer and model tide phasors reveals which extensometers appear coupled to εxy, although for at least two GTSMs, this shows that only negative coupling to εxx-εyy could reconcile the observed and model tides. Unfortunately, if all 4 extensometers require εxy coupling, there is a trade-off between the coupling parameters and an orientation correction. Allowing for vertical strain and εxy coupling, model and observed tides can be reconciled for strainmeters in California, where the tidal model is otherwise supported, as well as for some non-coastal strainmeters in Cascadia, where no other instruments have verified the strain tides. When neither vertical nor εxy coupling is significant, this approach will yield similar results to that of Hart et al. (JGR, 1996).
G21B-0691 INVITED
New Insights into Cascadia Slow Slip Events Using Plate Boundary Observatory Borehole Strainmeters
Until recently, tremor and slow slip events in the Cascadia subduction zone have been recorded only on seismic and GPS networks, respectively. The new Plate Boundary Observatory (PBO) borehole strainmeters record high-sensitivity, high-resolution (20Hz) time series of the horizontal strain fields that result from tremor and slow slip events, much higher than that resolvable from the current GPS network. We characterize and compare the strain time series of three successive northern Cascadia slow slip events in September 2005, January-February 2007, and May 2008. While the raw strainmeter gauge data are dominated by strains induced by atmospheric pressure changes, after correction for atmospheric pressure and conversion to horizontal shear strain components, the slow slip signal is very large- several times the tidal signal on several stations and more than an order of magnitude larger than the background noise level. With the occurrence of each ensuing event, PBO network coverage has densified such that comparisons can be made between all three slow slip events on one strainmeter (B004), and between the 2007 and 2008 events on six strainmeters. For the 2005 and 2007 events, B004 recorded similar sizes, durations, and patterns of differential extension, εEE-εNN, and east-north shear, εEN. The 2007 and 2008 slow slip events were recorded concurrently on more than one strainmeter, revealing both similarities and differences between the 2007 and 2008 events. We see the slipping zone propagate, as revealed by successive onsets at different stations, by sign reversals in individual strain time series, and by temporal changes in the ratio of εEE-εNN to εEN. For example, while the onset of the strain signal is similar in 2007 and 2008 on strainmeters B004 and B009, the rate of propagation of the slow slip event is slower in 2008. This difference is reflected in the duration and relative timing of the strain signal at the two strainmeters. For each event, we found the progression of the slow slip events inferred from the strainmeter data to be broadly consistent with that suggested by GPS data and the migration of tremor epicenters. Using the strainmeters, however, we can more clearly resolve the rise time and better constrain rupture propagation characteristics, giving us an improved estimate of the location, progression, and duration of the slow slip events.
G21B-0692
An Investigation on the Crustal Deformations in Istanbul after Eastern Marmara Earthquakes in 1999
Since the introduction of the GPS technique in mid 1970's there has been great advances in positioning activities. Today such Global Navigational Satellite Systems (GNSS) based positioning techniques are widely used in daily geodetic applications. High order geodetic network measurements are one of such geodetic applications. Such networks are established to provide reliable infrastructures for all kind of geodetic work from the production of cadastral plans to the surveying processes during the construction of engineering structures. In fact such positional information obtained in such engineering surveys could be useful for other studies as well. One of such fields is geodynamic studies where such positional information could be valuable to understand the characteristics of tectonic movements. In Turkey being located in a tectonically active zones and having major earthquakes quite frequently, the positional information obtained in engineering surveys could be very useful for earthquake related studies. In this paper an example of such engineering surveys is discussed. This example is the Istanbul GPS (Global Positioning System) Network, first established in 1997 and remeasured in 2005. Between these two measurement processes two major earthquakes took place, on August 17 and November 12, 1999 with magnitudes of 7.4 and 7.2, respectively. In the first measurement campaign in 1997, a network of about 700 points was measured, while in the second campaign in 2005 more than 1800 points were positioned. In these two campaigns are existing common points. The network covers the whole Istanbul area of about 6000 km2. All network points are located on the Eurasian plate to the north of the North Anatolian Fault Zone. In this study, the horizontal and vertical movements are presented and compared with the results obtained in geodynamic studies.
G21B-0693 INVITED
Sacks-Evertson Borehole Strainmeters: New Designs, Volcanic Activity and Slow Earthquakes
The quality of borehole strain data depends on a variety of factors, perhaps the most important being the character of rock in the immediate vicinity of the instrument. In tectonically active areas it is often difficult to find sites that provide suitable geometry for studying the activity and also have strong competent rock with few or no fractures. We have tested new designs, for both dilatometers and 3 component Sacks-Evertson hydraulic strainmeters, and have found that, in sites we would previously have rejected because of rock quality, we now obtain reliable data. The approach depends on two factors: the sensing components of the instruments have always been ~3m in length so that they integrate over that vertical interval of rock and additionally we can now have a weak inclusion so that we minimize the mechanical impedance contrast between rock and cement plus instrument. Our current three component design is radically different from the modified Sakata-type used previously. Numerical modeling of the design shows that the response to strain change is essentially perfect; compared with earlier designs this gives better shear response and avoids strain concentrations in the rock wall. This design also provides good data from a site with very low rock quality. Data recorded in Taiwan from the 'weak' single component system have been critically important in allowing us to identify and model slow earthquakes triggered by typhoons. During a 5 year interval we have observed 20 slow earthquakes (durations of hours to days), 11 of which are coincident with typhoons (30 during that time span). This part of Taiwan (south east) experiences extremely high deformation rates but has a paucity of large earthquakes. Our data and modeling indicate that the stressed region is segmented by slow relief of stress, reducing the likelihood of seismic failure over extended fault lengths. Borehole strain recordings of volcanic activity in Montserrat and in Iceland have been critical in allowing new insight into volcanic activity: we have found that the recent change in Hekla activity to small frequent (~ every 10 years) eruptions is likely due to a feeder conduit, from the ~10 km deep reservoir to close to the surface, remaining molten between eruptions such that a relatively small overpressure is sufficient to initiate an eruption. Additionally, from strain changes recorded due to magma movement before surface breakout, we are able to provide constraints on the reservoir bulk modulus. For Montserrat we find low bulk modulus, consistent with a few percent free gas; for Hekla the modulus is very high implying an absence of free gas in the reservoir.
G21B-0694 INVITED
Seismic Evidence for Remote Triggering of Fault-Strength Changes on the San Andreas Fault at Parkfield
Fault strength is a fundamental property of seismogenic zones, and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events. While changes in fault strength have been suggested to explain various phenomena, such as the remote triggering of microearthquakes, there has been, to our knowledge, no means of actually monitoring this important property in situ. We suggest that 20 years of observation (1987-2007) of the San Andreas Fault at Parkfield have revealed a means of monitoring fault strength, through the use of particularly well-recorded, tightly-clustered repeating microearthquakes. From the dramatically improved spatial and temporal resolutions of these repeating earthquakes provided by the borehole seismograms of the High-Resolution Seismic Network, we infer a long-term change in fault strength within the Parkfield segment of the San Andreas Fault that has been induced remotely by the dynamic stresses from the 26 December 2004 Mw 9.1 Sumatra-Andaman earthquake. The change possesses two manifestations. First, we observe temporal variations in the properties of seismic scatterers that likely reflect the stress-induced migration of fluids at seismogenic depth. We also observe an increase in the temporal variability of repeating-earthquake recurrence intervals that is coincident with the occurrence of the 2004 Sumatra-Andaman event. These fluctuations are strongly correlated with the variations in seismic moment and suggest a slip-predictable model for these repeating- earthquake sequences, with the important consequence that such fluctuations likely represent temporal changes in fault strength. We suggest that these two manifestations are linked by fluid flow, through the stress-induced redistribution of pore pressure, which in turn alters the distribution of fault strength. Given that a similar excursion in scatterer properties initiated ~1-4 months after the 28 June 1992 Mw 7.3 Landers earthquake, combined with a gradual reduction in seismic moment of repeating-microearthquake sequences, we hypothesize that the large dynamic stresses from the 1992 Landers event (indeed, nearly an order of magnitude larger than for the 2004 Sumatra-Andaman event) also altered fault strength, locally weakening the Parkfield segment of the San Andreas Fault, and triggered the 1993 Parkfield Aseismic Transient event and the cluster of four M4+ earthquakes at Parkfield that occurred between October of 1992 and December of 1994. The fault-strength changes produced by the distant 2004 Sumatra-Andaman earthquake are especially important, as they suggest that the very largest earthquakes may have a global, long-term influence on the strength of the Earth's fault systems. We thus predict that following these largest of seismic events, remotely-triggered changes in fault strength should produce a temporal clustering of global seismicity. It is thus noteworthy that 5 M≥8 earthquakes occurred in the three years following the 2004 Sumatra-Andaman earthquake, an unusually high number that happened only one other time since 1900, and which is expected to occur by chance alone 1% of the time.
G21B-0695
Characterizing Noise Levels on the PBO Borehole Seismometers and Strainmeters
To help search for signals from episodic tremor, we are analyzing the spectral and temporal characteristics of data in the seismic band (0.001 to 10 Hz) from the borehole strainmeters and seismometers installed by the Plate Boundary Observatory (PBO). This characterization will show what can be detected by these sensors, and also help to develop guidelines which can be used to search for unusual behavior. We summarize the spectrum by performing local regression (loess) in log-log space; this reduces the data volume by factors of 1000 or more, with little loss of information. We examine the resulting spectral summaries to establish ranges for the noise levels, and also look at temporal changes (such as daily or weekly fluctuations) to test for possible noise sources from human actions. Our analyses show some stations with persistent operational problems (B004 during early 2006); others (B010) show outside sources of noise possibly due to local equipment. The noise levels for seismometers within tens of kilometers show similar temporal changes but (apparently) different absolute levels; comparing records of teleseisms shows that there are significant differences in relative gain (compared to the nominal values provided by PBO). As expected for the short-period sensors used, the noise level for the borehole seismometers at frequencies below 0.1 Hz is set by instrument noise, not ground noise. The borehole strainmeters show noise levels that are dominated by instrumental noise (probably digitizer noise) at frequencies above the microseism band. These instruments do reach ground noise within the microseism band; while the noise at frequencies from 0.001 to 0.1 Hz is lower (increasing with decreasing frequency), in this band it is at levels higher than is seen on long-base laser strainmeters at the surface. Comparisons between seismometer and strainmeter noise allow us to determine which signals can be best observed on each type of instrument, as a function of frequency and slowness.
G21B-0696 INVITED
Japanese high-sensitivity borehole seismic observation network, Hi-net: overview and recent research results
The NIED has deployed a nationwide high-sensitivity seismograph network (Hi-net) comprising around 800 borehole stations with a spacing of 20|30km [Okada et al., 2004; Obara et al., 2005]. NIED Hi-net borehole stations are equipped with a three-component short-period velocity seismometer, three-component strong motion accelerometer and horizontal-component high-sensitivity accelerometer. The high-sensitivity accelerometer covers a wide response range from ground tilting to long-period seismic waves. There are four advantages of Hi-net; high sensitivity, high signal-to-noise ratio, broadband property of sensors and high density of stations. As a result, detection capability for micro earthquakes has been dramatically improved and some new geophysical phenomena have been discovered. One of remarkable discoveries from Hi-net is wide variety of slow earthquakes including non-volcanic deep low-frequency tremors [Obara, 2002], short- term slow slip events (SSE) [Obara et al., 2004], and very low-frequency (VLF) earthquakes [Ito et al., 2007]. These slow earthquakes lasting for several days occur simultaneously with a certain recurrence interval at the transition zone on the deeper plate interface along the strike of the subducting Philippine Sea plate, southwest Japan. The tremor is characterized by randomly wave trains having a predominant frequency of around 2 Hz. VLF earthquake has a predominant period of 20 s and occurs coincident with peak of tremors and SSEs. During the active stage, the source of these slow earthquakes migrates with a propagation velocity of around 10km/day along the strike of the plate geometry at the downdip side of the locked seismogenic zone. At the updip side of the seismogenic zone, another kind of VLF earthquake having a predominant period of 10s has been detected [Obara and Ito, 2005]. This shallow VLF earthquake mainly occurs inside the accretionary prism on the landward side of the Nankai trough. These slow earthquakes at updip and downdip sides of the seismogenic zone on the subducting plate interface reflect the stress relaxation process around the plate boundary. On the other hand, the Hi-net is a quite powerful tool to resolve the deep earth structure and the source process for huge earthquakes as an antenna or telescope. Receiver function analyses by using teleseismic signals from the Hi-net enabled to image the configuration of shallow subducting Philippine Sea plate [Shiomi et al., 2004] and the pathway for water into the deep mantle along the dipping Pacific plate [Kawakatsu and Watada, 2007]. The slant stack analysis of PKJKP phase detected by Hi-net from M7 shallow event in Mozambique derived constraints on inner-core compressional-wave velocity and shears attenuation and proposed a new model of shear-wave anisotropy in the inner core [Wookey and Helffrich, 2008]. Ishii et al. [2005] used the Hi-net seismic array data to map the progression of the 2004 Sumatra-Andaman earthquake rupture by monitoring the direction of high-frequency radiation. Matsuzawa et al. [2008] detected a migrating long-period microtremor excited by the ocean swells generated by a cyclone in the Atlantic from the Hi-net high-sensitivity accelerometer array data. As mentioned above, the Hi-net is useful for not only local seismology but also global geophysics.
G21B-0697
Drilling Boreholes and Installing Strainmeters in Yellowstone National Park.
In the fall of 2007 and the summer of 2008 UNAVCO, with the assistance of the NPS and YVO, installed five strainmeter/seismometer/tiltmeter sites and one seismometer/tiltmeter site. Due to the unique geology of Yellowstone; Unavco, the NPS, and the drilling contractor implemented a stringent drilling plan. Our primary concerns were the safety of the work crews, protecting the hydrothermal resources and finding the best zone in the bore hole to install a strainmeter. The plan while drilling involved sampling the temperature of the discharged fluids, sampling cuttings every five feet, and taking water samples when encountering increased flow Geophysical logging was done the night before casing was set and every night while drilling bellow the casing. In the mornings, a high resolution temperature survey was made of the hole. This allowed finding install zones while the drill rig was onsite. A blow out preventer was available at all times. A comprehensive plan to control and contain high pressure and high temperature steam was tailored for each site. The installation of strainmeters in relatively high temperature holes (65C) led to a change in our procedures. Unavco personnel devised techniques to temporarily cool the bore hole so that the grout the strainmeter is set in did not set up too quickly. The drilling plan was resource intensive, but it led to five successful strainmeter installations.
G21B-0698
Testing of Strainmeter Grouts: Observations on Workability vs. Strength of Expansive Grouts
UNAVCO has been conducting comprehensive testing of grouts for strainmeter installs between fall 2006 to spring 2008. These experiments have involved both workability and strength testing. Based on the results of these tests a final test was to install a strainmeter and observe data quality. The characteristics we looked for in the initial testing were a long working time, compatibility with our dump bailer and low viscosity. Test involved small scale mixes, flow cone testing, and full scale surface penetration testing. These were all performed over a range of water contents and temperatures. Samples from all testing were subject to compressive strength testing. Concerns we had with grouts were aggregate size and composition, fluidity, temperature sensitivity, and set time. The final result of this testing is the selection of a grout that has so far provided high quality data and shown to be workable over a broad range of install conditions.
G21B-0699
PBO Facility Construction: Borehole Network Completed
The Plate Boundary Observatory (PBO), part of the larger NSF-funded EarthScope program, will study the three-dimensional strain field resulting from active plate boundary deformation across the Western United States. The construction phase was completed on September 30, 2008. The strainmeter and seismic borehole network consisted of the reconnaissance, permitting, installation, documentation and maintenance of 80 deep boreholes and 16 shallow boreholes that have some combination of strainmeters, seismometers, tiltmeters, accelerometers, pore pressure transducers, GPS, and MET packs. Construction highlights will be presented including: Pacific Northwest, Yellowstone, Mendocino, Parkfield, Anza and others as well as summary of the technical and logistical hurdles along the way.