Borehole Strainmeters Research Articles

Preliminary Report on the 28 September 2004, M 6.0 Parkfield, California Earthquake

The Mw 6.0 Parkfield earthquake struck central California at 17:15:14 UTC on 28 September 2004. The epicenter was located 11 km southeast of the rural town of Parkfield, adjacent to Gold Hill and on the San Andreas Fault (Figure 1). The California Integrated Seismic Network (CISN) reported that the hypocenter was located at 35.819°N, 120.364°W at a depth of 8.8 km. From the distribution of aftershocks and from models of seismograms, strain changes, and geodetic displacements from the earthquake, it appears that the rupture propagated to the northwest along the San Andreas Fault from its hypocenter beneath Gold Hill to Middle Mountain. Because of the earthquake's moderate size and the low population density, only minimal damage was reported, but strong ground motions of approximately 1 g were recorded at a few isolated points (Shakal et al. , 2005, this issue). The 2004 Parkfield earthquake is the sixth of a series of historical events located on this reach of the San Andreas Fault; similar-sized earthquakes occurred in 1881, 1901, 1922, 1934, and 1966 (Bakun and McEvilly, 1984). Because of the frequency of moderate-sized earthquakes in the same locality, the Parkfield Prediction Experiment (Bakun and Lindh, 1985) was initiated in the mid-1980's, with cooperation between the U.S. Geological Survey (USGS), the California Geological Survey (CGS), and various universities to instrument the area densely to capture a future earthquake. In addition to enlarging the seismic network with more short-period, broadband, and strong-motion instruments, other instruments were installed that are unique to this experiment. These instruments include borehole strainmeters, Global Positioning System (GPS) receivers, a borehole seismic network, creepmeters, a telluric array, magnetometers, and fluid-pressure sensors (Roeloffs and Langbein, 1994). The goal of the experiment was to capture the complete earthquake cycle: the interseismic period, the actual earthquake, and the postseismic response. The …

Study on relationship between features of strain anomaly obtained by borehole strain meter at Wushi station in Xinjiang and strong earthquake: Taking Jiashi M s=6.8 earthquake as an example

The borehole strain meter at Wushi station recorded obvious anomaly before Jiashi M s=6.8 earthquake occurred on February 24, 2003. Its features are as follows. (1) Anomaly types are complete. The trend anomaly, short-term anomaly, short-imminent anomaly and exponential anomaly appeared 19 months, 56 days, 4 days, and more than 1 month before the event, respectively; (2) Anomaly is large in magnitude. The maximal magnitude of strain anomaly is 1.7×10−5, which is rare in the past 20-year observation records at Wushi station; (3) Strain rate fluctuates sharply with obvious alternation of tension and compression. According to the magnitude of strain anomaly, time of exponential anomaly appearance and regional features of recorded anomaly, we could predict the magnitude, occurrence time and potential region to a certain degree.

Developments of borehole strain observation outside China

Borehole strain observation is playing an increasingly important role in the study on the crustal movements. It has been used by many countries such as China, USA, Japan, Peru, Australia, South Africa, Iceland and Italy, in research fields of plate tectonics, earthquake, volcanic eruption, dam safety, oil field subsidence, mining collapse and so on. Borehole strainmeter has been improved rapidly and tends to get more and more components included in one probe. Based on observations by this kind of instruments, studies on seismic strain step, slow earthquake, earthquake precursor and volcanic eruption forecasting have made remarkable achievements. In the coming years, borehole strain observation is going to become one major goedetic means, together with GPS and InSAR.

Triggered Deformation and Seismic Activity under Mammoth Mountain in Long Valley Caldera by the 3 November 2002 Mw 7.9 Denali Fault Earthquake

The 3 November 2002 Mw 7.9 Denali fault earthquake triggered defor- mational offsets and microseismicity under Mammoth Mountain (MM) on the rim of Long Valley caldera, California, some 3460 km from the earthquake. Such strain offsets and microseismicity were not recorded at other borehole strain sites along the San Andreas fault system in California. The Long Valley offsets were recorded on borehole strainmeters at three sites around the western part of the caldera that includes Mammoth Mountain—a young volcano on the southwestern rim of the caldera. The largest recorded strain offsets were 0.1 microstrain at PO on the west side of MM, 0.05 microstrain at MX to the southeast of MM, and 0.025 microstrain at BS to the northeast of MM with negative strain extensional. High sample rate strain data show initial triggering of the offsets began at 22:30 UTC during the arrival of the first Rayleigh waves from the Alaskan earthquake with peak-to-peak dynamic strain amplitudes of about 2 microstrain corresponding to a stress amplitude of about 0.06 MPa. The strain offsets grew to their final values in the next 10 min. The associated triggered seismicity occurred beneath the south flank of MM and also began at 22:30 UTC and died away over the next 15 min. This relatively weak seismicity burst included some 60 small events with magnitude all less than M 1. While poorly constrained, these strain observations are consistent with triggered slip and intrusive opening on a north-striking normal fault centered at a depth of 8 km with a moment of 10 16 N m, or the equivalent of a M 4.3 earthquake. The cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller. These observations and this model resemble those for the triggered deformation and slip that occurred beneath the north side of MM following the 16 October 1999 M 7.1 Hector Mine, California, earthquake. However, in this case, we see little post-event slip decay reflected in the strain data after the Rayleigh-wave arrivals from the Denali fault earthquake and onset of triggered seismicity did not lag the triggered defor- mation by 20 min. These observations are also distinctly different from the more widespread and energetic seismicity and deformation triggered by the 1992 M 7.3 Landers earthquake in the Long Valley caldera. Thus, each of the three instances of remotely triggered unrest in Long Valley caldera recorded to date differ. In each case, however, the deformation moment inferred from the strain meter data was more than an order of magnitude larger than the cumulative moment for the associated triggered seismicity.

ボアホール歪計アレイが観測した2003年十勝沖地震波形

ボアホール歪計アレイが観測した2003年十勝沖地震波形

ボアホール歪計で観測された非定常的変化のGPSによる検証：産業技術総合研究所地質調査総合センター安富観測点での事例

ボアホール歪計で観測された非定常的変化のGPSによる検証：産業技術総合研究所地質調査総合センター安富観測点での事例

A New Instrument To Remotely Monitor Rock Mass Deformation

A new system for precise monitoring of surface and underground deformations in rock mass surrounding a mining operation has been developed and successfully deployed by CSIRO Exploration and Mining since 1993. The new system is based on a borehole strainmeter, which has the sensitivity to enable measurements remote (for example at distances of 1-5 km) from the mining operation, and long term stability to provide accurate measurements over periods of years. Strainmeters traditionally used for deformation measurement have neither the long-term stability nor the high sensitivity to perform measurements in these applications. This paper describes the new precision borehole strain monitoring system (GTS) currently deployed by CSIRO Exploration and Mining, which has the potential to solve these new issues in rock mass deformation monitoring. This technology was originally developed for use in hard rock mines (Gladwin 1977) and refined considerably for earthquake research (Gladwin et al. 1994, Gwyther et al., 1992). A case study of the use of the GTS in monitoring longwall coal mining over the period 1993-1999 is presented. Accurate measurement of rock mass response during the mining of massive underground ore-bodies is essential as mines become larger and deeper. The CSIRO GTS system is immediately applicable in circumstances such as: provision of quantitative data for optimal pit slope design and engineering; measurement of the stability of deep pit slopes, monitoring the long term integrity of shafts, large scale underground infrastructure and surface infrastructure, and long term measurements of deformation or rock mass creep following the mining process to provide data for environmental management.

Creep rate changes at Parkfield, California 1966–1999: Seasonal, precipitation induced, and tectonic

Measured fault creep at Parkfield contains information about the slip rate on the San Andreas fault but also is affected by rainfall and other seasonal effects. Seven creep meters record a repeatable seasonal variation that can be removed from the data. Four of the creep meters exhibit surges in creep rate during the rainy season. For these instruments a function of rainfall is developed that crosses a threshold within a few days of the onset of accelerated creep. In addition to the seasonal variations and surges, most Parkfield creep meters record higher creep rates more frequently during the wet season than during the dry season. Characterizing this behavior makes it possible to distinguish between rainfall‐influenced creep and creep rate changes of tectonic origin. Accelerated creep beginning in January 1983 was probably induced by rainfall, rather than by tectonic sources related to the Coalinga earthquake in May 1983. Four creep meters in the central part of the Parkfield fault segment are recording higher creep rates since 1993. A strong case can be made that the increased creep rate at CRR1 is tectonic, but for the other three creep meters an influence of rainfall is difficult to rule out. Strain rate changes recorded on three‐component borehole strain meters suggest that the rate of subsurface creep to the northwest of these instruments is even greater. The 1993 creep rate increase is more abrupt and farther south than the expected signal from accelerating slip near the hypocenter of the 1966 Parkfield earthquake but is in the same location as rapid fault slip reported prior to that event.

Strain Measurement by Double Coaxial Borehole Strainmeters at Odawara, Japan

Strain Measurement by Double Coaxial Borehole Strainmeters at Odawara, Japan

A model for possible crustal deformation prior to a coming large interplate earthquake in the Tokai district, central Japan

Abstract We perform numerical simulations of a seismic cycle by applying a laboratory-derived friction law to evaluate possible crustal deformations prior to the hypothesized Tokai earthquake, which is expected to occur in the near future along the Suruga trough, central Japan. The frictional force obeying a rate- and state-dependent friction law is assumed to act on the plate interface in a 2D model of the subduction zone. We test two versions of the friction law: the so-called slowness version and the slip version. We select model parameters on the basis of historical documents about past large earthquakes in the Tokai district and the present coupling state of the plate interface beneath the Tokai district estimated by GPS observations. Simulation results for both the slowness and the slip version are as follows: (1) The coupling region of the plate interface becomes narrower with time in the interseismic period. (2) The inland crust contracts at a nearly constant rate for most of the interseismic period. The contraction rate decreases or a change from contraction to extension takes place before the occurrence of a large interplate earthquake. (3) Significant abnormal crustal deformation precedes the earthquake, and maximum amplitudes about one day before the earthquake exceed the noise levels in the case of clear weather of borehole strainmeters installed in the Tokai district. (4) The spatiotemporal variation of aseismic sliding on the plate interface perturbs the regional stress field around the source volume of the coming earthquake, leading possibly to precursory seismic quiescence in the overriding plate and a change in focal mechanisms of small earthquakes in the subducting slab. The most significant difference in the results between the slowness version and the slip version of the friction law is that amplitudes of preslip and crustal deformation just before earthquakes are several times larger for the slowness version than for the slip version. Although some ambiguity remains in the present model mainly because of the incompleteness of the friction law and the deficiency of observational data constraining model parameters, the physical model can be improved through the comparison of predicted crustal deformation with observations. Continuous and concentrated observations of crustal deformation and seismic activity are thus important in the Tokai district to understand quantitatively the sliding process preceding a large interplate earthquake there.

Water-level changes in response to the 20 December 1994 earthquake near Parkfield, California

Abstract We analyze co-seismic changes of water level in nine wells near Parkfield, California, produced by an MD 4.7 earthquake on 20 December 1994 in order to test the hypothesis that co-seismic water-level changes are proportional to co-seismic volumetric strain. For each well, a quantitative relationship between water level and volumetric strain can be inferred from water-level fluctuations due to earth tides and barometric pressure. The observed co-seismic water-level changes, which ranged from −16 to +34 cm, can therefore be compared with volumetric strain recorded by borehole strainmeters or calculated using a dislocation model of the earthquake. We were able to find a dislocation model of the earthquake rupture that predicts volumetric expansion at five of the six wells where water level fell co-seismically, as well as volumetric contraction at one of the two sites where water level rose. Strain predicted by the dislocation model is in good quantitative agreement with the strain inferred from water-level changes observed at four of the well sites, as well as strain recorded by three borehole strainmeters. Water-level changes at two more well sites correspond to strain somewhat greater than predicted by the model but agree in sign with model-calculated strains. At three of the well sites, however, water-level changes took place that cannot be explained as responses to co-seismic volumetric strain for any plausible dislocation model of the earthquake rupture. At two of these sites, one in and one near the San Andreas fault, large water-level drops are probably influenced by co-seismic fault creep. The third site has a history of large water-level rises in response to earthquakes at distances up to several hundred kilometers. This data set shows that co-seismic water-level changes in many wells are proportional to volumetric strain but that other wells exist in which different mechanisms dominate co-seismic response.

Tidal calibration of borehole strain meters: Removing the effects of small‐scale inhomogeneity

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Piñon Flat. Small‐scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short‐baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small‐scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross‐coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.

A slow earthquake sequence on the San Andreas fault

EARTHQUAKES typically release stored strain energy on timescales of the order of seconds, limited by the velocity of sound in rock. Over the past 20 years, observations1–13 and laboratory experiments14 have indicated that rupture can also occur more slowly, with durations up to hours. Such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones. The detection of events with larger timescales requires near-field deformation measurements. In December 1992, two borehole strainmeters close to the San Andreas fault in California recorded a slow strain event of about a week in duration, and we show here that the strain changes were produced by a slow earthquake sequence (equivalent magnitude 4.8) with complexity similar to that of regular earthquakes. The largest earthquakes associated with these slow events were small (local magnitude 3.7) and contributed negligible strain release. The importance of slow earthquakes in the seismogenic process remains an open question, but these observations extend the observed timescale for slow events by two orders of magnitude.

Transient deformation during triggered seismicity from the 28 June 1992 Mw = 7.3 Landers earthquake at Long Valley volcanic caldera, California

Continuous records from a borehole strainmeter and a long baseline tiltmeter in the Long Valley caldera provide critical insights into the origin of at least one episode of minor seismicity in volcanic regions triggered by the 28 June 1992, M L 7.3 Landers, California, earthquake. A strain transient reaching a peak of 0.25 microstrain occurred in the few days following the Landers event and decayed over the next 20 days. A tilt perturbation during the same time reached a peak amplitude of 0.2 microradians. These signals correspond approximately in time to the primary seismic moment release across a 50 km 2 region of the south part of the caldera at depths between 2 and 10 km. Corresponding strain transients in 5-km geodetic lines across the south caldera are not apparent above the 95% confidence limits of about 0.4 microstrain in daily sampled data during this same period. These data rule out models involving single localized inflation sources within the upper crust beneath the caldera, including that responsible for the current rapid inflation of the resurgent dome. They also preclude models involving aseismic slip on single strike-slip or normal faults in the caldera. A single source in the form of a relaxing magma body at a depth of 50 km beneath the caldera can account for the deformation data, but whether the small stress changes are sufficient to drive the triggered seismicity is not clear. An alternate possibility involves distributed deformational sources triggered by the passage of the 10 microstrain peak amplitude surface waves from the earth- quake. This distributed deformational source could result either from rupturing of overpressured fluid or gas chambers commonly encountered in volcanic regions or from advective gas overpressure during release of gas bubbles in hydrothermal or magmatic fluids.

Mechanism of the 1991 eruption of Hekla from continuous borehole strain monitoring

VOLCANOES erupt when the pressure in a magma chamber several kilometres below the edifice overcomes the strength of the intervening rock. Seismic activity may accompany and precede eruptions, allowing (in favourable circumstances) the location and movement of magma to be traced. Ground deformation near volcanoes can provide more direct evidence for magma movement, but continuous monitoring is necessary to ensure that all the essential aspects of an eruption are recorded. Here we report dilatational strain data collected continuously during the January 1991 eruption of Hekla volcano by five borehole strainmeters located 15–45 km from the volcano. The data record the upward propagation of magma, as well as the deflation of a deep reservoir. In only 30 minutes the magma forced open a conduit to the surface from a depth of ∼4km. Although other volcanoes might behave differently, our results suggest the possibility of using continuous deformation measurements to monitor conduit formation at other sites, perhaps providing short-term warnings of impending eruptions.

Trial results of YRY-2 shallow borehole strainmeter at eight observation sites in North China

The analysis results of observations data indicate that this shallow-borehole (21 m – 52 m) strainmeter can record accurate crustal movements. Its records of earthquake and solid tide can be analysed. To use long period frequency data, null drift must be considered. To extend its application, the technique of installation and anti-lightning stroke should be improved.

Sensitivity of crustal deformation instruments to changes in secular rate

A variety of instruments (including borehole strainmeters, water wells, creepmeters, laser ranging and differential magnetometers) are used to monitor crustal deformation in areas that are prone to geologic hazards such as earthquakes and volcanic eruptions. In monitoring the deformation, one typically examines the data for either a change in rate, or a simple offset in the record. However, one needs to place a statistical confidence level that the detected signal differs from the background “noise”. Calculation of the statistical confidence level may be done using the formalism of the matched filter, whose output is the signal‐to‐noise ratio, ρ. Two ingredients are needed to form a matched filter: 1) The power density spectrum of the instrument and 2) the functional form of the signal that we desire to detect. Using the available crustal deformation data from the Parkfield, California network, the background noise for individual instruments as a function of frequency, f, is estimated using the traditional method of the power density spectra. Except for two‐color laser distance‐ranging data, the power spectra for most of the instruments have a frequency dependence of f−n where 2≤n≤3. The confidence level with which a hypothesized signal is present is determined directly from the signal‐to‐noise ratio, with the numerator being a function of the signal and the denominator being a function of the power spectrum. Using a creepmeter as an example, a 0.04‐mm change occurring over 1 hour, a 0.06‐mm occurring over 10 hours, or 0.20‐mm over 100 hours are all signals for which ρ=2 and therefore have only a 5% confidence that these signals could be background noise.

The Loma Prieta Earthquake, 1989 and Earth strain tidal amplitudes: An unsuccessful search for associated changes

The Loma Prieta earthquake of 1989 provided an opportunity for a sensitive test of suggestions that earthquakes may be preceded by variations in Earth tidal strain amplitudes. Such variations have been proposed as providing an advantageous technique for detecting precursory changes in elastic parameters in a seismogenic zone. We have analyzed data from two borehole strainmeters continuously operating within 40 km of the epicenter and within about 10 km of the southern end of the rupture. We are unable to identify any precursory changes in M2 and O1 tidal amplitudes and estimate that any large scale changes in Young's modulus must have been less than about 2%. If these results apply generally, we would conclude that variations in elastic material properties prior to earthquakes do not occur throughout substantial volumes of the subsequent hypocentral region.

The 1991 eruption of Hekla, Iceland

The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s−1. After this peak, the effusion rate declined rapidly to 10–20 m3 s−1, then more slowly to 1 m3 s−1, and remained at 1–12 m3 s−1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.

Stepwise Variations of Strain Observed by Two Strainmeters Installed in Nearby Boreholes in a Fracture Zone.

Stepwise variations of strain (hereafter called the strain step) were observed by a couple of borehole strainmeters installed 30 meters apart in a fracture zone of the southern Fossa Magna. Strain changes for a year after the installation carried at December, 1986—January, 1987 were apparent expansion indicating the cooling of grout and the relaxation of stress concentrated around the strainmeter. For two years after the installation the number of the strain steps decreased gradually. No clear relationships are seen between the occurrence of strain steps and the environmental conditions such as the rainfall, the atmospheric pressure, the groundwater temperature, and the groundwater level. The strain steps that occurred in the two boreholes are independent, which means that the spatial extent of the phenomenon is less than the distance between the strainmeters. The amplitude-frequency relationship for the strain steps is analogous to those of fracture processes such as earthquakes and acoustic emission in rocks. The decrease in the number of strain steps with time is represented by a formula similar to Ohmori's for the aftershock sequences of earthquakes. A laboratory experiment showed that the frequency of strain step occurrence depends on the stress levels in the surrounding media. Consequently it is inferred that the strain steps observed by our strainmeters are caused by local fracturing processes around the boreholes, the number of which indicates relaxation of the initial stress induced by the installation of the strainmeters.