Electronic Distance Meter Research Articles

Building infrastructure analysis using total station and unmanned aerial vehicle drone for surveying and modelling

Unmanned Aerial Vehicle (UAV) drones and Total Stations are two of some methods and tools used to conduct surveys on areas or building buildings. A UAV is a flying machine that functions with remote control by the pilot or can control itself, uses aerodynamic laws to lift itself, can reusable and can carry certain payloads. A total station is a device that is a combination of electronic theodolite, Electronic Distance Meter, and software that serves as a data collector and processor. The implementation of land surveying is one of the stages of the implementation of infrastructure development. Land survey itself is a way or method to evaluate land to get data directly from the field. Survey activities consist of field activities, making data analysis, interpretation of objectives and making survey reports. In this modern period, the technology to conduct land / land surveys often develops, namely with the total station and the use of UAV drone technology. The study aims to analyse and compare the capabilities of UAV Drones and Total Stations.

Transitional Characteristics of Ancient Vietnamese Housing in Mekong Delta

The aim of this paper is to explore the transition in the architectural characteristics of the ancient Vietnamese housing in Mekong Delta, in Southern Vietnam from 1858 to 1945. For this purpose, this study selects six ancient houses to determine the architectural characteristics that reveal the relationship between society and architecture during this period. Then, these ancient houses’ characteristics are analyzed based on six factors which are settlement characteristics, planning scheme, facade, structure, material, and interior decoration to show the transition in their architectural style. Six factors of ancient houses are surveyed by three methods: using of the electronic distance meter to determine the size of houses, taking photos and drawing of the houses to analysis the architectural characteristics and interviewing the landlord to get information about ancient houses. As the results, the study explored the difference in the ancient houses style which is strongly affected both by French style to their facade and Chinese style to their interior decoration, but the spatial structure is still remained to Vietnamese people. In conclusion, the characteristics of selected houses should be considered in detail as a reference to the current design goals.

Hassas Nokta Konumlama Yönteminin Doğruluğunun Mikro-Jeodezik Bir Ağda İncelenmesi

In this study, the accuracy of Precise Point Positioning (PPP) method was investigated. For this purpose, a micro-geodetic network containing 5 stations was established and 8-hour GNSS Global Navigation Satellite System (GNSS) sessions for three consecutive days were performed. At the same time, the baselines between the stations in this network were measured by Electronic Distance Meter (EDM) and the height differences are measured by the geometric leveling method. The results obtained from classical terrestrial measurements, which are considered to be highly accurate (3 mm in horizontal and few mm invertical), were compared with the results of PPP and the relative GNSS methods. As a result of the comparisons, it was seen that the values obtained from the relative GNSS analysis approximated with the classical method with the levels of 1-6 mm in the horizontal and 10-20 mm in the vertical direction. For the PPP method, it was seen that the same values varied between 1-28 mm in horizontal and 5-8 cm in the vertical component. The standard deviation values of these changes were determined as ± 2.20 mm in horizontal and ± 16.46 mm in vertical for the relative positioning technique and ± 11.43 mm in the horizontal and ± 59.48 mm in vertical component for PPP method.

Monitoring a Landslide with High Accuracy by Total Station: A DTM-Based Model to Correct for the Atmospheric Effects

For the monitoring of large landslides, total stations equipped with an Electronic Distance Meter (EDM) are widely used. To obtain the atmospheric parameters, required along the line of sight of every measure, the data collected by a weather station close to the instrument are usually adopted. Even after these corrections, the results obtained in the monitoring of areas with complex topography don’t reach the accuracies theoretically attainable by the high-end instruments. The article proposes a method for removing the errors due to the influence of microclimate on the measurements obtained by a high-end EDM, in order to get the maximum accuracy obtainable from such instruments. The method is based on an atmospheric model, set up by using the climatic data and a digital terrain model (DTM) of the landslide area. The methodology has been applied to a landslide in southern Italy. Over 38,000 distances, acquired for each monitored point, were used. The results demonstrate the effectiveness of the method: the standard deviations of the distances after their correction, show a reduction, ranging from 20% to 50%, with respect to the most diffused procedures; furthermore, the obtained accuracy equals the one declared by the manufacturer of the instrument for measurements in optimal conditions.

Estimating volcanic deformation source parameters with a finite element inversion: The 2001–2002 unrest at Cotopaxi volcano, Ecuador

AbstractDeformation at Cotopaxi was observed between 2001 and 2002 along with recorded seismicity beneath the northeast (NE) flank, despite the fact that the last eruption occurred in 1942. We use electronic distance meter deformation data along with the patterns of recorded seismicity to constrain the cause of this unrest episode. To solve for the optimum deformation source parameters we employ inverse finite element (FE) models that account for material heterogeneities and surface topography. For a range of source shapes the models converge on a shallow reservoir beneath the southwest (SW) flank. The individual best fit model is a small oblate‐shaped source, approximately 4–5 km beneath the summit, with a volume increase of roughly 20 × 106 m3. This SW source location contrasts with the NE seismicity locations. Subsequently, further FE models that additionally account for temperature‐dependent viscoelasticity are used to reconcile the deformation and seismicity simultaneously. Comparisons of elastic and viscous timescales allude to aseismic pressurization of a small magma reservoir in the SW. Seismicity in the NE is then explained through a mechanism of fluid migration from the SW to the NE along fault systems. We extend our analyses to further show that if future unrest crises are accompanied by measurable seismicity around the deformation source, this could indicate a higher magma supply rate and increased likelihood of a forthcoming eruption.

On accuracy specifications of electronic distance meter

The electronic distance meter (EDM) is an essential surveying tool. The principle of operation is the same among various EDM devices such as the stand alone EDM device, the theodolite mounted EDM unit and the coaxial design integrated with a total station. However, two interpretations exist for EDM accuracy specifications. This study investigates the meaning and cause of the two interpretations of identical accuracy specifications. Explanations from textbooks and manufacturers are reviewed. The procedures for deriving the value used in the accuracy specifications in practice are also detailed briefly. This research suggests that a unified interpretation of EDM accuracy specifications would be beneficial for EDM education and practice.

EDM을 활용한 지표변화율과 마그마 활동 양상 변화 연구

화산체 위에서 수 km 또는 십 수 km 떨어진 거리에 위치한 수준점 사이의 거리 측정은 때때로 마그마가 지표로 상승할 때, 그 시기와 장소를 정확하게 나타낼 수 있다. 지표로 상승하는 마그마는 그 상부 암층을 상방으로 밀어 올리거나 또는 화산체의 정상부를 옆으로 밀어내기도 하다. 어떤 경우이든 화산체의 특정한 한 지점이 다른 지점에 비교하여 수 mm에서 수 십 m 이상으로 수평으로 움직일 수 있다. 전자거리측정장치인 광파측거의(EDM)를 통한 이러한 변화의 측정에 대한 시도는 적절한 장소에서 수준점들을 설정하고 이들 수준점들을 짝을 이루어 수시로 측정하는 것이다. 광파측거의는 전자기파 신호를 보내고 수신하는 장비이다. 광파측거의와 반사경 사이의 거리에 따라 되돌아오는 신호의 주파수는 전송된 신호와 위상차를 나타낼 것이다. 전송된 신호와 수신된 신호의 위상을 비교하여 전자적으로 위상차로부터 거리를 측정한다. 측정범위와 정밀도에 따라 광파측거의의 종류는 다양하다. 화산모니터링의 목적으로 단거리(10 km 이내) 광파측거의와 중거리(50 km 이내) 광파측거의가 일반적으로 사용되고 있다. 단거리 광파측거의는 전자기 스펙트럼의 가시 적외선에 가까운 영역을 전송하고 수신하면서 5 mm 이내의 정확도를 가지고 있다. 본 연구에서는 지금까지 알려진 광파측거의의 특성과 작동 원리, 국외 화산지역의 지표 변화율 측정 실례를 알아보고, 백두산에서의 지표면 변형 측정에 활용 가능한 기법을 기술하였다. Measuring the distance between benchmarks placed on a volcano tens to thousands of meters apart can sometimes pinpoint where and when magma is rising toward the surface. Rising magma will sometimes push overlying rocks upward or shove them aside. In either case, one part of the volcano may actually move horizontally relative to another part from as little as a few millimeters to as much as several tens of meters. The challenge in measuring such changes with an electronic distance meter is putting benchmarks in the right places and making frequent measurements between pairs of benchmarks. An electronic distance meter is an instrument that both sends and receives an electromagnetic signal. Depending on the distance between the EDM and reflector, the wavelength of the returned signal will be out of phase with the transmitted signal. The instrument compares the phase of the transmitted and received signals and measures the phase difference electronically. There is a wide range of EDM capabilities in range and precision, but for volcano monitoring purposes, short-range (less than 10 km) to medium-range (less than 50 km) EDM's are typically used. Short-range EDM's transmit and receive the near visible infrared part of the electromagnetic spectrum for measuring distances with an accuracy of about 5 mm.

Wireless Beach Profiler

Abstract Gutierrez, J.; Gomez-Munoz, V.; Villa-Medina, F., and Porta-Gandara, M.A., 2012. Wireless beach profiler. A wireless beach profiler was designed using a vertically aligned accelerometer to estimate slopes along several points of the beach. The accelerometer output from each point was measured as a tilt angle by a microcontroller and transmitted to a personal digital assistant via a radio frequency wireless link. The digital assistant was used to store the data file for subsequent analysis. The device was mounted on a metal board and pulled along a perpendicular path to the beach, where the sampling points were located. The system included a global positioning system (GPS) receiver to record the actual path traversed. The results of this proposed profiling system were compared with an electronic distance meter and with the Emery method. The wireless beach profiler was also evaluated on a robotic vehicle to automate the process. Results showed that this profiler system is a reliable alternative bec...

Inter-eruptive volcanism at Usu volcano: Micro-earthquakes and dome subsidence

Post-eruptive crustal activity after the 2000 eruption of Usu volcano was investigated by seismic and geodetic field observations. Remarkable features of the magmatic eruptions that occur almost every 30 years include lava dome formation and strong precursory earthquakes. On the other hand, rapid dome subsidence was observed by electronic distance meter (EDM) measurement after the 1977–1982 summit eruption. Since the 2000 eruption, seismic activity at a shallow part under the summit crater has remained at a high level relative to that after the 1977–1982 eruption, although eruption occurred at the western foot of the volcano during the 2000 eruption. To reveal the shallow crustal activity in the inter-eruptive period around the summit area, seismicity and crustal deformation have been investigated since 2006. Dense temporary seismic observations and hypocenter relocation analysis using a three-dimensional velocity structure model revealed that the focal area is localized along the U-shaped fault that developed in the dome-forming stage of the 1977–1982 eruption. Three major focal clusters are distributed on the southwestern side of Usu-Shinzan cryptodome, which was built up during the 1977–1982 eruption. For the seven major events with magnitudes larger than 1, the focal mechanism was a large dip-slip component, which suggests the subsidence of Usu-Shinzan cryptodome. Interferomatetric satellite aperture radar (InSAR) image analysis and repeated GPS measurements revealed subsidence of the summit dome, which is almost centered at the Usu-Shinzan cryptodome. The area of rapid deformation is restricted to a small area around the summit crater. The estimated rate of dome subsidence relative to the crater floor is about 3 cm/year. These results strongly suggest that subsidence of Usu-Shinzan is associated with the small earthquakes along the U-shaped fault that surrounds the cryptodome. According to prior seismic and geodetic studies, it is thought that most of the magma rising under the summit crater during the 2000 eruption stopped around a depth of 2 km below sea level, which is sufficiently deep relative to the focal area of the present seismicity. A part of the magma intruded under the western foot and contributed to the 2000 eruption. We conclude that the 2000 eruption scarcely affected the shallow crustal activity under the summit crater, and that Usu-Shinzan cryptodome is continuing to subside just as it was before the 2000 eruption. The shallow volcanic earthquakes that began increasing from 1995 are closely related to the successive subsidence of the summit domes. Temporal change in fumarole temperature suggests a relationship between the shallow earthquakes and cooling of the magma that intruded under Usu-Shinzan during the 1977–1982 eruption.

Uplift and magma intrusion at Long Valley caldera from InSAR and gravity measurements

The Long Valley caldera (California) formed ~760,000 yr ago following the massive eruption of the Bishop Tuff. Postcaldera volcanism in the Long Valley volcanic field includes lava domes as young as 650 yr. The recent geological unrest is characterized by uplift of the resurgent dome in the central section of the caldera (75 cm in the past 33 yr) and earthquake activity followed by periods of relative quiescence. Since the spring of 1998, the caldera has been in a state of low activity. The cause of unrest is still debated, and hypotheses range from hybrid sources (e.g., magma with a high percentage of volatiles) to hydrothermal fluid intrusion. Here, we present observations of surface deformation in the Long Valley region based on differential synthetic aperture radar interferometry (InSAR), leveling, global positioning system (GPS), two-color electronic distance meter (EDM), and microgravity data. Thanks to the joint application of InSAR and microgravity data, we are able to unambiguously determine that magma is the cause of unrest.

Distribution of postseismic slip on the Calaveras fault, California, following the 1984 M6.2 Morgan Hill earthquake

Repeating earthquakes (REs) are sequences of events that have virtually identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth. To investigate the postseismic deformation after the 1984 M6.2 Morgan Hill earthquake, we identify RE sequences occurring on the central Calaveras fault between 1984 and 2005 using a combination of cross-correlation and spectral coherence techniques. Both the accelerated slip transients due to the earthquake as well as the return to interseismic background creep rates can be imaged from our dataset. A comparison between the regions of the fault that ruptured coseismically and the locations of the REs show that REs preferentially occur in areas adjacent to the coseismic rupture. Using calculated RE-derived subsurface slip distributions at 6 months and 18 months after the mainshock, we predict surface electronic distance meter (EDM) line length changes between stations near the Morgan Hill rupture area. The RE-derived slip model underpredicts a subset of the observed line-length changes. Inclusion of transient aseismic slip below the seismogenic zone is needed to better match the measured surface deformation.

Coseismic and Initial Postseismic Deformation from the 2004 Parkfield, California, Earthquake, Observed by Global Positioning System, Electronic Distance Meter, Creepmeters, and Borehole Strainmeters

Global Positioning System (gps), electronic distance meter, creepmeter, and strainmeter measurements spanning the M 6.0 Parkfield, California, earthquake are examined. Using these data from 100 sec through 9 months following the mainshock, the Omori’s law, with rate inversely related to time, 1/ t p and p ranging between 0.7 and 1.3, characterizes the time-dependent deformation during the postseismic period; these results are consistent with creep models for elastic solids. With an accurate function of postseismic response, the coseismic displacements can be estimated from the high-rate, 1-min sampling gps; and the coseismic displacements are approximately 75% of those estimated from the daily solutions. Consequently, fault-slip models using daily solutions overestimate coseismic slip. In addition, at 2 months and at 8 months following the mainshock, postseismic displacements are modeled as slip on the San Andreas fault with a lower bound on the moment exceeding that of the coseismic moment. Online material: Data description and supplementary figures, tables, and data used in models and time-series analysis.

Spatiotemporal evolution of a transient slip event on the San Andreas fault near Parkfield, California

In 1993 several baselines of the two‐color electronic distance meter (EDM) network at Parkfield, California, deviated from their long‐term rates, coincident with anomalous observations from nearby strain meters and a creep meter, as well as an increase in microseismicity. Between October 1992 and December 1994, three M ∼ 4.5 earthquakes occurred beneath Middle Mountain, near the hypocenter of the 1934 and 1966 Parkfield M6 events. We analyzed the two‐color EDM data using a Kalman‐filtering based technique to image the spatiotemporal evolution of slip on the fault at Parkfield between the mid‐1980s and 2003. This method accounts for localized random walk motion of the geodetic monuments and a prominent seasonal signal that affects many baselines. We find that a slip rate increase occurred between January 1993 and July 1996 on the upper 8 km of the fault near Middle Mountain. The peak estimated slip rate during this time was 49 mm/yr, which exceeds the long‐term geologic rate of ∼35 mm/yr. The slip rate evolution appears episodic, with an initial modest increase after the M4.3 earthquake and a much larger jump following the shallower M4.7 event in December 1994. This temporal correlation between inferred slip and seismicity suggests that the moderate earthquakes triggered the aseismic fault slip. The EDM data cannot resolve whether transient slip propagated across the nucleation zone of the 1934 and 1966 M6 Parkfield earthquakes. However, transient slip and its associated stress release in the hypocentral area of previous Parkfield events is consistent with the nucleation of the 2004 M6 Parkfield earthquake elsewhere on the fault.

Noise in two‐color electronic distance meter measurements revisited

Frequent, high‐precision geodetic data have temporally correlated errors. Temporal correlations directly affect both the estimate of rate and its standard error; the rate of deformation is a key product from geodetic measurements made in tectonically active areas. Various models of temporally correlated errors are developed and these provide relations between the power spectral density and the data covariance matrix. These relations are applied to two‐color electronic distance meter (EDM) measurements made frequently in California over the past 15–20 years. Previous analysis indicated that these data have significant random walk error. Analysis using the noise models developed here indicates that the random walk model is valid for about 30% of the data. A second 30% of the data can be better modeled with power law noise with a spectral index between 1 and 2, while another 30% of the data can be modeled with a combination of band‐pass‐filtered plus random walk noise. The remaining 10% of the data can be best modeled as a combination of band‐pass‐filtered plus power law noise. This band‐pass‐filtered noise is a product of an annual cycle that leaks into adjacent frequency bands. For time spans of more than 1 year these more complex noise models indicate that the precision in rate estimates is better than that inferred by just the simpler, random walk model of noise.

Deformation of the Long Valley Caldera, California: inferences from measurements from 1988 to 2001

Two periods of volcanic unrest occurred between 1989 and 1998 in the Long Valley Caldera, eastern California. Numerous earthquakes were recorded, and these periods of unrest were documented with high-precision geodetic measurements. The first round of unrest started rapidly in late 1989 and slowly decreased in rate through the early 1990s. For this interval there are both leveling and two-color electronic distance meter (EDM) measurements. The second round of unrest started slowly in mid-1997, climaxed in late 1997, and rapidly returned to quiescence by mid-1998. Deformation was recorded by both the two-color EDM and continuous GPS. Both episodes require inflation at 6–7 km beneath the resurgent dome, and both episodes had roughly 0.1 m extension across the resurgent dome. In addition, the data presented here suggest that there is a deeper, 10–20 km, inflation source beneath the south moat of the caldera. For both episodes, the better-resolved inflation beneath the resurgent dome is a near-vertical, prolate spheroid rather than an isotropic source, which suggests that magma came up through vertical cracks. However, the modeling suggests that the location changed with the depth from 6.0 to 6.7 km for the later episode. In contrast to the earlier episode, the 1997–1998 episode has additional deformation in the south moat, where the simplest model is that of a right-lateral slip on a steeply dipping plane that is defined by the location of earthquakes in the south moat. Models of the time-dependent behavior suggest that slip on this fault occurred from late November through December 1997, corresponding to the time of greatest moment release by the earthquake swarm in the south moat. Confounding the interpretation of these data is an active geothermal field near the center of the EDM network and adjacent to the south moat and resurgent dome. Additional modeling of leveling and EDM data within the geothermal field during a period of low rate of inflation of the dome suggests some methods of adjustments to the EDM data during the inflation episodes.

Deformation near the Casa Diablo geothermal well field and related processes Long Valley caldera, Eastern California, 1993–2000

Regional first-order leveling lines, which extend from Lee Vining, CA, to Tom’s Place, CA, have been surveyed periodically since 1957 by the U.S. Geological Survey (USGS), the National Geodetic Survey (NGS), and Caltrans. Two of the regional survey lines, or leveling networks, intersect at the Casa Diablo geothermal well field. These leveling networks, referenced to a distant bench mark (C916) near Lee Vining, provide time-series vertical control data of land-surface deformation that began around 1980. These data are also useful for delineating localized subsidence at Casa Diablo related to reservoir pressure and temperature changes owing to geothermal development that began in 1985. A comparison of differences in bench-mark elevations for five time periods between 1983 and 1997 shows the development and expansion of a subsidence bowl at Casa Diablo. The subsidence coincides spatially with the geothermal well field and temporally with the increased production rates and the deepening of injection wells in 1991, which resulted in an increase in the rate of pressure decline. The subsidence, superimposed on a broad area of uplift, totaled about 310 mm by 1997. The USGS established orthogonal tilt arrays in 1983 to better monitor deformation across the caldera. One tilt array (DBR) was established near what would later become the Casa Diablo geothermal well field. This array responded to magmatic intrusions prior to geothermal development, tilting away from the well field. With the start of geothermal fluid extraction in 1985, tilt at the DBR array reversed direction and began tilting into the well field. In 1991, geothermal power production was increased by a factor of four, and reservoir pressures began a period of steep decline. These changes caused a temporary three-fold increase in the tilt rate. The tilt rate became stable in 1993 and was about 40% lower than that measured in 1991–1992, but still greater than the rates measured during 1985–1990. Data from the local leveling networks spanning the well field and the bounding graben were analyzed for several 2-year periods (1993–1995, 1995–1997, and 1997–1999). Annual rates of change across the normal faults bounding the graben have steadily decreased for each 2-year period between 1993 and 1999, reflecting the slowing decline in geothermal reservoir pressure. Horizontal control data from a two-color electronic distance meter (EDM) defined the lateral extent of subsidence at Casa Diablo. The EDM and leveling data elucidate the localized effect of the shallow source of subsidence and the broader effect of the deeper magmatic inflation source. Data from bench marks common to both the vertical and the horizontal control networks were used to assess the effect of subsidence on the EDM base station (CASA). Modeling of geodetic data collected during periods of little or no magmatic inflation indicated that the CASA two-color EDM station is being drawn toward the well field at a rate of 3–5 mm/yr.

Slope instability and sagging reactivation at Maratea (Potenza, Basilicata, Italy)

The present work shows results collected, through a long-term monitoring, on slope movements in the Maratea Valley (Potenza, Italy). The investigated area shows large-scale gravitational phenomena and sagging type morphology that have been well-described in previous works. Electronic Distance Meter (EDM) measurements were first taken in 1983 and from these it would appear that since about 1991 there has been an acceleration in the movement occurring in the upper part of the valley, where sagging is located and which was previously motionless. Seismic and rainfall series for the last 20 years have been examined. The data collected show that this sagging destabilisation could not be related to rainfall and recent seismic activity; the latter reactivating later, in 1998. Global Positioning System (GPS) data on a network including benchmarks located outside of the landslide area, and comparison with EDM data indicate that movement affected the surrounding mountain slopes, widening the valley and destabilising the sagging: thus suggesting a relationship between the sagging and tectonic activity.

Use of differential SAR interferometry in monitoring and modelling large slope instability at Maratea (Basilicata, Italy)

Differential Synthetic Aperture Radar (SAR) interferometry (DiffSAR) allows, in principle, to measure very small movements of the ground and to cover in continuity large areas, so that it can be considered as a potentially ideal tool to investigate landslides and other slope instability. In this paper, we explore the use of this technique to improve our knowledge of the slope instability of a well-investigated area (the Maratea Valley), affected by continuous slow movements, producing an impressive “Sackung”-type phenomenon, which poses several unanswered questions. In particular, by using this technique, we analyse the time evolution of ground movements from 1997 to 2000. The slope movements during this same time interval have also been monitored in the past by using other techniques, such as electronic distance-meter (EDM) and GPS measurements: GPS and DiffSAR results are here compared. In our implementation of the DiffSAR technique, the problem of decorrelation noise is faced by using a phase unwrapping approach that allows to process sparse data, and the impact of atmospheric artefacts is reduced by performing a temporal analysis of the deformations observed in successive interferograms. In this study, we also show that it is possible to perform a temporal analysis of continuous slow landslide movements by using a limited number of ERS SAR data sets and low-precision topographic information. All the acquired data (EDM, GPS and DiffSAR) are consistent and allow a kinematic model of instability within the investigated time interval to be proposed. A map of slopes subject to different velocities of vertical displacements was delineated, modifying previous knowledge. Within the valley, progressive and almost linear displacements over time were confirmed.

Landslide monitoring by using ground-based SAR interferometry: an example of application to the Tessina landslide in Italy

An innovative technique, based on radar interferometry and implemented using ground-based instrumentation, has been applied for monitoring the Tessina landslide (Italy, Belluno). The technique has allowed us to derive multitemporal surface deformation maps of the entire depletion zone of the landslide with a high spatial resolution and accuracy. The portable device used in this application is known as Linear SAR (LISA), and it is able to provide measurements at 17 GHz with a synthetic aperture of up to 2.8 m. The results have been validated by comparing the recorded pixel displacements with independent measurements carried out by a motorized theodolite and Electronic Distance Meter (EDM) on two benchmarks.

Analysis of deformation data at Parkfield, California: Detection of a long‐term strain transient

Analysis of more than a decade of high‐quality data, particularly those from the two‐color electronic distance meter (EDM), in the Parkfield, California, area reveals a significant transient in slip rate along the San Andreas Fault. This transient consists of an increase in fault slip rate of 3.3±0.9 mm/yr during 1993.0 to 1998.0. The most reliable fault creep instruments show a comparable increase in slip rate, suggesting that the deformation is localized to the fault which breaks the surface. There was also an increase in precipitation around 1993. It is unlikely, however, that this anomaly is due directly to hydrology, as its spatial distribution is what would be expected for increased slip on the San Andreas Fault. The increase in slip rate corresponds temporally to a dramatic increase in seismicity, including the four largest earthquakes in the period 1984–1999 that occurred along a 6‐km segment of the fault just to the north of the EDM network. There was also a previously reported anomaly in borehole shear strain [Gwyther et al., 1996] that closely corresponds temporally to the transient in EDM data. Solely on the basis of EDM data the transient can be modeled as a slip event on a 10‐km‐long segment of the fault. The calculated shear strains from this model, however, are not consistent with the observed ones. A compatible model can be found if there is increased aseismic slip to the northwest in conjunction with the four earthquakes. Support for this northwestern slip is provided by a recent study of slip rate based on microearthquake activity. We speculate that this northwestern event served to load the fault to the southeast, with the stress being partially released by the observed slip.