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M.J.S. Johnston

Publications and source records attributed to M.J.S. Johnston.

49 records · Page 3Linked to original sources
USGS1979

Preliminary results from comparisons of redundant tiltmeters at three sites in central California

The U.S. Geological Survey has been operating a network of shallow-borehole tiltmeters in central California since June 1973. At six sites redundant instruments have been installed as a check on data coherency. These include the Sage Ranch, Tres Pinos, New Idria, Aromas, Bear Valley and San Juan Bautista tiltmeter sites. Preliminary results from the comparison of redundant data from the Aromas, Bear Valley and San Juan Bautista sites for periods of eight, three and seven months respectively, suggest that short period tilt signals in the range 5 min < T < 3–5 h and ranging in amplitude from 5 · 10 −8 to 10 −6 rad, but not including step offsets, show excellent agreement on closely spaced instruments. Agreement is not as good in this period range for instruments at San Juan Bautista with a separation of 200 m. Signals of interest observed in this period range include coseismic tilts, teleseisms and tilts associated with creep events. Tilt signals in the period range 3–5 h < T < 2– 5 weeks are not always coherent at all three of the redundant tilt sites studied. Tilt signals in this period range have amplitudes up to 5 · 10 −6 rad and wavelengths down to at least the instrument separation at the closely spaced sites (~several meters). Regarding longerterm coherency, the instruments at San Juan Bautista with 200-m spacing, agree within 0.5 μrad for the N-S component and 0.7 jurad for the E-W component for a period of two months. The closely spaced redundant instruments at Aromas agree within 2 μrad for the N-S component and 1 μrad for the E-W component for the eight-month period of operation. Data from the three sites have been checked for effects of temperature, atmospheric pressure and rainfall. The latter appears to be critically site dependent. The worst case tilts for 1 inch of rainfall can be more than 1 jurad with a duration of a few days to a week. Typical rain-induced tilts are less than 0.3 μrad for 1 inch of rain. The two instruments at the Sage Ranch site have been in operation for the longest period. However, they have shown local site or ground instability, high drift and lack of coherency since installation. Data are not yet available from the Tres Pinos or New Idria instruments. Deeper installation appears necessary for these instruments and two alternative methods of tiltmeter emplacement are currently being tested in an attempt to evaluate the depth, spatial and temporal dependency of surface tilt sources.

California↗
USGS1979

Dislocation modeling of creep-related tilt changes

Tilt changes associated with 1–5 mm of fault creep have been detected at several different locations on the San Andreas fault on tiltmeters within 500 m of the creep observation point. The creep-related tilts have amplitudes of ≤ 0̌.5 μrad and durations comparable to the creep events. No changes ≥ 10 −2 μrad have been observed on tiltmeters at distances ≥ 1 km from the fault at the time of the creep events. Dislocation models capable of replicating the creep-related tilt events have been constructed to examine the relationship of the model parameters to details of the tilt waveforms. The tilt time histories and bounded assumptions of the source-station configurations, and the displacement time history, can be used to infer the type and amount of displacement, the propagation direction and depth of the slip zone. The shallow depth and finite size of the slip zone indicated by these models contrasts with the horizontal extent.

Tectonophysics↗
USGS1978

Catalog of locations of U.S.G.S. instruments recording low-frequency data in California

During the past few years, the U.S. Geological Survey has installed many different types of instrumentation for the observation and detection of crustal deformation within the San Andreas fault system. In order to have an effective earthquake prediction program, it is important to have continuous data from these instruments collected at a central site. A second prerequisite is to provide adequate coverage throughout the large region of active faulting in this system.

California↗
USGS1978

Local magnetic field measurements, fault creep observations and local earthquakes on the San Andreas fault

Simultaneous records of local variations in magnetic field, fault creep and the occurrence times of local earthquakes have been obtained for the period early 1974 through mid-1977 along the San Andreas fault between the most southern extent of the 1906 earthquake fault break and the most northern extent of the 1857 break. The data utilized are primarily from stations located near the two ends of this section of the fault where strains accumulation is expected. The magnetic data show that local magnetic field variations up to 1.8 gammas with durations of a few minutes to several months have occurred. The creep data indicate both episodic events and changes in creep rate of up to 10 mm/yr lasting for six months or more. No clear magnetic transients or offsets are evident either simultaneous with or preceding the occurrence times of the episodic creep events by up to a day or so. Although some patterns of creep onset times at adjacent stations appear to correspond to some periods of longer term change in local magnetic field, these changes do not always occur and other groups of creep events have no corresponding changes in local magnetic field. Earthquakes with magnitudes less than 4.0 do not appear to correspond in time to local changes in magnetic field greater than 0.8 gamma or variations in the creep rate. In order to explain the observations presented in this study, it appears necessary to allow for a substantial amount of deep aseismic slip without any obvious attendant changes in the time distribution or size of the local earthquakes. Changes in stress related to the surface expression of aseismic slip on the San Andreas fault can be estimated from dislocation models fit to these data and to observations of simultaneous strains and tilts at points near the fault. These stress values are on average less than one bar near the surface but are probably more than 10 bars in localized regions at depths of a kilometer or so.

California↗
USGS1978

Continuous tilt, strain, and magnetic field measurements near four earthquakes (ML = 3.6 to 3.8) on the San Andreas fault, California

Four moderate earthquakes (M = 3.6 to 3.8) have occurred on the San Andreas fault in central California since October, 1977. These earthquakes are the first since 1974 to occur at this magnitude level within the array of tilt, strain, and magnetic instruments between Chalome (35.726N, 12.249 W) and San Francisco (37.79N, 122.23W) shown in Figure 1. They offer, therefore, one of the few opportunities to search for indications of precursive ground deformation. Furthermore, several other geophysical parameters such as resistivity, creek, radon, seismicity, geodetic strain, etc, are also monitored and allow comparisons for some of these events. This note reports comparative data and some possible implications in continuous strain, tilt, magnetic field and other measurements obtained from instruments within 10 km of the epicenters.

California↗
USGS1977

A multi-channel digital telemetry system for low frequency geophysical data

An inexpensive general purpose digital telemetry system for collection of low frequency geophysical data from U.S. Geological Survey instruments (eg. tilt, strain, gravity, creep, water level, radon, magnetic field, resistivity, telluric current, temperature, etc.) has been designed and built. This system provides data for a more general interactive data acquisition, retrieval and analysis system. The field stations are self-contained, battery operated and housed in weather proof containers. Each accepts up to 15 analog data inputs in the range of -5 to +5 volts. The dynamic range is 70db. The units transmit information as FSK (Frequency Shift Keyed) tones onto either a phone line or radio link with up to 150 transmitters sharing one line. The average power consumption is 0.06 nR watts where n is the 1 number of input channels transmitted and R is the sample rate in minutes -1 . The central receiver-recorder unit accepts and decodes the FSK tones and converts, formats and records the digital data together with time information and station identification on IBM combatible magnetic tape. The digital data are also converted and recorded in analog form for visual monitoring.

Open-File Report↗
USGS1976

A magnetic method for determining the geometry of hydraulic fractures

We propose a method that may be used to determine the spatial orientation of the fracture plane developed during hydraulic fracture. In the method, magnetic particles are injected into the crack with the fracturing fluid so as to generate a sheet of magnetized material. Since the magnetization of a body with extreme dimension ratios, such as a crack, exceeds that of an equidimensional body and since this magnetization is sensitive both to orientation and geometry, this could be used to obtain information about the crack. By measuring the vertical and horizontal components of the magnetic field and field gradients at the earth's surface surrounding the injection well with superconducting magnetometers having 10-4 gamma sensitivity and also by measuring field direction within the well itself, it should be possible to calculate the orientation and perhaps infer the approximate geometry of the fracture surface. Experiments on electric field potential operated in conjunction with this experiment could further constrain estimates of shape and orientation. ?? 1976 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1976

On simultaneous tilt and creep observations on the San Andreas Fault

THE installation of an array of tiltmeters along the San Andreas Fault 1 has provided an excellent opportunity to study the amplitude and spatial scale of the tilt fields associated with fault creep. We report here preliminary results from, and some implications of, a search for interrelated surface tilts and creep event observations at four pairs of tiltmeters and creepmeters along an active 20-km stretch of the San Andreas Fault. We have observed clear creep-related tilts above the instrument resolution (10 −8 rad) only on a tiltmeter less than 0.5 km from the fault. The tilt events always preceded surface creep observations by 2–12 min, and were not purely transient in character.

California↗
USGS1975

The nature of surface tilt along 85 km of the San Andreas fault-preliminary results form a 14-instrument array

The continuous monitoring of surface deformation near active faults is clearly necessary for an understanding of elastic strain accumulation and elastic and anelastic strain release associated with earthquakes. Fourteen 2-component tiltmeters have been installed in shallow boreholes along 85 km of the currently most active section of the San Andreas fault in the western United States. These instruments operate at a sensitivity of 10 −8 radians. Five of these tiltmeters, extending along one 35 km section of the fault, have been in operation since June 1973. The results indicate that regional tectonic tilting has occurred before more than ten individual earthquakes or groups of earthquakes with epicenters within ten earthquake source dimensions of one or more instruments. This tilting has a time scale of up to a month depending on earthquake magnitude. The amplitude of these tilts exceeds by almost an order of magnitude that expected from a dislocation model of the source using seismically determined parameters. No indication of rapid or accelerated tilt just prior to these earthquakes has been seen.

California↗
USGS1974

Preliminary results from a search for regional tectonomagnetic effects in California and western Nevada

Physical theory and laboratory experiments both indicate that tectonomagnetic effects in seismically active areas should be detected with highly sensitive drift-free differential magnetometers. By using a pair of synchronized 0.25 7 absolute magnetometers to measure precisely field differences between 70 adjacent sites with a 10–15 km separation, more than 1,000 km of faults in California and western Nevada have been monitored for anomalous changes in the local magnetic field. Over a nine-month period, four sets of mea surements have been completed along 350 km of the San Andreas and two sets along the Excelsior Mountains, Mono Lake, and Owens Valley. Preliminary results show that significant changes have occurred between each subsequent data set and that these changes appear to be related to tectonic structure and seismicity. This method looks promising as a simple inexpensive scheme for indicating some hazardous sections of active faults, particularly in countries where extensive geophysics programs are not feasible.

California, Nevada↗
USGS1974

Tilt precursors before earthquakes on the San Andreas fault, California

An array of 14 biaxial shallow-borehole tiltmeters (at 10-7 radian sensitivity) has been installed along 85 kilometers of the San Andreas fault during the past year. Earthquake-related changes in tilt have been simultaneously observed on up to four independent instruments. At earthquake distances greater than 10 earthquake source dimensions, there are few clear indications of tilt change. For the four instruments with the longest records (>10 months), 26 earthquakes have occurred since July 1973 with at least one instrument closer than 10 source dimensions and 8 earthquakes with more than one instrument within that distance. Precursors in tilt direction have been observed before more than 10 earthquakes or groups of earthquakes, and no similar effect has yet been seen without the occurrence of an earthquake.

California↗