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Jerry P. Eaton

Publications and source records attributed to Jerry P. Eaton.

At least 19 recordsLinked to original sources
USGS2010

Earthquake refraction profiles of the root of the Sierra Nevada

We examine the seismic structure of the Sierra Nevada using records of nine earthquakes and one explosion in and near the Sierra, recorded on stations in the Sierra. We first interpret travel times from these paths, which are confined to a single tectonic block, in terms of one‐dimensional structures. The most nearly reversed pair of earthquakes, the 1966 Truckee and 1983 Durrwood Meadows earthquakes, share refracted ( P n ) arrival times (corrected to surface focus) along a line t =8.75±0.25+Δ/8.0, suggesting that a nearly flat layer of 8.0 km/s mantle material lies at depths of 46–48 km. First arrivals from these events do not constrain velocities from ≈30 to 45 km depth. Secondary arrivals and some first arrivals from other earthquakes suggest that velocities in part of this region range between 6.9 and 7.8 km/s. The presence of this “7.x‐km/s” layer can help to explain previous contradictory observations. The 7.x‐km/s layer could be interpreted as either the mafic bottom of a silicic, Mesozoic magmatic arc or as accreted mafic underplating or rejuvenated mantle related to Cenozoic arc volcanism or Basin and Range spreading. Arrivals at stations in the foothills and the crest of the Sierra cannot be fit with a single longitudinal structure, indicating a lateral variation of velocity structure. These variations support previously inferred variations of lithospheric structure, with higher‐velocity, thinner crust to the west beneath the Sierran foothills and slower‐velocity crust (or possibly upper mantle) beneath the high mountains in the eastern Sierra. Rapid changes in arrival times between stations separated by short distances in the eastern Sierra suggest that a sharp boundary exists between the Sierra and the Basin and Range at Moho depths. We also present fresh evidence of the asymmetry of the root of the Sierra, wherein arrivals from earthquakes on the west of the Sierra are delayed within the Sierra and return to original values in the Basin and Range, while arrivals from earthquakes and explosions from the Sierra into the Great Valley. We suggest that if the 7.x‐km/s material occurs in a wedge above the Moho, then the asymmetry can be explained by arrivals from the west being delayed by the dipping 8.0‐km/s Moho, while those from the east may be entering the root along a 7.x‐km/s layer that is near the depth of the Basin and Range Moho.

New Hampshire↗
USGS1999

Calibration formulae and values for velocity seismometers used in the 1998 Santa Clara Valley, California seismic experiment

Eaton (1975), Bakun and Dratler (1976), Eaton (1977), Healy and O’Neil (1977), Asten (1977), Stewart and O'Neill (1980), Liu and Peselnick (1986), Eaton (1991), Rodgers et al. (1995), and many others (see Asten (1977) for a list of earlier references) have presented formulae for calculating the damped generator constant (or motor constant), and the damping constant (or fractional damping ratio) for magnetically damped velocity seismometers. Unfortunately the notation varies between authors, and not all the formulae allow for some of the significant variables -- differences in input impedance of the recording system in particular. This has become particularly relevant because the USGS seismic networks in California have traditionally set up their velocity sensors for the 10K Ohm impedance of the standard USGS analog telemetry systems (Eaton, 1977), but modern digital recording systems are usually set up with high input impedances, often of a megaohm or greater. Thus the nominal calibration values valid for USGS velocity sensors in their “normal” configuration are incorrect when they are recorded on other systems. In this short note we have collected the relevant formulae needed, and computed the seismometer responses for the various velocity sensors used in the recent Santa Clara Valley Seismic Experiment (SCVSE, see Lindh et al., 1999).

California↗
USGS1993

The Cape Mendocino, California, earthquakes of April 1992: Subduction at the triple junction

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America—Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity.

California↗
USGS1977

Factors limiting the sensitivity and dynamic range of a seismic system employing analog magnetic tape recording and a seismic amplifier with adjustable gain settings and several output levels

In the course of modernizing the low-speed-tape-recorder portable seismic systems and considering the possibilities for the design of a cassette-tape-recorder seismic refraction system, the factors that limit the sensitivity and dynamic range of such systems have been reviewed. These factors will first be stated briefly, and then their influence on systems such as the new 5-day-tape seismic system will be examined in more detail. To fix ideas, we shall assume that the system consists of the following elements: 1. A seismic sensor: usually a moving coil inertial seismometer with a period of about 1 second, a coil resistance of about 5000 ohms, and an effective motor constant of 1.0 V/cm/sec (across a 10K load terminating the seismometer sensitivity-and-damping-adjustment resistive network). 2. A seismic amplifier/voltage controlled oscillator unit made up of the following components: a) A fixed gain preamplifier with an input resistance of 10K and an internal noise level of 0.5 muVpp referred to the preamp input (0.1 Hz <= freq. <= 30 hz). b) An adjustable gain (0 to 42 db in 6 db steps) intermediate amplifier c) One or more fixed gain output amplifiers. d) Two sections of 6 db/octave bandpass filter serving to couple the 3 amplifier stages together. e) Voltage controlled oscillators for each output amplifier to produce modulated FM carriers for recording on separate tape tracks or modulated FM subcarriers for subsequent multiplexing and direct recording on tape in the California Network format. 3. An analog magnetic tape recorder: e.g. the PI 5100 (15/80 ips recording in the FM mode or in the direct mode with the 'broad-band' variant-of the Cal Net multiplex system, or 15/16 ips recording in the direct mode with the standard Cal Net multiplex system), or the Sony TC-126 cassette recorder operating in the direct record mode with the standard Cal Net multiplex system. 4. Appropriate magnetic tape playback equipment: e.g., the Bell and Howell 3700-B for the PI-5100 or the Sony TC 126 for its own tapes. 5. Appropriate discriminators (employing subtractive compensation, at least for the multiplexed systems) to restore the data signals to their original forms. 6. An A/D convertor to digitize the seismic signals for computer processing and/or a strip chart recorder (e.g., the Siemens Oscillomink) for playout of the data.

Open-File Report↗
USGS1976

15/16 ips Operation of the Precision Instrument Company Model P15100 tape recorder to record the standard (30 Hz) NCER seismic data multiplex system

In recent months the need has arisen to record special seismic networks consisting of a dozen or more standard NCER seismic systems telemetered to a central collection point on a reliable, portable, low-power tape recorder. Because of its simplicity and the ease with which it can be adapted for the purpose, the PI 5100 field recorder should be considered for such use. In the tests described here, a PI 5100 was speeded up to run at 15/16 inches per second (ips) and signals from the standard multiplex system test modulator bank were recorded on one tape track by means of a simple, improvised AM record amplifier. The results of these tests are extremely encouraging: the dynamic range of the system when played back on the Bell and Howell Model 3700 B reproduce machine, with subtractive compensation, is nearly as high as for the system employing the B&H 3700 B for recording. These notes indicate the principle employed to speed up the recorder, outline the circuit required to drive the tape heads in the AM record mode, and describe the tests carried out to evaluate the system's performance.

Open-File Report↗
USGS1976

Tests of the standard (30 hz) NCER FM multiplex telemetry system, augmented by two timing channels and a compensation reference signal, used to record multiplexed seismic network data on magnetic tape

The application of subtractive compensation to USGS seismic magnetic tape recording and playback systems was examined in a recent USGS Open-file report (1). It was found, for the standard (30 Hz) NCER multiplex system, that subtractive compensation utilizing a 4688 Hz reference signal multiplexed onto each data track was more effective than that utilizing a 3125 Hz reference signal recorded separately on a different track. Moreover, it was found that the portion of the spectrum between the uppermost data channel (3060 Hz + or - 125 Hz) and the compensation reference signal (4688 Hz) could be used to record an additional timing signal, with a center frequency of 3700 Hz and a broader playback bandwidth (ca 0 to 100 Hz) than that of the standard data channels. Accordingly, for the tests described in that report, the standard 8-datachannel multiplex system was augmented by one additional timing channel with a center frequency of 3700 Hz. The 3700 Hz discriminator used in those tests was not successfully set up to utilize subtractive compensation; so its output from a tape playback was quite noisy. Subsequently, further tests have been carried out on the application of subtractive compensation to a 4-channel broad-band multiplex system and to the standard multiplex system, both recorded on field tape recorders with relatively poor tape speed control (2), (3). In the course of these experiments, it was discovered that two separate timing channe1s, not just one, can be inserted between the uppermost data channel and the compensation reference signal, Furthermore, it was possible to adjust the discriminators used to playback these timing channels so that they profited significantly from subtractive compensation even though the playback bandwidth was 0 to 100 Hz (for short rise times of square wave timing signals). The advantages of recording two timing signals on each data track include: 1) one standard time signal to be used for critical timing, e.g. IRIG E, can be recorded with the data on each track, eliminating any problem that might arise from tape head misalignment if the timing base were recorded on a separate track from the data signals being timed, 2) other essential timing signals e.g. WWVB and IRIG C, can each be recorded on several tracks, to insure more reliable recording through redundancy, without displacing data from standard data channels, 3) the broader playback bandwidth of the special timing channels reproduces the sharp-edged timing codes with much less distortion than is obtained from the standard data channels. In order to implement subtractive compensation with the 4688 Hz multiplexed signal and to record timing signals on the proposed special timing channels, it was necessary to design and build a signal generator/multiplexer unit. The functions of this unit are: 1) generate a stable (x-tal controlled) compensation reference frequency (4688 Hz), 2) generate two timing channel subcarriers (3500 Hz and 3950 Hz) and provide for their modulation by appropriate timing signals, 3) separately, for each of th~14 tape tracks, adjust the relative levels of the timing and compensation subcarriers and multiplex them, at the appropriate level, with the incoming multiplexed data signals for introduction to the tape system direct record amplifiers. These units will be described in detail by Gray Jensen, who designed and built them, as well as being shown diagrammatically in this report. This report continues the work described in reference (1), and it should be read as a supplement to that report rather than as an independent effort. It introduces changes in the multiplex system and test circuits employed in the tests and then repeats the tests from the earlier report that are required to illustrate the characteristics of the modified system.

Open-File Report↗
USGS1976

Notes on a broad-band variant of the NCER seismic data multiplex system for use with field tape recorders

Tests of the standard NCER multiplex system recorded and played back on both the Bell and Howell 3700B (about 0.1% tape speed variation) and on the Sony TC-126 cassette recorder (about 1% tape speed variation) showed that subtractive compensation employing a reference frequency multiplexed on the data track was remarkably effective in reducing tape-speed-variation-induced noise 1 and, hence, in increasing the dynamic range of the record/playback system. Further tests suggested that the 0 to 30 Hz bandwidth of the standard system (8 data channels) might be increased substantially, at the 'price' of reducing the number of data channels to 3 or 4, without serious loss of dynamic range if subtractive compensation could be implemented effectively with the broader-band system.

Open-File Report↗
USGS1975

Notes on some experiments on the application of subtractive compensation to USGS seismic magnetic tape recording and playback systems

The purpose of these experiments is to lay the groundwork for the implementation of subtractive compensation of the USGS seismic network tape playbacks utilizing the Develco model 6203 discriminators at a x1 playback speed. Although the Develco discriminators were designed for this application and a matching Develco compensation discriminator was purchased, effective use of this system for subtractive compensation has been blocked by the inadequate (frequency dependent) matching of the phase of the compensation signal to that of the data signal at the point compensation is carried out in the data discriminators. John Van Schaack has ameliorated the phase mismatch problem by an empirical alteration of the compensation discriminator input bandpass filter. We have selected a set (of eight) Develco discriminators and adjusted their compensation signal input levels to minimize spurious signals (noise) originating from tape speed irregularities. The sensitivity of the data discriminators was adjusted so that deviations of +125 Hz and -125 Hz produced output signals of +2.00 volts and -2.00 volts, respectively. The eight data discriminators are driven by a multiplex signal on a single tape track (subcarriers 680, 1020, 1360, 1700, 2040, 2380, 2720, and 3060 Hz). The Develco-supplied compensation discriminator requires an unmodulated 3125 Hz signal on a separate tape track.

Open-File Report↗
USGS1974

Development and evaluation of a prototype global volcanic surveillance system utilizing the ERTS-1 satellite data collection system

The ERTS Data Collection System makes it feasible for the first time to monitor the level of activity at widely separated volcanoes and to relay these data rapidly to one central office for analysis. This capability opens a new era in volcanology where the hundreds of normally quiescent but potentially dangerous volcanoes near populated regions around the world can be economically and reliably monitored daily to warn when any one volcano is becoming active again. Before ERTS was launched only a few volcanoes in the world were monitored continuously because of the high cost of building and staffing volcano observatories. Yet it is known from data collected in this century, that while visible signs of pending eruptions may occur only minutes to days in advance, invisible but measurable signs may be detected days, weeks, months and even years before a major eruption. While prediction of specific eruptions is still an elusive goal, early warning of a reawakening of activity at quiescent volcanoes is now a distinct possibility. A prototype volcano surveillance system was established during the latter part of 1972 and early 1973 on 15 volcanoes in Alaska, Hawaii, Washington, California, Iceland, Guatemala, El Salvador, and Nicaragua. Nineteen seismic detectors that count four different sizes of earthquakes and six biaxial borehole tiltmeters that measure ground tilt with a resolution of 1 microradian have been installed. Data from these instruments are relayed through the ERTS satellite and through a teletype link to the U.S. Geological Survey Office in Menlo Park for rapid analysis. Only seismic and tilt data are collected because these have been shown in the past to indicate most reliably the level of volcanic activity and also because they can be measured relatively easily with available instrumentation. Experience during this project demonstrates the feasibility of building inexpensive, low power, reliable instruments that can be installed in remote locations and can be expected to run unattended for a few years. Comparison of the data from these new earthquake counters with data from nearby standard seismometers shows that the counters do normally indicate the level of seismic activity. During periods of high seismic background noise there may be a significant number of spurious counts but the existence and duration of such noisy periods are reliably indicated by other data collected by the earthquake counters. An eruption of Volcan Fuego in Guatemala was preceded by an order of magnitude increase in the number of seismic-event counts several days before. The tiltmeters operated stably in several different environments. A twenty-microradian collapse of the summit of Kilauea Volcano in Hawaii was observed on three tiltmeters. This initial experiment shows that now with the advent of inexpensive satellite telemetry it is both technologically and economically feasible to build a global volcano surveillance system. Several details in the design and deployment of appropriate low-power, inexpensive, and reliable instruments still need to be worked out. Work continues to evaluate the scientific feasibility of this system by collecting and analyzing data that clearly demonstrate the ability of this system to detect changes in volcanic activity.

Open-File Report↗