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At least 883 records · Page 49Linked to original sources
USGS1989

Three-dimensional records of surface displacement on the Superstition Hills fault zone associated with the earthquakes of 24 November 1987

Seven quadrilaterals, constructed at broadly distributed points on surface breaks within the Superstition Hills fault zone, were repeatedly remeasured after the pair of 24 November 1987 earthquakes to monitor the growing surface displacement. Changes in the dimensions of the quadrilaterals are recalculated to right-lateral and extensional components at millimeter resolution, and vertical components of change are resolved at 0.2 mm precision. The displacement component data for four of the seven quadrilaterals record the complete fault movement with respect to an October 1986 base. These data fit with remarkable agreement the power law

California↗
USGS1989

Inverting measurements of surface slip on the Superstition Hills fault

We derive and test a set of inversions of surface-slip measurements based on the empirical relation u(t) = uf/(1 + T/t)c proposed by Sharp and Saxton (1989) to estimate the final slip uf, the power-law exponent c, and the power-law duration T. At short times, Sharp's relation behaves like the simple power law, u(t) ∼ u1tc, where u1 is the initial slip, that is, the slip at 1 day after the earthquake. At long times, the slip approaches the final slip asymptotically. The inversions are designed in part to exploit the accuracy of measurements of differential slip; that is, measurements of surface slip which are made relative to a set of nails or stakes emplaced after the earthquake. We apply the inversions to slip measurements made at 53 sites along the Superstition Hills fault for the 11 months following the M = 6.2 and 6.6 earthquakes of 24 November 1987. In general, estimates of the initial slip and the power law exponent are well resolved, while estimates of the power-law durations and the final slip are less well resolved because the durations of the surface slip measurements are often less than the derived power-law durations. The slip on the three fault strands is a relatively smooth function of position; the initial slip and final slip are well correlated. The time dependence of surface slip at the 53 sites is roughly similar along the entire fault, where the power-law exponents are distributed as c = 0.14 ± 0.04 and the power-law durations range from 100 < T < 1000 days.

California↗
USGS1989

Pre-earthquake displacement and triggered displacement on the Imperial fault associated with the Superstition Hills earthquake of 24 November 1987

Two right-lateral slip events, about 3 weeks apart in November 1987, broke the surface discontinuously along probably similar, nearly 20 km lengths of the northern Imperial fault. The first displacement, at about the beginning of November, was accompanied by a surface tilt representing deep vertical motion or distributed strain. This movement may have been part of a more regional event that also involved the southern San Andreas fault, although the evidence there is questionable. The later surface offset was triggered, probably by the second main shock of the 24 November earthquakes located in the Superstition Hills, about 37 km northwest of the Imperial fault. The maximum observed displacement was less than 4 cm on both occasions; for the triggered movement the maximum slip occurred on a branch strand near the northern extremity of the fault.

California↗
USGS1989

Moment-tensor solutions for the 24 November 1987 Superstition Hills, California, earthquakes

The teleseismic long-period waveforms recorded by the Global Digital Seismograph Network from the two largest Superstition Hills earthquakes are inverted using an algorithm based on optimal filter theory. These solutions differ slightly from those published in the Preliminary Determination of Epicenters Monthly Listing because a somewhat different, improved data set was used in the inversions and a time-dependent moment-tensor algorithm was used to investigate the complexity of the main shock. The foreshock (origin time 01:54:14.5, mb 5.7, Ms 6.2) had a scalar moment of 2.3 × 1025 dyne-cm, a depth of 8 km, and a mechanism of strike 217°, dip 79°, rake 4°. The main shock (origin time 13:15:56.4, mb 6.0, Ms 6.6) was a complex event, consisting of at least two subevents, with a combined scalar moment of 1.0 × 1026 dyne-cm, a depth of 10 km, and a mechanism of strike 303°, dip 89°, rake −180°.

California↗
USGS1989

Rupture process of the Ms 6.6 Superstition Hills, California, earthquake determined from strong-motion recordings: application of tomographic source inversion

We analyze strong-motion recordings of the M s 6.6 Superstition Hills earthquake to determine the timing, location, spatial extent, and rupture velocity of the subevents that produced the bulk of the high-frequency (0.5 to 4 Hz) seismic energy radiated by this shock. The earthquake can be characterized by three principal subevents, the largest ones occurring about 3 and 10 sec after initiation of rupture. Timing relationships between pulses on the seismograms indicate that the three subevents are located within 8 km of each other along the northern portion of the Superstition Hills fault. The two largest subevents display different directivity effects. We apply a tomographic source inversion to the integrated accelerograms to determine the slip acceleration on the fault as a function of time and distance, based on a one-dimensional fault model. The azimuthal distribution of amplitudes for the second subevent can be largely explained by a rupture that propagated about 2 km to the southeast along the Superstition Hills fault at a velocity about equal to the P -wave velocity. An alternative model with rupture propagating to the northeast along a conjugate fault plane can also account for the observed directivity of this subevent, but it is not supported by the aftershock distribution. The third subevent ruptured to the southeast along an 8-km long portion of the Superstition Hills fault at about the shear-wave velocity. This rupture propagation caused the relatively large accelerations and velocities observed in strong-motion records for stations southeast of the hypocenter. The long time intervals between the subevents and their relative proximity to each other indicate a very slow component to the rupture development. The southern half of the Superstition Hills fault did not generate significant high-frequency strong ground motion, although it showed substantial co-seismic surface displacement. The subevents are situated along the same northern portion of the fault where most of the aftershocks are located. The locations of the subevents appear to be controlled by bends in the fault mapped at the surface and by changes in basement structure at depth.

California↗
USGS1989

Microearthquake spectra from the Anza, California, seismic network: Site response and source scaling

We analyzed spectra of local microearthquakes recorded by the Anza, California, seismic network to isolate the effects of site response and to investigate the scaling of source parameters for small earthquakes. Spectra of microearthquakes (M < 2; Mo < 1019 dyne-cm) at Anza have shapes characteristic of the receiver sites and are generally independent of the source region. Thus, the site response is a major conditioner of the observed spectral shape. To remove the effects of site response from the spectra of a M ∼ 3 event and isolate its source spectrum, we divided by the spectra of an adjacent aftershock used as an empirical Green's function event. The spectral ratios indicate that the apparent corner frequencies of small earthquakes (Mo < 1019dyne-cm) observed at even the high-fmax stations on hard rock are much lower than the source corner frequencies. The spectral ratios are consistent with stress drop remaining constant with decreasing seismic moment, for events with moments as small as 1018 dyne-cm. The spectral ratios display remarkable agreement between sites which showed vast differences in their original spectra, indicating that the spectral division effectively removed the site response. The source spectrum of the M ∼ 3 event has a high-frequency spectral fall-off of about ω−2. An apparent dependence of high-frequency fall-off with seismic moment in the original spectra can also be explained by the effects of site response. The difference between the P- and S-wave corner frequencies and high-frequency roll-offs in the observed spectra for these events is the result of the site response and is not a source property. The shapes of the spectra of microearthquakes at Anza can largely be explained by attenuation at shallow depth with a frequency-independent Q. For some sites, near-surface resonances are also apparent in the spectra of microearthquakes. It is indicated by t* values determined for each site that Qp ∼ Qs for the shallow low-Q layer. Further evidence of low near-surface Q is observed in the anomalous spectra of an unusually shallow earthquake (source depth ∼ 1 km) in the network. The spectra from this shallow event are depleted in high-frequency energy at most stations, relative to those of deeper events. This observation can be explained by a low-Q surficial zone. For stations of the network situated on alluvium, this low-Q layer has a maximum thickness of about 3 km and maximum P- and S-wave Q values of 30 to 50.

California↗
USGS1989

An exact anelastic model for the free-surface reflection of P and S-I waves

Exact anelastic solutions incorporating inhomogeneous waves are used to model numerically S -I and P waves incident on the free surface of a low-loss anelastic half-space. Anelastic free-surface reflection coefficients are computed for the volumetric strain and displacement components of inhomogeneous wave fields. For the problem of an incident homogeneous S -I wave in Pierre shale, the largest strain and displacement amplitudes for the reflected P wave occur at angles of incidence for which the particle motion for the reflected inhomogeneous P wave is elliptical (minor/major axis = 0.6), the specific absorption ( Q P −1 ) is greater (300 per cent) and the velocity is less (25 per cent) than those for a corresponding homogeneous P wave, the direction of phase propagation is not parallel to the free surface, and the amplitude of the wave shows a significant increase with depth (6 per cent in one wavelength). Energy reflection coefficients computed for this low-loss anelastic model show that energy flow due to interaction of the incident and reflected waves reach maxima (30 per cent of the incident energy) near large but nongrazing angles of incidence. For the problem of an incident homogeneous P wave in Pierre shale, the inhomogeneity of the reflected S wave is shown not to contribute to significant variations in wave field characteristics over those that would be expected for a homogeneous wave.

Bulletin of the Seismological Society of America↗
USGS1989

On the use of volumetric strain meters to infer additional characteristics of short-period seismic radiation

Volumetric strain meters (Sacks-Evertson design) are installed at 15 sites along the San Andreas fault system, to monitor long-term strain changes for earthquake prediction. Deployment of portable broadband, high-resolution digital recorders (GEOS) at several of the sites extends the detection band for volumetric strain to periods shorter than 5 × 10 −2 sec and permits the simultaneous observation of seismic radiation fields using conventional short-period pendulum seismometers. Simultaneous observations establish that the strain detection bandwidth extends from periods greater than 10 7 seconds to periods near 5 × 10 −2 sec with a dynamic range exceeding 140 dB. Measurements of earth-strain noise for the period band, 10 7 to 10 −2 sec, show that ground noise, not instrument noise, currently limits the measurement of strain over a bandwidth of more than eight orders of magnitude in period. Comparison of the short-period portion of earth-strain, noise spectra (20 to 5 × 10 −2 sec) with average spectra determined from pendulum seismometers, suggest that observed noise is predominantly dilatational energy. Recordings of local and regional earthquakes indicate that dilatometers respond to P energy but not direct shear energy and that straingrams can be used to resolve superimposed reflected P and S waves for inference of wave characteristics not permitted by either sensor alone. Simultaneous measurements of incident P - and S -wave amplitudes are used to introduce a technique for single-station estimates of wave field inhomogeneity, free-surface reflection coefficients and local material P velocity. Estimates of these parameters derived for the North Palm Springs earthquake ( M w 5.9) respectively for an incident P wave of 29° are −85°, 1.71, 2.9 km/sec, and for an incident S wave of 17° are 79°, 0.85, 2.9 km/sec. The empirical estimates of reflection coefficients are consistent with model estimates derived using an anelastic half-space model with incident inhomogeneous wave fields.

California↗
USGS1989

On the similarity of theories of anelastic and scattering attenuation

We point out basic parallels between theories of anelastic and scattering attenuation. We consider approximations to scattering effects presented by O'Doherty and Anstey (1971), Sato (1982), and Wu (1982). We use the linear theory of anelasticity. We note that the frequency dependence of Q can be related to a distribution of scales of physical properties of the medium. The frequency dependence of anelastic Q is related to the distribution of relaxation times in exactly the same manner as the frequency dependence of scattering Q is related to the distribution of scatterer sizes. Thus, the well-known difficulty of separating scattering from intrinsic attenuation is seen from this point of view as a consequence of the fact that certain observables can be interpreted by identical equations resulting from either of two credible physical theories describing fundamentally different processes.

Bulletin of the Seismological Society of America↗
USGS1990

The theory and prototype development of a stress-monitoring system

A new approach has been developed by which changes in rock stress can be measured directly. The measurement of stress change depends on the reaction of a slender, compliant cavity to the transient variations of rock stress about the cavity. A liquid-filled pressurized cell, emplaced in the cavity, provides the means by which changes in pressure can be monitored. The change in pressure in the cell is equivalent to the change in rock stress in the direction normal to the cell if the compliance contrast between cavity and rock is large and the aspect ratio of the cavity is small. Earthquake-related variations in the stress field in a highly fractured quartz-monzonite stock have been observed to occur in a characteristic way prior to a microseismic event. The precursory stress change (decompression followed by compression) lasted about 5 hours and led directly to a stress drop of 15 × 10−3 bar. Seismic noise monitored during the same period of time suggests that failure in the fractured rock mass resulted from rapid encroachment of meltwater and subsequent increase in fluid pressure.

Bulletin of the Seismological Society of America↗
USGS1990

The 1989 earthquake swarm beneath Mammoth Mountain, California: An initial look at the 4 May through 30 September activity

Mammoth Mountain is a 50,000- to 200,000-year-old cumulovolcano standing on the southwestern rim of Long Valley in eastern California. On 4 May 1989, two M = 1 earthquakes beneath the south flank of the mountain marked the onset of a swarm that has continued for more than 6 months. In addition to its longevity, noteworthy aspects of this persistent swarm include (1) an exponential-like increase in the rate of activity through the first month; (2) a vertically oriented, planar distribution of hypocenters at depths between 6 and 9 km with a north-northeast strike (roughly perpendicular to the average T-axis orientation for the swarm earthquakes); (3) recurring spasmodic bursts (rapid-fire sequences of similar-sized earthquakes with overlapping coda) and occasional earthquakes with enhanced low-frequency energy; (4) a uniform temporal distribution of the four largest (M ≈ 3) events over the first 4 months of the swarm with a cumulative seismic moment for the entire sequence through 30 September corresponding to a single M ≈ 4 earthquake; (5) a b-value of 1.2; and (6) submicrostrain perturbations on the nearby borehole dilatometer, the first of which led the onset of swarm activity by more than 2 weeks. These aspects of the swarm, together with its location along the southern extension of the youthful Mono-Inyo volcanic chain, which last erupted 500 to 600 years ago, point to a magmatic source for the modest but persistent influx of strain energy into the crust beneath Mammoth Mountain.

Bulletin of the Seismological Society of America↗
USGS1990

The 3 December 1988 Pasadena, California earthquake: Evidence for strike-slip motion on the Raymond Fault

The Pasadena earthquake (ML = 4.9) occurred on 3 December 1988, at a depth of 16 km. The hypocenters of the earthquake and its aftershocks define a east-northeast striking, steeply northwest-dipping surface that projects up to the active surficial trace of the Raymond fault. One of the nodal planes of the focal mechanism of the earthquake parallels the Raymond fault with left-lateral strike-slip movement on that plane, and is consistent with geomorphic and paleoseismic evidence that the Raymond fault is dominantly a left-lateral strike-slip fault. The existence of a component of sinistral slip along the Raymond fault had been suspected prior to the earthquake, but the northward dip of the fault and the prominent scarp along the western portion of its trace had led most workers to conclude that slip along the fault was dominantly reverse.

California↗
USGS1990

Source inversion of the 1988 Upland, California, earthquake: Determination of a fault plane for a small event

We examined short-period P waves to investigate if waveform data could be used to determine which of two nodal planes was the actual fault plane for a small (ML 4.6) earthquake near Upland, California. We removed path and site complications by choosing a small aftershock (ML 2.7) as an empirical Green function. The main shock P waves were deconvolved by using the empirical Green function to produce simple far-field displacement pulses. We used a least-squares method to invert these pulses for the slip distribution on a finite fault. Both nodal planes (strike 125°, dip 85° and strike 221°, dip 40°) of the first-motion focal mechanism were tested at various rupture velocities. The southwest trending fault plane consistently gave better fitting solutions than the southeast-trending plane. We determined a moment of 4.2 × 1022 dyne-cm. The rupture velocity, and thus the source area could not be well resolved, but if we assume a reasonable rupture velocity of 0.87 times the shear wave velocity, we obtain a source area of 0.97 km2 and a stress drop of 38 bars. Choice of a southwest-trending fault plane is consistent with the trend of the nearby portion of the Transverse Ranges frontal fault zone and indicates left-lateral motion. This method provides a way to determine the fault plane for small earthquakes that have no surface rupture and no obvious trend in aftershock locations.

California↗
USGS1990

Near-surface velocities and attenuation at two boreholes near Anza, California, from logging data

To investigate near-surface site effects in granite rock, we drilled 300-m-deep boreholes at two sites which are collocated with stations from the digital array at Anza, California. The first borehole was sited at station KNW (Keenwild fire station), which is located along a ridge line about 8.7 km east of the San Jacinto Fault zone. Station PFO (Piñon Flat Observatory), chosen for the second site, is another 6 km further to the east of station KNW and is located on a gently sloping hillside. We logged each borehole for P- and S-wave velocities, as well as for crack density and orientation.

California↗
USGS1990

Calculation of zero-offset vertical seismic profiles generated by a horizontal point force acting on the surface of an elastic half-space

Impulse responses including near-field terms have been obtained in closed form for the zero-offset vertical seismic profiles generated by a horizontal point force acting on the surface of an anelastic half-space. The method is based on the correspondence principle. Through transformation of variables, the Fourier transform of the elastic impulse response is put in a form such that the Fourier transform of the corresponding anelastic impulse response can be expressed as elementary functions and their definite integrals involving distance, angular frequency, phase velocities, and attenuation factors. These results are used for accurate calculation of shear-wave arrival rise times of synthetic seismograms needed for data interpretation of anelastic-attenuation measurements in near-surface sediment.

Bulletin of the Seismological Society of America↗
USGS1990

Source parameters and effects of bandwidth and local geology on high- frequency ground motions observed for aftershocks of the northeastern Ohio earthquake of 31 January 1986

A 10-station array (GEOS) yielded recordings of exceptional bandwidth (400 sps) and resolution (up to 96 dB) for the aftershocks of the moderate ( m b ≈ 4.9) earthquake that occurred on 31 January 1986 near Painesville, Ohio. Nine aftershocks were recorded with seismic moments ranging between 9 × 10 16 and 3 × 10 19 dyne-cm ( M w : 0.6 to 2.3). The two largest aftershocks (depth 5.3, 5.6 km; oblique right slip, rake ≈30°, strike ≈N25°E) yielded seismic signals above background noise at frequencies as high as 130 Hz at epicentral distances up to 17 km. The aftershock recordings at a site underlain by ≈8 m of lakeshore sediments show significant levels of high-frequency soil amplification of vertical motion at frequencies near 8, 20, and 70 Hz. Viscoelastic models for P and SV waves incident at the base of the sediments yield estimates of vertical P -wave response consistent with the observed high-frequency site resonances, but suggest additional detailed shear-wave logs are needed to account for observed S -wave response. Peak acceleration values obtained from the broadband recordings are about two and four times as large as those that would be recorded on strong-motion recorders or short-period networks with upper bandwidth limits of 30 and 15 Hz, respectively. Attenuation-corrected acceleration spectra are used to reduce the influence of high-frequency (up to 100 Hz) local site effects on corner frequency estimates. The moment versus source radius trend inferred for events with moments as small as 9 × 10 16 dyne-cm, based on the Brune source model, extends previous relations inferred for the central United States, shows little evidence for a minimum source radius, and suggests that stress drops for the smaller events ( M 0 < 10 19 dyne-cm) decrease with decreasing moment.

Ohio↗