Geology topics

Arthur D. Frankel

Publications and source records attributed to Arthur D. Frankel.

93 records · Page 6Linked to original sources

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

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↗