Tectonic Implications of Small Earthquakes in the Central Transverse Ranges

Fault-plane solutions for 22 small (local magnitude (M_L ≤ 4.6) earthquakes in the central Transverse Ranges were determined using an azimuthally varying crustal model. The dominant type of faulting observed is reverse faulting on east-striking planes, which suggests a regional stress field characterized by north-south compression. Some strike-slip faulting also occurs. There is some indication that strike-slip earthquakes may be more common than reverse-slip earthquakes during episodes of crustal dilatation. The rate of north-south crustal shortening attributable to small-0earthquake deformation during 1974-76 is two orders of magnitude smaller than the north-south contraction of 0.3 parts per million per year measured at the surface. The scatter in earthquake hypocenters and the general inconsistency of focal mechanisms with geologically determined motions on nearby major faults indicate that the small earthquakes in this region are not associated with large-scale block movements along major fault systems. Rather, they appear to represent fracturing along random minor zones of weakness in response to the regional stress field or, alternatively, small-scale block movements that are below the resolution of this study. Earthquakes in the San Gabriel Mountains north of the Santa Susana-Sierra Madre-Cucamonga frontal fault system tend to concentrate near the eastern and western ends of the range, where good evidence for late Quaternary movement along the frontal faults has been found. Seismicity is markedly lower north of the central section of the frontal fault system, where evidence for late Quaternary movement is lacking

Waveforms and spectra of preshocks and aftershocks of the 1979 Imperial Valley, California, Earthquake: evidence for fault hetergeneity

We have compared digitally-recorded waveforms of M_L 2.0–2.8 earthquakes that occurred in two small areas along the Imperial fault before and after it broke in the ML 6.6 Imperial Valley earthquake on October 15, 1979. Eight preshocks (1977–1979) from a 4½ by 1½ km area centered 4 km SE of the mainshock epicenter have strikingly similar waveforms over the entire record length (∼30 s), with an average peak cross correlation between seismograms of 0.74. The seismograms are well correlated at frequencies up to at least 4 Hz. This implies similar source mechanisms and hypocenters within ¼ of the 4-Hz wavelengths, i.e., <200–400 m. Five aftershocks from the same area show an average peak cross correlation between seismograms of only 0.23. Any associated changes in mechanism must be small because they are not reflected in the first motion data. Analysis of frequency content of these events using bandpass-filtering techniques showed no systematic temporal changes in spectral shape. Ten preshocks and 24 aftershocks from a 1½ by 2 km source area centered along the fault 16 km NW of the mainshock epicenter were also studied. First motion data suggest that all of the aftershocks and a swarm of six preshocks on December 7–9, 1978, were associated with the main fault but that four earlier preshocks were not. The six preshocks on December 7–9, 1978, were tightly clustered, as evidenced by the strong similarity of the waveforms (most peak cross correlations ≥0.6). During this swarm the 8- to 16-Hz spectral amplitude increased relative to the 1- to 2-Hz spectral amplitude over the whole record length by about a factor of 3, suggesting a systematic increase in stress drop. Groups of like events are also present among the aftershocks in this data set. The average peak correlation for pairs of aftershocks, 0.43, is almost the same as that for pairs of preshocks, 0.45, if all 10 preshocks are included. However, several sources appear to have been active simultaneously during the aftershock period so that no more than two to three consecutive aftershocks have maximum cross correlations ≥0.6. The highly localized sources characterized by waveform similarity may represent fault asperities or clusters of asperities. Our observations are consistent with a decrease in the number of these asperities as the weaker ones fail under increasing stress during the intervals between large earthquakes

Contemporary tectonics of the Wasatch front region, Utah, from earthquake focal mechanisms

We have completed a comprehensive study of focal mechanisms of digitally recorded earthquakes (M, -_< 4.4) that occurred in the Wasatch front region in Utah during 1980 to 1986. Single-event solutions for 24 events were determined using recently revised crustal models and a computerized grid-search technique. Overall, the mechanisms show predominantly normal faulting on N-S-striking nodal planes of moderate to steep dip (>30°). Tension-axis azimuths average 96 ° _+ 12% Thus, in general, the mechanisms indicate E-W to ESE-WNW crustal extension and vertical crustal shortening. Oblique slip, when observed, is characterized by left-lateral motion on planes striking N to NE or right-lateral motion on planes striking N to NW. Most of the mechanisms with significant amounts of oblique-slip motion occur in the southern part of the study area, where compression- axis orientations range from near vertical to near horizontal. Thus, the mechanisms suggest a possible change in stress regime from north to south along the Wasatch front. Despite geologic evidence for low-angle faults in the study area, shallowly dipping nodal planes are relatively uncommon.This research was supported by the U.S. Geological Survey, Department of the Interior, under award numbers 14-08-0001-Gl163 and 14-08-0001- G1349. Partial support also came from a scholarship awarded by the Society of Exploration Geophysicists and sponsored by the Sohio Petroleum Company.Peer Reviewe

Seismicity near Palmdale, California, and its relation to strain changes

We evaluate the relationships between the spatio-temporal patterns and faulting mechanisms of small earthquakes and the recent temporal changes in horizontal strain observed along the ‘big bend’ portion of the San Andreas fault near Palmdale, California. Microearthquake activity along the entire big bend of the San Andreas fault increased in November 1976 concurrent with the initiation of an earthquake swarm at Juniper Hills. This activity then decreased abruptly to the northwest and southeast of Juniper Hills during the beginning of 1979. This drop in seismic activity occurred around the time that crustal dilatation was observed on the U. S. Geological Survey Palmdale trilateration network. Focal mechanisms from the study region are predominantly thrust. There are two time periods when the mechanisms are closer to strike slip than to thrust. The first period (December 1976 to February 1977) corresponds to the beginning of the Juniper Hills swarm. The second period (November 1978 to April 1979) approximately coincides with a change in trend of the strain data from uniaxial N-S compression to dilatation

Waveforms and spectra of preshocks and aftershocks of the 1979 Imperial Valley, California, earthquake: Evidence for fault heterogeneity?

We have compared digitally‐recorded waveforms of M_L 2.0–2.8 earthquakes that occurred in two small areas along the Imperial fault before and after it broke in the M_L 6.6 Imperial Valley earthquake on October 15, 1979. Eight preshocks (1977–1979) from a 4½ by 1½ km area centered 4 km SE of the mainshock epicenter have strikingly similar waveforms over the entire record length (∼30 s), with an average peak cross correlation between seismograms of 0.74. The seismograms are well correlated at frequencies up to at least 4 Hz. This implies similar source mechanisms and hypocenters within ¼ of the 4‐Hz wavelengths, i.e., <200–400 m. Five aftershocks from the same area show an average peak cross correlation between seismograms of only 0.23. Any associated changes in mechanism must be small because they are not reflected in the first motion data. Analysis of frequency content of these events using bandpass‐filtering techniques showed no systematic temporal changes in spectral shape. Ten preshocks and 24 aftershocks from a 1½ by 2 km source area centered along the fault 16 km NW of the mainshock epicenter were also studied. First motion data suggest that all of the aftershocks and a swarm of six preshocks on December 7–9, 1978, were associated with the main fault but that four earlier preshocks were not. The six preshocks on December 7–9, 1978, were tightly clustered, as evidenced by the strong similarity of the waveforms (most peak cross correlations ≥0.6). During this swarm the 8‐ to 16‐Hz spectral amplitude increased relative to the 1‐ to 2‐Hz spectral amplitude over the whole record length by about a factor of 3, suggesting a systematic increase in stress drop. Groups of like events are also present among the aftershocks in this data set. The average peak correlation for pairs of aftershocks, 0.43, is almost the same as that for pairs of preshocks, 0.45, if all 10 preshocks are included. However, several sources appear to have been active simultaneously during the aftershock period so that no more than two to three consecutive aftershocks have maximum cross correlations ≥0.6. The highly localized sources characterized by waveform similarity may represent fault asperities or clusters of asperities. Our observations are consistent with a decrease in the number of these asperities as the weaker ones fail under increasing stress during the intervals between large earthquakes

On the performance of ML-MC as a depth discriminant for small seismic events recorded at local distances in Yellowstone, Oklahoma, and Italy

A recent study by Koper et al. (2016) found that the difference between local magnitude (ML) and coda duration magnitude (MC) successfully distinguished shallow seismic events (mining blasts, mining-induced seismicity, and shallow tectonic earthquakes) from deeper seismic events (tectonic earthquakes) in the Utah region and could therefore be helpful for blast discrimination. Here we present tests of the performance of ML-MC as a depth discriminant in three regions and show that it is effective in all of them. Initially, we investigated ML-MC as a function of depth for seismicity in and around Yellowstone National Park recorded by the University of Utah Seismograph Stations. For 2,845 Yellowstone earthquakes with well-constrained depths varying from 0-25 km, we found that ML-MC decreases 0.036 ± 0.014 magnitude units (m.u.) per 1 km in depth over the depth range of 0-8 km. Then, we examined ML-MC values for anthropogenic seismicity recorded by the National Earthquake Information Center in northern Oklahoma and southern Kansas. We found that for 1,692 events with well-constrained depths, the slope of ML-MC for the shallowest 10 km in depth is 0.037 ± 0.016 m.u. per 1 km depth. Finally, we analyzed ML-MC for 28,721 well-located earthquakes in Italy and Sicily recorded by Istituto Nazionale di Geofisica e Vulcanologia. This region showed an increase of 0.017 ± 0.001 m.u. per 1 km depth, up to 30 km in depth. In each case, the quoted error bounds represent 99% confidence regions. We performed several robustness tests in which we varied the depth bin size, the criterion used to define a well-constrained depth, and the depth range used in the linear fit. In nearly all cases we found a positive slope for ML-MC vs. depth at a confidence level above 99%. Our results provide further evidence that ML-MC is useful as a depth discriminant for events recorded at local distances in different physiographic regions.peer-reviewe