The foreshock activity of the 1971 San Fernando earthquake, California

All of the earthquakes which occurred in the epicentral area of the 1971 San Fernando earthquake during the period from 1960 to 1970 were relocated by using the master-event method. Five events from 1969 to 1970 are located within a small area around the main shock epicenter. This cluster of activity is clearly separated spatially from the activity in the surrounding area, so these five events are considered foreshocks. The wave forms of these foreshocks recorded at Pasadena are, without exception, very complex, yet they are remarkably similar from event to event. The events which occurred in the same area prior to 1969 have less complex wave forms with a greater variation among them. The complexity is most likely the effect of the propagation path. A well located aftershock which occurred in the immediate vicinity of the main shock of the San Fernando earthquake has a wave form similar to that of the foreshocks, which suggests that the foreshocks are also located very close to the main shock. This complexity is probably caused by a structural heterogeneity in the fault zone near the hypocenter. The seismic rays from the foreshocks in the inferred heterogeneous zone are interpreted as multiple-reflected near the source region which yielded the complex wave form. The mechanisms of the five foreshocks are similar to each other but different from either the main shock or the aftershocks, suggesting that the foreshocks originated from a small area of stress concentration where the stress field is locally distorted from the regional field. The number of small events with S-P times between 3.8 to 6 sec recorded at Mt. Wilson each month suggests only a slight increase in activity of small earthquakes near the epicentral area during the 2-month period immediately before the main shock. However, because of our inability to locate these events, the evidence is not definitive. Since the change in the wave forms is definite the present result suggests that detailed analyses of wave forms, spectra, and mechanism can provide a powerful diagnostic method for identifying a foreshock sequence

Focal mechanisms and aftershock locations of the Songpan earthquakes of August 1976 in Sichuan, China

The precursory swarm, three mainshocks (M = 7.2,6.7, 7.2), and aftershocks of the Songpan earthquakes have been reanalyzed using both local and teleseismic data. The three mainshocks of this sequence occurred on the Huya fault over a 7-day period. Relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence. Each mainshock occurred on a separate strand, each one south of the strand activated in the previous mainshock, and the aftershock zones of each mainshock appear to abut rather than overlap. Fault plane solutions determined by matching teleseismic P waveforms at World-Wide Standard Seismograph Network stations with synthetic seismograms are consistent with the observed aftershock zones. The first and third mainshocks (M_0 = 1.3 ×10^(19) and 8.4 × 10^(18) N m, respectively) showed almost identical senses of motion, a combination of reverse and left-lateral strike-slip motion, on parallel strands, striking N15°W, that were separated by a large rightstepping en echelon offset. The second mainshock (M_0 = 4.0 × 10^(18) N m), occurred in this offset on a fault at a steep angle (∼125°) to the other two strands and showed almost pure reverse motion. Differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock. These changes in normal stresses may have given rise to the longer time between the first and second events (5 days) as compared with the time between the second and third events (30 hours). A precursory swarm that preceded the Songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone. The time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in Soviet Central Asia

Machine-Learning Reveals Aftershock Locations for Three Idaho Earthquake Sequences

I explore spatial and temporal aftershock patterns related to three instrumentally recorded earthquakes in Idaho -- the Sulphur Peak, the Challis, and the Stanley earthquakes. These three M \u3e 5 earthquakes border the eastern Snake River Plain and lie within the Intermountain Seismic Belt and Centennial Tectonic Belt. Using machine learning for event detection and phase picking from local and regional seismic networks, I generate new aftershock catalogs. I locate more aftershocks than in the USGS catalog due to lower signal-to-noise detections. Using my phase picks, I locate aftershocks using a range of velocity models and select a catalog that represents the smallest residuals in hypocenter locations. I compare my results with handpicked phases and previously published velocity models. My 2014-2017 Challis catalog is consistent with the work of Pang et al. (2018), with more high-quality events with similar average vertical error. My one-month aftershock catalog for the 2017 Sulphur Peak earthquake is spatially consistent with the results of Koper et al. (2018); however, I show that my machine-learning approach produced relatively few aftershocks because afterslip events were not matched using a coseismic training dataset. Finally, I locate a factor of five more aftershocks from the 2020 Stanley earthquake when compared to the USGS catalog. I relocate the mainshock using biases computed by differencing my aftershock epicenters with the same aftershocks in the USGS catalog. The revised mainshock location now lies within a large and pronounced aftershock zone. My catalog suggests no motion along the active Sawtooth Fault, but instead I map a new N10W trending fault that accommodated the mainshock and much of the aftershock slip. I conclude that aftershock catalogs derived from a machine-learning approach can enhance seismic detection and aid in determining the driving mechanisms responsible for a coseismically driven earthquakes

Source parameters of the 11 June 1909, Lambesc (Provence, southeastern France) earthquake: a reappraisal based on macroseismic, seismological and geodetic observations

Destructive earthquakes are rare in France yet pose a sizable seismic hazard, especially when critical infrastructures are concerned. Only a few destructive events have occurred within the instrumental period, the most important being the 11 June 1909, Lambesc (Provence) earthquake. With a magnitude estimated at 6.2 [Rothé, 1942], the event was recorded by 30 observatories and produced intensity IX effects in the epicentral area, ~30 km north of Marseille. We collected 30 seismograms, leveling data and earthquake intensities to assess the magnitude and possibly the focal mechanism of this event. Following this multidisciplinary approach, we propose a source model where all relevant parameters are constrained by at least two of the input datasets. Our reappraisal of the seismological data yielded Mw 5.8-6.1 (6.0 preferred) and Ms 6.0, consistent with the magnitude from intensity data (Me 5.8) and with constraints derived from modeling of coseismic elevation changes. Hence, we found the Lambesc earthquake to have been somewhat smaller than previously reported. Our datasets also constrain the geometry and kinematics of faulting, suggesting that the earthquake was generated by reverse-right lateral slip on a WNW-striking, steeply north-dipping fault beneath the western part of the Trévaresse fold. This result suggests that the fold, located in front of the Lubéron thrust, plays a significant role in the region’s recent tectonic evolution. The sense of slip obtained for the 1909 rupture also agrees with the regional stress field obtained from earthquake focal mechanisms and microtectonic data as well as recent GPS data

Magnitude Uncertainties Impact Seismic Rate Estimates, Forecasts and Predictability Experiments

The Collaboratory for the Study of Earthquake Predictability (CSEP) aims to prospectively test time-dependent earthquake probability forecasts on their consistency with observations. To compete, time-dependent seismicity models are calibrated on earthquake catalog data. But catalogs contain much observational uncertainty. We study the impact of magnitude uncertainties on rate estimates in clustering models, on their forecasts and on their evaluation by CSEP's consistency tests. First, we quantify magnitude uncertainties. We find that magnitude uncertainty is more heavy-tailed than a Gaussian, such as a double-sided exponential distribution, with scale parameter nu_c=0.1 - 0.3. Second, we study the impact of such noise on the forecasts of a simple clustering model which captures the main ingredients of popular short term models. We prove that the deviations of noisy forecasts from an exact forecast are power law distributed in the tail with exponent alpha=1/(a*nu_c), where a is the exponent of the productivity law of aftershocks. We further prove that the typical scale of the fluctuations remains sensitively dependent on the specific catalog. Third, we study how noisy forecasts are evaluated in CSEP consistency tests. Noisy forecasts are rejected more frequently than expected for a given confidence limit. The Poisson assumption of the consistency tests is inadequate for short-term forecast evaluations. To capture the idiosyncrasies of each model together with any propagating uncertainties, the forecasts need to specify the entire likelihood distribution of seismic rates.Comment: 35 pages, including 15 figures, agu styl

Seismicity patterns (1963-1983) as stress indicators in the Shumagin seismic gap, Alaska

Earthquakes (1963 to 1983) of magnitude 5.0 to 6.5 form a rim of high seismic activity around a central region of relative seismic quiescence that coincides with the Shumagin seismic gap. This pattern, which can be inferred both from local network and teleseismic earthquake locations, shows high activity near the eastern and western ends of the seismic gap and along the down-dip end of the main thrust zone. The rim of seismicity surrounding an area of seismic quiescence may reflect strong couplign along the elastic-brittle part of the plate boundary where the great earthquakes occur. The temporal behavior of the microseismicity recorded by the Shumagin network is characterized by a burst of activity in 1978 and 1979, and a low or average level of activity from 1980 to January 1983. The increased microearthquake activity during 1978-1979 is located mainly near the down-dip end of the main thrust zone and along the Benioff zone down to 100 to 200 km depth. Composite fault plane solutions of earthquakes occurring in 1978-1979 show down-dip tension between 50 to 120 km depth in the upper plane of the Benioff zone. Composite fault plane solutions of 1981 earthquakes, however, indicate in-plate compression in the same region. Hence, the rate of occurrence and focal mechanisms of microearthquakes located in the Benioff zone below the main thrust both show coincident temporal and spatial variations that may reflect fluctuations in local stresses

What We Can Learn From Japan\u27s Early Earthquake Warning System

Japan\u27s combination of high technology and cultural adaptation to its natural setting makes their earthquake detection systems a model for the rest of the world

The October 2012 magnitude (Mw) 7.8 earthquake offshore Haida Gwaii, Canada

Alison L. Bird et al. report on the Mw 7.8 earthquake offshore Haida Gwaii, Canada, from 2012 for the Summary of the Bulletin of the International Seismological Centre