摘要:

This issue of the Geophysical Research Letters contains a collection of 15 reports derived mainly from papers presented at a special session on the same theme at the 1980 Fall Meeting of the American Geophysical Union held in San Francisco during December 8‐12 (EOS Trans. AGU, 61(46), 1032‐1035, 1980). The purpose of the session was to gather together all U.S. scientists known to be working in this study area to review research progress since 1978, when a similar session was convened (EOS Trans. AGU, 59(12), 1195‐1197, 1978). Some of the papers presented in 1978 have since been published in a special issue of the Journal of Geophysical Research (85(B6), 3051‐3121

ISSN:0094-8276    

DOI:10.1029/GL008i005p00421

年代:1981

数据来源: WILEY

摘要:

Water level, temperature, salinity, electric conductivity, and pH have been measured periodically for several years at three water wells located along a 17‐km segment of the San Andreas fault between San Juan Bautista and Cienega Winery in central California. Water samples were collected at the same time for subsequent chemical analyses in the laboratory. Some sudden large changes in salinity and conductivity were recorded in early March 1980 at the two wells near San Juan Bautista. These changes coincided approximately with the beginning of an episode of increased local seismicity, including a magnitude 4.8 earthquake on April 13. Analyses of water samples revealed corresponding changes in ion concentrations, especially of Na+, Ca++, Mg++, SO4−−, HCO3−, F−, and Cl−. The observed changes may be the result of mixing of waters from different aquifers through cracks developed in the water barriers by a possible crustal strain episode that may have occurred in the

ISSN:0094-8276    

DOI:10.1029/GL008i005p00425

年代:1981

数据来源: WILEY

摘要:

Measurements of D and18O concentrations of ground waters in seismically active regions are potentially useful in earthquake prediction and in elucidating mechanisms operative during earthquakes. Principles of this method are discussed and some preliminary data regarding a magnitude 5.7 earthquake at the Oroville Dam in 1975 and a series of events near San Juan Bautista in 1980 are presented to support the utility of such measurements. After earthquakes, the D content of nearby ground waters increased by several permil while the18O content remained constant. This increase implies that H2O may have either decomposed or reacted to form molecular H2at depth. It is emphasized that many areas must be investigated for these effects in order to find a sufficient number of "sensitive" water wells and springs to permit a truly effective program of earthquake research.

ISSN:0094-8276    

DOI:10.1029/GL008i005p00429

年代:1981

数据来源: WILEY

摘要:

Decreases in the helium concentration of soil‐gas have been observed to precede six of eight recent central California earthquakes. Ten monitoring stations were established near Hollister, California and along the San Andreas Fault to permit gas collection. The data showed decreases occurring a few weeks before the earthquakes and concentrations returned to prequake levels either shortly before or after the earthquake

ISSN:0094-8276    

DOI:10.1029/GL008i005p00433

年代:1981

数据来源: WILEY

摘要:

Since late 1976 abandoned water wells have been monitored along the San Andreas and San Jacinto faults to detect water‐level changes which may be premonitory to earthquakes. An M5.5 earthquake occurred 25 February 1980 within the San Jacinto fault zone near Anza. The water level in a continuously recorded well in Borrego Valley 35 km from the epicenter rose 0.5 m and returned to its prior level during a four‐hour period beginning 88 hours before the earthquake. The water level in this well had been remarkably steady compared to other wells being monitored. Another well in Borrego Valley showed a much smaller rise and fall in water level at the same time. Because these changes are unique for the long‐term records of these wells, they may represent precursors to the earthquake.Several wells along the Palmdale‐Valyermo segment of the San Andreas fault showed possibly anomalous, long‐term water‐level changes which are not clearly related to rainfall. Between early 1979 and early 1980, water levels in six wells were .6 to 4.5 m higher and levels in two wells were about 3 m lower than would have been predicted based on the previous records. The water‐level increases could have been the result of delayed response to seasonal rainfall, following a long drought. However, it is interesting that the possibly anomalous water‐level changes first occurred at the same time as the reported changes in strain pattern from compression to expansion along the San Andreas fault i

ISSN:0094-8276    

DOI:10.1029/GL008i005p00437

年代:1981

数据来源: WILEY

摘要:

Recent measurements in the Central Transverse Ranges of southern California suggest possible correlations of changes in groundwater radon content with occurrences of nearby earthquakes. Since measurements began in 1974, three radon anomalies have been accompanied by subsequent nearby seismic events. Two of these anomalies were associated with moderate‐sized earthquakes and one with a swarm. Within a 60‐day window prior to the seismicity, groundwater radon increased in each case at sites close to the earthquake epicenters. Before the Big Bear earthquake of June 30, 1979 (M=4.8), radon anomalies were found at three nearby monitoring sites. Groundwater radon content at one site near the January 1, 1979 Malibu earthquake (M=5.0) showed negative as well as positive anomalies both prior to and following the earthquake. A radon anomaly occurred at a nearby spring prior to the fall 1976 Palmdale swarm. The observed pattern is similar to pre‐earthquake anomalies reported from Russia, China and

ISSN:0094-8276    

DOI:10.1029/GL008i005p00441

年代:1981

数据来源: WILEY

摘要:

Several wells and springs near active faults in southern California have been monitored for the concentration of Rn, Na+, K+, Mg+2, and Cl−for periods of up to 24 months in an effort to see if any of these constituents show variations which may be related to seismic activity or environmental phenomena. A few of these sites have shown significant fluctuations.One site in the San Gabriel Mountains has shown relatively high radon in the summer and low radon in the winter, apparently following the annual precipitation cycle. However, this site showed no corresponding change in its chemistry. A second site near Wrightwood, California has shown an annual cycle in its chemical composition, but shows no change in radon. Ten days prior to the Big Bear earthquake (M ∼ 4.9), 70 km to the east, one sample from this site showed a 50% increase in radon, but no change in chemistry. A third site, 20 km north of the Malibu earthquake (M ∼ 5.0) showed radon variation ranging from 0.4 to 4 times the normal concentration for two months preceding and two months following the event. The chemistry at this site did not change.Within this study area, radon appears to exhibit anomalous variations within several tens of kilometers of impending earthquakes. The mechanism creating these signals has not been elucidated, but does not influence the composition of important ionic constituents. Groundwater radon concentrations may also reflect variations in precipitation but do not appear to reflect variations in atmospheric pressure. Data from cold springs are consistent with a model in which ionic constituents are controlled by reactions in the soil zone and radon concentrations are controlled by flow rates in the aq

ISSN:0094-8276    

DOI:10.1029/GL008i005p00445

年代:1981

数据来源: WILEY

摘要:

Fast Fourier analysis of the near‐real‐time radon data collected since 1977 by the Caltech automated radon‐thoron monitoring system has been carried out in order to determine if any characteristic frequency components are present that can be associated either with precursors to seismicity or with environmental factors. Preliminary results indicate that during "quiet" periods with low seismicity and no rainfall the spectral power is distributed as 1/f. Before four local earthquakes a departure from this 1/f behavior was observed at low frequency. During periods of heavy rainfall an increase in both low and high frequency power was observed. The spectral power of the large radon anomaly observed prior to the October 15, 1979 Imperial Valley earthquake was found to have a 1/f distribution but with power at all frequencies about four times greater than that of data from "quiet" pe

ISSN:0094-8276    

DOI:10.1029/GL008i005p00449

年代:1981

数据来源: WILEY

摘要:

In order to assess the utility of uranium isotopes as fluid phase earthquake precursors, uranium concentrations and234U/238U activity ratios have been monitored on a monthly or bimonthly basis in water from 24 wells and springs associated with Southern California fault zones. Uranium concentrations vary from 0.002 ppb at Indian Canyon Springs on the San Jacinto fault to 8.3 ppb at Lake Hughes well on the San Andreas fault in the Palmdale area.234U/238U activity ratios vary from 0.88 at Agua Caliente Springs on the Elsinore fault to 5.4 at Niland Slab well on the San Andreas fault in the Imperial Valley. There was one large earthquake in the study area during 1979, the 15 October 1979 M=6.6 Imperial Valley earthquake. Correlated with this event, uranium concentrations varied by a factor of more than 60 and activity ratios by a factor of 3 at the Niland Slab site, about 70 km from the epicenter. At the other sites monitored, uranium concentrations varied in time, but with no apparent pattern, while uranium activity ratios remained essentially constant throughout the monitoring period.

ISSN:0094-8276    

DOI:10.1029/GL008i005p00453

年代:1981

数据来源: WILEY

摘要:

As part of the radon and helium monitoring program for earthquake prediction studies in Southern California, concentrations of radium‐226 in groundwaters have been measured along the major fault zones including the Palmdale area. Radium was measured in order to obtain information on its spatial variation as well as baseline levels. Several measurements were made at selected sites to examine their temporal variations. Radium concentrations are used to define the portion of radium supported radon in the observed groundwater radon. Radium levels vary spatially by 4 orders of magnitude (0.015 to 146 dpm/kg), but temporally by a factor of 2 or less. Similar spatial and temporal variations are observed in groundwater radon (60 to 46,700 dpm/kg). The radon/radium activity ratio ranges from 10 to 105. These values indicate that radon is in large excess over radium in groundwaters because of radon diffusion from the ambient rocks. The spatial variations of radon, radium and the activity ratio are probably due to different chemical and physical properties of the rocks through which groundwaters flow.The dependence of groundwater radium concentrations on the water temperature and conductivity is explored. It appears that radium increases in general with conductivity but not necessarily with temperature. Radium is probably leached more readily into the groundwater when the water is more conductive with more dissolved chemical constituent

ISSN:0094-8276    

DOI:10.1029/GL008i005p00457

年代:1981

数据来源: WILEY