The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).
Comparison between local variations in magnetic field, long-term changes in creep rate, and local earthquakes have been made for the seismically active and creeping section of the San Andreas fault between the most southern extent of the 1906 earthquake fault break and the most northern extent of the 1857 break, for the period early 1974 through mid-1977. The data utilized are from stations located near the two ends of this section of the San Andreas fault where strain accumulation is expected. The proton precession magnetometer stations included in this study have recorded local magnetic field variations up to 1.8γ with durations of a few minutes to several months. The creep data indicated changes in creep rate of up to 10mm/year lasting for 6 months or more and a close similarity between the changes in creep rate on two adjacent creepmeters about 7km apart. Earthquakes with magnitudes less than 4.0 do not appear to correspond in time to local changes in magnetic field greater than 0.75γ or variations in the creep rate. There is no general correspondence between creep events and magnetic field variations. There is, however, an approximate correspondence, in both space and time, between the long-term changes in creep rate and the variations in magnetic field. In order to explain the observations presented in this study, it appears necessary to allow for a substantial amount of deep aseismic slip without any obvious attendant changes in the time distribution or size of the local earthquakes.
In order to probe the subsurface dynamics associated with geyser eruptions, we measured ground deformation at Old Faithful Geyser of Calistoga, CA. We present a physical model in which recharge during the period preceding an eruption is driven by pressure differences relative to the aquifer supplying the geyser. The model predicts that pressure and ground deformation are characterized by an exponential function of time, consistent with our observations. The geyser's conduit is connected to a reservoir at a depth of at least 42 m, and pressure changes in the reservoir can produce the observed ground deformations through either a poroelastic or elastic mechanical model.
Using 1‐second magnetometer data recorded 67 km from the epicenter of the 1993 M w 7.7 Guam earthquake, Hayakawa et al. (1996) and Miyahara et al. (1999) identify anomalous precursory changes in ultra‐low frequency magnetic polarization (the ratio of vertical to horizontal field components). In a check of their results, we compare their data (GAM) with 1‐second data from the Kakioka observatory (KAK) in Japan and the global magnetic activity index Kp . We also examine log books kept by USGS staff working on the Guam magnetic observatory. We find (1) analysis problems with both Hayakawa et al. and Miyahara et al., (2) significant correlation between the GAM, KAK, and Kp data, and (3) an absence of identifiable localized anomalous signals occurring prior to the earthquake. The changes we do find in polarization are part of normal global magnetic activity; they are unrelated to the earthquake.
Five to ten years of data from an array of 34 total field magnetometers are used to define the temporal and spatial characteristics of secular variation throughout central and southern California. For this period, well‐determined rates of secular variation are obtained at each site. These rates are temporally linear but spatially variable, ranging from −45 nT/a near San Francisco to −54 nT/a near the Mexican border. Least squares analysis of all data indicates secular variation decreases in a general southeasterly direction according to , where is in nanoteslas per year, θ and ϕ are the geographic latitude and longitude, and k 1 , k 2 , and K are 1.66±0.13 nT/a deg, −0.13±0.10 nT/a deg, and −123.2±0.2 nT/a, respectively. Deviations of as much as 1 nT/a occur on scales of a few tens of kilometers. These apparent small‐scale secular variation anomalies result, in part, from differences in local induction and remanent magnetization and may be reduced by determination of a site transfer function. A planar surface fit to the corrected data has the form , where k 1 k 2 , and K are now 1.50±0.08 nT/a deg, −0.23±0.06 nT/a deg, and −129.2±0.1 nT/a, respectively. Residual field variations obtained after correction of all data for secular variation are most apparent on the San Andreas fault in southern California between Palm Springs and the Salton Sea and marginally so along the recent Coyote ( M L = 5.9 of August 6, 1979) and Morgan Hill ( M L = 5.8 of April 24, 1984) aftershock zones. These residuals could be explained by stress localization in these regions and, particularly in the case of the southern San Andreas anomaly, may indicate the location of a future damaging earthquake. Incomplete correction for complex site effects may be an alternative explanation. In the first large‐scale test of global secular variation models we find that the models for this region do not predict either the amplitudes or the mean isogram directions of these data to better than several tens of nanoteslas per year and several tens of degrees, respectively. This may result from a failure to correct for site response effects at some observatories before using the data in global spherical harmonic expansions. Local magnetization response can therefore bias estimates of secular variation and yield apparent impulsive behavior when external fields are perturbed.
