“FAUST”

A volume containing publications on topics related to

FAUST project.

Prepared for the partners of the project:

“FAUST”

EC contract ENV 4970528 (1998-Jan-01—2000-Dec-31)

This collection is to be used by the FAUST contract partners only as references for the ongoing project.

The image in the front page shows an example of earthquake interaction.

King, G. C. P., R. S. Stein, and J. Lin, Static stress changes and the triggering of earthquakes, Bull. Seismol. Soc. Am., 84, 935-953, 1994. (Figure 9)

One goal of this project is to perform seismic hazard studies using different level of accuracy of fault models (a line in a plane, a plane in space, 3-d structures interacting in time) coupled with different statistical models (stationary and time-dependent) in order to assess the variation both in absolute value and in accuracy of the estimates, for each sample area.

In order to assess how much different fault models change the image of seismic hazard, several techniques will be used and compared, and namely:

• creation of a reference set of maps, based on the full exploitation of historical data only, including observed intensity at each site, but without any input from geology;
• time-independent seismic hazard estimate substituting to the Cornell's seismogenic areas used for a national estimate the 2-d sources derived from seismogenic faults (test on the influence of geometry);
• as the above, but including where available characteristic models of faulting and information on return times (test on first level time-dependence);
• deterministic (scenario-like) estimate of ground motion imposing the maximum expected event on each fault, using an hybrid model for 3-d faults (test on second level of geometry influence)
• study of variability of earthquake occurrence probability on a fault as a consequence of interaction with pattern of the stress released in space and time by the surrounding faults (test on a full-scale time dependence)

The analysis of static stress changes due to coseismic dislocations has been widely applied in recent years to study the variation in failure stress on well known segmented seismogenic faults. These studies assert that earthquakes induce changes in static stress on neighbouring faults that may delay, advance or trigger impending earthquakes [Nostro et al., 1997]. There is now an internationally distributed set of published works about static stress changes, generated by large earthquakes, influencing the timing and locations of subsequent earthquakes [Harris, 1998].

Stress change calculations have been performed for earthquakes in the Asal Rift of northeastern Africa [Jacques et al., 1996], in Chile [Delouis et al., 1998], in Greece [Hubert et al., 1996], in Italy [Nostro et al., 1997; Troise et al., 1998], in Japan [Yamashina, 1978, 1979; Rybicki et al., 1985; Kato et al., 1987; Yoshioka and Hashimota, 1989a, b; Okada and Kasahara, 1990; Pollitz and Sacks, 1995, 1997], along the Macquarie Ridge [Das, 1992], in Mexico [Singh et al., 1998], in New Zealand [Robinson, 1994], in Turkey [Roth, 1988; Nalbant et al., 1996; Stein et al., 1997], and in the United States in California [Smith and Van de Lindt, 1969; Rybicki, 1971, 1973; Das and Scholz, 1981; Mavko, 1982; Stein and Lisowski, 1983; Mavko et al., 1985; Li et al., 1987; Simpson et al., 1988; Oppenheimer et al., 1988; Hudnut et al., 1989; Michael, 1991; Reasenberg and Simpson, 1992; Harris and Simpson, 1992; Jaumé and Sykes, 1992; Stein et al., 1992; Du and Aydin, 1993; Oppenheimer et al., 1993; Stein et al., 1994; Simpson and Reasenberg, 1994; King et al., 1994; Bennett et al., 1995; Harris et al., 1995; Harris and Simpson, 1996; Deng and Sykes, 1996; Jaumé and Sykes, 1996; Deng and Sykes, 1997a, b; Burgmann et al., 1997; Reasenberg and Simpson, 1997] and Nevada [Hodgkinson et al., 1996; Caskey and Wesnousky, 1997].

Many of the authors have used calculations of a Coulomb stress increment calculated from an elastic dislocation model of the mainshock [e.g., Chinnery, 1963; Weertman and Weertman, 1964; Okada, 1992] and have examined the geographical pattern of subsequent earthquakes relative to the pattern of change in Coulomb failure stress. Almost all of these studies profess finding a positive correlation between the number (or rate) of aftershocks, or the occurrence of subsequent mainshocks, and regions of calculated stress increase. Many of the studies also show a deficiency (or rate decrease) of aftershocks, or subsequent mainshocks, in regions of calculated stress decrease [Harris, 1998].

This collection primarily contains reprints of some of the many publications on earthquake interaction studies and stress triggers, and is designed to facilitate understanding of this specific research field; at the end of the volume there is a detailed list of references.

Bennett, R. A., R. E. Reilinger, W. Rodi, Y. Li, and M. N. Toksöz, Coseismic fault slip associated with the 1992 M_W 6.1 Joshua Tree, California, earthquake: Implications for the Joshua Tree-Landers earthquake sequence, J. Geophys. Res., 100, 6443-6461, 1995.

Burgmann, R., P. Segall, M. Lisowski, and J. Svarc, Postseismic strain following the 1989 Loma Prieta earthquake from GPS and leveling measurements, J. Geophys. Res., 102, 4933-4955, 1997.

Caskey, S. J., and S. G. Wesnousky, Static stress changes and earthquake triggering during the 1954 Fairview Peak and Dixie Valley earthquakes, central Nevada, Bull. Seismol. Soc. Am., 87, 521-527, 1997.

Chinnery, M. A., The stress changes that accompany strike-slip faulting, Bull Seismol. Soc. Am., 53, 921-932, 1963.

Das, S., Reactivation of an oceanic fracture by the Macquarie Ridge earthquake of 1989, Nature, 357, 150-153, 1992.

Das, S., and C. Scholz, Off-fault aftershock clusters caused by shear stress increase?, Bull. Seismol. Soc. Am., 71, 1669-1675, 1981.

Delouis, B., H. Philip, L. Dorbath, and A. Cisternas, Recent crustal deformation in the Antofagasta region (northern Chile) and the subduction process, Geophys. J. Int., 132, 302-338, 1998.

Deng, J., and L. R. Sykes, Triggering of 1812 Santa Barbara earthquake by a great San Andreas shock: Implications for future seismic hazards in southern California, Geophys. Res. Lett., 23, 1155-1158, 1996.

Deng, J., and L. R. Sykes, Evolution of the stress field in southern California and triggering of moderate-size earthquakes: A 200-year perspective, J. Geophys. Res., 102, 9859-9886, 1997a.

Deng, J., and L. R. Sykes, Stress evolution in southern California and triggering of moderate-, small-, and micro-size earthquakes, J. Geophys. Res., 102, 24,411-24,435, 1997b.

Du, Y., and A. Aydin, Stress transfer during three sequential moderate earthquakes along the central Calaveras fault, California, J. Geophys. Res., 98, 9947-9962, 1993.

Harris R. A., Introduction to special section: Stress triggers, stress shadows, and implications for seismic hazard, J. Geophys. Res., 103 , 10 , 24,347-24,358, 1998.

Harris, R. A., and R. W. Simpson, Changes in static stress on southern California faults after the 1992 Landers earthquake, Nature, 360, 251-254, 1992.

Harris, R. A., and R. W. Simpson, In the shadow of 1857—The effect of the great Ft. Tejon earthquake on subsequent earthquakes in southern California, Geophys. Res. Lett., 23, 229-232, 1996.

Harris, R. A., R. W. Simpson, and P. A. Reasenberg, Influence of static stress changes on earthquake locations in southern California, Nature, 375, 221-224, 1995.

Hodgkinson, K. M., R. S. Stein, and G. C. P. King, The 1954 Rainbow Mountain-Fairview Peak-Dixie Valley earthquakes: A triggered normal faulting sequence, J. Geophys. Res., 101, 25,459-25,471, 1996.

Hubert, A., G. King, R. Armijo, B. Meyer, and D. Papanastasiou, Fault re-activation, stress interaction, and rupture propagation of the 1981 Corinth earthquake sequence, Earth Planet. Sci. Lett., 142, 573-585, 1996.

Hudnut, K. W., L. Seeber, and J. Pacheco, Cross-fault triggering in the November 1987 Superstition Hills earthquake sequence, southern California, Geophys. Res. Lett., 16, 199-202, 1989.

Jacques, E., G. C. P. King, P. Tapponnier, J. C. Ruegg, and I. Manighetti, Seismic activity triggered by stress changes after the 1978 events in the Asal Rift, Djibouti, Geophys. Res. Lett., 23, 2481-2484, 1996.

Jaumé, S. C., and L. R. Sykes, Change in the state of stress on the southern San Andreas fault resulting from the California earthquake sequence of April to June 1992, Science, 258, 1325-1328, 1992.

Jaumé, S. C., and L. R. Sykes, Evolution of moderate seismicity in the San Francisco Bay region, 1850 to 1993: Seismicity changes related to the occurrence of large and great earthquakes, J. Geophys. Res., 101, 765-789, 1996.

Kato, T., K. Rybicki, and K. Kasahara, Mechanical interaction between neighboring active faults—An application to the Altera fault, central Japan, Tectonophysics, 144, 181-188, 1987.

King, G. C. P., R. S. Stein, and J. Lin, Static stress changes and the triggering of earthquakes, Bull. Seismol. Soc. Am., 84, 935-953, 1994.

Li, V. C., S. H. Seale, and T. Cao, Postseismic stress and pore pressure readjustment and aftershock distributions, Tectonophysics, 144, 37-54, 1987.

Mavko, G. M., Fault interaction near Hollister, California, J. Geophys. Res., 87, 7807-7816, 1982.

Mavko, G. M., S. Schulz, and B. D. Brown, Effects of the 1983 Coalinga, California, earthquake on creep along the San Andreas fault, Bull. Seismol. Soc. Am., 75, 475-489, 1985.

Michael, A. J., Spatial variations of stress within the 1987 Whittier Narrows, California, aftershock sequence: New techniques and results, J. Geophys. Res., 96, 6303-6319, 1991.

Nalbant, S. S., A. A. Barka, and O. Alptekin, Failure stress change caused by the 1992 Erzincan earthquake (Ms = 6.8), Geophys. Res. Lett., 23, 1561-1564, 1996.

Nostro, C., M. Cocco, and M. E. Belardinelli, Static stress changes in extensional regimes: An application to southern Apennines (Italy), Bull. Seismol. Soc. Am., 87, 234-248, 1997.

Okada, Y., Internal deformation due to shear and tensile faults in a half-space, Bull. Seismol. Soc. Am., 82, 1018-1040, 1992.

Okada, Y., and K. Kasahara, Earthquake of 1987, off Chiba, central Japan and possible triggering of eastern Tokyo earthquake of 1988, Tectonophysics, 172, 351-364, 1990.

Oppenheimer, D. H., P. A. Reasenberg, and R. W. Simpson, Fault plane solutions for the l984 Morgan Hill, California, earthquake sequence: Evidence for the state of stress on the Calaveras fault, J. Geophys. Res., 93, 9007-9026, 1988.

Oppenheimer, D. H., et al., The Cape Mendocino, California, earthquakes of April 1992: Subduction at the triple junction, Science, 261, 433-438, 1993.

Pollitz, F. F., and I. S. Sacks, Consequences of stress changes following the 1891 Nobi earthquake, Japan, Bull. Seismol. Soc. Am., 85, 796-807, 1995.

Pollitz, F. F., and I. S. Sacks, The 1995 Kobe, Japan, earthquake: A long-delayed aftershock of the offshore 1944 Tonankai and 1946 Nankaido earthquakes, Bull. Seismol. Soc. Am., 87,1-10, 1997.

Reasenberg, P. A., and R. W. Simpson, Response of regional seismicity to the static stress change produced by the Loma Prieta earthquake, Science, 255, 1687-1690, 1992.

Reasenberg, P. A., and R. W. Simpson, Response of regional seismicity to the static stress change produced by the Loma Prieta earthquake, in The Loma Prieta, California Earthquake of October 17, 1989—Aftershocks and Postseismic Effects, edited by P. A. Reasenberg, U.S. Geol. Surv. Prof. Pap., 1550-D, D49-D71, 1997.

Robinson, R., Shallow subduction tectonics and fault interaction: The Weber, New Zealand, earthquake sequence of 1990-1992, J. Geophys. Res., 99, 9663-9679, 1994.

Roth, F., Modelling of stress patterns along the western part of the North Anatolian fault zone, Tectonophysics, 152, 215-226, 1988.

Rybicki, K., The elastic residual field of a very long strike-slip fault in the presence of a discontinuity, Bull. Seismol. Soc. Am., 61. 79-82, 1971.

Rybicki, K., Analysis of aftershocks on the basis of dislocation theory, Phys. Eanh Planet. Inter., 7, 409-422, 1973.

Rybicki, K., T. Kato, and K. Kasahara, Mechanical interaction between neighboring active faults—Static and dynamic stress field induced by faulting, Bull. Earthquake Res. Inst. Univ. Tokyo, 60, 1-21, 1985.

Simpson, R. W., and P. A. Reasenberg, Earthquake-induced static stress changes on central California faults, in The Loma Prieta, California Earthquake of October 17, 1989—Tectonic processes and models, edited by R. W. Simpson, U.S. Geol. Surv. Prof. Pap., 1550-F, F55-F89, 1994.

Simpson, R. W., S. S. Schulz, L. D. Dietz, and R. O. Burford, The response of creeping parts of the San Andreas fault to earthquakes on nearby faults: Two examples, Pure Appl. Geophys., 126, 665-685, 1988.

Singh, S. K., J. G. Anderson, and M. Rodriguez, Triggered seismicity in the Valley of Mexico from major earthquakes, Geofis. Int., 37, 3-15, 1998.

Smith, S. W., and W. Van de Lindt, Strain adjustments associated with earthquakes in southern California, Bull. Seismol. Soc. Am., 59, 1569-1589, 1969.

Stein, R. S., A. A. Barka, and J. H. Dieterich, Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering, Geophys. J. Int., 128, 594-604,1997.

Stein, R. S., and M. Lisowski, The 1979 Homestead Valley earthquake sequence, California: Control of aftershocks and postseismic deformation, J. Geophys. Res., 88, 6477-6490, 1983.

Stein, R. S., G. C. P. King, and J. Lin, Change in failure stress on the southern San Andreas fault system caused by the 1992 magnitude = 7.4 Landers earthquake, Science, 258, 1328-1332. 1992.

Stein, R. S., G. C. P. King. and J. Lin, Stress triggering of the 1994 M=6.7 Northridge, California, earthquake by its predecessors, Science, 265, 1432-1435, 1994.

Troise, C., G. De Natale, F. Pingue, and S. M. Petrazzuoli, Evidence for static stress interaction among earthquakes in south-central Apennines (Italy), Geophys. J. Int., 134, 809-817, 1998.

Weertman, J., and J. Weertman, Elastic Dislocation Theory, 213 pp., MacMillan, New York, 1964.

Yamashina, K., Induced earthquakes in the Izu Peninsula by the Izu-Hanto-Oki earthquake of 1974, Japan, Tectonophysics, 51, 139-154, 1978.

Yamashina, K., A possible factor which triggers shallow intra-plate earthquakes, Phys. Earth Planet. Inter., 18, 153-164, 1979.

Yoshioka, S., and M. Hashimoto, A quantitative interpretation on possible correlations between intraplate seismic activity and interplate great earthquakes along the Nankai trough, Phys. Earth Planet. Inter., 58, 173-191, 1989a.

Yoshioka, S., and M. Hashimoto, The stress field induced from the occurrence of the 1944 Tonankai and the 1946 Nankaido earthquakes, and their relation to impending earthquakes, Phys. Earth Planet. Inter., 56, 349-370, 1989b.

Harris R. A.,

J. Geophys. Res., 103 , 10 , 24,347-24,358, 1998.

Paper included in this collection.

Bowman, D. D., G. Ouillon, C. G. Sammis, A. Sornette, and D. Sornette,

J. Geophys. Res., 103 , 10 , 24,359-24,372, 1998.

We test the concept that seismicity prior to a large earthquake can be understood in terms of the statistical physics of a critical phase transition. In this model, the cumulative seismic strain release increases as a power law time to failure before the final event. Furthermore, the region of correlated seismicity predicted by this model is much greater than would be predicted from simple elastodynamic interactions.

We present a systematic procedure to test for the accelerating seismicity predicted by the critical point model and to identify the region approaching criticality, based on a comparison between the observed cumulative energy (Benioff strain) release and the power law behavior predicted by theory. This method is used to find the critical region before all earthquakes along the San Andreas system since 1950 with M 6.5. The statistical significance of our results is assessed by performing the same procedure on a large number of randomly generated synthetic catalogs.

The null hypothesis, that the observed acceleration in all these earthquakes could result from spurious patterns generated by our procedure in purely random catalogs, is rejected with 99.5% confidence. An empirical relation between the logarithm of the critical region radius (R) and the magnitude of the final event (M) is found, such that log Rµ 0.5M, suggesting that the largest probable event in a given region scales with the size of the regional fault network.

Crider, J. G., and D. D. Pollard,

J. Geophys. Res., 103, 10, 24,373-24,392, 1998.

Field observations of two overlapping normal faults and associated deformation document features common to many normal-fault relay zones: a topographic ramp between the fault segments, tapering slip on the faults as they enter the overlap zone, and associated fracturing, especially at the top of the ramp. These observations motivate numerical modeling of the development of a relay zone. A three- dimensional boundary element method numerical model, using simple fault-plane geometries, material properties, and boundary conditions, reproduces the principal characteristics of the observed fault scarps.

The model, with overlapping, semicircular fault segments under orthogonal extension, produces a region of high Coulomb shear stress in the relay zone that would favor fault linkage at the center to upper relay ramp. If the fault height is increased, the magnitude of the stresses in the relay zone increases, but the position of the anticipated linkage does not change. The amount of fault overlap changes the magnitude of the Coulomb stress in the relay zone: the greatest potential for fault linkage occurs with the closest underlapping fault tips. Ultimately, the mechanical interaction between segments of a developing normal-fault system promote the development of connected, zigzagging fault scarps.

Freed, A. M., and J. Lin,

J. Geophys. Res., 103, 10, 24,393-24,410, 1998.

Two-dimensional viscoelastic finite element models were used to calculate the time-dependent changes in Coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its down-dip extension. Results suggest that thrust earthquakes cause a coseismic increase in Coulomb stress along antithetic lobes normal to the slip plane. Following a quake, creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust. Under certain conditions, transfer of stress may lead to a further build-up of high Coulomb stress along the base of the upper crust where it may influence other faults in the region.

Calculations suggest that the 1971 San Fernando earthquake may have caused a 0.2 MPa increase in coseismic Coulomb stress at the location of the 1994 Northridge hypocenter on a 2-D cross-section linking the two quakes. Postseismic expansion of a lobe of high stress may have doubled the level of failure stress in certain areas of this region. The conditions under which an antithetic lobe of high Coulomb stress are favored to expand postseismically within a few decades include: the lower crust or upper mantle has an effective viscosity not greater than 10¹⁹Pa-s; the thrust fault has a moderate dip angle (40-50 degrees); the brittle/ductile transition is deep enough to provide a corridor for expansion; and the crust has a low apparent coefficient of friction (< 0.2). Postseismic increases in Coulomb stress within the upper crust may also be caused by aseismic creep on the fault. Stress changes due to this mechanism are maximized with a high apparent coefficient of friction.

Analysis of experimentally determined non-Newtonian flow laws suggests that wet granitic, quartz, and feldspar aggregates may have a viscosity on the order of 10¹⁹ Pa-s. The calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate. Results of this 2-D analysis suggest a potentially important role of viscous flow in controlling time-dependent postseismic stress changes that warrant further investigation using 3-D viscoelastic analysis.

Gomberg, J., N. M. Beeler, M. L. Blanpied, and P. Bodin,

J. Geophys. Res., 103, 10, 24,411-24,426, 1998.

Hardebeck, J. L., J. J. Nazareth, and E. Hauksson,

J. Geophys. Res., 103, 10, 24,427-24,438, 1998.

Static stress change has been proposed as a mechanism of earthquake triggering. We quantitatively evaluate this model for the apparent triggering of aftershocks by the 1992 7.3 Landers and 1994 6.7 Northridge earthquakes. Specifically, we test whether the fraction of aftershocks consistent with static stress change triggering is greater than the fraction of random events which would appear consistent by chance. Although static stress changes appear useful in explaining the triggering of some aftershocks, the model's capability to explain aftershock occurrence varies significantly between sequences. The model works well for Landers aftershocks. Approximately 85% of events between 5 and 75 km distance from the mainshock fault plane are consistent with static stress change triggering, compared to ~50% of random events. The minimum distance is probably controlled by limitations of the modeling, while the maximum distance may be because static stress changes of <0.01 MPa trigger too few events to be detected.

The static stress change triggering model, however, can not explain the first month of the Northridge aftershock sequence significantly better than it explains a set of random events. The difference between the Landers and Northridge sequences may result from differences in fault strength, with static stress changes being a more significant fraction of the failure stress of weak Landers-area faults. Tectonic regime, regional stress levels, and fault strength may need to be incorporated into the static stress change triggering model before it can be used reliably for seismic hazard assessment.

Harris, R. A., and R. W. Simpson,

J. Geophys. Res., 103, 10, 24,439-24,452, 1998.

Stress shadows generated by California's two most-recent great earthquakes (1857 Ft. Tejon and 1906 San Francisco) substantially modified 19th and 20th century earthquake history in the Los Angeles Basin and in the San Francisco Bay Area. Simple Coulomb-failure calculations, which assume that earthquakes can be modeled as static dislocations in an elastic halfspace, have done quite well at approximating how long the stress shadows, or relaxing effects, should last, and at predicting where subsequent large earthquakes will not occur. There has, however, been at least one apparent exception to the predictions of such simple models. The 1911 M>6.0 earthquake near Morgan Hill, California occurred at a relaxed site on the Calaveras fault.

We examine how the more complex rate-and-state friction-formalism based on laboratory experiments might have allowed the 1911 earthquake. Rate-and-state time-to- failure calculations are consistent with the occurrence of the 1911 event just 5 years after 1906 if the Calaveras fault was already close to failure before the effects of 1906. We also examine the likelihood that the entire 78- years of relative quiet (only four M≥6 earthquakes) in the Bay area after 1906 is consistent with rate-and-state assumptions, given that the previous 7 decades produced 18 M≥6 earthquakes. Combinations of rate-and-state variables can be found that are consistent with this pattern of large Bay area earthquakes, assuming that the rate of earthquakes in the 7-decades before 1906 would have continued had 1906 not occurred. These results demonstrate that rate-and-state offers a consistent explanation for the 78-year quiescence and the 1911 anomaly, although they do not rule out several alternate explanations.

Paper included in this collection in chapter 2.

Kagan, Y. Y., and D. D. Jackson,

J. Geophys. Res., 103, 10, 24,453-24,468, 1998.

Nalbant, S. S., A. Hubert, and G. C. P. King,

J. Geophys. Res., 103, 10, 24,469-24,486, 1998.

Paper included in this collection.

Nostro, C., R. S. Stein, M. Cocco, M. E. Belardinelli, and W. Marzocchi,

J. Geophys. Res., 103, 10, 24,487-24,504, 1998.

Paper included in this collection.

Seeber, L., J. G. Armbruster, W.-Y. Kim, C. Scharnberger, and N. Barstow,

J. Geophys. Res., 103, 10, 24,505-24,522, 1998.

Taylor, M. A. J., R. Dmowska, and J. R. Rice,

J. Geophys. Res., 103, 10, 24,523-24,542, 1998.

We investigate upper plate stressing during the earthquake cycle in a subduction segment, using 3D elastic models to address the effects of strongly heterogeneous coupling along strike of the interplate interface. We show how heterogeneity controls the locations and mechanisms of seismicity in the upper plate. Oblique subduction segments, two from the Aleutians (Andreanof Islands 1986 and Rat Islands 1965) and one from Indonesia (Biak 1996) are studied. All examples of upper-plate seismicity from the Aleutians represent events occurring toward the beginning of a new cycle, while in Biak, Indonesia, the examined events occur both towards the end of one cycle and the beginning of the next.

In the majority of cases studied the location and mode of the upper-plate seismicity is consistent with space- and time-dependent stressing as predicted by modeling. This confirms earlier observations that seismicity in the vicinity of large/great subduction earthquakes (towards the outer rise, at intermediate depth, and now in the upper plate) depends, in an interpretable manner, on the stage in the earthquake cycle as well as on distribution of coupling along the interplate interface.

Toda, S., R. S. Stein, P. A. Reasenberg, J. H. Dieterich, and A. Yoshida,

J. Geophys. Res., 103, 10, 24,543-24,566, 1998.

Paper included in this collection.

Vidale, J., D. Agnew, M. Johnston, and D. Oppenheimer,

J. Geophys. Res., 103, 10, 24,567, 1998.

Because the rate of stress change from the Earth tides exceeds that from tectonic stress accumulation, tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level.

We analyze the tidal stresses and stress rates on the fault planes and at the times of 13,042 earthquakes which are so close to the San Andreas and Calaveras faults in California that we may take the fault plane to be known. We find that the stresses and stress rates at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times. While the rate of earthquakes when the tidal stress promotes failure is 2% higher than when the stress does not, this difference in rate is not statistically significant. This lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0.15 bar/hr just preceding seismic failure, much above the the long-term tectonic stress rate of 0.0001 bar/hr.