Seismic Study of an Oceanic Ridge Earthquake Swarm in the Gulf of California

Detailed seismic investigation of an unusually intense earthquake swarm which occurred in the northern Gulf of California during March 1969 has provided new information about seismic processes which occur on actively spreading oceanic ridges and has placed some constraints on the elastic wave velocities beneath them. Activity during this swarm was similar to that of a foreshock-mainshock-aftershock sequence, but with a ‘mainshock’ composed of over 70 events with magnitudes between 4 and 5.5 occurring in a 6-hr period about a day after swarm activity was initiated. ‘Aftershocks’, including many events greater than magnitude 5, continued for over two weeks. Near-source travel-time data indicate all sources located are within 5–10 km of each other and that hypocentres are confined to the upper crust. Teleseismic P-delays for rays travelling beneath this ridge may be interpreted in terms of an upper mantle with compressional velocities 5–10 per cent less than normal mantle to a depth of 200 km. Average apparent stresses for all swarm events studied are very similar, show no consistent pattern as a function of time, and are close to values obtained from other ridges. The focal mechanism solution shows a large component of normal faulting. An apparent non-orthogonality of nodal planes common to this mechanism solution and to normal faulting events on other ridges disappears when the indicated low upper mantle velocities beneath the source are taken into account. A survey of recent seismicity (post 1962) in the northern Gulf suggests seismic coupling across about 200 km between adjacent inferred spreading ridge segments. Surface waves from these Gulf Swarm earthquakes have amplitudes from one to two orders of magnitude greater than Northern Baja California events with similar short period body wave excitation

Surface Wave Propagation and Source Studies in the Gulf of California Region

A number of aspects of seismic surface wave propagation and earthquake mechanism in the Gulf of California region are investigated in this thesis. In addition, several associated problems raised by this study are also explored in some detail. Surface wave dispersion and P-wave travel time delays are measured to delineate the crust and upper mantle structure in the Imperial Valley-Gulf of California region. Crustal thicknesses beneath Baja California and Sonora are comparable and near 25 km, while within the Gulf crustal structure varies laterally from nearly oceanic on the western side to continental shelf thicknesses (~20 km) towards the north and east. Love wave group velocities for Baja California paths are unusually high and were not used to determine structure. Pn and teleseismic P-wave delays are used in a reconnaissance survey of crustal structure in the Imperial Valley and across the Peninsular Range batholith. The data are consistent with an increase in crustal thickness of 12 km from flank to crest in the Peninsular Ranges, and a decrease of 8 km across the Imperial Valley. The high Love wave group velocities measured across Baja California are shown to be similar to velocities of the first higher mode. It is also demonstrated that higher Love modes can have group velocities very close to fundamental mode velocities for a range of wave periods and realistic earth models. The mode interference which is a consequence of this intertwining of group velocity curves has a significant effect on measured phase velocities, and this problem is investigated in detail. An important conclusion of this study is that anomalously high Love wave phase velocities reported for the United States midcontinent and Japan are straightforwardly explained by mode interference without appealing to complex or anisotropic models, as had been done previously. Seismic processes associated with actively spreading oceanic rises are examined in the study of a strong swarm of earthquakes located near an inferred spreading center in the Northern Gulf of California. Close-in travel time data constrain the origin times of swarm events and demonstrate that the epicenters are confined to the upper crust. Teleseismic P-delays suggest unusually low seis1nic velocities beneath the source. The previously suspected normal faulting nature of swarm earthquakes is also confirmed. Seismic coupling across 200 km between adjacent spreading centers in the Northern Gulf is indicated by a survey of recent seismicity. It is noted in the study of the Gulf swarm that these sources have significantly higher surface wave amplitudes than events with similar assigned magnitudes in Northern Baja California. In the final chapter of this thesis a detailed analysis is made of the Baja earthquakes and it is concluded that as a group they have distinctly smaller source dimensions and larger stress drops than events within the Gulf of California. These differences are quite marked and are often very clearly seen even on records from band-limited seismographs. Several examples exist where propagation paths are very similar but the visual appearance of records differs considerably, suggesting that near-source or path effects are not likely explanations of the observed differences. For small magnitude North Baja earthquakes, both source dimensions and long period surface excitation average only about a factor or two larger than corresponding quantities previously measured for underground nuclear explosions of similar magnitude.</p

Temporal evolution of continental lithospheric strength in actively deforming regions,GSA

ABSTRACT It has been agreed for nearly a century that a strong, loadbearing outer layer of earth is required to support mountain ranges, transmit stresses to deform active regions, and store elastic strain to generate earthquakes. However, the depth and extent of this strong layer remain controversial. Here we use a variety of observations to infer the distribution of lithospheric strength in the active western United States from seismic to steady-state time scales. We use evidence from post-seismic transient and earthquake cycle deformation, reservoir loading, glacio-isostatic adjustment, and lithosphere isostatic adjustment to large surface and subsurface loads. The nearly perfectly elastic behavior of Earth&apos;s crust and mantle at the time scale of seismic wave propagation evolves to that of a strong, elastic crust and weak, ductile upper mantle lithosphere at both earthquake cycle (EC, ~10 0 to 10 3 yr) and glacio-isostatic adjustment (GIA, ~10 3 to 10 4 yr) time scales. Topography and gravity field correlations indicate that lithosphere isostatic adjustment (LIA) on ~10 6 -10 7 yr time scales occurs with most lithospheric stress supported by an upper crust overlying a much weaker ductile substrate. These comparisons suggest that the upper mantle lithosphere is weaker than the crust at all time scales longer than seismic. In contrast, the lower crust has a chameleon-like behavior, strong at EC and GIA time scales and weak for LIA and steady-state deformation processes. The lower crust might even take on a third identity in regions of rapid crustal extension or continental collision, where anomalously high temperatures may lead to large-scale ductile flow in a lower crustal layer that is locally weaker than the upper mantle. Modeling of lithospheric processes in active regions thus cannot use a one-size-fits-all prescription of rheological layering (relation between applied stress and deformation as a function of depth) but must be tailored to the time scale and tectonic setting of the process being investigated

Seismicity and Tectonics of the Northern Gulf of California Region, Mexico. Preliminary Results

Three new seismographic statioms have been established· in the northern Gulf of California region; Mexico. Seismicity ,during a representative period in April and May of 1969 was concentrated on the Imperial, San Jacinto, Sierra Juarez, .and San Miguel faults, and the spread of epicentral locations was m\!Ch less than had previously been indicated. An intense earthquake swarm in March of· 1969 occurred near Consag Rock in the northern Gulf, and its study contributes to our understanding of the regional tectonics. In the northern Gulf of California and adjacent Salton trough, the tectonic framework may be. approximated by a series of six transform faults connected by five spreading centers (ridge, segments) evidenced by geothermal areas, recent .. volcanic activity, earthquake swarms, and submarine topographic depressions. Complexities in the fault pattern may be related to a northward decrease in spreading rates along the ridge segment,s. Five new high; quality seismographic stations around the •Gulf of California are now under construction in order to understand in more detail the pattern of sea-floor spreading in this unique, and important region

Experimental application to a water delivery canal of a distributed MPC with stability constraints

In this work, a novel distributed MPC algorithm, denoted D-SIORHC, is applied to upstream local control of a pilot water delivery canal. The D-SIORHC algorithm is based on MPC control agents that incorporate stability constraints and communicate only with their adjacent neighbors in order to achieve a coordinated action. Experimental results that show the effect of the parameters configuring the local controllers are presented

Probabilities of Large Earthquakes in the San Francisco Bay Region, California

In 1987 a Working Group on California Earthquake Probabilities was organized by the U.S. Geological Survey at the recommendation of the National Earthquake Prediction Evaluation Council (NEPEC). The membership included representatives from private industry, academia, and the U.S. Geological Survey. The Working Group computed long-term probabilities of earthquakes along the major faults of the San Andreas fault system on the basis of consensus interpretations of information then available. Faults considered by the Working Group included the San Andreas fault proper, the San Jacinto and Imperial-faults of southern California, and the Hayward fault of northern California. The Working Group issued a final report of its findings in 1988 (Working Group, 1988) that was reviewed and endorsed by NEPEC. As a consequence of the magnitude 7.1 Loma Prieta, California, earthquake of October 17, 1989, a second Working Group on California Earthquake Probabilities was organized under the auspices of NEPEC. Its charge was to review and, as necessary, revise the findings of the 1988 report on the probability of large earthquakes in the San Francisco Bay region. In particular, the Working Group was requested to examine the probabilities of large earthquakes in the context of new interpretations or physical changes resulting from the Loma Prieta earthquake. In addition, it was to consider new information pertaining to the San Andreas and other faults in the region obtained subsequent to the release of the 1988 report. Insofar as modified techniques and improved data have been used in this study, the same approach might also, of course, modify the probabilities for southern California. This reevaluation has, however, been specifically limited to the San Francisco Bay region. This report is intended to summarize the collective knowledge and judgments of a diverse group of earthquake scientists to assist in formulation of rational earthquake policies. A considerable body of information about active faults in the San Francisco Bay region leads to the conclusion that major earthquakes are likely within the next tens of years. Several techniques can be used to compute probabilities of future earthquakes, although there are uncertainties about the validity of specific assumptions or models that must be made when applying these techniques. The body of this report describes the data and detailed assumptions that lead to specific probabilities for different fault segments. Additional data and future advances in our understanding of earthquake physics may alter the way that these probabilities are estimated. Even though this uncertainty must be acknowledged, we emphasize that the findings of this report are supported by other lines of argument and are consistent with our best understanding of the likelihood for the occurrence of earthquakes in the San Francisco Bay region

Long-Term Time-Dependent Probabilities for the Third Uniform California Earthquake Rupture Forecast (UCERF3)

The 2014 Working Group on California Earthquake Probabilities (WGCEP 2014) presents time-dependent earthquake probabilities for the third Uniform California Earthquake Rupture Forecast (UCERF3). Building on the UCERF3 time-independent model published previously, renewal models are utilized to represent elastic-rebound-implied probabilities. A new methodology has been developed that solves applicability issues in the previous approach for unsegmented models. The new methodology also supports magnitude-dependent aperiodicity and accounts for the historic open interval on faults that lack a date-of-last-event constraint. Epistemic uncertainties are represented with a logic tree, producing 5760 different forecasts. Results for a variety of evaluation metrics are presented, including logic-tree sensitivity analyses and comparisons to the previous model (UCERF2). For 30 yr M ≥ 6.7 probabilities, the most significant changes from UCERF2 are a threefold increase on the Calaveras fault and a threefold decrease on the San Jacinto fault. Such changes are due mostly to differences in the time-independent models (e.g., fault-slip rates), with relaxation of segmentation and inclusion of multifault ruptures being particularly influential. In fact, some UCERF2 faults were simply too long to produce M 6.7 size events given the segmentation assumptions in that study. Probability model differences are also influential, with the implied gains (relative to a Poisson model) being generally higher in UCERF3. Accounting for the historic open interval is one reason. Another is an effective 27% increase in the total elastic-rebound-model weight. The exact factors influencing differences between UCERF2 and UCERF3, as well as the relative importance of logic-tree branches, vary throughout the region and depend on the evaluation metric of interest. For example, M ≥ 6.7 probabilities may not be a good proxy for other hazard or loss measures. This sensitivity, coupled with the approximate nature of the model and known limitations, means the applicability of UCERF3 should be evaluated on a case-by-case basis