Probabilistic prediction of rupture length, slip and seismic ground motions for an ongoing rupture: implications for early warning for large earthquakes

Earthquake EarlyWarning (EEW) predicts future ground shaking based on presently available data. Long ruptures present the best opportunities for EEW since many heavily shaken areas are distant from the earthquake epicentre and may receive long warning times. Predicting the shaking from large earthquakes, however, requires some estimate of the likelihood of the future evolution of an ongoing rupture. An EEW system that anticipates future rupture using the present magnitude (or rupture length) together with the Gutenberg-Richter frequencysize statistics will likely never predict a large earthquake, because of the rare occurrence of ‘extreme events’. However, it seems reasonable to assume that large slip amplitudes increase the probability for evolving into a large earthquake. To investigate the relationship between the slip and the eventual size of an ongoing rupture, we simulate suites of 1-D rupture series from stochastic models of spatially heterogeneous slip. We find that while large slip amplitudes increase the probability for the continuation of a rupture and the possible evolution into a ‘Big One’, the recognition that rupture is occurring on a spatially smooth fault has an even stronger effect.We conclude that anEEWsystem for large earthquakes needs some mechanism for the rapid recognition of the causative fault (e.g., from real-time GPS measurements) and consideration of its ‘smoothness’. An EEW system for large earthquakes on smooth faults, such as the San Andreas Fault, could be implemented in two ways: the system could issue a warning, whenever slip on the fault exceeds a few metres, because the probability for a large earthquake is high and strong shaking is expected to occur in large areas around the fault. A more sophisticated EEW system could use the present slip on the fault to estimate the future slip evolution and final rupture dimensions, and (using this information) could provide probabilistic predictions of seismic ground motions along the evolving rupture. The decision on whether an EEW system should be realized in the first or in the second way (or in a combination of both) is user-specific

Long-Period Building Response to Earthquakes in the San Francisco Bay Area

This article reports a study of modeled, long-period building responses to ground-motion simulations of earthquakes in the San Francisco Bay Area. The earthquakes include the 1989 magnitude 6.9 Loma Prieta earthquake, a magnitude 7.8 simulation of the 1906 San Francisco earthquake, and two hypothetical magnitude 7.8 northern San Andreas fault earthquakes with hypocenters north and south of San Francisco. We use the simulated ground motions to excite nonlinear models of 20-story, steel, welded moment-resisting frame (MRF) buildings. We consider MRF buildings designed with two different strengths and modeled with either ductile or brittle welds. Using peak interstory drift ratio (IDR) as a performance measure, the stiffer, higher strength building models outperform the equivalent more flexible, lower strength designs. The hypothetical magnitude 7.8 earthquake with hypocenter north of San Francisco produces the most severe ground motions. In this simulation, the responses of the more flexible, lower strength building model with brittle welds exceed an IDR of 2.5% (that is, threaten life safety) on 54% of the urban area, compared to 4.6% of the urban area for the stiffer, higher strength building with ductile welds. We also use the simulated ground motions to predict the maximum isolator displacement of base-isolated buildings with linear, single-degree-of-freedom (SDOF) models. For two existing 3-sec isolator systems near San Francisco, the design maximum displacement is 0.5 m, and our simulations predict isolator displacements for this type of system in excess of 0.5 m in many urban areas. This article demonstrates that a large, 1906-like earthquake could cause significant damage to long-period buildings in the San Francisco Bay Area

Will Performance-based Earthquake Engineering Break the Power Law?

It seems that the entire community of earthquake professionals was stunned by the number of fatalities (approximately 300,000 dead or missing and presumed dead) in the 2004 Sumatran-Andaman earthquake and tsunami. It took us by surprise and seemed so out of proportion with anything that occurred in the decades prior. It was a rare confluence of circumstances that led to such massive loss. If, through our earthquake studies, we had been able to prevent just 5% of those deaths, then we would have saved more lives than have been lost in all other tsunamis for many decades. One clear lesson stands out from this tragedy: We must do a better job on tsunami hazard mitigation efforts for very large earthquakes (M > 9). While these events are rare, they account for most of the total hazard

Seismology in the United States, 1986-1990

In this volume, seven highly respected seismologists attempt to summarize seismological research in the U.S. in the past four years. This is indeed a daunting task; the mere compilation and classification of the overwhelming volume of new seismological literature is difficult enough, but to confidently provide an overview of our new knowledge and understanding is almost impossible. Although the study of vibrations in the Earth (that is, seismology) may at first seem to be a scientific field of limited scope, it encompasses a vast range of observations and problems in mathematical physics. Seismologists currently study waves varying in frequencies from 10^2 Hz to 10^(-4) Hz (20 octaves) and in acceleration amplitudes from 1 g to 10^(-12) g (240 dB). These waves are observed at a wide variety of distances as they travel through a very complex medium, the interior of the Earth. Furthermore the waves are excited by numerous and often complex mechanisms, including earthquakes, man-made explosions, landslides, volcanoes, and atmospheric disturbances. Seismologists study waves from earthquakes that range in energy over 15 orders of magnitude. These waves are studied to reveal the physical properties of the Earth, the kinematics and dynamics of Earth deformation, the characteristics of destructive earthquakes and volcanoes, and the occurrence of man-made explosions. When viewed from this perspective, it is little wonder that any individual seismologist can feel overwhelmed by the sheer volume of seismological research in the past four years. Several thousand seismological papers were published in the past four years; Langston (this issue) alone lists 800 papers pertaining to wave propagation problems. I confess that I have only read a small fraction of these, and even if I had read them all, I would not attempt to choose those having the greatest significance. Instead, I can only summarize current trends in seismological research

Looking Back From the Year 3,000

Many people have asked me how I can justify living with the earthquake threat in the Los Angeles area. My answer is usually that we have some reasonably strict building codes and that the threat of earthquakes to life safety is minimized if our buildings survive our coming quakes. The current building code calls for buildings to sustain at most repairable damage from the strongest shaking that is anticipated with a 10% probability in 50 years. If the building is a critical structure, such as a hospital, then the requirement is increased to 10% in 100 years. Furthermore, buildings of both classes should not collapse for the strongest ground shaking that can be anticipated at the location of the building. If the building code works as it's supposed to, then we endure far greater risks from other factors than from earthquakes

Inertial Forces from Earthquakes on a Hyperloop Pod

High‐speed transit (1300  km/hr) using pods traveling in evacuated tubes has been proposed. This Short Note addresses how earthquake ground shaking is changed when it is experienced by a high‐speed pod that is confined to a track. In particular, earthquake motions can cause lateral deformations of the tube that cause centripetal forces in the pod. I discuss the nature of these forces for the cases of (1) a tube that crosses a fault offset, (2) a tube that is deformed by traveling waves in the Earth, and (3) a tube that resonates between fixed points (e.g., a simple bridge). I suggest several schemes to control the peak centripetal accelerations of the pod

Tidal triggering of earthquakes

Analysis of the tidal stress tensor at the time of moderate to large earthquakes fails to confirm an earlier hypothesis that the origin times of shallow dip-slip earthquakes correlate with solid-earth tidal shear stress. Furthermore, no correlation is seen for either tidal shear stress or tidal normal-to-the-fault compressive stress with shallow strike-slip earthquakes or with deep earthquakes

The 1971 San Fernando earthquake: A double event?

Evidence is presented which suggest that the 1971 San Fernando earthquake may have been a double event that occurred on two separate, subparallel thrust faults. It is postulated that the initial event took place at depth on the Sierra Madre fault zone which runs along the base of the San Gabriel Mountains. Rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km. A second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the Sierra Madre fault zone. The surface trace of this fault coincides with the San Fernando fault zone which was the principal fault associated with surface rupture. It is postulated that rupture propagated from a depth of 8 km to the free surface. The moments of the first and second events are approximately 0.7 × 10^(26) dyne-cm and 1.0 × 10^(26) dyne-cm, respectively. This model is found to explain the combined data sets of strong ground motions, teleseismic P and S waveforms, and static offsets better than previous models, which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth. Nevertheless, many features of the observed motions remain unexplained, and considerable uncertainty still exists regarding the faulting history of the San Fernando earthquake