Steven N. Ward

Cumbre Vieja Volcano -- Potential collapse and tsunami at La Palma, Canary Islands

Geological evidence suggests that during a future eruption, Cumbre Vieja Volcano on the Island of La Palma may experience a catastrophic failure of its west flank, dropping 150 to 500 km³ of rock into the sea. Using a geologically reasonable estimate of landslide motion, we model tsunami waves produced by such a collapse. Waves generated by the run-out of a 500 km³ (150 km³) slide block at 100 m/s could transit the entire Atlantic Basin and arrive on the coasts of the Americas with 10–25 m (3–8 m) height.

Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls

Large uplifts and tilts occurred on the Sumatran outer arc islands between 0.5° and 3.3°S during great historical earthquakes in 1797 and 1833, as judged from relative sea level changes recorded by annually banded coral heads. Coral data for these two earthquakes are most complete along a 160-km length of the Mentawai islands between 3.2° and 2°S. Uplift there was as great as 0.8 m in 1797 and 2.8 m in 1833. Uplift in 1797 extended 370 km, between 3.2° and 0.5°S. The pattern and magnitude of uplift imply megathrust ruptures corresponding to moment magnitudes (M_w) in the range 8.5 to 8.7. The region of uplift in 1833 ranges from 2° to at least 3.2°S and, judging from historical reports of shaking and tsunamis, perhaps as far as 5°S. The patterns and magnitude of uplift and tilt in 1833 are similar to those experienced farther north, between 0.5° and 3°N, during the giant Nias-Simeulue megathrust earthquake of 2005; the outer arc islands rose as much as 3 m and tilted toward the mainland. Elastic dislocation forward modeling of the coral data yields megathrust ruptures with moment magnitudes ranging from 8.6 to 8.9. Sparse accounts at Padang, along the mainland west coast at latitude 1°S, imply tsunami runups of at least 5 m in 1797 and 3–4 m in 1833. Tsunamis simulated from the pattern of coral uplift are roughly consistent with these reports. The tsunami modeling further indicates that the Indian Ocean tsunamis of both 1797 and 1833, unlike that of 2004, were directed mainly south of the Indian subcontinent. Between about 0.7° and 2.1°S, the lack of vintage 1797 and 1833 coral heads in the intertidal zone demonstrates that interseismic submergence has now nearly equals coseismic emergence that accompanied those earthquakes. The interseismic strains accumulated along this reach of the megathrust have thus approached or exceeded the levels relieved in 1797 and 1833.

Prehistoric earthquake history revealed by lacustrine slump deposits

Five strong paleoseismic events were recorded in the past 15 k.y. in a series of slump deposits in the subsurface of Lake Lucerne, central Switzerland, revealing for the first time the paleoseismic history of one of the most seismically active areas in central Europe. Although many slump deposits in marine and lacustrine environments were previously attributed to historic earthquakes, the lack of detailed three-dimensional stratigraphic correlation in combination with accurate dating hampered the use of multiple slump deposits as paleoseismic indicators. This study investigated the fingerprint of the well-described A.D. 1601 earthquake (I = VII–VIII, Mw ∼ 6.2) in the sediments of Lake Lucerne. The earthquake triggered numerous synchronous slumps and megaturbidites within different subbasins of the lake, producing a characteristic pattern that can be used to assign a seismic triggering mechanism to prehistoric slump events. For each seismic event horizon, the slump synchronicity was established by seismic-stratigraphic correlation between individual slump deposits through a quasi-three-dimensional high-resolution seismic survey grid. Four prehistoric events, dated by accelerator mass spectrometry, 14C measurements, and tephrochronology on a series of long gravity cores, occurred at 2420, 9770, 13,910, and 14,560 calendar yr ago. These recurrence times are essential factors for assessing seismic hazard in the area. The seismic hazard for lakeshore communities is additionally amplified by slump-induced tsunami and seiche waves. Numerical modeling of such tsunami waves revealed wave heights to 3 m, indicating tsunami risk in lacustrine environments.

Paleogeodetic records of seismic and aseismic subduction from central Sumatran microatolls, Indonesia

We utilize coral microatolls in western Sumatra to document vertical deformation associated with subduction. Microatolls are very sensitive to fluctuations in sea level and thus act as natural tide gauges. They record not only the magnitude of vertical deformation associated with earthquakes (paleoseismic data), but also continuously track the long-term aseismic deformation that occurs during the intervals between earthquakes (paleogeodetic data). This paper focuses on the twentieth century paleogeodetic history of the equatorial region. Our coral paleogeodetic record of the 1935 event reveals a classical example of deformations produced by seismic rupture of a shallow subduction interface. The site closest to the trench rose 90 cm, whereas sites further east sank by as much as 35 cm. Our model reproduces these paleogeodetic data with a 2.3 m slip event on the interface 88 to 125 km from the trench axis. Our coral paleogeodetic data reveal slow submergence during the decades before and after the event in the areas of coseismic emergence. Likewise, interseismic emergence occurred before and after the 1935 event in areas of coseismic submergence. Among the interesting phenomenon we have discovered in the coral record is evidence of a large aseismic slip or “silent event” in 1962, 27 years after the 1935 event. Paleogeodetic deformation rates in the decades before, after, and between the 1935 and 1962 events have varied both temporally and spatially. During the 25 years following the 1935 event, submergence rates were dramatically greater than in prior decades. During the past four decades, however, rates have been lower than in the preceding decades, but are still higher than they were prior to 1935. These paleogeodetic records enable us to model the kinematics of the subduction interface throughout the twentieth century.

San Francisco Bay Area Earthquake Simulations: A Step Toward a Standard Physical Earthquake Model

Earthquakes in Californias San Francisco Bay Area are likely to be more strongly affected by stress interaction than earthquakes in any other place in the world because of the regions closely spaced, subparallel distribution of faults. I believe, therefore, that meaningful quantification of earthquake probability and hazard in the Bay Area can be made only with the guidance provided by physically based and regionwide earthquake models that account for this interaction. This article represents a first step in developing a standard physical earthquake model for the San Francisco Bay Area through realistic, 3000-year simulations of earthquakes on all of the areas major faults. These simulations demonstrate that a standard physical earthquake model is entirely feasible, they illustrate its application, and they blueprint its construction. A standard physical earthquake model provides the mechanism to integrate fully the diverse disciplines within the earthquake research community. As a platform for data utilization and verification, a physical earthquake model can employ directly any earthquake property that is measurable in the field or in the laboratory to tune and test its seismicity products. As a platform for probability forecasts, a physical earthquake model can supply rational estimates of every imaginable earthquake statistic while simultaneously satisfying all slip and earthquake rate constraints. As a platform for hazard analysis, a physical earthquake model can compute earthquake shaking intensity from first principles by convolving a full suite of rupture scenarios with site-specific dislocation Greens functions. Physical earthquake models have advanced greatly in the last decade. Simulations of earthquake generation and recurrence are now sufficiently credible that such calculations can begin to take substantial roles in scientific studies of earthquake probability and hazard. Manuscript received 9 March 1999.

Crustal deformation at the Sumatran Subduction Zone revealed by coral rings

Analyses of coral rings grown in the interval 1970–1997 reveal a geographically distinct pattern of interseismic uplift off Sumatras western coast. At distances less than 110 km from the Sumatran trench, coral reefs are submerging as fast as 5 mm/y. At 130 and 180 km distance from the trench, they are emerging at similar rates. We suggest that a locked, or partially locked patch, located above 30 km depth on the upper surface of the subducting oceanic plate, generates this pattern.

An application of synthetic seismicity in earthquake statistics: The Middle America Trench

This paper demonstrates how synthetic seismicity calculations which are based on the concept of fault segmentation and incorporate the physics of faulting through static dislocation theory can improve earthquake recurrence statistics and hone the probabilities of hazard. Compared to forecasts constructed from a handful of earthquake recurrence intervals, forecasts constructed from synthetic seismicity are more robust in that they embody regional seismicity information over several units of magnitude, they can extrapolate seismicity to higher magnitudes than have actually been observed, and they are formulated from a catalog which can be extended as long as needed to be statistically significant. Synthetic seismicity models can also be used to judge the stability of common rate estimates and the appropriateness of idealizations to the earthquake cycle. I find that estimates of fault slip rate are unbiased regardless of sampling duration, while estimates of earthquake recurrence time are strongly biased. Recurrence intervals estimated from seismicity samples less than about 10 times the actual recurrence interval will almost certainly be too short. For the Middle America Trench (MAT), it would take 200 and 400 years of monitoring to constrain slip and recurrence rates to ±10%. Events M ≥ 6 have as much as a 60% probability of recurrence within 5 years due to the clustering of small earthquakes in foreshocks and aftershocks. This probability drops to less than 15% for M ≥ 7 events. Increasing gap time generally increases conditional probability of earthquake occurrence, but the effect is weak. For the MAT, the spread parameters of the best fitting lognormal or Weibull distributions (≈0.75) are much larger than the 0.21 intrinsic spread proposed in the Nishenko-Buland hypothesis. Stress interaction between fault segments disrupts time or slip predictability and causes earthquake recurrence to be far more aperiodic than has been suggested.

Tsunami Hazard Evaluation of the Eastern Mediterranean: Historical Analysis and Selected Modeling

Seismic sea waves in the eastern Mediterranean have been reported since written history first emerged several thousand years ago. We collected and investigated these ancient and modern reports to understand and model the typical tsunamigenic sources, with the ultimate purpose of characterizing tsunami hazard along the Levant coasts. Surprisingly, only 35% of the tsunami reports could be traced back to primary sources, with the balance remaining questionable. The tsunamis varied in size, from barely noticeable to greatly damaging, and their effects ranged from local to regional. Overall, we list 21 reliably reported tsunamis that occurred since the mid second century b.c. along the Levant coast, along with 57 significant historical earthquakes that originated from the “local” continental Dead Sea Transform (dst) system. An in-depth evaluation shows that 10 tsunamis are clearly associated with on-land dst earthquakes, and therefore, as formerly suggested, they probably originated from offshore, seismogenically induced slumps. Eight tsunamis arrived from the “remote” Hellenic and Cypriot Arcs, one from Italy, and two are left with as yet unrecognized sources. A major conclusion from this work is that onshore earthquakes commonly produce tsunamis along the Levant coastline, and that analogous situations are present elsewhere in the Mediterranean, as well as along the California coast and in another regions with active faults near the coast. We modeled three typical scenarios, and in light of the Sumatra experience, we examined the more likely severe magnitudes. This of course leads us toward the upper range of expected run-ups. The models show that sooner than five minutes after a strong earthquake produces an offshore slump, which occurs after close to a third of the large dst earthquakes, a 4- to 6-m run-up may flood part of the Syrian, Lebanese, and Israeli coasts. Tsunamis from remote earthquakes, however, arrive later and produce only 1- to 3-m run-ups, but are more regional in extent. Online material: Tsunami modeling and reports.

The Palos Verdes terraces, California: Bathtub rings from a buried reverse fault

Uplift of the Palos Verdes peninsula has long been associated with a northwest trending, southwest dipping, reverse fault. Unfortunately, the Palos Verdes Hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension, orientation, or earthquake potential. In this paper we investigate the tectonic style and slip rate of the Palos Verdes Hills fault and the uplift history of the Palos Verdes Hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration. Elevations of 211 terrace remnants constrain a fault model with 3.0 to 3.7 mm yr−1 of oblique, dextral/reverse slip on a fault dipping 67° at 6 to 12 km depth beneath the peninsula. If the rate was constant through time, fault inception would have occurred 2.4–3.0 Ma. We propose that the largest credible earthquakes on the fault have magnitude ≈6¾ and could revisit every 2000 years.

A synthetic seismicity model for southern California: Cycles, probabilities, and hazard

The absence of a long historical catalog of observed seismicity with which to constrain earthquake recurrence behaviors is a fundamental stumbling block to earthquake prediction in California. Conceding that this limitation is not likely to relax in the foreseeable future, alternative approaches must be sought to extend the catalog artificially. In this article, I evaluate the long-term behaviors of earthquakes on a map-like set of southern California faults through computer simulations that incorporate the physics of earthquake stress transfer and are constrained by excellent, but restricted, bodies of geological and seismological data. I find that model seismicity fluctuates on both short (decades) and long (centuries) timescales but that it possesses a well-defined mean and standard deviation. Seismicity fluctuations correlate across different magnitudes, and the long-term cycles of smaller events seem to lead cycles of larger events. Short-period seismicity fluctuations do not exhibit this tendency, and short-term changes in low-magnitude (M5 +) seismicity are not likely to be an effective predictor of future large events, at least for the region as a whole. As do real faults, the model faults produce characteristic and power law quakes in variable ratios with diverse periodic and nonperiodic behaviors. Generally, larger events tend to occur quasi-periodically, and smaller ones tend to cluster; however, only for a few earthquake classes and certain locations is recurrence notably non-Poissonian. An important use of synthetic seismicity is in the construction of earthquake hazard maps because it firmly grounds previously ad hoc assumptions regarding frequency-magnitude distributions, multiple-segment failure statistics, and rupture extents, while satisfying a spectrum of geological constraints such as fault slip rate, segment recurrence interval, and slip per event. With its depth of temporal and spatial coverage, synthetic seismicity also provides a means to investigate the time dependence of seismic hazard. Because hazard likelihood is a concatenation of the recurrence statistics from many seismic sources, in only about 40–50% of the regions near the major faults do sequences of 0.1 g or 0.2 g exceedances differ from Poissonian.