To understand whether the 1992 M = 7.4 Landers earthquake changed the proximity to failure on the San Andreas fault system, we examine the general problem of how one earthquake might trigger another. The tendency of rocks to fail in a brittle manner is thought to be a function of both shear and confining stresses, commonly formulated as the Coulomb failure criterion. Here we explore how changes in Coulomb conditions associated with one or more earthquakes may trigger subsequent events. We first consider a Coulomb criterion appropriate for the production of aftershocks, where faults most likely to slip are those optimally orientated for failure as a result of the prevailing regional stress field and the stress change caused by the mainshock. We find that the distribution of aftershocks for the Landers earthquake, as well as for several other moderate events in its vicinity, can be explained by the Coulomb criterion as follows: aftershocks are abundant where the Coulomb stress on optimally orientated faults rose by more than one-half bar, and aftershocks are sparse where the Coulomb stress dropped by a similar amount. Further, we find that several moderate shocks raised the stress at the future Landers epicenter and along much of the Landers rupture zone by about a bar, advancing the Landers shock by 1 to 3 centuries. The Landers rupture, in turn, raised the stress at site of the future M = 6.5 Big Bear aftershock site by 3 bars. The Coulomb stress change on a specified fault is independent of regional stress but depends on the fault geometry, sense of slip, and the coefficient of friction. We use this method to resolve stress changes on the San Andreas and San Jacinto faults imposed by the Landers sequence. Together the Landers and Big Bear earthquakes raised the stress along the San Bernardino segment of the southern San Andreas fault by 2 to 6 bars, hastening the next great earthquake there by about a decade.

/pdf/static-stress-changes-and-the-triggering-of-earthquakes-4r29mmr407.pdf

Static stress changes and the triggering of earthquakes

Bones of recent mammals in the Amboseli Basin, southern Kenya, exhibit distinctive weathering characteristics that can be related to the time since death and to the local conditions of temperature, humidity and soil chemistry. A categorization of weathering characteristics into six stages, recognizable on descriptive criteria, provides a basis for investigation of weathering rates and processes. The time necessary to achieve each successive weathering stage has been calibrated using known-age carcasses. Most bones decompose beyond recognition in 10 to 15 yr. Bones of animals under 100 kg and juveniles appear to weather more rapidly than bones of large animals or adults. Small-scale rather than widespread environmental factors seem to have greatest influence on weathering characteristics and rates. Bone weathering is potentially valuable as evidence for the period of time represented in recent or fossil bone assemblages, in- cluding those on archeological sites, and may also be an important tool in censusing populations of animals in modern ecosystems.

Taphonomic and Ecologic Information Form Bone Weathering

The importance of strike-slip faulting was recognized near the turn of the century, chiefly from investigations of surficial offsets associated with major earthquakes in New Zealand, Japan, and California. Extrapolation from observed horizontal displacements during single earthquakes to more abstract concepts of long-term, slow accumulation of hundreds of kilometers of horizontal translation over geologic time, however, came almost simultaneously from several parts of the world, but only after much regional geologic mapping and synthesis. Strike-slip faults are classified either as transform faults which cut the lithosphere as plate boundaries, or as transcurrent faults which are confined to the crust. Each class of faults may be subdivided further according to their plate or intraplate tectonic function. A mechanical understanding of strike-slip faults has grown out of laboratory model studies which give a theoretical basis to relate faulting to concepts of pure shear or simple shear. Conjugate sets of strike-slip faults form in pure shear, typically across the strike of a convergent orogenic belt. Fault lengths are generally less than 100 km, and displacements along them are measurable in a few to tens of kilometers. Major strike-slip faults form in regional belts of simple shear, typically parallel to orogenic belts; indeed, recognition of the role strike-slip faults play in ancient orogenic belts is becoming increasingly commonplace as regional mapping becomes more detailed and complete. The lengths and displacements of the great strike-slip faults range in the hundreds of kilometers. The position and orientation of associated folds, local domains of extension and shortening, and related fractures and faults depend on the bending or stepping geometry of the strike-slip fault or fault zone, and thus the degree of convergent or divergent strike-slip. Elongate basins, ranging from sag ponds to rhombochasms, form as result of extension in domains of divergent strike slip such as releasing bends; pull-apart basins evolve between overstepping strike-slip faults. The arrangement of strike-slip faults which bound basins is tulip-shaped in profiles normal to strike. Elongate uplifts, ranging from pressure ridges to long, low hills or small mountain ranges, form as a result of crustal shortening in zones of convergent strike slip; they are bounded by an arrangement of strike-slip faults having the profile of a palm tree. Paleoseismic investigations imply that earthquakes occur more frequently on strike-slip faults than on intraplate normal and reverse faults. Active strike-slip faults also differ from other types of faults in that they evince fault creep, which is largely a surficial phenomenon driven by elastic loading of the crust at seismogenic depths. Creep may be steady state or episodic, pre-seismic, co-seismic, or post-seismic, depending on the constitutive properties of the fault zone and the nature of the static strain field, among a number of other factors which are incompletely understood. Recent studies have identified relations between strike-slip faults and crustal delamination at or near the seismogenic zone, giving a mechanism for regional rotation and translation of crustal slabs and flakes, but how general and widespread are these phenomena, and how the mechanisms operate that drive these detachment tectonics are questions that require additional observations, data, and modeling. Several fundamental problems remain poorly understood, including the nature of formation of en echelon folds and their relation to strike-slip faulting; the effect of mechanical stratigraphy on strike-slip-fault structural styles; the thermal and stress states along transform plate boundaries; and the discrepancy between recent geological and historical fault-slip rates relative to more rapid rates of slip determined from analyses of sea-floor magnetic anomalies. Many of the concepts and problems concerning strike-slip faults are derived from nearly a century of study of the San Andreas fault and have added much information, but solutions to several remaining and new fundamental problems will come when more attention is focused on other, less well studied strike-slip faults.

Strike-Slip faults

SUMMARY This paper is concerned with the relationship between the overall motion across a zone of distributed continental deformation and the seismic moment tensors of earthquakes that occur within it. The overall deformation in the zone is described by the deformation gradient tensor L, which may be split into a symmetric part, S, and an antisymmetric part, A. S is the strain tensor, and can always be determined from the sum of the moment tensors, following the result of Kostrov (1974). A corresponds to a rigid body rotation, and is in principle unobservable seismically: the moment tensors contain no information about A, regardless of whether the ambiguity between fault and auxiliary planes is resolved. From S the integrated rates of motion normal and parallel to the zone boundary, as well as vertically, can be calculated. Of these, only the motion normal to the zone is specified by the motion across its boundaries. In general, S (and hence L) is not specified by the motion of the plates bounding the zone. Only if some a priori assumptions about L are made, can information about A be recovered from the seismic moment tensors. Otherwise A must be determined independently from paleomagnetic or geodetic measurements. These results are applied to the Mediterranean region to see whether the motion between the relatively rigid regions of central Iran, Turkey, Arabia, Africa, the Adriatic Sea and Eurasia is accommodated seismically within the upper crust of the wide deforming zones that bound these regions. In NE Iran, the North Anatolian Fault Zone and the Aegean Sea all or most of the deformation is probably taken up seismically. In the Zagros, Caucasus, Hellenic Trench and western Mediterranean probably 10 per cent or less of the upper-crustal deformation is seismic and the rest must be accommodated by creep. The Cyprus arc, the East Anatolian and Dead Sea Fault Zones have had insufficient seismicity this century for any conclusions to be drawn. The seismicity in central Italy and Yugoslavia accounts for velocities of about 2 mm yr-' normal to the deforming zones, but there is no independent estimate of the velocities on the borders of the Adriatic Sea with which to compare these. The seismicity in the Aegean region indicates very high stretching velocities (c. 60 mm yr-') and strain rates (c. 4 x s-l). These in turn require correspondingly high subduction rates (c. 100 mm yr-') in the Hellenic Trench. If these rates have been constant in time, it is unlikely that the tip of the sinking slab beneath the southern Aegean began to be subducted more than 5Ma ago. These high-stretching rates in the Aegean, if extrapolated back to the Pliocene, are compatible with observed finite strains and paleomagnetic rotations. They are also likely to have raised to a shallower depth the isotherm corresponding to the seismic-aseismic transition in the crust, perhaps accounting for the relatively shallow focal depths of normal faulting earthquakes in the Aegean compared with those in other areas of continental extension where strain rates are lower. It is possible that the reason for the dominantly aseismic deformation in the Zagros and Hellenic Trench is related to the great thickness of sediments, partly decoupled from the basement by salt, in both places. This may lead to elevated basement temperatures and inhibit upward fault propagation, thus restricting the size of seismogenic fault planes (and hence seismic moment) and causing the sedimentary cover to deform independently from the basement, partly by folding. However, this explanation has serious drawbacks and is not easily applicable to other areas, notably the western Mediterranean.

The relationship between plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and Middle East

Neotectonics of Turkey – a synthesis

The temporal distribution of earthquakes in the Dead Sea Graben is studied through a 50,000-year paleoseismic record recovered in laminated sediments of the Late Pleistocene Lake Lisan (paleo-Dead Sea). The Lisan represents more than 10 times the 4000 years of historical earthquake records. It is the longest and most complete paleoseismic record along the Dead Sea Transform and possibly the longest continuous record on Earth. It includes unique exposures of seismite beds (earthquake-induced structures) associated with slip events on syndepositional faults. The seismites are layers consisting of mixtures of fragmented and pulverized laminae. The places where the seismites abut syndepositional faults are interpreted as evidence for their formation at the sediment-water interface during slip events on these faults. Thicker sediment accumulation above the seismites in the downthrown blocks indicates that a seismite formed at the water-sediment interface on both sides of the fault scarps. Modern analogs and the association with surface ruptures suggest that each seismite formed during a M L ≥5.5 earthquake. The 230 Th- 234 U ages of a columnar section, obtained by thermal ionization mass spectrometry, give a mean recurrence time of ∼1600 years of M L ≥5.5 earthquakes in the Dead Sea Graben. The earthquakes cluster in ∼10,000-year periods separated by quiet periods of similar length. This distribution implies that a long-term behavior of the Dead Sea Transform should be represented by a mean recurrence of at least 20,000 year record. This observation has ramifications for seismic hazard assessment based on shorter records.

Long-term earthquake clustering: A 50,000-year paleoseismic record in the Dead Sea Graben

The magnitude and sense of block rotations depicted from such structural data as fault spacing and slip are noted to agree with values obtained from independent paleomagnetic determinations. The agreement between paleomagnetic rotation data and those inferred from offset and spacing data in northern Israel is excellent, suggesting that the faults and intervening blocks were rotated with progressive deformation along the levant transform. It is suggested that the rotation of blocks and the strike-slip displacement are two qualitative and quantitative contemporaneous aspects of a single deformation process.

Block rotation by strike-slip faulting: Structural and paleomagnetic evidence

In many strike-slip tectonic settings, large rotations (up to 100°) of crustal blocks have been inferred from paleomagnetic data. These blocks are bounded by sets of parallel faults, which accommodate the relative motion between the blocks as regional deformation progresses. Simple geometrical considerations require that the faults must also rotate. In this paper we show that on the basis of mechanical considerations, the amount of fault rotation permissible under a stationary stress field is limited to 20° to 45°. Consequently, block rotations that are larger than 40° or 45° require more than one set of accompanying faults to accommodate the block rotation. Examples of such multiple sets with 40° to 45° between them, as predicted by the model, were recognized in Sistan, Iran; in Yerington, the Lake Mead area, Nevada; and in southern California.

Fault mechanics and the kinematics of block rotations

SUMMARY

We define the Sinai subplate, from a seismotectonic perspective, as a distinct component in the plate tectonics of the eastern Mediterranean region. This is based on the tectonic characteristics of a comprehensive listing of all ML≥ 4 recorded seismicity in the region during the 20th century, on newly calculated and recalculated fault plane mechanisms of first P-wave arrivals and on published solutions based on waveform inversion of broad-band data.



The low seismicity level and scarcity of strong events in the region required a thorough search for useful data and a careful examination of the reliability of the focal solutions. We gathered all available records of first P-wave onsets from the ISS and ISC Bulletins and the local seismic networks. Altogether, we were able to calculate 48 new focal mechanisms and 33 recalculated ones of events that occurred during the years 1940–1992. With the increasing number of teleseismic and regional broad-band stations in the later years, we added 37 solutions based on teleseismic and regional waveform inversions of events that occurred during 1977–2001.



These mechanisms enabled us to examine the seismotectonic character of the Sinai subplate. The strike and rake directions of the calculated mechanisms usually reflect the geometry and the large-scale type of deformation observed along the boundaries of the Sinai subplate—the Dead Sea Transform, the Cypriot Arc convergent zone and the Suez Rift. Nevertheless, along each of these boundaries we found anomalous solutions that attest to the complexity of the deformation processes along plate margins.



Earthquakes along the Dead Sea Transform exhibit mainly sinistral transtension and transpression, reflecting its leaky manner and local change in the transform geometry. The presence of other unexpected mechanisms near the transform, however, reflects the heterogeneous deformation it induces around.



As expected, thrust mechanisms along the Cypriot Arc mirror its convergent nature and typical curved geometry. Transtension and transpressional solutions in the eastern segment of the arc reflect the sinistral shear motion between Anatolia and Sinai there. However, shear mechanisms found between Cyprus and the Eratosthenes Seamount pose a problem regarding its collision process.



Most intriguing of all are ML≥ 4 thrust and shear solutions found in the Gulf of Suez. They are associated with predominantly normal mechanisms within a rift zone and therefore constitute a unique phenomenon, yet to be deciphered.

/pdf/seismotectonics-of-the-sinai-subplate-the-eastern-4h0i8bpmf7.pdf

Hagai Ron

Papers

Long-term earthquake clustering: A 50,000-year paleoseismic record in the Dead Sea Graben

Block rotation by strike-slip faulting: Structural and paleomagnetic evidence

Fault mechanics and the kinematics of block rotations

Seismotectonics of the Sinai subplate – the eastern Mediterranean region

Seismicity of the eastern Mediterranean region: Perspective from the Sinai subplate