A combined sedimentological and high-resolution petrographic analysis was conducted on a glacial-age (20,000-65,000 cal yr BP) sediment core from Baldwin Lake, Southern California. The results of this research represent the most complete glacial-age, terrestrial climate record from Southern California to date. These results are used to characterize the different sediment types and to investigate the difference in depositional processes and environments between the core's three predominant sediment types: massive, semi-laminated, and laminated sediments. Massive sediments are commonly associated with a blocky texture and/or desiccation cracks, are organic-poor, have high magnetic susceptibility values, and are coarser-grained. Thin-sections from massive sediments reveal a homogenous sediment fabric. Sub-centimeter-scale laminated and centimeter-scale semi-laminated sediments are generally organic-rich, have low magnetic susceptibility values, and are finer grained. Thin-sections from laminated and semi-laminated sediments reveal diffuse sub-millimeter- to millimeter-scale laminae. This combination of sedimentological and high-resolution petrographic data enabled us to characterize four sediment facies, each related to specific depositional processes and environments: (1) a play a lake: (2) a perennial shallow lake; (3) an intermediate lake with variable lake level; and, (4) a perennial deep lake. At centennial-to millennial-timescales, lower lake levels are represented by deposition of massive to semi-laminated sediments in a play a to a perennial shallow lake environment. At similar timescales, higher lake levels are recorded by semi-laminated to laminated sediments deposited in an intermediate lake to a perennial deep lake environment. These results provide an additional sedimentological study for comparison to similar and environment basins, and for comparison to existing regional paleoclimatic reconstructions. (C) 2009 Elsevier B.V. All rights reserved.

In this work, we provide a joint study of the stress accumulation method (SAM) (King and Bowman, 2003) and the Pattern Informatics (PI) index (Tiampo et al., 2002b). We examine the theoretical underpinnings for the similarities between the two techniques, as well as the differences in their application. The SAM technique is employed to determine likely mechanisms for smaller areas of increased probability identified by the PI index, while a modified version of the PI index can be used to locate regions where the smaller magnitude associated with the anomaly is below the resolution of the SAM. Finally, we present three case studies from different regions of the San Andreas fault system to illustrate both their complementary nature, as well as the advantages to combining them in one synthesized analysis.

Paleoseismological data suggest the occurrence of four bursts of seismic moment release in the Los Angeles region during the past 12,000 yr. The historic period appears to be part of an ongoing lull that has persisted for about the past 1000 yr. These periods of rapid seismic displacement in the Los Angeles region have occurred during the lulls between similar bursts of activity observed on the eastern California shear zone in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the greater San Andreas system suppress activity on faults in the eastern California shear zone, and vice versa, can explain the apparent switching of activity between the two fault networks. Combined with the observation that short-term geodetic and longer-term geologic rates co-vary on major southern California fault systems, this suggests that either (1) a temporal cluster of seismic displacements on upper-crustal faults increases ductile deformation on their downward extensions, or (2) rapid ductile slip in the lower crust beneath faults loads the upper crust, driving a seismic cluster. We suggest that alternating periods of rapid seismic displacement may be the expected mode of seismicity when two fault systems accommodate the same plate-boundary motion, and slip on one system suppresses slip on the other.

The stress field that existed before a large earthquake can be calculated based on the known source parameters of the event. This stress field can be used to define a region that shows greater seismic moment rate changes prior to the event than arbitrarily shaped regions, allowing us to link two previously unrelated subjects: Coulomb stress interactions and accelerating seismicity before large earthquakes. As an example, we have examined all M•_6.5 earthquakes in California since 1950. While we illustrate the model using seismicity in California, the technique is general and can be applied to any tectonically active region. We show that where sufficient knowledge of the regional tectonics exists, this method can be used to augment current techniques for seismic

Large earthquakes can be preceded by a period of accelerating seismic activity of moderate- sized earthquakes. This phenomenon, usually termed accelerating moment release, has yet to be clearly understood. A new mathematical formulation of accelerating moment release is obtained from simple stress transfer considerations, following the recently proposed stress accumulation model. This model, based on the concept of elastic rebound, simulates accelerating seismicity from theoretical stress changes during an idealized seismic cycle. In this view, accelerating moment release is simply the consequence of the decrease, due to loading, of the size of a stress shadow due to a previous earthquake. We show that a power law time- to- failure equation can be expressed as a function of the loading rate on the fault that is going to rupture. We also show that the m value, which is the power law exponent, can be defined as m = D/3, with D a parameter that takes into account the geometrical shape of the stress lobes and the distribution of active faults. In the stress accumulation model, the power law is not due to critical processes.

We test the Bowman and King [Bowman, D.D., King, G.C.P., 2001a, Accelerating seismicity and stress accumulation before large earthquakes. Geophys. Res. Lett., 28 (21), 4039–4042, Bowman, D.D., King, G.C.P., 2001b. Stress transfer and seismicity changes before large earthquakes. C. R. Acad. Sci. Paris, 333, 591–599] Stress Accumulation model by examining the evolution of seismicity rates prior to the 1992 Landers, California earthquake. The Stress Accumulation (SA) model was developed to explain observations of accelerating seismicity preceding large earthquakes. The model proposes that accelerating seismicity sequences result from the tectonic loading of large fault structures through aseismic slip in the elasto-plastic lower crust. This loading progressively increases the stress on smaller faults within a critical region around the main structure, thereby causing the observed acceleration of precursory activity. A secondary prediction of the SA model is that the precursory seismicity rates should increase first at the edges of the critical region, with the rates gradually rising over time at closer distances to the main fault. We test this prediction by examining year-long seismicity rates between 1960 and 2004, as a function of distance from the Landers rupture. To quantify the significance of trends in the seismicity rates, we auto-correlate the data, using a range of spatial and temporal lags. We find weak evidence for increased seismicity rates propagating towards the Landers rupture, but cannot conclusively distinguish these results from those obtained for a random earthquake catalog. However, we find a strong indication of periodicity in the rate fluctuations, as well as high correlation between activity 130–170 km from Landers and seismicity rates within 50 km of the Landers rupture temporally offset 1.5–2 years. The implications of this spatio–temporal correlation will be addressed in future studies.

A promising approach to assessing seismic hazards has been to combine the concept of seismic gaps with Coulomb-stress change modeling to refine short-term earthquake probability estimates. However, in practice the large uncertainties in the seismic histories of most tectonically active regions limit this approach since a stress increase is only important when a fault is already close to failure. In contrast, recent work has suggested that Accelerated Moment Release (AMR) can help to identify when a stretch of fault is approaching failure without any knowledge of the seismic history of the region. AMR can be identified in the regions around the Sumatra Subduction system that must have been stressed before the 26 December 2004 and 28 March 2005 earthquakes. The effect is clearest for the epicentral regions with less than a 2% probability that it could occur in a random catalogue. Less clear AMR is associated with the regions north of Sumatra around the Nicobar and Andaman islands where rupture in the December 2004 earthquake was less vigorous. No AMR is found for the region of the 1833 Sumatran earthquake suggesting that an event in this region in the near future is unlikely. AMR similar to that before the December 2004 and March 2005 events is found for a 750 kin stretch of the southeastern Sumatra and western Java subduction system suggesting that it is close to failure. Should the whole of this stretch break in a single event the magnitude could be similar to the December 2004 earthquake. (c) 2006 Elsevier B.V. All rights reserved.

Slip-partitioned fault breaks have been mapped for a 70-km stretch of the 450-km surface rupture of the 14 November 2001 Kokoxili earthquake. Simultaneous dip-slip and strike-slip motion on parallel faults has been proposed before, but the new observations demonstrate unequivocally that it can occur in a single earthquake and allows the mechanical processes to be scrutinized. Observed normal fault offsets were between 0.5 and 1 m and strike-slip offsets were 3-5 m. The partitioned stretch of faulting has a strike that differs by 2-3 degrees from the pure strike-slip faulting to the east and west. This allows a horizontal opening vector of 0.25 m to be determined for the partitioned region. The distance between the two faults is greatest (similar to 2 km) at the center of the partitioned portion and diminishes toward the ends.The faulting is modeled to result from strains due to a buried oblique slip-fault dipping at 80 degrees to the south. The depth to the top of the buried fault is shown to vary commensurately with the separation of the surface faults. Clear surface rupture is observed where the predicted model mechanisms are colinear and where substantial faults can develop into a kinematically stable partitioned system. In a few interesting examples fragmentary, oblique surface ruptures occur where the predicted mechanisms are not colinear, but they are not associated with long-term surface faulting.The proposed mechanism for slip partitioning requires that rupture propagates upward from depth. For the Kokoxili surface breaks this is a consequence of coseismic, dynamic rupture traveling faster at depth than near the surface, leaving the surface deformation to catch up. While the mechanism we propose requires slip weakening and localization to create faults or shear zones, it does not require that faults with different mechanisms have different frictional behavior.

— In recent years there has been renewed interest in observations of accelerating moment release before large earthquakes, as well as theoretical descriptions of seismicity in terms of statistical physics. Most aspects of these works are encompassed by a concept called intermittent criticality in which a region alternately approaches and retreats from a critical -point. From this perspective, the evolution of seismicity in a region is described in terms of the growth and destruction of correlation in the stress field over the course of the seismic cycle. In this paper we test the concept of intermittent criticality by investigating the temporal evolution of the Gutenberg-Richter distribution before and after two successive M ≥ 5.0 earthquakes in western Washington State. The largest event in this distribution, Mmax, is observed to systematically increase before each event, producing accelerating moment release, and then to subsequently decrease. Associated variations in the b-value are minimal. This is the predicted result if Mmax is a measure of the correlation length of the regional stress field.

— An algorithm recently developed by RUNDLE et al. (2002) to find regions of anomalous seismic activity associated with large earthquakes identified the location of an M w = 5.6 earthquake near Calexico, Mexico. In this paper we analyze the regional seismicity before this event, and a nearby M w = 5.7 event, using time-to-failure algorithms developed by BOWMAN et al. (1998) and BOWMAN and KING (2001a,b). The former finds the radius of a circular region surrounding the epicenter that optimizes the time-to-failure acceleration of seismic release. The latter optimizes acceleration based on the expected stress accumulation pattern for a dislocation source. Both methods found a period of accelerating seismicity in an optimal region, the size of which agrees with previously proposed scaling relations. This positive result suggests that the Rundle algorithm may provide a useful technique to identify regions of accelerating seismicity, which can then be analyzed using signal optimization time-to-failure techniques.

Oblique motion along tectonic boundaries is commonly partitioned into slip on faults with different senses of motion. The origin of slip partitioning is important to structural geology, tectonophysics, and earthquake mechanics. Partitioning can be explained by the upward elastoplastic propagation of oblique slip from a fault or shear zone at depth. The strain field ahead of the propagating fault separates into zones of predominantly normal, reverse, and strike-slip faulting. The model successfully predicts the distribution of fault types along parts of the San Andreas and Haiyuan faults.

We describe a simple model that links static stress (Coulomb) modeling to the regional seismicity around a major fault. Unlike conventional Coulomb stress techniques, which calculate stress changes, we model the evolution of the stress field relative to the failure stress. Background seismicity is attributed to inhomogeneities in the stress field which are created by adding a random field that creates local regions above the failure stress. The inhomogeneous field is chosen such that when these patches fail, the resulting earthquake size distribution follows a Gutenburg‐Richter law. Immediately following a large event, the model produces regions of increased seismicity (aftershocks) where the overall stress field has been elevated and regions of reduced seismicity where the stress field has been reduced (stress shadows). The high stress levels in the aftershock regions decrease due to loading following the main event. Combined with the stress shadow from the main event, this results in a broad seismically quiet region of lowered stress around the epicenter. Pre‐event seismicity appears as the original stress shadows finally fill as a result of loading. The increase in seismicity initially occurs several fault lengths away from the main fault and moves inward as the event approaches. As a result of this effect, the seismic moment release in the region around the future epicenter increases as the event approaches. Synthetic catalogues generated by this model are virtually indistinguishable from real earthquake sequences in California and Washington.

In recent years, observational and theoretical descriptions of spatio-temporal patterns of seismicity have focused on two fundamental (and controversial) observations: static stress (Coulomb) interactions between earthquakes and accelerating seismic moment release before large earthquakes. While there have been several documented examples of static stress changes influencing the space-time pattern of seismicity following great earthquakes (main shocks and aftershocks), there have been few attempts to link this method to the evolution of seismicity before great earthquakes (precursory seismicity and foreshocks). In this paper, we describe a simple physical model that links static stress modeling to accelerating moment release before a large event. For practical reasons, it is not straightforward to apply this technique as a method of forecasting future large earthquakes. However, after the large event has occurred, the region of stress accumulation can be calculated with precision based on the known source parameters of the earthquake. This region can then be examined for seismic moment rate changes prior to the event. As examples, we have examined all M greater than or equal to 6.5 earthquakes in California since 1950 in regions defined by their pre-event stress fields, and find a period of accelerating moment release before all of these events. While we illustrate the model using seismicity in California, the technique is general and can be applied to any tectonically active region. Where sufficient knowledge of the regional tectonics exists, this method can be used to augment current techniques for seismic hazard estimation. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

The stress field that existed before a large earthquake can be calculated based on the known source parameters of the event. This stress field can be used to define a region that shows greater seismic moment rate changes prior to the event than arbitrarily shaped regions, allowing us to link two previously unrelated subjects: Coulomb stress interactions and accelerating seismicity before large earthquakes. As an example, we have examined all M ≥ 6.5 earthquakes in California since 1950. While we illustrate the model using seismicity in California, the technique is general and can be applied to any tectonically active region. We show that where sufficient knowledge of the regional tectonics exists, this method can be used to augment current techniques for seismic hazard estimation.

anyone who has ever spent any time in California can attest, much public attention is being focused on the great earthquake-prediction debate. Unfortunately, this attention focuses on deterministic predictions on the day-to-week timescale. But as some of the participants in this debate have pointed out 1,2 current efforts to identify reliable short-term precursors to large earthquakes have been largely unsuccessful, suggesting that earthquakes are such a complicated process that reliable (and observable) precursors might not exist. That is not to say that earthquakes do not have some 'preparatory phase', but rather that this phase might be not be consistently observable by geophysicists on the surface. But does this mean that all efforts to determine the size, timing and locations of future earthquakes are fruitless? Or are we being misled by human scales of time and distance?

Abstract A promising approach,to assessing seismic hazards has been to combine,the concept of seismic gaps with Coulomb-stress change modeling to refine short-term earthquake probability estimates. However, in practice the large uncertainties in the seismic histories of most tectonically active regions limit this approach since a stress increase is only important when a fault is already close to failure. In contrast, recent work has suggested that Accelerated Moment Release (AMR) can help to identify when a stretch of fault is approaching,failure without any knowledge,of the seismic history of the region. AMR can be identified in the regions around,the Sumatra Subduction system,that must,have,been stressed before the 26 December,2004 and,28 March 2005 earthquakes. The effect is clearest for the epicentral regions with less than a 2% probability that it could occur in a random catalogue. Less clear AMR is associated with the regions north of Sumatra around the Nicobar and Andaman,islands where rupture in the December 2004 earthquake was less vigorous. No AMR is found for the region of the 1833 Sumatran earthquake suggesting that an event in this region in the near future is unlikely. AMR similar to that before the December 2004 and March 2005 events is found for a 750 km,stretch of the southeastern Sumatra and western Java subduction system suggesting that it is close to failure. Should the whole of this stretch break in a single event the magnitude,could be similar to the December 2004 earthquake. © 2006 Elsevier B.V. All rights reserved. Keywords: earthquake stress interactions; seismicity and tectonics; subduction zones; earthquake forecasting and prediction; accelerating moment