Earthquake Generation Process Research Articles

Correction [to “Geoelectric signals in China and the earthquake generation process” by Jean J. Chu, Xietai Gui, Jingan Dai, Chris Marone, Marc W. Spiegelman, Leonardo Seeber, and John G. Armbruster

Correction [to “Geoelectric signals in China and the earthquake generation process” by Jean J. Chu, Xietai Gui, Jingan Dai, Chris Marone, Marc W. Spiegelman, Leonardo Seeber, and John G. Armbruster

Geoelectric signals in China and the earthquake generation process

We present original apparent resistivity records for stations located in the Chinese provinces of Hebei and Sichuan, along with seismicity patterns, fault maps, and limited hydrologic data. These records are part of a 30‐year database of geoelectric signals that have been recorded in China by a nationwide network of more than 100 stations set up by the State Seismological Bureau of the People's Republic of China. The database includes the time period during which the M 7.8 1976 Tangshan and M 7.2 1976 Songpan earthquakes occurred. Resistivity data recorded in and around the epicentral region of the Tangshan event show coherent changes, both temporal and in areal distribution, years and months before and after the earthquakes. A regional change in water table occurred more than 3 years prior to the Tangshan earthquake. The resistivity and water table signals appear to correlate in the 2‐ to 3‐year period prior to the earthquake. Rainfall records from the region around Tangshan, extending to the cities of Beijing and Tianjin, do not appear to account for the resistivity changes. Regional seismicity and active fault patterns in north China indicate that deformation occurs mainly along the borders of large tectonic blocks. The source region for the geoelectric signals appears to lie within a large shear zone, at the edge of a tectonic block undergoing rotation. The Tangshan epicentral area is thus only part of a larger zone of deformation. We suggest that dilational strain associated with the opening of fractures may lower resistivity and water table. For shallow crustal depths, such as those monitored by the Chinese geoelectric network, dilation and resistivity changes are expected to begin at stress levels well below the peak values of the seismic cycle. Our study suggests that in some cases the areal extent of geoelectric signals may be significantly larger than that of the eventual earthquake source dimension. The Chinese geoelectric database offers an opportunity to improve our understanding of possible geoelectric precursors to earthquakes.

Nonuniformity of the constitutive law parameters for shear rupture and quasistatic nucleation to dynamic rupture: a physical model of earthquake generation processes.

Based on the recent high-resolution laboratory experiments on propagating shear rupture, the constitutive law that governs shear rupture processes is discussed in view of the physical principles and constraints, and a specific constitutive law is proposed for shear rupture. It is demonstrated that nonuniform distributions of the constitutive law parameters on the fault are necessary for creating the nucleation process, which consists of two phases: (i) a stable, quasistatic phase, and (ii) the subsequent accelerating phase. Physical models of the breakdown zone and the nucleation zone are presented for shear rupture in the brittle regime. The constitutive law for shear rupture explicitly includes a scaling parameter Dc that enables one to give a common interpretation to both small scale rupture in the laboratory and large scale rupture as earthquake source in the Earth. Both the breakdown zone size Xc and the nucleation zone size L are prescribed and scaled by Dc, which in turn is prescribed by a characteristic length lambda c representing geometrical irregularities of the fault. The models presented here make it possible to understand the earthquake generation process from nucleation to unstable, dynamic rupture propagation in terms of physics. Since the nucleation process itself is an immediate earthquake precursor, deep understanding of the nucleation process in terms of physics is crucial for the short-term (or immediate) earthquake prediction.

非地震性すべりの発生機構

Aseismic sliding in the earth plays an important part in time-dependent stress concentration and relaxation, which significantly affect the occurrence of earthquakes. I review 1) observations of fault creep, slow and silent earthquakes, and pre- and post-seismic sliding in the earth, 2) experimental studies on stable sliding, episodic sliding, and preseismic sliding on artificial faults in rocks, and 3) constitutive laws of rock friction derived from laboratory studies and its application to the modeling of aseismic sliding in the earth. It is concluded that the dependence of rock friction on slip and/or slip-rate is essentially important for the mechanism of aseismic sliding. Detection of the occurrence of aseismic sliding is effective in understanding the earthquake generation process.

Deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of Kilauea

Dense microearthquake swarms occur in the upper south flank of Kilauea, providing multiplets composed of hundreds of events. The similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters. A simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically. Relocation of these events greatly improves the definition of the seismic image of the fault that generates them. Indeed, relative relocations define a plane dipping about 6° northward, although corresponding absolute locations are widely dispersed in the swarm. A composite focal mechanism, built from events providing a correct spatial sampling of the multiplet, also gives a well‐constrained northward dip of about 5° to the near‐horizontal plane. This technique thus collapses the clouds of hypocenters of single‐event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies. We cannot conclude that all south flank earthquakes collapse to a single plane. There may locally be several planes, perhaps with different dips and depths throughout the south flank volume. The 6° northward‐dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of Hawaii. This plane is probably an important feature that characterizes the basal slip layer below the upper south flank of Kilauea volcano. Differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane. The present work illustrates how high precision relative relocations of similar events in dense swarms, combined with the analysis of geodetic measurements, can help to describe deep fault plane geometry. Systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well‐instrumented, highly seismic areas to provide reliable basic information, especially useful for understanding of earthquake generation processes.

Earthquake prediction lessons from Parkfield Experiment

In 1985, the National Earthquake Prediction Evaluation Council (NEPEC), and later its California counterpart (CEPEC), approved an earthquake prediction for the first time. This was the Bakun and Lindh [1985] forecast, with 95% confidence, that an M = 5.5 to 6 earthquake would rupture along the San Andreas fault near the town of Parkfield before 1993. This forecast was an important factor in selecting the Parkfield area for a major geophysical and geological monitoring study. The experiments installed there are aimed at producing a better understanding of the earthquake generation process and earthquake effects, gaining experience with real‐time monitoring of a variety of geophysical data, and, if possible, making a short‐term prediction of the future earthquake. Twenty‐eight different observational networks now exist in the Parkfield area. The forecast also generated interest that led researchers to reexamine past Parkfield earthquakes with current knowledge and to develop new models of the earthquake cycle.

Influence of source region properties on scaling relations forM<0 events

Excavation induced seismic events with moment magnitudesM<0 are examined in an attempt to determine the role geology, excavation geometry, and stress have on scaling relations. Correlations are established based on accurate measurements of excavation geometry and methodology, stress regime, rock mass structure, local tectonics, and seismic locations. Scaling relations incorporated seismic moments and source radii obtained by spectral analysis, accounting for source, propagation, and site effects, and using Madariaga's dynamic circular fault model. Observations suggest that the interaction of stresses with pre-existing fractures, fracture complexity and depth of events are the main factors influencing source characteristics and scaling behaviour. Self-similar relationships were found for events at similar depths or for weakly structured rock masses with reduced clamping stresses, whereas a non-similar behaviour was found for events with increasing depth or for heavily fractured zones under stress confinement. Additionally, the scaling behaviour for combined data sets tended to mask the non-similar trends. Overall, depth and fracture complexity, initially thought as second order effects, appear to significantly influence source characteristics of seismic events withM<0 and consequently favour a non-similar earthquake generation process.

Frequency‐magnitude distribution of earthquakes in Vrancea: Relevance for a discrete model

A high‐resolution analysis of a homogeneous and complete data set of intermediate depth microearthquakes (h &gt; 60 km), extending over an interval of 9.3 years emphasizes a significant deviation at low magnitudes (ML &lt; 4) in a linear frequency‐magnitude distribution. It appears as a distinct change in the slope of the cumulative distribution and as a seismicity deficit followed by a relative enhancement in the corresponding noncumulative curve. This is in agreement with the presence of two characteristic mechanisms and allows the identification of both the magnitude threshold of asperity‐like earthquakes and the transition zone (ML = 3.3 ‐ 3.9) from crack‐like earthquakes (background seismicity) to asperity‐like events. These features are better pointed out on well individualized active zones. The background seismicity shows a tremendeous decrease in its b slope, from 1.03 to 0.57, during a 6‐year interval before the occurrence of a major earthquake in 1986 ( Mw = 7.3), followed by a fast recovery to 1.03 in 2 years after this event. Such a behavior could be correlated to the continuous growth of the shear stress‐free surface on the fault, as in a percolation process, followed by its sudden diminution due to the locking of the fault. The above results provide relevance for the discrete character of faulting and enable a coherent modeling of the earthquake generation process from microearthquake to major event scale lengths.

Numerical simulation of the earthquake generation process

A numerical algorithm is proposed for the simulation of the earthquake process during a seismic cycle. The algorithm is based on a heterogeneous discrete model of the fault plane and assumes there are two kinds of seismicity: background crack-like earthquakes and asperity-like events. An active zone of the fault contains an asperity distribution with a characteristic elementary area. The background seismicity randomly develops shear stress-free surfaces which tend to surround the asperities as in a 2D percolation process. The model parameters are taken from observations on the Vrancea (Romania) intermediate depth seismic region. The results emphasize the significant role of the geometry in the mechanism of the seismic failure. The algorithm predicts the nonlinear behavior in the frequency-magnitude distribution, the decrease of theb-slope associated with the asperity-like events, the magnitude range of major earthquakes, and their recurrence times.

Hydrologic precursors to earthquakes: A review

This review summarizes reports of anomalous flow rates or pressures of groundwater, oil, or gas that have been interpreted as earthquake precursors. Both increases and decreases of pressure and flow rate have been observed, at distances up to several hundred kilometers from the earthquake epicenter, with precursor times ranging from less than one day to more than one year. Although information that might rule out nontectonic causes does not appear in many published accounts of hydrologic anomalies, several recent studies have critically evaluated the possible influences of barometric pressure, rainfall, and groundwater or oil exploitation. Anomalies preceding the 1976 Tangshan, China, and the 1978 Izu-Oshima-Kinkai, Japan, earthquakes are especially well-documented and worthy of further examination. Among hydrologic precursors, pressure head changes in confined subsurface reservoirs are those most amenable to quantitative interpretation in terms of crustal strain. The response of pressure head to earth tides determines coefficients of proportionality between pressure head and crustal strain. The same coefficients of proportionality should govern the fluid pressure response to any crustal strain field in which fluid flow in the reservoir is unimportant. Water level changes in response to independently recorded tectonic events, such as earthquakes and aseismic fault creep, provide evidence that a calibration based on response to earth tides may be applied to crustal strains of tectonic origin. Several models of earthquake generation predict accelerating stable slip on part of the future rupture plane. If precursory slip has moment less than or equal to that of the impending earthquake, then the coseismic volume strain is an upper bound for precursory volume strain. Although crustal strain can be only crudely estimated from most reported pressure head anomalies, the sizes of many anomalies within 150 kilometers of earthquake epicenters appear consistent with this upper bound. In contrast, water level anomalies at greater epicentral distances appear to be larger than this bound by several orders of magnitude. It is clear that water level monitoring can yield information about the earthquake generation process, but progress higes on better documentation of the data.

Future earthquakes

While significant progress toward earthquake prediction has occurred in the past four years, the current guarded optimism for continued progress reflects in part revised goals and new definitions. First, recognition [e.g., Evernden, 1982] of the failures of the search during the 1970s for a reliable readily‐observable earthquake precursor has reemphasized the need to understand the fundamental physics of the earthquake generation process [e.g., Stuart, 1984/85] and brought acceptance of a probabilistic rather than a deterministic approach to earthquake prediction. That is, we now accept the notion that earthquake predictions must be couched in terms of probability gain and likelihood estimates. Second, a prediction terminology [Wallace et al., 1984] has been largely adopted that separates and expands the tasks into long‐term earthquake potential (no specific time window) and long‐term prediction (time windows of a few years to a few decades), where progress has been significant, and intermediate‐term prediction (time windows of a few weeks to a few years) and short‐term prediction (time windows up to a few weeks), where progress has lagged. In the 1970s, earthquake prediction usually was taken to mean a deterministic short‐term or intermediate‐term warning; now, probabilistic estimates of earthquake potential [e.g., Lindh, 1983; Sykes and Nirhenko, 1984] are accepted as the most reasonable and valid form for expressing the likelihood of a future earthquake and its hazards, as well as the uncertainties.

Introduction to the Special Section on Fault Behavior and the Earthquake Generation Process

This special section is a direct outgrowth of an AGU Chapman Conference on Fault Behavior and the Earthquake Generation Process that was convened in Snowbird, Utah, from October 11 to 15, 1982. The body of information concerning the behavior of faults in space and time, especially with regard to earthquake recurrence, fault zone geometry, and the mechanical and physical properties of fault zones that control rupture processes has been increasing at an enormous rate. And, perhaps more importantly, our understanding of these subjects has direct societal application in the mitigation of earthquake hazards, especially the identification of potential seismogenic sources and the characterization of these sources in terms of quantitative estimates of the location and probability of earthquake occurrence. The motivation behind the conference was the desire to pause and reflect on the recent progress in the field and to evaluate, and perhaps identify, potentially important new areas of research.

Physical-mechanical properties of heterogeneous media at pressures up to 16 kilobars in the intervals of filler melting

The velocities of longitudinal and shear waves, and the density and compressibility of heterogeneous media were determined in the interval of fusible filler melting (softening) at pressures to 16 kbar. A high-pressure vessel of cylinder-piston type with outer heating was used. Sharp changes in elastic wave velocities were observed in the initial and final stages of the filler melting (solidification) process. The phenomenon of a sharp structure rearrangement and redistribution of stresses in a two-phase media was determined at certain values of thermodynamic parameters. The results of these experiments are discussed in relation to problems of the physics of earthquake generation processes, the nature of the low-velocity zone, and magma segregation.

Estimating the probability of occurrences of surface faulting earthquakes on the Wasatch fault zone, Utah

abstract Although geological and geomorphic evidence strongly suggests that the Wasatch fault zone has generated large-magnitude earthquakes in late Quaternary time, the fault zone has not been associated with earthquakes greater than magnitude 512 in the past 133 yr. Therefore, realistic estimates of the likelihood of future damaging earthquakes must be based on more than historical seismicity data. The data base can be expanded by collecting site-specific geological information on earthquake recurrence and fault slip rates and by using this information in a model of the earthquake generation process. Uncertainties in both the physical basis for the model and in the geological parameters dictate a probabilistic approach. A semi-Markov model provides real-time probabilities of occurrence of at least one moderate to large (magnitude 612 or larger) earthquake at either of two sites for given elapsed times. Probabilities derived for the entire Wasatch fault zone are based on earthquake recurrence data on individual fault segments and are especially sensitive to elapsed times on individual segments.