Earthquake Failure Research Articles

Spatial patterns of aftershocks of deep focus earthquakes

This paper presents precise relative relocations for aftershocks of 59 intermediate and deep earthquakes, using P, pP, and PKP arrival times read by us or reported in the literature. These aftershocks included 36 “rupture subevents,” occurring within 1 min of the initial event, and 71 “true aftershocks,” occurring later and identified statistically as being related to the initial event. We compare the relative locations with the orientations of the Wadati‐Benioff zones and with the available focal mechanisms using statistical methods not previously applied to earthquake data. Surprisingly, neither true aftershocks nor rupture subevents cluster along the nodal planes of intermediate or deep earthquake focal mechanisms. Aftershocks more than 20 km from the initial events occur preferentially in the plane of the Wadati‐Benioff zone, while those lying closer have isotropically distributed directions. Rupture subevents occur after the travel time of the S wave from the initial event. Larger‐magnitude earthquakes generally possess rupture subevents lying farther from the initial event, whereas true aftershocks can occur 50 km or more from initial events having magnitudes as small as 5.3. Except following a few unusually large earthquakes with focal depths of about 100 km, we find no rupture subevents or true aftershocks more than about 80 km from the initial hypocenter. The existence of many aftershocks far from nodal planes does not favor models in which deep earthquake failure is simply slip along a planar fault. Rather, the aftershocks may occur in response to a general redistribution of stress caused either by the occurrence of the initial event or, possibly, by nearby continuing phase transitions. Alternatively, the failure zones of deep earthquakes may be surfaces which are frequently curved by 40° or more from the orientation of the initial nodal plane, perhaps due to inhomogeneous stress distributions.

Constitutive relations between dynamic physical parameters near a tip of the propagating slip zone during stick-slip shear failure

Constitutive relations between physical parameters in the cohesive zone during stick-slip shear failure are experimentally investigated. Stick-slip was generated along a 40 cm long precut fault in Tsukuba granite samples using a servocontrolled biaxial loading apparatus. Dynamic behavior during local breakdown processes near a tip of the slipping zone is revealed; the slip velocity and acceleration are given as a function of the slip displacement and the cohesive (or breakdown) shear stress as a function of the slip velocity. A cycle of the breakdown and restrengthening process of stick-slip is composed of five phases characterized in terms of the cohesive strength and the slip velocity. The cohesive strength can degrade regardless of the slip velocity during slip instabilities. The maximum slip acceleration u ̈ max and the maximum slip velocity u ̇ max are obtained experimentally as: u ̈ max = 2 u c u ̇ max 2 and u ̇ max = Δτ b G v where u c is the critical displacement, Δτ b the breakdown stress drop, G the rigidity and v the rupture velocity. These relations are consistent with Ida's theoretical estimation based on the cohesive zone model. The above formula gives good estimates for the maximum slip acceleration of actual earthquakes. The cutoff frequency ƒ max of the power spectral density of the slip acceleration increases with increasing normal stress; in particular, ƒ max is found to be directly proportional to the normal stress σ n within the normal stress range less than 17 MPa as: ƒ max (kHz) = 4.0σ n(MPa) σ n <17(MPa) ƒ max increase with an increase in u ̇ max or u ̈ max . All these results lead to the conclusion that u ̈ max , u ̇ max and ƒ max increase with increasing normal stress. This is consistent with a previous observation that τ b increases with increasing normal stress. The above empirical linear relation between ƒ max and σ n can be explained by a linear dependence of Δτ b on σ n . The size-scale dependence of physical parameters is discussed, and such size-scale dependent parameters as u ̈ max and apparent fracture energy are scaled to the breakdown zone size regarded as a characteristic length, to extend the results obtained in the laboratory to an earthquake failure in the earth.

Machine stiffness appropriate for experimental simulation of earthquake processes

Loading machine stiffness is considered in an attempt to simulate strain accumulation processes prior to failure in earthquakes. We impose the similarity condition that the inelastic deformation in a test specimen relative to the loading machine corresponds with that in a spherical seismic region relative to the surrounding earth. From this constraint the appropriate ratio between the stiffnesses of machine and specimen is determined to be 1.5 ∼ 2.3. Preliminary experiments were carried out for sandstone samples. A qualitative difference between acoustic emission sequences was observed depending on whether the conditions were similar or dissimilar to the earth based on the scaling assumption. In the former case, a subsidence in acoustic emission activity prior to the main fractures was clearly and systematically observed.

Seismic radiation from the sudden creation of a spherical cavity in an arbitrarily prestressed elastic medium

Summary. We solve the general problem of seismic radiation from the sudden creation of a spherical cavity in an arbitrarily prestressed elastic medium. This problem has direct application to tectonic release due to the creation of a shatter zone by large underground explosions. In addition, however, the problem has essential features in common with the more general earthquake source. Specifically, it can provide an understanding of how an inhomogeneous prestress can affect radiated seismic energy from a tectonic source. The problem is solved through the use of a Green’s tensor integral equation in which all quantities are expressed in terms of vector spherical harmonics. We obtain an exact solution and an approximate solution to this problem. The approximate solution may be useful for the study of other failure geometries. The results agree with previous solutions for the case of pure shear. The case of localized inhomogeneous prestress is examined in detail. The primary result of the inhomogeneous prestress is the addition of considerable energy at frequencies above the usual corner frequency. This causes an increase in the corner frequency, a change in the slope of the spectrum near the corner frequency and in some cases a strong, low frequency, far field spectral peak. Peaked spectra always exist, however, near the usual quadrupole nodes. Further, the angular distribution of radiation in general will not be pure quadrupole in nature. As an example of a strongly inhomogeneous prestress case, stress concentrations near the source due to a point dislocation and a point centre of compression are considered. The radiated waveforms and spectra then vary greatly with angle with the spectra peaked strongly at all azimuths in some cases. For these cases the angular distribution of first motions is still relatively simple, but can be deceptive if used lo determine a focal mechanism. Because of the canonical relation of this problem to the more general earthquake failure problem, it is to be expected that similar stress concentration effects will occur for earthquakes. Thus, similar changes in corner frequency due to stress concentration effects could lead to errors in earthquake source dimension estimates and the related existence of spectral peaks could lead to errors in seismic moment measurements.