Underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding along the plate interface. We have determined the depth of this stability transition for the circurn‐Pacific subduction zones of: Honshu, Kuriles, Kamchatka, Aleutians, Alaska, Mexico, and Chile. These subduction zones have experienced great interplate earthquakes and the aftershock regions are well‐located. Depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling. For an average P wave velocity of 6.7 km s−1 above the plate interface, we find that for most subduction zones the stability transition occurs at 40 ± 5 km depth. There are, however, several exceptions. At the Hokkaido trench junction, where the Japan trench and the Kurile trench intersect, seismic coupling is deep and extends down to 52–55 km. Deep coupling was also found in the Coquimbo region in central Chile. The Mexico subduction zone has shallow coupling: the transition occurs at 20–30 km depth. Previous studies of micro‐earthquakes in Honshu, Hokkaido, the Aleutians, and Alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find. Given our measurements of seismic coupling depth, we then explore the mechanism that may determine coupling depth. The concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments, thus we first test whether a critical temperature can explain our results. Temperatures at the plate interface are dependent on many variables; but two that are poorly determined are shear stress and radiogenic heat generation. Shear stress has been constrained by inversion of heat flow data. Assuming a crustal radiogenic heat production rate of 3.1 exp−z/8.5 μWm−3 and a constant coefficient of friction, we find two critical temperatures of about 400 ° C and 550 ° C. The lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust. On the other hand, one single critical temperature of about 250 ° C can explain the coupling depths if shear stresses are constant with depth.

Underthrusting at subduction zones can cause large earthquakes at shallow depths but is accommodated by aseismic creep below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding. We have determined the maximum depth of the coupled zone and its variability along the Chilean subduction zone. The maximum depth of seismic coupling is defined by the depth of large (M > 6) underthrusting earthquakes that have occurred at the downdip edge of the coupled plate interface. Earthquake depth is determined with omnilinear waveform inversion of longperiod P waves. The statistical uncertainty in the depth is estimated using bootstrapping. Omnilinear inversion formally accounts for the scaling incompatibility between the P waveforms and decreases the uncertainty in the depth estimate. We have found the depths of 27 earthquakes in the time period from 1961 to 1987. Seismic coupling in Chile extends down to 48-53 km. There is a resolvable change in the maximum depth of coupling around 28øS. The region immediately north of this latitude has a coupled zone that extends no deeper than 36-41 km, while south of this latitude coupling extends down to 48-53 km. This transition is not simply related to the oceanic lithospheric age and plate convergence rate but coincides with a change in several geophysical and geological phenomena. With respect to shallower (north) and deeper (south) coupling there is a change from active volcanos to no volcanos, the dip of the deep slab changes from steep to shallow, the thickness of trench sediments changes from thin to thick, and the depth of the oceanic basement changes from deeper to shallower at 28øS. Due to the multiplicity of changes at 28øS there is no clear unique interpretation of what physical mechanism controls the variability in the maximum depth of the seismically coupled zone in Chile. of a set of azimuthally well-distributed seismograms. To assess the statistical uncertainty of the depth estimate, we use a resampling technique called "bootstrapping" [Efron, 1979, 1982]. This allows us to estimate a central confidence interval, even though focal depth is a complicated, nonlinear parameter in waveform inversion. We determined focal depth and its 80% central confidence interval for 27 interplate earthquakes along the Chilean subruction zone. These 27 events span the time interval from 1962 to 1987 and are those earthquakes with M > 6 that are farthest downdip from the trench axis and thus can be used for a first-order mapping of the maximum depth of the coupled zone. The results show that coupling in the Chilean subruction zone does not extend deeper than 48-53 km. Furthermore, we find resolvable differences in the maximum depth of coupling along the subruction zone. The seismically coupled zone is the depth range on the plate interface that is capable of producing an underthrusting 11,997 11,998 TICItELAAR AN•> Ru•: SEISMIC COUPLING ALONG CHILE ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß Yoshii, T., A detailed cross-se...

Many techniques have been developed to extract a model from data. In general, these techniques are based on minimization of the misfit between measured data and predicted “data.” The model is connected to the predicted “data” by a physical theory. To know how good the model is, one must evaluate model variance. Since the data variance, or alternatively the misfit, is generally nonzero, model variance is generally nonzero. In many cases, the model is a linear function of the data, and model variance can be estimated by formally mapping the data variance to model space [e.g., Menke, 1984].

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The fault plane and overall coseismic slip of the 1989 Loma Prieta, California, earthquake (Ms=7.1) are well determined [Plafker and Galloway, 1989]. Teleseismic waves can be used to determine the time history of moment release. We invert a data set of ten broadband P and SH waves for the most general point source description: the five moment tensor rate functions. The linear inversion also provides formal estimates of model uncertainty. While the moment tensor rate functions suggest a different focal mechanism for the first few seconds of the rupture process, it is not statistically significant at the 95% level. We can thus proceed to invert for one single time function (the source time function), and five scalars (the moment tensor). The major double couple that we find (strike 138°±6° clockwise from North, dip 76°±5°, slip angle 120°±10°) agrees with the results of Plafker and Galloway [1989]. The minor double couple is small (1%). The best point source depth is about 10 km. Several broadband P waves show that a small precursor occurred a few seconds before the main pulse of moment release. The duration of the source time function is 9 s, with a moment of 2±0.5 ×1019 Nm. A bilateral rupture with velocity of 2.8 km/s and 9 s duration encompasses the aftershock region.