Jeanne L. Hardebeck

The tectonic history of the Tasman Sea: A puzzle with 13 pieces

We present a new model for the tectonic evolution of the Tasman Sea based on dense satellite altimetry data and a new shipboard data set. We utilized a combined set of revised magnetic anomaly and fracture zone interpretations to calculate relative motions and their uncertainties between the Australian and the Lord Howe Rise plates from 73.6 Ma to 52 Ma when spreading ceased. From chron 31 (67.7 Ma) to chron 29 (64.0 Ma) the model implies, transpression between the Chesterfield and the Marion plateaus, followed by strike-slip motion. This transpression may have been responsible for the formation of the Capricorn Basin south of the Marion Plateau. Another major tectonic event took place at chron 27 (61.2 Ma), when a counterclockwise change in spreading direction occurred, contemporaneous with a similar event in the southwest Pacific Ocean. The early opening of the Tasman Sea cannot be modeled by a simple two-plate system because (1) rifting in this basin propagated from south to north in several stages and (2) several rifts failed. We identified 13 continental blocks which acted as microplates between 90 Ma and 64 Ma. Our model is constrained by tectonic lineaments visible in the gravity anomaly grid and interpreted as strike-slip faults, by magnetic anomaly, bathymetry and seismic data, and in case of the South Tasman Rise, by the age and affinity of dredged rocks. By combining all this information we derived finite rotations that describe the dispersal of these tectonic elements during the early opening of the Tasman Sea.

A New Method for Determining First-Motion Focal Mechanisms

We introduce a new method for determining earthquake focal mecha- nisms from P-wave first-motion polarities. Our technique differs from previous meth- ods in that it accounts for possible errors in the assumed earthquake location and seismic-velocity model, as well as in the polarity observations. The set of acceptable focal mechanisms, allowing for the expected errors in polarities and takeoff angles, is found for each event. Multiple trials are performed with different source locations and velocity models, and mechanisms with up to a specified fraction of misfit po- larities are included in the set of acceptable mechanisms. The average of the set is returned as the preferred mechanism, and the uncertainty is represented by the dis- tribution of acceptable mechanisms. The solution is considered adequately stable only if the set of acceptable mechanisms is tightly clustered around the preferred mechanism. We validate the method by demonstrating that the well-constrained mechanisms found for clusters of closely spaced events with similar waveforms are indeed very similar. Tests on noisy synthetic data, which mimic the event and station coverage of real data, show that the method accurately recovers the mechanisms and that the uncertainty estimates are reasonable. We also investigate the sensitivity of focal mechanisms to changes in polarities, event depth, and seismic-velocity model, and we find that mechanisms are most sensitive to changes in the vertical velocity gradient.

The static stress change triggering model: Constraints from two southern California aftershock sequences

Static stress change has been proposed as a mechanism of earthquake triggering. We quantitatively evaluate this model for the apparent triggering of aftershocks by the 1992 M_W 7.3 Landers and 1994 M_W 6.7 Northridge earthquakes. Specifically, we test whether the fraction of aftershocks consistent with static stress change triggering is greater than the fraction of random events which would appear consistent by chance. Although static stress changes appear useful in explaining the triggering of some aftershocks, the models capability to explain aftershock occurrence varies significantly between sequences. The model works well for Landers aftershocks. Approximately 85% of events between 5 and 75 km distance from the mainshock fault plane are consistent with static stress change triggering, compared to ∼50% of random events. The minimum distance is probably controlled by limitations of the modeling, while the maximum distance may be because static stress changes of <0.01 MPa trigger too few events to be detected. The static stress change triggering model, however, can not explain the first month of the Northridge aftershock sequence significantly better than it explains a set of random events. The difference between the Landers and Northridge sequences may result from differences in fault strength, with static stress changes being a more significant fraction of the failure stress of weak Landers-area faults. Tectonic regime, regional stress levels, and fault strength may need to be incorporated into the static stress change triggering model before it can be used reliably for seismic hazard assessment.

Implications for prediction and hazard assessment from the 2004 Parkfield earthquake

Obtaining high-quality measurements close to a large earthquake is not easy: one has to be in the right place at the right time with the right instruments. Such a convergence happened, for the first time, when the 28 September 2004 Parkfield, California, earthquake occurred on the San Andreas fault in the middle of a dense network of instruments designed to record it. The resulting data reveal aspects of the earthquake process never before seen. Here we show what these data, when combined with data from earlier Parkfield earthquakes, tell us about earthquake physics and earthquake prediction. The 2004 Parkfield earthquake, with its lack of obvious precursors, demonstrates that reliable short-term earthquake prediction still is not achievable. To reduce the societal impact of earthquakes now, we should focus on developing the next generation of models that can provide better predictions of the strength and location of damaging ground shaking.

Crustal stress field in southern California and its implications for fault mechanics

We present a new, high spatial resolution image of stress orientation in southern California based on the inversion of earthquake focal mechanisms. We use this image to study the mechanics of faulting in the plate boundary region. The stress field contains significant spatial heterogeneity, which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause. Temporal changes in the stress field are also observed, primarily related to major earthquakes. The observed 15° (±10°) rotation of the stress axes due to the 1992 M7.3 Landers mainshock implies that the deviatoric stress magnitude in the crust is low, of the order of 10 MPa. This suggests that active faults in southern California are weak. The maximum principal stress axis near the San Andreas Fault is often at ∼50° to the fault strike, indicating that the shear stress on the fault is comparable to the deviatoric stress. The San Andreas in southern California may therefore be a weak fault in a low-strength crust.

Three-dimensional compressional wavespeed model, earthquake relocations, and focal mechanisms for the Parkfield, California, region

We present a new three-dimensional (3D) compressional wavespeed ( V p) model for the Parkfield region, taking advantage of the recent seismicity associated with the 2003 San Simeon and 2004 Parkfield earthquake sequences to provide increased model resolution compared to the work of Eberhart-Phillips and Michael (1993) (epm93). Taking the epm93 3D model as our starting model, we invert the arrival-time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast–southwest by 120 km northwest–southeast. We include catalog picks and cross-correlation and catalog differential times in the inversion, using the double-difference tomography method of Zhang and Thurber (2003). The principal V p features reported by epm93 and Michelini and McEvilly (1991) are recovered, but with locally improved resolution along the San Andreas Fault (saf) and near the active-source profiles. We image the previously identified strong wavespeed contrast (faster on the southwest side) across most of the length of the saf, and we also improve the image of a high V p body on the northeast side of the fault reported by epm93. This narrow body is at about 5- to 12-km depth and extends approximately from the locked section of the saf to the town of Parkfield. The footwall of the thrust fault responsible for the 1983 Coalinga earthquake is imaged as a northeast-dipping high wavespeed body. In between, relatively low wavespeeds (<5 km/sec) extend to as much as 10-km depth. We use this model to derive absolute locations for about 16,000 earthquakes from 1966 to 2005 and high-precision double-difference locations for 9,000 earthquakes from 1984 to 2005, and also to determine focal mechanisms for 446 earthquakes. These earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure. The aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre-2004 seismicity, confirming earlier observations (Waldhauser et al. , 2004) that the seismicity pattern at Parkfield is long lived and persists through multiple cycles of mainshocks. Online material : 3D V p model and earthquake relocations.

Using S/P Amplitude Ratios to Constrain the Focal Mechanisms of Small Earthquakes

We test whether S-wave/P-wave amplitude ratio data can improve the computed focal mechanisms of small earthquakes, using events from two southern California aftershock sequences. The observed S/P ratios are generally consistent with the expected mechanisms, implying that S/P ratios can in fact be useful in constraining the focal mechanisms of small events. However, we also find that noise in the observations leads to scatter in the S/P ratios of factors of 2, and sometimes higher. This scatter limits the usefulness of the S/P ratios in two ways: (1) the focal mechanism cannot simply be fit to S/P amplitude data alone without accounting for the noise in a more sophisticated focal mechanism inversion process; (2) while the amplitude ratios may improve poorly constrained mechanisms, they are less useful in refining solutions that are already relatively well constrained.

Stress Orientations Obtained from Earthquake Focal Mechanisms: What Are Appropriate Uncertainty Estimates?

Crustal stress orientations provide important information about the mechanics of regional deformation. Numerous methods exist for inverting earthquake focal mechanisms for stress orientation, and the more widely used methods usually obtain similar results for similar data sets. However, error estimates are highly variable, complicating the interpretation of results. The southern California stress field, for example, contains much statistically significant spatial and temporal variability according to the error estimates of one method (Michael, 1984, 1987b), but very little according to those of another (Gephart and Forsyth, 1984). To resolve whether the southern California stress field is generally homogeneous or heterogeneous, we must determine which of the error estimates best reflects the true inversion uncertainty. To do this, we tested both methods on a suite of synthetic focal mechanism data sets containing random errors. The method of Gephart and Forsyth (1984) usually provides more accurate estimates of stress orientation, especially for high-quality data sets, but its confidence regions are in most cases too large. The method of Michael (1984, 1987b) is more accurate for very noisy data sets and provides a more appropriate estimate of uncertainty, implying that the stress field in southern California is probably heterogeneous.

A California Statewide Three-Dimensional Seismic Velocity Model from Both Absolute and Differential Times

We obtain a seismic velocity model of the California crust and uppermost mantle using a regional-scale double-difference tomography algorithm. We begin by using absolute arrival-time picks to solve for a coarse three-dimensional (3D) P ve- locity (VP) model with a uniform 30 km horizontal node spacing, which we then use as the starting model for a finer-scale inversion using double-difference tomography applied to absolute and differential pick times. For computational reasons, we split the state into 5 subregions with a grid spacing of 10 to 20 km and assemble our final statewide VP model by stitching together these local models. We also solve for a state- wide S-wave model using S picks from both the Southern California Seismic Network and USArray, assuming a starting model based on the VP results and a VP=VS ratio of 1.732. Our new model has improved areal coverage compared with previous models, extending570kmintheSW-NEdirectionand1320kmintheNW-SEdirection.Italso extends to greater depth due to the inclusion of substantial data at large epicentral distances. Our VP model generally agrees with previous separate regional models for northern and southern California, but we also observe some new features, such as high-velocity anomalies at shallow depths in the Klamath Mountains and Mount Shasta area, somewhat slow velocities in the northern Coast Ranges, and slow anoma- lies beneath the Sierra Nevada at midcrustal and greater depths. This model can be applied to a variety of regional-scale studies in California, such as developing a unified statewide earthquake location catalog and performing regional waveform modeling. Online Material: Smoothing and damping trade-off analysis, a priori Moho depth, resolution tests, and map-view slices and cross sections through the 3D VP and VS models.

Homogeneity of Small-Scale Earthquake Faulting, Stress, and Fault Strength

Small-scale faulting at seismogenic depths in the crust appears to be more homogeneous than previously thought. I study three new high-quality focal- mechanism datasets of small ( M < ∼3) earthquakes in southern California, the east San Francisco Bay, and the aftershock sequence of the 1989 Loma Prieta earthquake. I quantify the degree of mechanism variability on a range of length scales by comparing the hypocentral distance between every pair of events and the angular difference between their focal mechanisms. Closely spaced earthquakes (interhypocentral distance <∼2 km) tend to have very similar focal mechanisms, often identical to within the 1-sigma uncertainty of ∼25°. This observed similarity implies that in small volumes of crust, while faults of many orientations may or may not be present, only similarly oriented fault planes produce earthquakes contemporaneously. On these short length scales, the crustal stress orientation and fault strength (coefficient of friction) are inferred to be homogeneous as well, to produce such similar earthquakes. Over larger length scales (∼2–50 km), focal mechanisms become more diverse with increasing interhypocentral distance (differing on average by 40–70°). Mechanism variability on ∼2- to 50-km length scales can be explained by relatively small variations (∼30%) in stress or fault strength. It is possible that most of this small apparent heterogeneity in stress or strength comes from measurement error in the focal mechanisms, as negligible variation in stress or fault strength (<10%) is needed if each earthquake is assigned the optimally oriented focal mechanism within the 1- sigma confidence region. This local homogeneity in stress orientation and fault strength is encouraging, implying it may be possible to measure these parameters with enough precision to be useful in studying and modeling large earthquakes.