Andrew J. Michael

Seismicity Remotely Triggered by the Magnitude 7.3 Landers, California, Earthquake

The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

Three-dimensional velocity structure, seismicity, and fault structure in the Parkfield Region, central California

This study examines the three-dimensional velocity structure in a 60- by 80-km region containing the Parkfield segment of the San Andreas fault. We use local earthquake and shot P arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters. Using the three-dimensional model, we relocated 5251 events that occurred from 1969 to 1991, as well as the 1966 aftershocks, and computed 664 fault plane solutions. The San Andreas fault (SAF), characterized by a sharp across-fault velocity gradient, is the primary feature in the velocity solution. There is a 5–20% lateral change in velocity over a 4-km width, the contrast being sharper where there is better resolution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. The largest across fault velocity difference is below Middle Mountain, where a large volume of low-velocity material impinges on the SAF from the northeast. This material is inferred to be overpressured and may be key to understanding the unusual behavior in the Parkfield preparation zone. A 20-km-long high-velocity slice is imaged northeast of the SAF near Gold Hill. Its along-fault length corresponds to the length of the maximum slip in 1966. The relocated seismicity shows that the San Andreas fault is a planar vertical fault zone at seismogenic depths. Ninety percent of the fault plane solutions that are on, or near, the SAF were right-lateral strike-slip on subvertical fault planes that parallel the SAF. Thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at Parkfield. At Parkfield, variations in material properties play a key role in fault segmentation and deformation style. Our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure.

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.

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.

Relations among fault behavior, subsurface geology, and three-dimensional velocity models.

The development of three-dimensional P-wave velocity models for the regions surrounding five large earthquakes in California has lead to the recognition of relations among fault behavior and the material properties of the rocks that contact the fault at seismogenic depths; regions of high moment release appear to correlate with high seismic velocities whereas rupture initiation or termination may be associated with lower seismic velocities. These relations point toward a physical understanding of why faults are divided into segments that can fail independently, an understanding that could improve our ability to predict earthquakes and strong ground motion.

Seismotectonics of the Loma Prieta, California, region determined from three‐dimensional V p , V p / V s , and seismicity

Three-dimensional Vp and Vp/Vs velocity models for the Loma Prieta region were developed from the inversion of local travel time data (21,925 P arrivals and 1,116 S arrivals) from earthquakes, refraction shots, and blasts recorded on 1700 stations from the Northern California Seismic Network and numerous portable seismograph deployments. The velocity and density models and microearthquake hypocenters reveal a complex structure that includes a San Andreas fault extending to the base of the seismogenic layer. A body with high Vp extends the length of the rupture and fills the 5 km wide volume between the Loma Prieta mainshock rupture and the San Andreas and Sargent faults. We suggest that this body controls both the pattern of background seismicity on the San Andreas and Sargent faults and the extent of rupture during the mainshock, thus explaining how the background seismicity outlined the along-strike and depth extent of the mainshock rupture on a different fault plane 5 km away. New aftershock focal mechanisms, based on three-dimensional ray tracing through the velocity model, support a heterogeneous postseismic stress field and can not resolve a uniform fault normal compression. The subvertical (or steeply dipping) San Andreas fault and the fault surfaces that ruptured in the 1989 Loma Prieta earthquake are both parts of the San Andreas fault zone and this section of the fault zone does not have a single type of characteristic event.

Long-Term Time-Dependent Probabilities for the Third Uniform California Earthquake Rupture Forecast (UCERF3)

The 2014 Working Group on California Earthquake Probabilities (WGCEP 2014) presents time-dependent earthquake probabilities for the third Uniform California Earthquake Rupture Forecast (UCERF3). Building on the UCERF3 time-in- dependent model published previously, renewal models are utilized to represent elastic- rebound-implied probabilities. A new methodology has been developed that solves applicability issues in the previous approach for unsegmented models. The new meth- odology also supports magnitude-dependent aperiodicity and accounts for the historic open interval on faults that lack a date-of-last-event constraint. Epistemic uncertainties are represented with a logic tree, producing 5760 different forecasts. Results for a variety of evaluation metrics are presented, including logic-tree sensitivity analyses and comparisons to the previous model (UCERF2). For 30 yr M ! 6:7 probabilities, the most significant changes from UCERF2 are a threefold increase on the Calaveras fault and a threefold decrease on the San Jacinto fault. Such changes are due mostly to differences in the time-independent models (e.g., fault-slip rates), with relaxation of segmentation and inclusion of multifault ruptures being particularly influential. In fact, some UCERF2 faults were simply too long to produce M 6.7 size events given the segmentation assumptions in that study. Probability model differences are also influential, with the implied gains (relative to a Poisson model) being generally higher in UCERF3. Accounting for the historic open interval is one reason. Another is an effective 27% increase in the total elastic-rebound-model weight. The exact factors influencing differences between UCERF2 and UCERF3, as well as the relative im- portance of logic-tree branches, vary throughout the region and depend on the evalu- ation metric of interest. For example, M ! 6:7 probabilities may not be a good proxy for other hazard or loss measures. This sensitivity, coupled with the approximate nature of the model and known limitations, means the applicability of UCERF3 should be evaluated on a case-by-case basis.

The 1987 Whittier Narrows Earthquake in the Los Angeles Metropolitan Area, California

The Whittier Narrows earthquake sequence (local magnitude, ML = 5.9), which caused over

Seismic Velocity Structure and Seismotectonics of the Eastern San Francisco Bay Region, California

358-million damage, indicates that assessments of earthquake hazards in the Los Angeles metropolitan area may be underestimated. The sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east-west length of the northern margin of the Los Angeles basin. Peak horizontal accelerations from the main shock, which were measured at ground level and in structures, were as high as 0.6g (where g is the acceleration of gravity at sea level) within 50 kilometers of the epicenter. The distribution of the modified Mercalli intensity VII reflects a broad north-south elongated zone of damage that is approximately centered on the main shock epicenter.

Subsurface Structure and Kinematics of the Calaveras–Hayward Fault Stepover from Three-Dimensional Vp and Seismicity, San Francisco Bay Region, California

The Hayward Fault System is considered the most likely fault system in the San Francisco Bay Area, California, to produce a major earthquake in the next 30 years. To better understand this fault system, we use microseismicity to study its structure and kinematics. We present a new 3D seismic-velocity model for the eastern San Francisco Bay region, using microseismicity and controlled sources, which reveals a ∼10% velocity contrast across the Hayward fault in the upper 10 km, with higher velocity in the Franciscan Complex to the west relative to the Great Valley Sequence to the east. This contrast is imaged more sharply in our localized model than in previous regional-scale models. Thick Cenozoic sedimentary basins, such as the Livermore basin, which may experience particularly strong shaking during an earthquake, are imaged in the model. The accurate earthquake locations and focal mechanisms obtained by using the 3D model allow us to study fault complexity and its implications for seismic hazard. The relocated hypocenters along the Hayward Fault in general are consistent with a near-vertical or steeply east-dipping fault zone. The southern Hayward fault merges smoothly with the Calaveras fault at depth, suggesting that large earthquakes may rupture across both faults. The use of the 3D velocity model reveals that most earthquakes along the Hayward fault have near-vertical strike-slip focal mechanisms, consistent with the large-scale orientation and sense of slip of the fault, with no evidence for zones of complex fracturing acting as barriers to earthquake rupture. Online material: Velocity model validation experiments and additional seismicity plots.