Paul Somerville

THE CAPE MENDOCINO, CALIFORNIA, EARTHQUAKES OF APRIL 1992 : SUBDUCTION AT THE TRIPLE JUNCTION

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America—Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity.

Simulation of Near-Fault Strong-Ground Motion Using Hybrid Green's Functions

The recently proposed hybrid Greens function method is designed to combine the advantages of both deterministic and stochastic approaches to simulating broadband ground motion when records of small events are not available. The method has the flexibility of incorporating complexities in the source, wave path, and local- site effects into strong ground motion simulations. In this article we analyze its effectiveness at simulating near-fault ground motions by comparisons with the em- pirical source time function method, empirical ground-motion-attenuation relations, and recorded near-fault ground motion. We present a simple model for introducing the effect of the radiation pattern to the stochastic Greens functions in the inter- mediate frequency range (1-3 Hz). The numerical test results of the method and the generally good agreement between simulated and recorded ground motion from the 17 January 1995 Kobe earthquake shown in this study indicate that the technique has the capability of reproducing the main characteristics of near-fault ground mo- tion.

The influence of site conditions on the spatial incoherence of ground motions

Abstract We have found evidence for large differences in the spatial incoherence of ground motions under different site conditions. At sites located on flat-lying alluvial sequences, such as the Imperial Valley, California and Lotung, Taiwan, the spatial incoherence increases smoothly as a function of station separation and wave frequency. This uniform behavior is consistent with wave scattering in an otherwise laterally homogeneous seismic velocity structure. In contrast, on sites located on folded sedimentary rocks, such as the Coaling anticline in California, the spatial incoherence does not show a strong dependence on station separation and wave frequency, and is higher at close station spacing and low frequency than in flat-lying alluvial sites. This chaotic behavior is consistent with wave propagation in a medium having strong lateral heterogeneities in seismic velocity. We have modeled this behavior using synthetic seismograms calculated for a basin structure in which overlying sedimentary material is separated from basement rock by an undulating interface. Even simple undulations in a smooth interface produce caustics, giving rise to ground motions whose spatial incoherence characteristics resemble those of the Coalinga data. We conclude that the spatial incoherence measured on flat-lying sedimentary sites such as the Imperial Valley and Lotung may not provide a good description of the spatial incoherence at sites where there is significant lateral heterogeneity. These include folded sedimentary rocks exposed at the surface, basins generated by the folding of sedimentary rocks, and alluviated river valleys. At present, we do not have measurements of spatial incoherence at enough sites to know whether the spatial incoherence characteristics of ground motions can be readily correlated with simplified categories of site conditions. In order to obtain reliable estimates of the spatial incoherence of ground motion at a particular site, it may be necessary to use accelerograms of a small earthquake or explosion recorded on a small array at the site.

Engineering applications of strong ground motion simulation

Abstract The formulation, validation and application of a procedure for simulating strong ground motions for use in engineering practice are described. The procedure uses empirical source functions (derived from near-source strong motion recordings of small earthquakes) to provide a realistic representation of effects such as source radiation that are difficult to model at high frequencies due to their partly stochastic behavior. Wave propagation effects are modeled using simplified Greens functions that are designed to transfer empirical source functions from their recording sites to those required for use in simulations at a specific site. The procedure has been validated against strong motion recordings of both crustal and subduction earthquakes. For the validation process we choose earthquakes whose source models (including a spatially heterogeneous distribution of the slip of the fault) are independently known and which have abundant strong motion recordings. A quantitative measurement of the fit between the simulated and recorded motion in this validation process is used to estimate the modeling and random uncertainty associated with the simulation procedure. This modeling and random uncertainty is one part of the overall uncertainty in estimates of ground motions of future earthquakes at a specific site derived using the simulation procedure. The other contribution to uncertainty is that due to uncertainty in the source parameters of future earthquakes that affect the site, which is estimated from a suite of simulations generated by varying the source parameters over their ranges of uncertainty. In this paper, we describe the validation of the simulation procedure for crustal earthquakes against strong motion recordings of the 1989 Loma Prieta, California, earthquake, and for subduction earthquakes against the 1985 Michoacan, Mexico, and Valparaiso, Chile, earthquakes. We then show examples of the application of the simulation procedure to the estimatation of the design response spectra for crustal earthquakes at a power plant site in California and for subduction earthquakes in the Seattle-Portland region. We also demonstrate the use of simulation methods for modeling the attenuation of strong ground motion, and show evidence of the effect of critical reflections from the lower crust in causing the observed flattening of the attenuation of strong ground motion from the 1988 Saguenay, Quebec, and 1989 Loma Prieta earthquakes.

Physics-Based Earthquake Source Characterization and Modeling with Geostatistics

Abstract Physics-based ground-motion simulation requires the development of physically self-consistent source modeling tools to emulate the essential physics of earthquake rupture. Because of the high computational demand of full-dynamic rupture modeling, the kinematic description of earthquake source processes provides the most practical way of covering a wide range of rupture and wave propagation scenarios. We apply 2D spatial data analysis tools, commonly used in geostatistics, to characterizing earthquake rupture process and developing an effective source modeling tool for strong-motion prediction. The earthquake source process is described by key kinematic source parameters, such as static slip, rupture velocity, and slip duration. The heterogeneity of each source parameter is characterized with autocoherence while the linear dependency (coupling) between parameters is characterized with cross coherence. Both zero- and nonzero-offset spatial coherence can be considered in the form of cross coherence. We analyzed both synthetic and real dynamic rupture models to demonstrate the efficiency of these new techniques and found that many important features of earthquake rupture can be captured in this way, which may be difficult to analyze, or even detect by zero-offset coherence only. For instance, the correlation maximum between slip and rupture velocity can be shifted from the zero offset, that is, large slip may generate faster rupture velocity ahead of the current rupture front, which may be important for rupture directivity. We demonstrate that we can generate a number of realizations of earthquake source models to reproduce the target coherence using stochastic modeling techniques (e.g., sequential Gaussian simulation) once coherence structures in earthquake rupture are well understood. This type of coherence analysis may provide the potential for improved understanding of earthquake source characteristics and how they control the characteristics of near-fault strong ground motions.

Time-domain determination of earthquake fault parameters from short-period P-waves

abstract Source parameters of two shallow earthquakes have been determined by the time-domain analysis of short-period teleseismic recordings. For each event, the effect of the receiver crust was deconvolved from a set of globally distributed recordings using the homomorphic method. The resulting seismograms were compared with the form of the elastic-wave radiation computed from Savage9s model of radially spreading rupture on a plane elliptical fault surface. This time-domain approach has permitted the determination of several kinematic parameters pertaining to the dynamics of rupture that are not ordinarily evaluated from spectral analysis. These parameters are rupture velocity, the direction of farthest rupture propagation, and the duration of a ramp dislocation time function which was prescribed to be the same everywhere on the fault surface. The application of a general linear inverse scheme has shown that the model parameters (notably rupture velocity and dimension) are only weakly coupled. Inversion is also used to determine the range of acceptable parameter values and indicates the importance of array recordings in constraining the models. A consistent discrepancy between the observed and model seismograms during the first half-cycle of motion is attributed to the incorrect prescription of the dislocation time function. It is suggested that a space-dependent function determined theoretically by Kostrov in 1964 would tend to remove this discrepancy.

Moment Tensor and Rupture Model for the 1949 Olympia, Washington, Earthquake and Scaling Relations for Cascadia and Global Intraslab Earthquakes

We reanalyzed the 13 April 1949 Olympia, Washington, earthquake by using digitized records and first-motion polarities from long-period seismograms. The moment-tensor mechanism is normal faulting with a down-dip-trending T axis similar in style to other Cascadia intraslab earthquakes. The total seismic moment is 1.3 × 1026 dyne cm ( M w 6.7) and the hypocenter depth is 60 km. Additional inverse modeling for the kinematic rupture process assuming the steeply east-dipping fault plane from the moment tensor resulted in a slightly higher total moment of 1.9 × 1026 dyne cm ( M w 6.8). The earthquake ruptured to the south with at least two subevents. The combined area of asperities and seismic moment for the 1949 earthquake was compiled with those from the 1965 Seattle-Tacoma and the 2001 Nisqually earthquakes and with those from Japan and Mexico to develop a source-scaling relation separate from shallow global strike-slip earthquakes. We infer that deeper intraslab earthquakes have a significantly smaller combined area of asperities than those compiled for shallower strike-slip earthquakes with the same seismic moment. This difference in rupture area leads to a 3- to 5-fold increase in stress drop for earthquakes with seismic moments between 1024 and 1028 dyne cm.

The Whittier Narrows, California Earthquake of October 1, 1987—Simulation of Recorded Accelerations

We have simulated accelerograms from many of the strong motion stations close to the mainshock of the 1987 Whittier Narrows earthquake using a semi-empirical Greens function summation technique. This method allows gross aspects of the source rupture process to be treated deterministically using a kinematic model based on first motion studies, teleseismic modeling and the distribution of aftershocks. Stochastic aspects of the rupture process are then included to simulate irregularity in both rupture and slip velocity. Gross aspects of wave propagation are modeled using theoretical Greens functions calculated with generalized rays. Detailed aspects of the source radiation at high frequencies, as well as unmodeled propagational aspects such as scattering, are included empirically by using multiple recordings of a smaller Imperial Valley earthquake as empirical source functions. Our main objective is to see how well we can predict the peak ground accelerations, time histories and response spectra of ground motions of a moderate sized earthquake within the Los Angeles Basin having limited detailed source information. We find that the simulations predict the observations accurately enough to identify which phases and amplitudes in the observed data may be due to local site response rather than source or radiation effects. Comparisons between observed and simulated accelerograms for all the stations modeled are made using peak ground acceleration, and using time histories and response spectra for the stations that have been hand-digitized to date. The Bright Avenue Whittier station has the largest simulated peak acceleration, in agreement with the recorded peak acceleration data.

Estimating Subsurface Shear Velocity with Radial to Vertical Ratio of Local P Waves

Online Material: Code for computing V S by the r/z ratio method; seismogram plots. In contrast to the western United States where seismicity is high and seismic networks are dense, the central and eastern United States (CEUS) is covered with sparse seismic networks. The relatively low seismicity in the CEUS has also led to a limited dataset of waveform data from local earthquakes, hindering the development of reliable ground‐motion prediction equations in the CEUS. This situation has been changing with the Earthscope Transportable Array (USArray) program that started installing about 1000 broadband and short‐period stations in the CEUS starting in 2009, and will fully cover the CEUS soon. Such large numbers of stations provide a rich dataset for ground‐motion studies in the United States. Also, the increasing number of Advanced National Seismic System (ANSS) broadband stations with high‐sampling rate substantially augments the waveform data. Site responses at each seismic station need to be obtained before the waveform data can be used for ground‐motion studies. For example, Boore (2003) and Atkinson and Boore (2006) find that site amplification needs to be taken into account for ground‐motion modeling in eastern North America, especially for high frequencies. Site response is controlled by the subsurface shear‐velocity profile, and the velocity structure of the top ten to hundreds of meters is particularly important (Wald and Mori, 2000; Boore, 2006). Therefore an essential part of site‐response modeling is determining subsurface velocity structure. A detailed review of methods for determining subsurface velocity structure was made by Boore (2006). Theoretically, invasive methods such as borehole logging provide the most accurate measurement of velocity structure, but their cost prohibits extensive application in site‐response studies. Noninvasive geophysical exploration methods are more often used in determining shallow velocity structure, and these include spectral analysis of surface waves (SASW), multichannel …

Ground-Motion Attenuation from the 1995 Kobe Earthquake Based on Simulations Using the Hybrid Green's Function Method

Because of the limited number of strong-motion stations in the Kobe area at the time of the 1995 Kobe earthquake, information about the characteristics of the near-fault ground-motion acceleration in bedrock is sparse. In this study we estimated the near-fault ground motion and derived characteristics of its attenuation on rock, using an hybrid broadband technique and a source model that have been validated against data. We found that at high frequencies the near-fault ground motion produced by the Kobe earthquake was of the same level as that predicted by the empirical attenuation relation for Japanese crustal earthquakes. The areas with the largest peak horizontal acceleration are located at the extremities of the fault and include most of the Kobe city.