Southern Surface Rupture Associated with the M 7.3 1992 Landers, California, Earthquake

Although most evidence suggests that the 28 June 1992 M 7.3 Landers earthquake ruptured unilaterally north, significant surface rupture was mapped on the Eureka Peak and Burnt Mountain faults, to the south of the Landers epicenter. An eyewitness account reports that surface rupture occurred on the northern Eureka Peak fault within approximately 35 sec of the mainshock initiation. Array analysis of the Landers mainshock provides evidence in support of this report; a significant southern subevent in the early mainshock coda. I also analyze dense array recordings of a M 5.6 aftershock that occurred 3 min after the mainshock at 34°7.65′N, 116°23.82′W and show that there is strong evidence that this event was also associated with significant rupture on the Eureka Peak fault. This analysis thus suggests that the Eureka Peak fault rupture was not caused by direct bilateral mainshock rupture but instead was initially triggered less than a minute after the mainshock and reruptured by the M 5.6 aftershock. Results for the evolution of the Landers sequence suggest that mainshock subevents may in some cases be accurately described as aftershocks (i.e., disjoint triggered events) that occur within the duration of mainshock strong ground motion

Empirical Green’s Function Analysis of Recent Moderate Events in California

I use seismic data from portable digital stations and the broadband Terrascope network in southern California to investigate radiated earthquake source spectra and discuss the results in light of previous studies on both static stress drop and apparent stress. Applying the empirical Green's function (EGF) method to two sets of M 4–6.1 events, I obtain deconvolved source-spectra estimates and corner frequencies. The results are consistent with an ω^2 source model and constant Brune stress drop. However, consideration of the raw spectral shapes of the largest events provides evidence for a high-frequency decay more shallow than ω^2. The intermediate (≈f^(–1)) slope cannot be explained plausibly with attenuation or site effects and is qualitatively consistent with a model incorporating directivity effects and a fractional stress-drop rupture process, as suggested by Haddon (1996). However, the results obtained in this study are not consistent with the model of Haddon (1996) in that the intermediate slope is not revealed with EGF analysis. This could reflect either bandwidth limitations inherent in EGF analysis or perhaps a rupture process that is not self-similar. I show that a model with an intermediate spectral decay can also reconcile the apparent discrepancy between the scaling of static stress drop and that of apparent stress drop for moderate-to-large events

Scientific overview and historical context of the 1811-1812 New Madrid earthquake sequence

The central and eastern United States has experienced only 5 historic earthquakes with Mw 7.0, four during the New Madrid sequence of 1811-1812: three principal mainshocks and the so-called «dawn aftershock» following the first mainshock. Much of the historic earthquake research done in the United States has focused on the New Madrid Seismic Zone (NMSZ), because the largest New Madrid earthquakes may represent the archetype for the most damaging earthquakes to be expected in intraplate regions. Published magnitude values ranging from 7.0 to 8.75 have generally been based on macroseismic effects, which provide the most direct constraint on source size for the events. Critical to the interpretation of these accounts is an understanding of their historic context. Early settlments clustered along waterways, where substantial amplification of seismic waves is expected. Analyzing the New Madrid intensity values with a consideration of these effects yields preferred values of Mw 7.2-7.3, 7.0, and 7.4-7.5 for the December, January, and February mainshocks, respectively, and of 7.0 for the «dawn aftershock». These values are consistent with other lines of evidence, including scaling relationships. Finally, I show that accounts from the New Madrid sequence reveal evidence for remotely triggered earthquakes well outside the NMSZ. Remotely triggered earthquakes represent a potentially important new wrinkle in historic earthquake research, as their ground motions can sometimes be confused with mainshock ground motions

Source parameters of the 23 April 1992 M 6.1 Joshua Tree, California, earthquake and its aftershocks: Empirical Green's function analysis of GEOS and TERRAscope data

Source parameters of the M 6.1 23 April 1992 Joshua Tree mainshock and 86 M 1.8 to 4.9 aftershocks are determined using an empirical Green's function methodology. For the aftershocks, deconvolved P- and S-wave spectra are calculated for 126 pairs of closely spaced events recorded on portable GEOS stations; S-wave spectra from the two horizontal components are averaged. The deconvolved spectra are fit by a ratio of omega-square source models, yielding an optimal (least-squares) corner frequency for both the large and the small event in each pair. We find no resolved difference between the inferred P- and S-wave corner frequencies. Using the standard Brune model for stress drop, we also find no resolved nonconstant scaling of stress drop with moment, although we also conclude that detailed scaling systematics would be difficult to resolve. In particular, a weak increase of stress drop with moment over a limited moment/magnitude cannot be ruled out. For magnitudes smaller than M 3 to 3.5, the inferred stress-drop values will be limited by the maximum observable corner frequency value of 60 Hz. For the mainshock, source-time functions are obtained from mainshock recordings at three TERRAscope stations (PFO, PAS, and GSC) using an M 4.3 foreshock as an empirical Green's function. The results indicate a fairly simple, single-pulse source-time function, with clear south-to-north directivity and an inferred rupture radius of 5 to 6 km. The deconvolved source-time functions are inverted to obtain a finite-rupture model that gives a robust estimate of rupture dimension. Early aftershocks are found to lie along the perimeters of regions with high mainshock slip. The inferred mainshock stress-drop value, 56 bars, is within the range determined for the aftershocks. Our derived mainshock source spectra do not show resolvable deviation from the omega-square model

On the Coherence of Ground Motion in the San Fernando Valley

We present an analysis of the coherence of seismic ground motion recorded on alluvial sediments in the San Fernando Valley, California. Using aftershocks of the 17 January 1994 M_w6.7 earthquake recorded at a quasi-dense array of portable stations, we analyze the coherence of three well-recorded magnitude 3.7 to 4.0 events over the frequency range 0.5 to 15 Hz and a distance range of 0.5 to 5.3 km. All stations are located at sites with broadly similar near-site geology, characterized by medium to fine-grain Quaternary alluvial sediments. On average, relatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 Hz; coherence drops sharply at frequencies near and above 3 Hz. Although average coherence functions are described reasonably well by a log-linear relationship with frequency, the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at hardrock sites. The distance decay of coherence is also markedly less strong, with high coherence values observed over station separations corresponding to multiple wavelengths. This may reflect fundamental differences in shallow-wave propagation in the two environments, with high-frequency scattering relatively more dominant in regions of hard-rock near-surface geology. Within a sedimentary basin or valley, the site response itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions. However, previous studies have demonstrated the existence of pathological focusing and amplification effects within complex sedimentary basin environments such as the greater Los Angeles region; our results undoubtedly do not quantify the full range of ground-motion variability at all sites, but rather represent the level of that variability that can be expected, and quantified, for typical source/receiver paths

The 1998 Earthquake Sequence South of Long Valley Caldera, California: Hints of Magmatic Involvement

A significant episode of seismic and geodetic unrest took place at Long Valley Caldera, California, beginning in the summer of 1997. Activity through late May of 1998 was concentrated in and around the south moat and the south margin of the resurgent dome. The Sierran Nevada block (SNB) region to the south/southeast remained relatively quiet until a M 5.1 event occurred there on 9 June 1998 (UT). A second M 5.1 event followed on 15 July (UT); both events were followed by appreciable aftershock sequences. An additional, distinct burst of activity began on 1 August 1998. The number of events in the August sequence (over the first week or two) was similar to the aftershock sequence of the 15 July 1998 M 5.1 event, but the later sequence was not associated with any events larger than M 4.3. All of the summer 1998 SNB activity was considered tectonic rather than magmatic; in general the SNB is considered an unlikely location for future eruptions. However, the August sequence—an “aftershock sequence without a mainshock”—is suggestive of a strain event larger than the cumulative seismotectonic strain release. Moreover, a careful examination of waveforms from the August sequence reveals a small handful of events whose spectral signature is strikingly harmonic. We investigate the waveforms of these events using spectral, autocorrelation, and empirical Green's function techniques and conclude that they were most likely associated with a fluid-controlled source. Our observations suggest that there may have been some degree of magma or magma-derived fluid involvement in the 1998 SNB sequence

A Spectropolarimetric Atlas of Seyfert 1 Galaxies

We present optical spectropolarimetry of the nuclei of 36 Seyfert 1 galaxies, obtained with the William Herschel and the Anglo-Australian Telescopes from 1996 to 1999. In 20 of these, the optical emission from the active nucleus is intrinsically polarized. We have measured a significant level of polarization in a further 7 objects but these may be heavily contaminated by Galactic interstellar polarization. The intrinsically polarized Seyfert 1s exhibit a variety of characteristics, with the average polarization ranging from < 0.5 to 5 per cent and many showing variations in both the degree and position angle of polarization across the broad H alpha emission line. We identify a small group of Seyfert 1s that exhibit polarization properties similar to those of Seyfert 2 galaxies in which polarized broad-lines have been discovered. These objects represent direct observational evidence that a Seyfert 2-like far-field polar scattering region is also present in Seyfert 1s. Several other objects have features that can be explained in terms of equatorial scattering of line emission from a rotating disk. We propose that much of the diversity in the polarization properties of Seyfert galaxies can be understood in terms of a model involving both equatorial and polar scattering, the relative importance of the two geometries as sources of polarized light being determined principally by the inclination of the system axis to the line-of-sight.Comment: Accepted for publication in MNRAS (28 pages, 25 figures

On the variability of aftershock ground motions in the San Fernando Valley

We analyze aftershocks of the 1/17/94 Mw6.7 Northridge earthquake recorded at a 3-element small-aperture array within the town of Northridge, above the mainshock rupture plane. Many of the M4-5 aftershocks are observed to have a prolonged shaking duration, up to ~8 seconds, with conspicuous longer period (≈1 s) arrivals in the latter part of the wave train. Recordings of a M4.0 aftershock that occurred at 23:49 GMT on 1/17 show the origin of these waves. A slant-stack cross-correlation method on each of the three components shows that the late arrivals are characterized by low apparent velocities and a back-azimuth that is approximately 10 degrees off that of the direct arrivals. Based on the inferred apparent velocities and consideration of studies in other sedimentary basins, we conclude that these later arrivals consist of surface waves generated within the San Fernando Valley. Similar results are obtained for a M3.4 event recorded across the array. The surface waves are not, however, a ubiquitous feature of the aftershock recordings. We show that other M~4 events recorded at the same site are characterized by simple displacement pulses and durations that are typical for their magnitude, suggesting that 3-dimensional site response may be difficult to predict in cases where the sources are close to a valley or basin and/or the basin structure is complex

Rupture process of the June 28, 1992 Big Bear Earthquake

The June 28, 1992 Big Bear earthquake in southern California was assumed to have ruptured along a northeast-trending plane, as suggested by long-term aftershock distribution. No surface rupture was found, however, and mainshock locations determined from both strong motion and TERRAscope data are mutually consistent and do not lie on the assumed fault plane. An integrated study involving waveform modeling, directivity and seismicity analyses suggests a complex rupture pattern, with significant short- and long-period energy propagating northwest along the presumed conjugate fault-plane

Fault-Zone Waves Observed at the Southern Joshua Tree Earthquake Rupture Zone

Waveform and spectral characteristics of several aftershocks of the M 6.1 22 April 1992 Joshua Tree earthquake recorded at stations just north of the Indio Hills in the Coachella Valley can be interpreted in terms of waves propagating within narrow, low-velocity, high-attenuation, vertical zones. Evidence for our interpretation consists of: (1) emergent P arrivals prior to and opposite in polarity to the impulsive direct phase; these arrivals can be modeled as headwaves indicative of a transfault velocity contrast; (2) spectral peaks in the S wave train that can be interpreted as internally reflected, low-velocity fault-zone wave energy; and (3) spatial selectivity of event-station pairs at which these data are observed, suggesting a long, narrow geologic structure. The observed waveforms are modeled using the analytical solution of Ben-Zion and Aki (1990) for a plane-parallel layered fault-zone structure. Synthetic waveform fits to the observed data indicate the presence of NS-trending vertical fault-zone layers characterized by a thickness of 50 to 100 m, a velocity decrease of 10 to 15% relative to the surrounding rock, and a P-wave quality factor in the range 25 to 50