David P. Schaff

Large‐scale relocation of two decades of Northern California seismicity using cross‐correlation and double‐difference methods

[1] We simultaneously reanalyzed two decades (1984–2003) of the digital seismic archive of Northern California using waveform cross-correlation (CC) and double-difference (DD) methods to improve the resolution in hypocenter locations in the existing earthquake catalog generated at the Northern California Seismic Network (NCSN) by up to three orders of magnitude. We used a combination of 3 billion CC differential times measured from all correlated pairs of events that are separated by less than 5 km and 7 million P wave arrival-time picks listed in the NCSN bulletin. Data were inverted for precise relative locations of 311,273 events using the DD method. The relocated catalog is able to image the fine-scale structure of seismicity associated with active faults and revealed characteristic spatiotemporal structures such as streaks and repeating earthquakes. We found that 90% of the earthquakes have correlated P wave and S wave trains at common stations and that 12% are colocated repeating events. An analysis of the repeating events indicates that uncertainties at the 95% confidence level in the existing network locations are on average 0.7 km laterally and 2 km vertically. Correlation characteristics and relative location improvement are remarkably similar across most of Northern California, implying the general applicability of these techniques to image high-resolution seismicity caused by a variety of plate tectonic and anthropogenic processes. We show that consistent long-term seismic monitoring and data archiving practices are key to increase resolution in existing hypocenter catalogs and to estimate the precise location of future events on a routine basis.

Postseismic response of repeating aftershocks

The recurrence intervals of repeating earthquakes on the San Andreas Fault in the Loma Prieta aftershock zone follow the characteristic 1/t decay of Omoris law. A model in which these earthquakes occur on isolated patches of the fault that fail in stick-slip with creep around them can explain this observation. In this model the recurrence interval is inversely proportional to the loading rate due to creep. Logarithmic velocity strengthening friction predicts 1/t decay in creep rate following the mainshock. The time dependence of recurrence is inconsistent with a simple viscous constitutive relationship, which predicts an exponential decay of loading rate. Thus, our observations imply postseismic slip at seismogenic depth under a power law rheology. The time dependence of postseismic deformation measured geodetically may be diagnostic of whether postseismic deformation is caused by creep or possible viscoelastic deformation at greater depths.

Optimizing Correlation Techniques for Improved Earthquake Location

Earthquake location using relative arrival time measurements can lead to dramatically reduced location errors and a view of fault-zone processes with un- precedented detail. There are two principal reasons why this approach reduces lo- cation errors. The first is that the use of differenced arrival times to solve for the vector separation of earthquakes removes from the earthquake location problem much of the error due to unmodeled velocity structure. The second reason, on which we focus in this article, is that waveform cross correlation can substantially reduce measurement error. While cross correlation has long been used to determine relative arrival times with subsample precision, we extend correlation measurements to less similar waveforms, and we introduce a general quantitative means to assess when correlation data provide an improvement over catalog phase picks. We apply the technique to local earthquake data from the Calaveras Fault in northern California. Tests for an example streak of 243 earthquakes demonstrate that relative arrival times with normalized cross correlation coefficients as low as 70%, interevent separation distances as large as to 2 km, and magnitudes up to 3.5 as recorded on the Northern California Seismic Network are more precise than relative arrival times determined from catalog phase data. Also discussed are improvements made to the correlation technique itself. We find that for large time offsets, our implementation of time- domain cross correlation is often more robust and that it recovers more observations than the cross spectral approach. Longer time windows give better results than shorter ones. Finally, we explain how thresholds and empirical weighting functions may be derived to optimize the location procedure for any given region of interest, taking advantage of the respective strengths of diverse correlation and catalog phase data on different length scales.

Waveform Cross-Correlation-Based Differential Travel-Time Measurements at the Northern California Seismic Network

We processed the complete digital seismogram database for northern California to measure accurate differential travel times for correlated earthquakes observed at common stations. Correlated earthquakes are earthquakes that occur within a few kilometers of one another and have similar focal mechanisms, thus generating similar waveforms, allowing measurements to be made via cross-corre- lation analysis. The waveform database was obtained from the Northern California Earthquake Data Center and includes about 15 million seismograms from 225,000 local earthquakes between 1984 and 2003. A total of 26 billion cross-correlation measurements were performed on a 32-node (64 processor) Linux cluster, using improved analysis tools. All event pairs with separation distances of 5 km or less were processed at all stations that recorded the pair. We computed a total of about 1.7 billion P-wave differential times from pairs of waveforms that had cross- correlation coefficients (CC) of 0.6 or larger. The P-wave differential times are often on the order of a factor of ten to a hundred times more accurate than those obtained from routinely picked phase onsets. 1.2 billion S-wave differential times were mea- sured with CC 0.6, a phase not routinely picked at the Northern California Seismic Network because of the noise level of remaining P coda. We found that approxi- mately 95% of the seismicity includes events that have cross-correlation coefficients of CC 0.7 with at least one other event recorded at four or more stations. At some stations more than 40% of the recorded events are similar at the CC 0.9 level, indicating the potential existence of large numbers of repeating earthquakes. Large numbers of correlated events occur in different tectonic regions, including the San Andreas Fault, Long Valley caldera, Geysers geothermal field and Mendocino triple junction. Future research using these data may substantially improve earthquake lo- cations and add insight into the velocity structure in the crust.

Lg-Wave Cross Correlation and Double-Difference Location: Application to the 1999 Xiuyan, China, Sequence

A surprising discovery has been made that in some cases the complex, highly scattered Lg wave is found to be similar for clusters of events. We analyze in detail a subset of 28 out of 90 events from the 1999 Xiuyan sequence. Cross correlations provide highly accurate differential travel-time measurements. Their error estimated from the internal consistency is about 7 msec. These travel-time differences are then inverted by the double-difference technique to obtain epicenter estimates that have location precision on the order of 150 m. The locations are computed with waveform data from four to five regional stations 500 to 1000 km away. The epicenter estimates are not substantially affected by the sparseness of stations or large azimuthal gaps. Comparison with a surface trace a few kilometers away and location estimates based on much more dense networks led us to conclude that the absolute positions are accurate to the 5-km level. Regional event locations must often be based on a small number of phases and stations due to weak signal-to-noise ratios and sparse station coverage. This is especially true for monitoring work that seeks to locate smaller magnitude seismic events with a handful of regional stations. Two primary advantages of using Lg for detection and location are that it is commonly the largest amplitude regional wave (enabling detection of smaller events) and it propagates more slowly than P waves or Sn (resulting in smaller uncertainty in distance, for a given uncertainty in travel time).

Lop Nor Revisited: Underground Nuclear Explosion Locations, 1976-1996, from Double-Difference Analysis of Regional and Teleseismic Data

We have used seismic signals recorded at regional and teleseismic distances to determine precise locations of 19 underground nuclear explosions (13 in vertical shafts, and 6 in horizontal tunnels) carried out between October 1976 and July 1996 at the Lop Nor test site in the southern Xinjiang province of China. In addition to first- and later-arriving phase-pick data from the International Seismological Centre and Chinese bulletins, we use waveform cross-correlation methods to measure relative arrival times between the explosions with an accuracy of about 10 msec. We adapted the double-difference algorithm to work with regional and global network data, and applied it to the combined Lop Nor data set to remove the effects of uncertainty in the Earth model. Specifically, we determined locations of more recent tests to the accuracy of the cross-correlation data while simultaneously determining the relative locations of the older tests, for which digital waveforms are not available, to the accuracy of the phase-pick data. In general, our locations are consistent, at the 90% confidence level, with previously published locations (when compared in a common reference frame), with anthropogenic features from satellite imagery, and with high-resolution elevation data. For four explosions, however, our results indicate that a particular explosion may have been carried out in a different shaft than previously noted, or previously associated features must be ruled out because of their location outside a particular error ellipse. Mislocations of explosions from associated satellite features are less than 1 km for all 13 shaft events. The pattern of tunnel-explosion locations falls within a region of suitable overburden required for containment. Eighteen Lop Nor locations have solution qualities at the GT2 level or better, and are well suited to calibrate IMS stations for the purpose of monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty. Manuscript received 4 September 2003.

Splay faults imaged by fluid-driven aftershocks of the 2004 Mw 9.2 Sumatra-Andaman earthquake

High-precision teleseismic double-difference locations and focal mechanisms of aftershocks of the A.D. 2004 M w 9.2 Sumatra-Andaman earthquake illuminate an active imbricate fault system in the accretionary prism offshore northern Sumatra. They reveal repeated failure of a shallow northeast-dipping thrust fault above the megathrust, which we interpret to be the reactivation of a splay fault that rises from the megathrust at ∼55 km depth and cuts through the overriding Sunda plate. The projected intersection of the splay fault with the seafloor correlates with a recently active thrust fault seen in postseismic bathymetry data west of the Aceh basin. A spatiotemporal analysis of the streaky aftershock distribution indicates that ascending fluids released from the subducting oceanic crust along inherited seafloor fabric may control brittle fracture in the overriding plate. We speculate that if the splay fault was active coseismically, it may have led to amplified vertical uplift of the forearc ridge and contributed to generating the cataclysmic near-field tsunami that struck the northwest Sumatra coast following the 2004 rupture.

Seismological Constraints on Proposed Low-Yield Nuclear Testing in Particular Regions and Time Periods in the Past, with Comments on “Radionuclide Evidence for Low-Yield Nuclear Testing in North Korea in April/May 2010” by Lars-Erik De Geer

We have attempted to detect seismic signals from small explosions in North Korea on five specific days in 2010 that feature in scenarios proposed by De Geer. We searched the seismic data recorded by station MDJ in northeastern China, applying three-component cross-correlation methods using signals from known explosions as templates. We assess the capability of this method of detection, and of simpler methods, all of which failed to find seismic signals that would be expected if De Geers scenarios were valid. We conclude that no well-coupled underground explosion above about a ton occurred near the North Korea test site on these five days and that any explosion would have to be very small (local magnitude less than about 2) to escape detection.

Multistation Validation of Waveform Correlation Techniques as Applied to Broad Regional Monitoring

Abstract Waveform correlation is garnering attention as a method for detecting, locating, and characterizing similar seismic events. To explore the opportunities for using waveform correlation in broad regional monitoring, we applied the technique to a large region of central Asia over a three‐year period, monitoring for events at regional distances using three high‐quality stations. We discuss methods for choosing quality templates and introduce a method for choosing correlation detection thresholds, tailored for each template, for a desired false alarm rate. Our SeisCorr software found more than 10,000 detections during the three‐year period using almost 2000 templates. We discuss and evaluate three methods of confirming detections: bulletin confirmation, high correlation with a template, and multistation validation. At each station, 65%–75% of our detections could be confirmed, most by multistation validation. We confirmed over 6500 unique detections. For monitoring applications, it is of interest that a significant portion of the Comprehensive Nuclear‐Test‐Ban Treaty Organization’s Late Event Bulletin (LEB) catalog events was detected and that adding our confirmed detections for the LEB catalog would more than double the catalog size. Waveform correlation also allows for relative magnitude calculation, and we explore the magnitudes of detected events. The results of our study suggest that doing broad regional monitoring using historical and real‐time‐generated templates is feasible and will increase detection capabilities.

Possible precursory signals in damage zone foreshocks

Foreshocks may provide a precursory signal of an impending earthquake, but their role in nucleation of the mainshock is unclear. One way to further our understanding of foreshock failure mechanisms is to determine where they occur in the fault zone. However, earthquake locations commonly include uncertainties large enough to allow rupture on either the main fault interface or on subsidiary fractures within a surrounding damage zone. Here we obtain precise earthquake locations, with ~10 m uncertainty, for foreshocks and aftershocks of an Mw5.0 near Prague, OK, USA. Repeating earthquakes imply that some precursory slow slip occurred before the mainshock. In addition, we show that foreshocks initially rupture faults and fractures throughout the 300-meter-thick fault damage zone, and later localize onto a narrower zone (<100 m thick) nearer the mainshock hypocenter. Focal mechanisms corroborate that foreshocks occur in the surrounding damage zone as well as on the mainshock rupture interface. These results highlight that earthquake nucleation is most likely a complex feedback between frictional failure processes on the fault interface and deformation in the surrounding damaged rock, not just nucleation on a single surface.