Analysis of the data produced by BSL networks begins as the waveforms are acquired by computers at UC Berkeley, and ranges from automatic processing for earthquake response to analyst review for earthquake catalogs and quality control.
Over the last 9 years, the BSL has invested in the development of the hardware and software necessary for an automated earthquake notification system (Gee et al., 2002; Gee et al., 1996). The Rapid Earthquake Data Integration (REDI) project is a research program at the BSL for the rapid determination of earthquake parameters with three major objectives: to provide near real-time locations and magnitudes of northern and central California earthquakes; to provide estimates of the rupture characteristics and the distribution of ground shaking following significant earthquakes, and to develop better tools for the rapid assessment of damage and estimation of loss. A long-term goal of the project is the development of a system to warn of imminent ground shaking in the seconds after an earthquake has initiated but before strong motions begin at sites that may be damaged.
In 1996, the BSL and USGS began collaboration on a joint notification system for northern and central California earthquakes. The current system merges the programs in Menlo Park and Berkeley into a single earthquake notification system, combining data from the NCSN and the BDSN.
This year, significant progress was made in the development of the California Integrated Seismic Network.
Figure 12.1 illustrates the distributed nature of the current joint notification system in northern California.
On the USGS side, incoming analog data from the NCSN are digitized, picked, and associated as part of the Earthworm system (Johnson et al., 1995). Preliminary locations, based primarily on phase picks from the NCSN, are available within seconds, while final locations and preliminary coda magnitudes are available within 2-4 minutes. Earthworm reports events - both the "quick-look" 25 station hypocenters (without magnitudes) and the final (unreviewed) solutions (with coda magnitudes) to the Earlybird alarm module in Menlo Park. This system sends the Hypoinverse archive file to the BSL for additional processing, generates pages to USGS and UC Berkeley personnel, and distributes information via the Quake Data Distribution System (QDDS).
Once an event is declared, additional Earthworm processing at the USGS generates ground motion amplitudes from NCSN and NSMP stations and loads them into a database. A process known as ShakeMapFeeder extracts amplitudes from the database and pushes them to the ShakeMap system (Wald et al., 1999) implemented in Menlo Park.
On the UC Berkeley side, the Hypoinverse archive file is normally used to drive the REDI processing system. The REDI processing is divided into two systems - routine or "standard" processing (local magnitude, ground motion amplitudes, and moment tensors) - and the finite-fault processing added last year (Figure 12.2). Each REDI system provides several "stages" of processing, and the attributes of events such as magnitude, the "age" (time since origin), and number of associated phases are used to determine the appropriate processing. The REDI stage structure allows processing to be scheduled (for example, wait 5 minutes after the origin time before scheduling a moment tensor computation) as well as prioritized (for example, process the magnitude 6 before the magnitude 2).
In "abnormal" situations, such as when communication links between the BSL and the USGS are disrupted, the BSL can drive the REDI system using events detected based on BDSN data alone. The BSL has implemented the same association algorithm used in the Earthworm system in Menlo Park, using Murdock-Hutt phase detections and/or picks from an Earthworm picker.
Stage 0 of the "standard" processing provides initial event handling. It can accept either Hypoinverse files from the USGS (the normal source of event information) or events generated from the local associator. If the preliminary magnitude estimate is less than 3.0, no additional processing is performed and event information is distributed if appropriate.
Stage 1 is initiated for all events with preliminary magnitudes greater than 3.0 and for events with no preliminary magnitude. In this stage, broadband waveforms are processed to produce Wood-Anderson synthetics and estimates of local magnitude are generated. This stage uses the preliminary magnitude and a distance criterion to decide which channels to analyze.
Stage 2 generates ground-motion amplitudes for use in ShakeMap and other applications. Stage 2 currently generates estimates of peak ground acceleration, peak ground velocity, and peak ground displacement from BDSN acceleration records, but does not produce estimates of spectral acceleration.
Stage 3 performs the automated moment tensor analysis. In this stage of REDI processing, both the waveform modeling method of Dreger and Romanowicz (1994) and the surface wave inversion technique of Romanowicz et al. (1993) are run for every qualifying event (earthquakes with greater than 3.5). Each algorithm produces an estimate of the seismic moment, the moment tensor solution, the centroid depth, and solution quality. The REDI system uses the individual solution qualities to compute a weighted average of moment magnitude, to compare the mechanisms using normalized root-mean-square of the moment tensor elements (Pasyanos et al., 1996), and to determine a "total" mechanism quality.
In 2000-2001, two new stages were added to standard REDI processing. Stage 4 extracts the waveform data required for the finite-fault processing and Stage 5 "packs" the event up and ships it to the REDI finite-fault system running on aramis.
In Stage 0, waveform data are prepared for inversion and rough estimates of the fault dimensions are derived using the empirical scaling relationships of Wells and Coppersmith (1994). Using these parameters to constrain the overall dimensions of the extended source, the stage tests the two possible fault planes obtained from the moment tensor inversion over a range of rupture velocities by performing a series of inversions using a line-source representation. In addition to the identification of the fault plane and apparent rupture velocity, this stage yields preliminary estimates of the rupture length, dislocation rise time, and the distribution of slip in one dimension.
Stage 1 combines the results of the line-source inversion with the directivity-corrected attenuation relationships of Somerville et al. (1997) to simulate ground motions in the near-source region. "FFShake" computes peak ground acceleration, peak ground velocity, and spectral response at 0.3, 1.0, and 3.0 sec period, which are the values used in ShakeMap, for a grid of pseudo-stations in the vicinity of the epicenter. The predicted ground motions are automatically incorporated in ShakeMap updates as described below.
In Stage 2, the second component of the finite-fault parameterization uses the best-fitting fault plane and rupture velocity from Stage 0 to obtain a more refined image of the fault slip through a full two-dimensional inversion. If line-source inversion fails to identify the probable fault (due to insufficient separation in variance reduction), the full inversion is computed for both fault planes. In the present implementation, the full inversion requires 20-30 minutes per plane, depending on the resolution, on a Sun UltraSPARC1/200e.
Stage 3 completes the cycle by simulating the near-fault strong ground motion parameters by convolving the velocity structure response with the finite-fault slip distribution. As in Stage 1, "FFShake" computes peak ground acceleration, peak ground velocity, and spectral response at 0.3, 1.0, and 3.0 sec period for a grid of pseudo-stations in the vicinity of the epicenter and pushes these ground motions to the ShakeMap system.
The motivation for this effort is the desire to integrate the ground motions predicted from the finite-fault inversions into the ShakeMap generation. The goal is to provide updated ShakeMaps as more information about the earthquake source is available. Versions 2.0 and higher of ShakeMap are structured to allow the use of different "estimates" files, that is, to incorporate ground motions predicted by alternate means.
As shown in Figure 12.2, the REDI processing system is integrated with the ShakeMap software at several levels. "Event.txt" files are generated at several stages - these files tell the ShakeMap software to wake-up and process an event. A ShakeMap is generated following Stage 2 in the Standard processing and updated if a revised estimate of magnitude is obtained following Stage 3.
For events which trigger the finite-fault processing, estimates of ground motions based on the results of the line-source computation and the full 2D inversion are produced in the "FFShake" stages. "Estimates.xml" files are generated and pushed to the ShakeMap package. The output of the line source computation produces what we call an "Empirical ShakeMap", while output from the 2D inversion produces a "Conservative ShakeMap".
Figure 12.3 illustrates the three different methodologies with examples from an M6 earthquake which occurred in the Mammoth Lakes region in May 1999. Very few data were available to constrain these maps. This event is somewhat small for this methodology, but the impact of the successive improvements in the ground motion estimates is clearly illustrated.
Future plans include the continued testing and refinement of the procedure, working with the USGS group toward integration into the authoritative ShakeMap method for northern California and other regions, and development of additional capabilities based on the incorporation of BARD GPS data. Figure 12.4 shows the typical processing times associated with the current implementation.
This structure has greatly expedited automatic earthquake processing in northern California. The dense network and Earthworm-Earlybird processing environment of the NCSN provides rapid and accurate earthquake locations, low magnitude detection thresholds, and first-motion mechanisms for smaller quakes. The high dynamic range data loggers, digital telemetry, and broadband and strong-motion sensors of the BDSN and REDI analysis software provide reliable magnitude determination, moment tensor estimation, peak ground motions, and source rupture characteristics. Robust preliminary hypocenters are available about 25 seconds after the origin time, while preliminary coda magnitudes follow within 2-4 minutes. Estimates of local magnitude are generally available 30-120 seconds later, and other parameters, such as the peak ground acceleration and moment magnitude, follow within 1-4 minutes (Figure 12.4).
Earthquake information from the joint notification system is distributed by pager, e-mail, and the WWW. The first two mechanisms "push" the information to recipients, while the current Web interface requires interested parties to actively seek the information. Consequently, paging and, to a lesser extent, e-mail are the preferred methods for emergency response notification. The recenteqs site has enjoyed enormous popularity since its introduction and provides a valuable resource for information whose bandwidth exceeds the limits of wireless systems and for access to information which is useful not only in the seconds immediately after an earthquake, but in the following hours and days as well.
In order to ensure that automatic systems are operating correctly, BSL staff and students participate in alarm response. Each week, two people are on duty in order to respond to earthquakes - or computer problems. One person is designated as primary and is responsible for earthquake-related issues. The second individual serves as a backup to the second and addresses more operational problems. For earthquake notification, the alarm response team receives pages from several sources including an automatic monitoring of the ground velocity at BKS (known as the SEismic ALarm or SEAL), a human-initiated alarm from the UCB Police Department, and separate notifications from REDI and the USGS. For operational monitoring, we have developed a component of the REDI system for tracking the heartbeats and data flow from critical systems and processes. Because this is inherently a distributed system, it is critical to monitor the "health" of every component. In the REDI system, the monitor program is a master scheduler that can perform several types of monitoring. As presently implemented, the monitor program watches critical computers and network components, disk resources, specified processes, and particular subsystems, such as data acquisition.
This modification provided the opportunity to enhance the way REDI selects waveforms for processing. The "redi.avail" file allows for control of which channels are used for REDI modules and makes it easy to turn channels off and on if sensors or data loggers fail or telemetry problems are experienced.
We have taken steps to use automatically and hope to complete this work in the early fall of 2002. In parallel, this work will allow both the USGS and the BSL components of the joint notification system to report earthquake information to Web independently (currently, only the USGS component distributes information to the Web using QDDS). As a result of this development, both the USGS and BSL components will distribute information to the Web, enhancing the robustness of the Northern California operations.
Figure 12.1 illustrates the current organization of the two systems. As described above, an Earthworm/Earlybird component is tied to a REDI component and the pair form a single "joint notification system". Although this approach has functioned reasonably well over the last 5 years, there are a number of potential problems associated with the separation of critical system elements by 30 miles of San Francisco Bay.
Recognizing this, we intend to redesign the northern California operations so that a single independent system operates at the USGS and at UC Berkeley. Figures 12.5 and 12.6 illustrate the planned configuration. Our discussions have proceeded to the stage of establishing specifications and determining the details required for design. In the last year, the BSL and the USGS Menlo Park have met several times to discuss designs for the proposed system. In October, the BSL and the USGS Menlo Park asked representatives from the USGS Golden, USGS Pasadena, and Caltech to participate in a 2-day meeting.
On a daily basis, the BSL continues to locate and determine the magnitude of earthquakes in northern California and adjacent regions. As a general rule, events are analyzed if their magnitude is greater than 2.8 in the Central Coast ranges, greater than 3.0 in all of northern California, or greater than 3.8 in the bordering regions. Traditionally, these events were located using hand-picked arrival times from the BDSN stations in conjunction with P-arrival times from the NCSN using the program st-relp. Over the past several years, the BSL has made a transition in the daily analysis to take advantage of the automatic processing system. As part of this transition, events which have been processed by the automatic system are not generally relocated, although phase arrivals are still hand-picked and the synthetic Wood-Anderson readings are checked. Instead, analysts are focusing on the determination of additional parameters, such as the seismic moment tensor, phase azimuth, and measures of strong ground shaking.
From July 2001 through June 2002, BSL analysts reviewed nearly 200 earthquakes in northern California and adjoining areas, ranging from M2.8 to 5.9. Reviewed moment tensor solutions were obtained for 28 events (through 6/30/2002). Figure 12.7 and Table 12.1 displays the earthquakes located in the BSL catalog and the moment tensor solutions.
In addition to the routine analysis of local and regional earthquakes, the BSL also processes teleseismic earthquakes. Taking advantage of the CNSS catalog, analysts review teleseisms of magnitude 5.8 and higher. All events of magnitude 6 and higher are read on the quietest BDSN station, while events of magnitude 6.5 and higher are read on the quietest station and BKS. Earthquakes of magnitude 7 and higher are read on all BDSN stations.
The locations and magnitude determined by the BSL are cataloged on the NCEDC. The phase and amplitude data are provided to the NEIC, along with the locations and magnitudes, as contributions to the global catalogs, such as that of the ISC.
Lind Gee leads the development of the REDI system and directs the routine analysis. Peter Lombard, Doug Neuhauser, and Jim Yan contribute to the development of software. Rick McKenzie, Doug Dreger, Hrvoje Tkalcic, and Dennise Templeton contribute to the routine analysis. Lind Gee, Doug Neuhauser, and Dennise Templeton contributed to the writing of this chapter.
Partial support for the develop of the REDI system is provided by the USGS.
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