Analysis of the data produced by the BDSN and NHFN begins as soon as the waveforms are acquired by BSL computers and ranges from automatic processing for earthquake response to analyst review for earthquake catalogs and quality control.
The BSL collaborates with the USGS Menlo Park to provide automatic earthquake information for northern and central California. Event processing at UC Berkeley is performed by the Rapid Earthquake Data Integration (REDI) system and includes the determination of local magnitude, the seismic moment tensor and moment magnitude, peak ground motions, and finite-fault analysis. Together with Caltech, UC Berkeley and the USGS provide earthquake information to the State of California and a number of public and private agencies.
On a daily basis, analysts at UC Berkeley review and read events, making phase and amplitude readings from the broadband data, and determine the mechanism and moment. In addition to reading local and regional earthquakes, BSL staff routinely read larger teleseisms and report these observations to the NEIC and ISC.
Over the last 8 years, the BSL has invested in the development of the hardware and software necessary for an automated earthquake notification system (Gee et al., 2001). The 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.
On April 18, 1996, the BSL and the USGS announced the formation of a joint notification system for northern and central California earthquakes (Gee et al., 1996b). The system merges the programs in Menlo Park and Berkeley into a single earthquake notification system, combining data from the NCSN and the BDSN. 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 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 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 updates the northern California earthquake WWW server.
On the UC Berkeley side, the Hypoinverse archive file is used to drive the REDI processing system. This is a modification of the original REDI design, which identified and located events using raw phase data from the BDSN and NCSN (Gee et al., 1996a). In the revised REDI processing, the magnitude of each hypocenter is assessed. If the coda magnitude is greater than or equal to 3.0, waveforms from the BDSN are analyzed to estimate local magnitude. Once an updated magnitude is obtained, the event information is paged to USGS and UC Berkeley personnel, notification is sent to emergency response agencies, and the revised magnitude is transmitted to the Earthworm/Earlybird system in Menlo Park. The earthquake is then evaluated for the next level of REDI processing. If the local magnitude is greater than 3.5, waveforms from the BDSN strong-motion instruments are analyzed to determine peak ground acceleration, velocity, and displacement and to estimate the duration of strong shaking. When the strong-ground motion processing is complete, these values are distributed by email and pager and the event is scheduled for moment tensor estimation. 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. During 2000-2001, several new additions to REDI processing were introduced and these are described below.
At present, two Earthworm-Earlybird systems in Menlo Park feed two REDI processing systems at UC Berkeley (Figure 9.1). One of these systems is the production or paging system; the other is set up as a hot backup. The second system is frequently used to test new software developments before migrating them to the production environment. In addition, the BSL operates a third REDI system, which performs additional processing as described below. A fourth system is installed in Sacramento as a stand-alone operation in order to provide a redundant notification facility outside of the Bay Area.
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 9.4).
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.
The "Standard" REDI processing system (associated with the normal data acquisition computers) had two new stages added. Stage 4 extracts the waveform data required for the finite-fault processing and Stage 5 "packs" the event up and ships it to a second computer system. The second computer system (aramis) is running a REDI system comprised of 4 stages - two associated with the determination of finite-fault parameters and two associated with the prediction of ground motion parameters, based on the finite-fault information.
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.
With the current distribution of stations in northern California, data gaps are common. The ShakeMap software attempts to augment this coverage by predicting ground motions in areas where data are not available. The V2.0 package uses attenuation curves from Joyner, Boore, and Fumal (1997) and Joyner and Boore (1988) and the point source location and magnitude to produce ground motion estimates.
As part of the development of the finite-fault project, the BSL worked with the USGS Menlo Park to install ShakeMap V2.0 at UC Berkeley. Although USGS personnel had done most of the work to adapt the program to northern California, development was required to integrate the ShakeMap package into the REDI environment. In the process, BSL staff identified and fixed some minor bugs in the software.
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. The V2.0 software is structured to allow the use of different "estimates" files, that is, to incorporate ground motions predicted by alternate means.
As shown in Figure 9.2, 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 V2.0 map 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 9.3 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 9.4 shows the typical processing times associated with the current implementation.
The finite-fault/ShakeMap packages in the REDI system were implemented in April of 2001. Since that time, we have been involved in fixing bugs and cleaning up some aspects of the codes. We currently have the system configured so that any event of M 5.0 and higher will trigger the finite-fault processing system in order to test the software. The largest event which has occurred since the implementation was an earthquake in the Portola area of the Sierra Nevada. Although only a moderate event, it provides an example of the implementation as well as the newly developed Analyst Interface.
On August 10, 2001 at 20:19:26UTC a 5.5 event occurred 15 km west of Portola, California (39.893, -120.638). This event was processed by the automatic system and a seismic moment tensor was obtained within 8 minutes, indicating a strike-slip mechanism (strike=328., rake=-170., dip=84) with scalar seismic moment of 4.39e+23 dyne cm. This is a small event, and not expected to deviate significantly from a point-source. As expected for such a small event, the finite-fault results were marginal.
Stage 0 yielded a rupture velocity of 1 km/s, the lowest allowed. The low value reflects the desire of the code to attempt to map slip close to the hypocenter. Although the line source results indicated a slight preference for the NW trending plane, the difference was so slight that both planes were tested during the full 2D inversion. The Stage 2 results were a variance reduction measure of goodness of fit of 10.9% for the SW-trending plane and 10.4% for the NW trending plane, indicating the difficulty with the small event.
The inversion results were reviewed using the Analyst Interface. A number of iterations were made investigating different combinations of stations, fault dimensions, and values of dislocation rise time and rupture velocity. Based on historical seismicity in the region, the NW plane was assumed to be the actual plane. Reasonably good results were found using 6 three-component stations (CMB, KCC, MOD, ORV, WDC, YBH) located in the 180-300 km distance range from the source. Figure 9.6a shows the seismic moment tensor solution for the event and the locations of stations used in the inversion. Figure 9.6b shows the slip distribution for the NW striking plane, which was found to locate close to the hypocenter when a 0.5s delay was applied to the Green's functions. The delay is necessary to account for possible errors in event origin time. The rupture velocity was set to a more realistic value of 2 km/s. The variance reduction for the NW and SW striking planes were 20 and 25%, respectively. Both fit considerably better than the automatic result. The scalar seismic moment from the reviewed inversion is 2.46e+23 dyne cm corresponding to 4.9. Figure 9.6c compares the peak ground acceleration ShakeMap produced by V2.0 software, and the ShakeMap produced from the analyst reviewed inversion results described above. The two maps are seen to be very similar in terms of the extent of the strong shaking, and peak values, however the location of the peak is offset in the Berkeley map due to the contribution from the finite-fault results.
Both examples of the ShakeMaps in this section (Mammoth Lakes and Portola) illustrate the importance of this methodology. In areas where there are limited number of stations, the methods used to predict ground motions will be central to use of ShakeMap as an emergency response tool. Although progress has been made in instrumentation in the San Francisco Bay area, large portions of northern California are without stations. This methodology will be critical to improving ShakeMaps through seismic remote sensing.
In order to improve our capabilities on the edges of the network, we have continued efforts to establish mechanisms of data exchange with neighboring networks.
As described last year, we are working with the University of Nevada, Reno, to enhance the earthquake monitoring capabilities in northern California and Nevada. As part of this agreement, we agreed to exchange waveform data. At the present, three-component data from BK stations CMB, WDC, MOD, and ORV and vertical component data from YBH, JCC, HOPS, WENL, SAO, and KCC are being sent to UNR. In exchange, the BSL is receiving three-component data from NN stations BEK, OMM, PAH, and WCN. In addition, UNR is forwarding data from the NSN stations WVOR, MNV, DAC, and ELK. The UNR sensors are Guralp 40Ts and these stations will enhance the REDI capabilities in eastern California and western Nevada.
We initially established this waveform exchange using the Earthworm import/export mechanisms, but experienced problems with unexplained timeouts and loss of socket connections. While BSL and UNR staff were working to resolve these problems with the Earthworm modules, IRIS negotiated a license with BRTT that allowed member universities to use components of the Antelope software system. Since UNR is using the Antelope software to drive their real-time earthquake processing system, BSL staff installed the appropriate components at UCB. The real-time waveform exchange has been migrated to the Antelope system and we are experiencing fewer problems with the exchange. This has been relatively stable for the last several months. This waveform exchange is a critical first step to improving the monitoring efforts at both UNR and UCB/USGS.
As part of this effort, we also modified our data exchange with UCSD to use the Antelope client. Figure 8.4 illustrates the current dataflow in the REDI environment.
We installed the "Quake Data Delivery Service" or QDDS software at the BSL in the last year. This software was developed by the USGS to allow for the exchange of parametric earthquake data, such as locations and magnitudes. QDDS provides the earthquake information for the "recenteqs" maps as well as for other applications. We are currently running QDDS and the "recenteqs" software.
At present, the REDI system is not contributing data to QDDS. The USGS Menlo Park pushes information from the joint notification system to the two QDDS hubs. In the long run, we intend to modify the REDI system so that it provides information to one hub and the USGS provides information to the other. However, this modification requires restructuring some aspects of the way REDI tracks information (essentially, the use of version numbers). We have worked with the USGS to define a plan and intend to implement this during the fall 2001.
Future plans for development in the REDI system include replacement of the current ground-motion processing module with an adaptation of the module developed for the Earthworm system. We are also considering adapting the local magnitude module. These replacements of our existing modules insure consistency of results as well as allowing to migrate away from research codes which were adapted into the operational environment.
During the past several years, we have worked toward coordinating our earthquake monitoring activities with southern California and on improving the collaboration in northern California. These efforts span the range of activities from real-time earthquake information to catalog generation to long-term archive and distribution.
This effort to coordinate is in part an outgrowth from the interest of the State of California in uniform earthquake reporting and in part a response of the development of the Advanced National Seismic System or ANSS. As the ANSS moves forward, national earthquake monitoring will be coordinated on a regional basis. California is one of those regions through the development of the California Integrated Seismic Network (CISN). Participating organizations are the Berkeley Seismological Laboratory, the Caltech Seismological Laboratory, the California Division of Mines and Geology, the USGS Pasadena, and the USGS Menlo Park.
In practical terms, the development of the CISN is leading to better coordination among the participating agencies. Progress is being made in terms of sharing data and software. One of the most important components of the CISN is the Standards committee, which meets frequently to work on issues of coordination.
In the coming year, the CISN effort for forming a statewide system will move beyond rhetoric into reality. The CISN partners have agreed to move toward the standardized and robust distribution of earthquake parameters statewide. As part of this design, dedicated connections will be established among the CISN partners and exchanges of waveform and parametric data will be established. The CISN has defined two centers for routine statewide earthquake processing - the Northern California Management Center, operated by the USGS Menlo Park and UC Berkeley, and the Southern California Management Center, operated by the USGS Pasadena and Caltech. These centers will operate in parallel for statewide earthquake reporting. The Engineering Management Center, operated by CDMG and the USGS NSMP, has the lead responsibility for producing engineering data products.
As part of this effort within the CISN, the BSL and the USGS Menlo Park have begun to plan for the next generation of the northern California joint notification system. In the fall of 2000, the BSL and the USGS committed to merge their systems more completely, moving beyond the current situation where the systems exchange information but are not highly integrated.
Figure 9.1 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. Figure 9.7 illustrates the planned configuration. Our discussions have proceeded to the stage of establishing specifications and determining the details required for design.
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 2000 through June 2001, BSL analysts reviewed nearly 200 earthquakes in northern California and adjoining areas, ranging from M2.7 to 5.3. Reviewed moment tensor solutions were obtained for 32 events (through 8/31/2001). Figure 9.8 and Table 9.1 displays the earthquakes located in the BSL catalog and the moment tensor solutions. For contrast with the automatic solutions, the figure also shows the comparison between the two estimates of and the correlation between the two methods when available.
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.
We thank Howard Bundock and Jack Boatwright of the USGS for assisting us with the installation of the ShakeMap software. Partial funding for this project was provided by PG&E, through PEER.
Under Barbara Romanowicz's general supervision, Lind Gee leads the development of the REDI system and directs the routine analysis. Asya Kaverina, Peter Lombard, Doug Neuhauser, Doug Dreger, and Lind Gee were part of the team that completed the implementation of the finite-fault processing, ShakeMap integration, and development of the analyst interface. Rick McKenzie, Sayaka Araki, Doug Dreger, and Hrvoje Tkalcic contribute to the routine analysis. Lind Gee, Doug Neuhauser, Doug Dreger, and Hrvoje Tkalcic contributed to the writing of this chapter.
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