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Northern California Earthquake Monitoring

Subsections

Introduction

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.

Joint Notification System Overview

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.

Figure 12.1: Schematic diagram illustrating the connectivity between the real-time processing systems at the USGS Menlo Park and UC Berkeley. This figure also illustrates the newly added finite-fault and ShakeMap capability, which is handled on a separate system, as well as the independent processing system in Sacramento.
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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.

Standard Processing

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 $M_{L}$ 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.

Figure 12.2: Diagram showing the two levels of REDI processing. The "Standard" processing is conducted on the two main data acquisition systems and includes the computation of $M_{L}$, ground-motion processing, and the determination of the seismic moment tensor. The "Finite-fault" system is an expansion of REDI processing. Items in parentheses are planned expansions.
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Finite-Fault Processing

During 2000-2001, the estimation of finite-fault parameters was migrated from the development platform to the REDI operational environment, new modules were developed to use the finite-fault parameters to simulate near-fault strong ground motions, and the results integrated into the generation of ShakeMap (Dreger and Kaverina, 2000; Dreger and Kaverina, 1999).

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.

ShakeMap

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. 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.

Figure 12.3: Summary of the three levels of ShakeMaps produced by the REDI system, with an example for an M6 earthquake in the Mammoth Lakes region. Note that the contour intervals vary from plot to plot.
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Figure 12.4: Illustration of the current (solid lines) and planned/proposed (dotted lines) development of real-time processing in northern California. The Finite Fault I and II are fully implemented within the REDI system at UC Berkeley and are integrated with ShakeMap. The resulting maps are still being evaluated and are not currently available to the public.
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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.

Implementation

At present, two Earthworm-Earlybird systems in Menlo Park feed two "standard" REDI processing systems at UC Berkeley (Figure 12.1). 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. The REDI finite-fault processing is installed on a third system, which is always "fed" from the production system. 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 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.

System Monitoring

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.

2001-2002 Activities

Queueing of event messages

Working with the USGS, we have implemented a mechanism to queue event messages transmitted from the USGS to the REDI system. Previously, Earlybird opened a socket connection to the REDI computers. This approach was vulnerable to loss of data during times of connectivity problems. This socket-based approach has been replaced with a "file flinger", which provides the capability to queue or store event messages if connectivity between Berkeley and Menlo Park is lost.

Support for SNCL

Over the last year, we completed the implementation of full SEED channel names. In the past, the REDI system had used Station/Network/Channel (SNC) to describe a unique waveform channel. However, the evolution of the BDSN and experience with other networks led to the implementation of Location code or the full SNCL convention. This involved revisiting a number of REDI modules which handle waveform data.

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.

Moment Tensor codes

As part of the changes for supporting SNCL, BSL staff put considerable time into recasting some of the moment tensor codes. Pete Lombard worked with Doug Dreger to modify the complete waveform codes. Changes included the ability to handle acceleration as well as velocity data, generalizing assumptions about data rates and channel orientation, and enhancements to how channels are selected, based on the distribution of stations.

$M_{w}$

The REDI system has routinely produced automatic estimates of moment magnitude ($M_{w}$) for many years. However, these estimates have not routinely used as the "official" magnitude, due in part to questions about the reliability of the automatic solutions. However, in response to the 05/14/2002 Gilroy earthquake ($M_{w}$ 4.9, $M_{L}$ 5.1) and the complications created by the publication of multiple magnitudes, the BSL and USGS Menlo Park have agreed to use automatically determined moment magnitudes, when available, to supplement estimates of local magnitude ($M_{L}$).

We have taken steps to use $M_{w}$ 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.

Collaboration with UNR

As part of our collaboration with UNR Seismological Laboratory, we implemented the capability to "forward" event messages from UNR over the REDI paging system in January. UNR sends Qpager format messages to the BSL via email, where a program extracts the message, evaluates the location, and then sends a page if the event is inside the UNR paging polygon. As part of this collaboration, the USGS Menlo Park and the BSL modified their paging polygon to align with the California/Nevada border.

ShakeMap

During 2001-2002, the ShakeMap software at the BSL was upgraded to version 2.4, in order to stay synchronized with the implementation in Menlo Park. V2.5 is currently being tested in Menlo Park and we anticipate upgrading in the next quarter.

Northern California Management Center

As part of ongoing efforts to improve the monitoring systems in northern California, the BSL and the USGS Menlo Park have begun to plan for the next generation of the northern California joint notification system.

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.

Figure 12.5: Future design of the Northern California Earthquake Notification System. In contrast with the current situation (Figure 12.1), the system is being redesigned to integrate the Earthworm/Earlybird/REDI software into a single package. Parallel systems will be run at the Berkeley and Menlo Park facilities of the Northern California Operations Center.
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Figure 12.6: Illustration of the design currently being considered for the development of the Northern California Management Center.
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Routine Earthquake Analysis

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.

Figure 12.7: Map comparing the reviewed moment tensor solutions from the joint notification system. Solutions from the complete waveform inversion are plotted in black.
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Table 12.1: Moment tensor solutions for significant events from July 1, 2001 to June 30, 2002 using both regional methodologies. Epicentral information from the UC Berkeley/USGS Northern California Earthquake Data Center. Moment is in dyne-cm and depth is in km. Key to methods: (1) Complete waveform fitting inversion; (2) Regional surface wave inversion.

Location Date Time Lat. Lon. MT Dep. $M_{L}$ $M_{w}$ $M_{o}$ Str. Dip Rake Meth.
Tres Pinos 07/02/2001 17:33:53.0 36.694 -121.333 8.0 4.1 4.1 1.80e22 60 89 -6 1
11.0 4.1 4.1 1.69e22 133 87 -158 2
Tres Pinos 07/03/2001 19:02:50.0 36.696 -121.329 6.0 4 4.2 2.30e22 221 85 28 1
11.0 4 4.1 2.05e22 138 77 -176 2
Tres Pinos 07/03/2001 19:07:16.0 36.683 -121.319 6.0 3.9 4.0 1.10e22 36 70 -35 1
11.0 3.9 4.0 1.23e22 133 76 -154 2
Rohnert Park 07/07/2001 15:07:31.0 38.344 -122.630 11.0 3.7 3.7 3.34e21 133 90 -166 2
Coso Junction 07/17/2001 12:07:25.0 36.017 -117.878 8.0 4.9 5.2 6.89e23 352 83 171 2
Coso Junction 07/17/2001 12:59:58.0 36.013 -117.893 5.0 4.7 4.9 2.76e23 344 66 -176 2
Portola 08/10/2001 20:19:26.0 39.893 -120.638 24.0 5.1 5.1 4.60e23 139 89 161 1
24.0 5.1 5.2 6.80e23 326 81 -173 2
San Benito 08/26/2001 05:03:02.0 36.820 -121.550 5.0 3.9 4.0 1.12e22 135 80 -179 2
Trinidad 11/17/2001 02:17:00.0 41.234 -125.915 21.0 3.8 3.9 8.18e21 292 90 -167 2
Oregon 11/20/2001 16:58:11.0 41.854 -125.900 24.0 3.8 4.1 1.40e22 225 74 13 2
Oregon 11/21/2001 02:01:28.0 41.844 -125.878 24.0 3.9 4.1 1.48e22 218 84 10 2
Punta Gorda 11/30/2001 07:46:56.0 40.490 -126.850 18.0 4.7 4.9 2.20e23 269 79 145 1
11.0 4.7 5.1 4.69e23 265 88 -144 2
Ukiah 12/07/2001 14:29:08.0 39.043 -123.117 8.0 4.1 4.3 3.09e22 326 81 162 2
Ukiah 12/14/2001 09:41:05.0 39.045 -123.120 11.0 4 4.4 3.92e22 328 76 179 2
Tres Pinos 12/28/2001 21:14:01.0 36.641 -121.252 5.0 4.7 4.6 9.99e22 320 80 174 2
Punta Gorda 01/07/2002 20:23:15.0 40.309 -127.108 11.0 4.7 5.0 3.38e23 345 72 -79 2
Geysers 04/18/2002 11:35:40.0 38.791 -122.774 5.0 3.5 4.0 9.97e21 162 74 -175 2
San Jose 04/23/2002 11:59:20.0 36.866 -121.607 8.0 3.6 3.7 3.45e21 324 89 171 2
Ukiah 04/26/2002 11:01:26.0 39.040 -123.125 8.0 3.5 3.8 5.48e21 228 85 -2 2
Ferndale 04/29/2002 00:43:29.0 40.609 -124.462 27.0 4.4 4.6 1.00e23 166 68 179 2
Punta Gorda 05/04/2002 12:17:00.0 40.305 -124.501 5.0 3.9 4.0 9.85e21 160 73 44 2
Punta Gorda 05/04/2002 12:54:23.0 40.313 -124.549 8.0 3.7 3.8 5.69e21 347 74 2 2
Punta Gorda 05/04/2002 13:56:31.0 40.319 -124.611 5.0 4.3 4.4 4.54e22 353 85 -8 2
Fairfield 05/08/2002 14:59:36.0 38.229 -121.845 14.0 3.7 3.7 3.90e21 353 62 160 2
Geysers 05/09/2002 11:07:55.0 38.797 -122.728 5.0 3.6 3.9 8.11e21 155 43 -147 2
Gilroy 05/14/2002 05:00:29.0 36.967 -121.600 8.0 5.2 4.9 2.86e23 212 87 -6 2
Nevada 06/14/2002 12:40:49.0 36.715 -116.300 8.0 4.4 4.6 7.86e22 2 75 -121 2
Eureka 06/17/2002 16:55:07.0 40.829 -124.606 21.0 5.1 5.3 8.98e23 238 88 12 2


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.

Acknowledgements

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.

References

Dreger, D. and A. Kaverina, Seismic remote sensing for the earthquake source process and near-source strong shaking: A case study of the October 16, 1999 Hector Mine earthquake, Geophys. Res. Lett., 27, 1941-1944, 2000.

Dreger, D., and A. Kaverina, Development of procedures for the rapid estimation of ground shaking, PGE-PEER Final Report, 1999.

Dreger, D., and B. Romanowicz, Source characteristics of events in the San Francisco Bay region, USGS Open-File-Report 94-176 , 301-309, 1994.

Gee, L., D. Neuhauser, D. Dreger, M. Pasyanos, R. Uhrhammer, and B. Romanowicz, The Rapid Earthquake Data Integration Project, Handbook of Earthquake and Engineering Seismology, IASPEI, in press, 2002.

Gee, L., D. Neuhauser, D. Dreger, M. Pasyanos, B. Romanowicz, and R. Uhrhammer, The Rapid Earthquake Data Integration System, Bull. Seis. Soc. Am., 86, 936-945,1996.

Johnson, C., A. Bittenbinder, B. Bogaert, L. Dietz, and W. Kohler, Earthworm: A flexible approach to seismic network processing, IRIS Newsletter, XIV (2), 1-4, 1995.

Kanamori, H., P. Maechling, and E. Hauksson, Continuous monitoring or ground-motion parameters, Bull. Seis. Soc. Am., 89, 311-316, 1999.

Murdock, J., and C. Hutt, A new event detector designed for the Seismic Research Observatories, USGS Open-File-Report 83-0785, 39 pp., 1983.

Romanowicz, B., D. Dreger, M. Pasyanos, and R. Uhrhammer, Monitoring of strain release in central and northern California using broadband data, Geophys. Res. Lett., 20, 1643-1646, 1993.

Pasyanos, M., D. Dreger, and B. Romanowicz, Toward real-time estimation of regional moment tensors, Bull. Seis. Soc. Am., 86, 1255-1269, 1996.

Somerville, P., N. Smith, R. Graves, N. Abrahamson, Modification of empirical strong ground motion attenuation results to include the amplitude and duration effects of rupture directivity, Seismol. Res. Lett., 68, 199-222, 1997.

Wald, D., V. Quitoriano, T. Heaton, H. Kanamori, C. Scrivner, and C. Worden, TriNet "ShakeMaps": Rapid generation of peak ground motion and intensity maps for earthquakes in southern California, Earthquake Spectra, 15, 537-556, 1999.



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