Subsections


Northern California Earthquake Monitoring

Introduction

Routine 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 10 years, the BSL has invested in the development of the hardware and software necessary for an automated earthquake notification system (Gee et al., 1996; 2003a). 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.

Today, the BSL and USGS system forms the Northern California Management Center (NCMC) of the California Integrated Seismic Network (Chapter 2).

Northern California
Management Center

The details of the Northern California processing system and the REDI project have been described in past annual reports. In this section, we will describe how the Northern California Management Center fits within the CISN system, detail recent developments, and discuss plans for the future development.

Figure 2.3 in Chapter 2 illustrates the NCMC as part of the the CISN communications ring. The NCMC is a distributed center, with elements in Berkeley and Menlo Park. The 35 mile separation between these two centers is in sharp contrast to the Southern California Management Center, where the USGS Pasadena is located across the street from the Caltech Seismological Laboratory. As described in Chapter 2, the CISN partners are connected by a dedicated T1 communications link, with the capability of falling back to the Internet. In addition to the CISN ring, the BSL and the USGS Menlo Park have a second dedicated communication link to provide bandwidth for shipping waveform data and other information between their processing systems.

Figure 9.1 provides more detail on the current system at the NCMC. At present, two Earthworm-Earlybird systems in Menlo Park feed two "standard" REDI processing systems at UC Berkeley. 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 Earthworm-Earlybird-REDI systems perform the standard detection, location, estimation of $M_{d}$, $M_{L}$, and $M_{w}$, as well as processing of ground motion data. The computation of ShakeMaps is also performed on two systems, one in Menlo Park and one in Berkeley, as described below. An additional system performs finite-fault processing and the computation of higher level ShakeMaps.

Figure 9.1: Detailed view of the current Northern California processing system, showing the two Earthworm-Earlybird-REDI systems, the two ShakeMap systems, and the finite-fault system.
\begin{figure}\begin{center}
\epsfig{file=nc_jns.ps, width=8cm, bbllx=54,bblly=233,bburx=570,bbury=690}\end{center}\end{figure}

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 9.2).

Earthquake information from the joint notification system is distributed by pager/cellphone, 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.

Figure 9.2: 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.
illustration

2003-2004 Activities

Most of the effort in the last year has gone toward designing the Northern California Seismic System.

System Development

As part of ongoing efforts to improve the monitoring systems in northern California, the BSL and the USGS Menlo Park have started to develop the next generation of the northern California joint notification system or the Northern California Seismic System (NCSS).

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 eight years, there are a number of potential problems associated with the separation of critical system elements by $\sim$35 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. In FY01/02, our discussions proceeded to the stage of establishing specifications and determining the details required for design. However, in the last year, most of the development effort focused on CISN activities and specific plans for the "next generation" Northern California system were put on hold. This enforced wait provided the opportunity for some ideas to mature and the current plans for the NCMC are somewhat different from those envisioned in 2001.

The current design draws strongly on the experience in Southern California for the development of TriNet (Figure 9.3), with some modifications to allow for local differences (such as very different forms of data acquisition and variability in network distribution). In addition, the BSL and the USGS want to minimize use of proprietary software in the system. The TriNet software uses three forms of proprietary software: Talerian Smart Sockets (TSS) for inter-module communication via a "publish and subscribe" method; RogueWave software for database communication, and Oracle as the database management system. As part of the development of the Northern California Earthquake Data Center, the USGS and BSL have worked extensively with Oracle databases and extending this to the real-time system is not viewed as a major issue. However, we did take the opportunity to review options for replacing Smart Sockets and RogueWave with Southern California, resulting in joint agreement on replacement packages and shared development effort.

Figure 9.3: Schematic diagram of the planned NCSS system. The design combines elements of the Earthworm, TriNet, and REDI systems
\begin{figure*}\begin{center}
\epsfig{file=nc_box.ps, width=12cm}\end{center}\end{figure*}

In the last two years, BSL staff, particularly Pete Lombard, have become extremely familiar with portions of the TriNet software. We have begun to adapt the software for Northern California, making adjustments and modifications along the way. For example, Pete Lombard has adapted the TriNet magnitude module to northern California, where it is running on a test system. Pete made a number of suggestions on how to improve the performance of the magnitude module and has worked closely with Caltech and the USGS/Pasadena on modifications. One of the recent discoveries with the magnitude module was related to differences in time references as implemented in the database schema.

More recently, the BSL and the USGS Menlo Park undertook the effort to develop and test a design to exchange "reduced amplitude timeseries". One of the important innovations of the TriNet software development was the concept of continuous processing (Kanamori et al., 1999), where waveform data are processed to produce Wood Anderson synthetic amplitudes and peak ground motions constantly. A program called rad produces a reduced timeseries, sampled every 5 secs, and stores it in a memory area called an "Amplitude Data Area" or ADA. Other modules can access the ADA to retrieve amplitudes to calculate magnitude and ShakeMaps as needed. In the the past year, the BSL and the USGS Menlo Park have collaborated to establish the tools for the ADA-based exchange. As part of the software development in northern California, several modules have been developed:

The first, ada2ring, reads from an ADA, creates an EW message, and plops it into a ring where it can be picked up and transferred between computers using the standard EW import/export. The second, ring2ada, will take the EW amplitude message and put it into the ADA. More recently, some development in northern California now allows multiple rads to work on the same time base and feed a single ADA (solving the problem of multiple rads working on the same channels).

This system is currently being tested in northern California, with ADAs in Menlo Park and Berkeley feeding an ADA in Berkeley that is being used to test the magnitude codes.

Additional capability needed in the future includes the capability to filter channels in the ADA (so that NoCal does not send CI timeseries back to SoCal, for example), and the ability to handle location codes (currently in the NC version but not in the SC version).

More information on the Northern California software development efforts is available at http://www.cisn.org/ncmc/.

Figure 9.4: Left: Comparison of the estimates of $M_{w}$ from the complete waveform (CW) method (Dreger and Romanowicz, 1994) with other regional moment tensor (RMT) methods (Thio and Kanamori, 1995; Ritsema and Lay, 1995; Inchinose et al., 2003). Middle: Difference between the estimates of $M_{w}$, plotted against $M_{w}$. Right: Histogram of the magnitude differences.
alt

Figure 9.5: Left: Comparison of the estimates of $M_{w}$ from the CW method with the Harvard CMT method (Dziewonski and Woodhouse, 1981). Middle: Difference between the estimates of $M_{w}$, plotted against $M_{w}$. Right: Histogram of the magnitude differences, showing the very tight distribution.
alt

Figure 9.6: Comparison between $M_{w}$ and $M_{L}$ for earthquakes in northern California. a) & b) Plots of the $M_{L}$ versus $M_{w}$ for all events. The magnitude difference shows a broad spread, with an average of 0.06. c) Map showing the distribution of the magnitude residual, illustrating the strong geographic signal of the Cape Mendocino, Geysers, and eastern California regions. d)-h) Plots showing the distribution of the magnitude residual for different areas.
\begin{figure*}\begin{center}
\epsfig{file=nc_mw3.ps, width=16cm}\end{center}\end{figure*}

$M_{w}$

The REDI system has routinely produced automatic estimates of moment magnitude ($M_{w}$) for many years. However, these estimates were not routinely used as the "official" magnitude until after the 05/14/2002 Gilroy earthquake ($M_{w}$ 4.9, $M_{L}$ 5.1), motivated by the complications created by the publication of multiple magnitudes.

In last year's annual report, we discussed the issue of when to report $M_{w}$. As currently implemented, solutions that meet a minimum quality criterion are automatically reported (a variance reduction of 40% or higher). This criterion appears to work very well and screens out events contaminated by teleseisms. Over the last few years, nearly all events over 4.5 have met this criterion, as have a number of events in the M3.5-4.5 range.

As part of the effort to establish a statewide magnitude reporting hierarchy, we have looked more closely at the estimates of $M_{w}$ (Gee et al., 2003b; 2004b) and the comparison between $M_{w}$ and $M_{L}$.

Two methods of determining regional moment tensor (RMT) solutions have been automated as part of the REDI system - the complete waveform modeling technique (CW) of Dreger and Romanowicz (1994) and the surface wave inversion (SW) of Romanowicz et al. (1993). Comparison between the CW method and other regional moment tensor studies in northern California and the western United States show excellent agreement in the estimate of seismic moment and $M_{w}$ (Figure 9.4). Over 128 events, the average difference in $M_{w}$ is 0.002 magnitude units.

There is also very good agreement between the regional complete waveform method and global methods such as the Harvard Centroid Moment Tensor (CMT) (Dziewonski and Woodhouse, 1981). Figure 9.5 shows the excellent agreement between the CW and CMT solutions, with an apparent bias: the CMT estimates of $M_{w}$ are on average 0.09 higher than the CW estimates. There is a slight suggestion that the residual is increasing at the lower magnitude range, which may be attributed to the global methods reaching the end of their resolution for small earthquakes.

Comparison of the CW estimates of $M_{w}$ with other regional methods and the CMT solutions indicate the robustness of the procedures and the continuity between the regional and global estimates. The difference between the CW and CMT estimates of $M_{w}$ shows more scatter for events in the early 1990s (red crosses in Figure 9.5), reflecting the limited distribution of regional broadband stations at that time. However, the bias is consistent over time. We do not observe a correlation with this difference and the difference in depth, and are currently investigating the influence of different velocity models.

In comparing regional estimates of $M_{w}$ to $M_{L}$ in northern California, we observe an interesting pattern. Overall, the average magnitude difference is quite small (-0.06). However, there is a strong geographic signal in the data. In Figure 9.6, earthquakes with $M_{L} > M_{w}$ are drawn in red; events with $M_{w} > M_{L}$ are drawn in blue. In general, $M_{L}$ is consistently larger than $M_{w}$ with the exception of two regions - the North Bay/Geysers and the Cape Mendocino/Gorda plate areas. In the North Bay Geysers area, the mean difference between $M_{w}$ and $M_{L}$ is 0.26 and in Cape Mendocino, the mean difference is 0.19 (although there are 9 events with a difference greater than 0.5). In both areas, there is evidence that this is due to source processes.

At this point in time, we believe that the variation in the magnitude residuals is due to path effects (particularly for events on the east side of the Sierra).

Routine Earthquake Analysis

In fiscal year 2003-2004, over 12,500 earthquakes were detected by the automatic systems in northern California. This compares with over 8,300 events last year. The 50% increase in the number of events can be attributed to the 2003 San Simeon earthquake. Of those 12,500+ events, over 560 had preliminary magnitudes greater than 3. 46 events had $M_{L}$ greater than 4. The largest event recorded by the system was the $M_{w}$6.5 San Simeon earthquake, which is discussed below.

During the last year, the BSL modified its routine analysis procedures, both in response to the increase in seismicity and to a change in personnel.

In the past, BSL analysts routinely located and determined the magnitude of earthquakes in northern California and adjacent regions. As a general rule, events were 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.

In the past year, we migrated to a system where the analyst imports and exports data from the database for review. A graphical user interface allows the analyst to select events for review, export picks and amplitudes, modify and update those readings, and import new readings as well as new hypocenters and magnitudes. While this new system is an improvement in many ways (in particular, it has allowed us to stop depending on an older PC database), there are several components that are difficult to work with since Seistool is not designed to work in a database environment. This system is an interim solution while the development of the new northern California system is underway.

During this transition, the M6.5 San Simeon earthquake occurred. The significant increase in seismicity associated with its vigorous aftershock sequence, combined with a reduction in staff, presented significant challenges for the earthquake analysis at the BSL (see, for example, Chapter 5). Recognizing that the BSL efforts to locate local earthquakes duplicates efforts of the USGS, the BSL changed their policy for local and regional reading. As of March 2004, BSL staff are no longer reading BDSN records for local and regional earthquakes. Instead, the BSL analysis effort is being redirected to help with the HRSN readings.

The BSL continues to focus on the unique contributions that can be made from the broadband network. From July 2003 through June 2004, BSL analysts reviewed nearly 179 earthquakes in northern California and adjoining areas of magnitude 3.5 and higher. Reviewed moment tensor solutions were obtained for 65 events (through 6/30/2004). Figure 9.7 and Table 9.1 display the earthquakes located in the BSL catalog and the moment tensor solutions.


Table 9.1: Moment tensor solutions for significant events from July 1, 2003 to June 31, 2004 using a complete waveform fitting inversion. Epicentral information from the UC Berkeley/USGS Northern California Earthquake Data Center. Moment is in dyne-cm and depth is in km.

Location Date UTC Time Lat. Lon. MT $M_l$ $M_w$ Mo Str. Dip Rake
Depth
Petrolia 07/04/03 20:52:57.30 40.318 -124.584 21 3.7 4.3 3.69e22 189 88 14
Healdsburg 07/30/03 04:50:06.00 38.679 -122.910 8 4.0 4.0 1.08e22 311 86 176
Geysers 08/03/03 12:00:53.00 38.796 -122.767 5 3.8 4.2 2.18e22 345 64 -153
Ferndale 08/15/03 09:15:13.00 40.984 -125.596 8 5.0 5.1 5.75e23 42 73 -19
Punta Gorda 08/26/03 02:29:55.80 40.451 -124.648 24 3.9 4.4 4.48e22 124 83 170
Orinda 09/05/03 01:39:54.00 37.846 -122.222 14 4.1 4.0 1.15e22 141 76 -177
Petrolia 09/10/03 21:19:24.00 40.493 -125.657 11 3.9 4.2 2.08e22 360 87 -34
Tres Pinos 09/25/03 14:33:45.70 36.820 -121.354 8 3.5 3.5 2.15e21 137 89 176
Geysers 10/03/03 16:56:34.00 38.839 -122.810 5 3.9 4.3 2.73e22 27 51 -95
Eureka 10/09/03 06:11:33.00 41.073 -125.238 8 4.3 4.5 6.70e22 47 49 -5
Lafayette 10/19/03 15:32:52.00 37.907 -122.150 5 3.5 3.5 2.12e21 245 77 -6
Morgan Hill 11/06/03 22:04:13.00 37.209 -121.661 8 3.9 3.8 4.98e21 134 55 89
Nevada 11/15/03 20:11:59.00 38.222 -117.873 8 4.5 4.1 1.60e22 181 81 -160
Nevada 11/15/03 21:19:36.00 38.219 -117.870 8 4.5 4.2 2.21e22 183 81 -168
San Simeon 12/22/03 19:15:56.00 35.706 -121.102 8 6.4 6.5 5.93e25 290 58 78
San Simeon 12/22/03 21:31:36.00 35.713 -121.069 5 4.3 4.2 2.03e22 103 65 64
Templeton 12/22/03 23:52:36.00 35.529 -120.890 5 4.1 4.1 1.58e22 92 50 71
San Simeon 12/23/03 02:06:55.00 35.689 -121.113 8 4.2 4.1 1.64e22 315 59 113
San Simeon 12/23/03 02:56:49.00 35.709 -121.072 8 4.1 3.9 7.98e21 352 63 160
Paso Robles 12/23/03 03:46:00.00 35.699 -121.116 14 4.3 4.3 3.07e22 162 90 -162
San Simeon 12/23/03 05:30:19.00 35.667 -121.110 5 4.2 4.1 1.61e22 116 54 91
Cambria 12/23/03 18:17:11.00 35.654 -121.054 5 4.9 4.7 1.30e23 293 60 98
San Simeon 12/25/03 05:20:13.00 35.706 -121.102 5 4.2 3.9 7.03e21 282 51 79
San Simeon 12/25/03 05:21:59.00 35.652 -121.094 5 4.2 4.0 1.05e22 293 58 91
San Simeon 12/25/03 06:34:49.00 35.657 -121.091 5 4.1 3.9 9.05e21 295 50 73
San Simeon 12/25/03 09:45:55.00 35.666 -121.091 5 4.1 3.9 7.31e21 308 49 99
Templeton 12/25/03 11:50:01.00 35.553 -120.839 8 4.4 4.5 5.30e22 313 58 131
San Simeon 12/27/03 21:29:39.00 35.668 -121.109 5 4.0 4.0 1.00e22 331 42 46
San Simeon 01/02/04 10:44:51.00 35.705 -121.153 5 3.9 4.1 1.69e22 322 49 105
San Simeon 01/02/04 10:47:39.00 35.682 -121.180 5 4.2 4.2 2.38e22 327 56 112
San Martin 01/31/04 02:11:23.00 37.099 -121.568 8 3.7 3.7 3.46e21 136 87 133
Bakersfield 02/14/04 12:43:11.00 35.052 -119.125 5 4.6 4.6 8.50e22 50 60 49
Geysers 02/18/04 20:37:46.00 38.834 -122.765 5 4.4 4.5 5.46e22 14 50 -100
San Juan Bautista 03/16/04 06:38:32.00 36.806 -121.521 5 4.6 4.3 3.01e22 42 82 13
San Simeon 03/17/04 23:53:07.00 35.735 -121.075 5 4.8 4.5 5.50e22 298 49 88
San Simeon 06/05/04 02:46:51.00 35.548 -121.298 5 3.6 3.6 2.63e21 322 81 170
San Simeon 06/06/04 08:40:52.00 35.552 -121.284 8 3.8 3.8 6.13e21 238 90 -1
Mendocino 06/17/04 18:43:55.00 40.754 -125.076 5 3.7 3.8 5.64e21 149 79 -165
Offshore Baja Calif. 06/15/04 22:28:49.00 32.383 -117.847 5 5.3 5.0 3.14e23 231 74 -18
Lakeview,OR 06/27/04 07:00:13.00 42.208 -120.318 5 5.0 4.1 1.64e22 168 67 -105
Lakeview,OR 06/30/04 12:21:50.00 42.175 -120.295 5 5.0 4.7 1.09e23 165 61 -114


Figure 9.7: Map comparing the reviewed moment tensor solutions determined by the BSL in the last 11 years (blue) and those from the last fiscal year (red).
alt

San Simeon Earthquake

Figure 9.8: Left: Map showing the seismicity from 1966-2003 in the region of the San Simeon earthquake. Earthquakes with magnitude less than 3 are plotted as points; events with magnitude greater than 3 are plotted as squares. Moment tensors solutions over the last 10 years are plotted in green. The location and mechanism of the M6.5 event are shown in red. Also shown is the location and first-motion mechanisms of the 1952 Bryson earthquake (Dehlinger and Bolt, 1987). Right: Earthquakes and moment tensors in the region of the San Simeon earthquake since the 12/22/03 mainshock. The aftershock region extends from the mainshock to the southeast. The solid line indicates the extent of the line source determined on the 22nd for improving the ShakeMap. Triangles indicate the location of stations used in the ShakeMap - yellow indicates near real time stations; grey indicates stations without communications that were not available in near real time. Stations mentioned in the text - Cambria, Templeton, and Parkfield - are labelled.
\begin{figure*}\begin{center}
\epsfig{file=nc_setting.ps, width=3.1in}\epsfig{file=nc_stations.ps, width=3.1in}\end{center}\end{figure*}

The December 22, 2003 M6.5 San Simeon earthquake is the largest event in California since the 1999 M7.1 Hector Mine earthquake and resulted in 2 deaths and over 50 injuries (Figure 9.8). Preliminary reports suggest that the most severe damage was to unreinforced masonry structures that had not yet been retrofitted (e.g., EERI, 2004). Significant damage to water tanks has also been reported and a number of wineries suffered significant loss of wine barrels and their contents. In the following description, we draw upon the San Simeon report of the CISN (Gee et al., 2004a).

The automated procedures of earthquake location and magnitude determination worked well (Tables 9.2 and 9.3). A preliminary location was available within 30 seconds, and a final location with a saturated duration magnitude ($M_{d}$) of 5.6 was released approximately 4 minutes after the event occurred. An updated and more reliable local magnitude ($M_{L}$) of 6.4 was released 30 seconds later, and the final moment magnitude ($M_{w}$) of 6.5 was released 6.5 minutes after the earthquake origin time. The automatically determined first motion mechanism and moment tensor solution each showed a reverse mechanism, in excellent agreement with the reviewed mechanisms.

One of the most challenging aspects of this event was the lack of ShakeMap-quality stations in the vicinity of the earthquake, particularly stations with communications capability. The closest such station to the epicenter with continuous telemetry was the UC Berkeley station PKD, in Parkfield, CA, at a distance of 56 km. The California Geological Survey (CGS) operates three stations in the area - Cambria at 13 km, San Antonio Dam at 22 km, and Templeton at 38 km from the epicenter. However, since these stations did not have telemetry, their data were not available until hours after the earthquake. Caltech/USGS Pasadena operate stations to the south of the event, but their nearest station was 60 km from the epicenter.

Figure 9.9: Left: Map of instrumental intensity for the M6.5 San Simeon earthquake, after correction for the fault distance calculation. Right: Close-up of the Community Internet Intensity Map for the San Simeon earthquake.
\begin{figure*}\begin{center}
\epsfig{file=nc_intensity.eps, width=2.5in}\epsfig{file=nc_ciim.ps, width=2.5in}\end{center}\end{figure*}

Figure 9.10: The temporal evolution of ShakeMaps for the San Simeon earthquake, as illustrated through the intensity maps. All times are local.
\begin{figure*}\begin{center}
\epsfig{file=nc_shakemap.eps, width=6.5in}\end{center}\end{figure*}

The first automatic ShakeMap was posted 8 minutes after the event, based on the $M_{L}$ of 6.4 and with 29 stations contributing. The first update occurred 6 minutes later based on the revised $M_{w}$ of 6.5 and the addition of 45 stations (mostly distant). Throughout December 22nd and 23rd, the ShakeMap was updated multiple times with additional data (including the observations from the CGS stations at Templeton and Cambria) and as more information about the earthquake rupture (fault orientation and length) became available.

The San Simeon event provided an important proving ground for the finite fault processing (see section III.13) for more details. The automatic codes performed correctly, although a configuration mistake caused the inversion to use the lower quality of the two moment tensor solutions obtained. As a result, the finite-fault system did not obtain optimal results. The computations proved to be relatively fast in this implementation, with the line source inversion completed approximately eight minutes after the event occurred and the resulting predicted ground motions available six minutes later. The 2-D inversion and the predicted ground motions were completed 30 minutes after the earthquake.

Although the automated system had a configuration error, the processed data were available for rapid review by the seismic analyst. Using available strong motion and broadband displacement waveforms, both line-source and planar-source analyses indicated that this event ruptured nearly horizontally to the SE from the epicenter, essentially in the null-axis direction of the NE dipping reverse mechanism. Because of this nearly horizontal, along dip rupture, it was not possible to uniquely determine the causative fault plane, although there was a slight preference for the NE dipping plane which is consistent with the aftershock distribution. The southeast rupture produced directivity-amplified ground motions toward the SE that is consistent with felt reports and the damage in Paso Robles. The preliminary results from the reviewed finite source analysis were included in the ShakeMap system approximately 4 hours after the earthquake.

Only a few ShakeMaps have made use of finite source information in the past - the 1999 Hector Mine and 2001 Denali earthquakes are examples. As noted earlier, the use of finite source information is not automatically included in the ShakeMaps available to the public. Because this component of the system has been seldom exercised, the San Simeon earthquake uncovered a problem in the code used to compute distances to a rupture segment. As a result, the ShakeMaps in Figure 9.10c-f underestimate ground motions near the middle of the fault trace. Figure 9.9 displays the revised intensity map, which shows a broader area of intensity VIII than observed in Figure 9.10f.

The lack of nearby ShakeMap-quality stations resulted in maps with an overwhelming reliance on theoretically predicted ground motions. Figure 9.10 illustrates the evolution of the intensity map with time. In Figure 9.10a and b, the source is modeled as a point source and the maps show areas of significant ground motions south and north of the epicenter. Four hours after the earthquake, information about the fault rupture was added (c), based on the inversion results of Dreger et al. (2004). The addition of the finite fault information (in this case, limited to the linear extent and orientation of the fault) focused the higher ground motions to the southeast and showed more damaging shaking in the vicinity of Paso Robles. However the most significant change in the ShakeMap came with the addition of data from the Templeton station, seven hours after the earthquake (d). The high shaking observed at Templeton (47% g), raised all the intensity levels significantly. Maps (e) and (f) show the intensity level after the addition of the Cambria data and the map as of January 5, 2004.

As seen in Figure 9.10c, the addition of information about the fault length and orientation was an important addition to the ShakeMap, particularly given the sparseness of instrumentation. This methodology provides an important tool in areas with limited station distribution to improve ShakeMaps.


Table 9.2: Timing of earthquake information for the San Simeon earthquake.
CISN Timing
Earthquake Information UTC Time Elasped time
    (HH:MM:SS)
Origin Time (OT) 12/22 19:15:56 00:00:00
Quick Look hypocenter 12/22 19:16:20 00:00:24
Final hypocenter & $M_{d}$ 12/22 19:20:25 00:04:29
Local Magnitude 12/22 19:20:58 00:05:02
First Motion mechanism 12/22 19:21:36 00:05:40
Moment Tensor mechanism & $M_{w}$ 12/22 19:22:40 00:06:44
1st ShakeMap completed ($M_{L}$ 6.4) 12/22 19:24:13 00:08:17
Analyst review/1st aftershock probabilty 12/22 19:32:00 00:16:04
2nd ShakeMap completed ($M_{w}$ 6.5) 12/22 19:38:28 00:22:32
Analyst review of moment tensor 12/22 20:16:49 01:00:53
1st Internet Quick Report at cisn-edc.org 12/22 20:30:- 01:14:-
Analyst review of line source 12/22 21:54:- 02:38:-
ShakeMap update with line source 12/22 23:33:- 04:17:-
ShakeMap update with Templeton data 12/23 02:34:- 07:18:-
Earthquake Report at cisn.org 12/23 17:34:- 22:18:-
Updated aftershock probabilty 12/23 22:54:- 27:38:-
ShakeMap update with Cambria data 12/24 00:28:- 29:12:-
Preliminary science report at cisn.org 12/24 23:44:- 52:28:-



Table 9.3: Comparison of parameters as determined by the automatic earthquake processing system with those obtained after analyst review. Note that the value of $M_{d}$ is lower than the $M_{L}$ or $M_{w}$ as the duration magnitude estimate generally saturates around M4.0-4.5. FM - first motion; MT - moment tensor.
Parameters of the Dec 22, 2003 San Simeon Earthquake
  Automatic Reviewed
Origin Time (UTC) 19:15:56.24 19:15:56.20
Location (latitude longitude) 35.7058 -121.1013 35.7043 -121.1032
Depth (km) 7.59 7.34
$M_{d}$ 5.62 5.35
$M_{L}$ 6.43 6.44
$M_{w}$ 6.50 6.50
FM Mechanism (strike/dip/rake) 297/56/97 105/35/80 305/60/71 160/35/120
MT Mechanism (strike/dip/rake) 294/59/83 128/32/102 290/58/78 131/34/108
MT Depth (km) 8.0 8.0


Other Events of Interest

Other significant events in fiscal year 2003-2004 include a small swarm in the Livermore area (largest magnitude M3.5) in January 2004, a M4.4 near the Geysers on February 18th, an M4.3 near San Juan Bautista on March 16th, an M4.2 near King's Beach (Lake Tahoe) on June 3rd, and a sequence in southeastern Oregon which began on June 23rd and continues as of this writing.

Teleseisms

In addition to the routine analysis of local and regional earthquakes, the BSL also processes teleseismic earthquakes. Taking advantage of the ANSS 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 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 and Doug Neuhauser contribute to the development of software. Rick McKenzie, Doug Dreger, Dennise Templeton, Peggy Hellweg, and David Dolenc 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

Dehlinger, P., and B. Bolt, Earthquakes and associated tectonics in a part of coastal central California, Bull. Seis. Soc. Am., 77, 2056-2073, 1987.

Dreger, D., Lombard, O., Boatwright, J., Wald D., and L. Gee, Finite source models of the December 22, 2003 San Simeon earthquake and applications to ShakeMap (abstract), Seismol. Res. Lett., 75, 293, 2004.

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

Dziewonski, A.M., T.-A. Chou and J.H. Woodhouse, Determination of earthquake source parameters from waveform data for studies of global and regional seismicity, J. Geophys. Res., 86, 2825-2852, 1981.

Earthquake Engineering Research Institute, Preliminary observations on the December 22, 2003, San Simeon earthquake, http://www.eeri.org/lfe/usa_san_simeon.html, 2004.

Gee, L., D. Oppenheimer, T. Shakal, D. Given, and E. Hauksson, Performance of the CISN during the 2003 San Simeon Earthquake, http://www.cisn.org/docs/CISN_SanSimeon.pdf, 2004a.

Gee, L., J. Polet, R. Uhrhammer, and K. Hutton, Earthquake Magnitudes in California, Seism. Res. Lett., 75(2), 272, 2004b.

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, 1261-1273, 2003a.

Gee, L., D. Dreger, G. Wurman, Y, Gung, B. Uhrhammer, and B. Romanowicz, A Decade of Regional Moment Tensor Analysis at UC Berkeley, Eos Trans. AGU, 84(46) , Fall Meet. Suppl., Abstract S52C-0148, 2003b.

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.

Hardebeck, J., Boatwright, J., Dreger, D., Goel, R., Graizer, V., Hudnut, K., Ji, C., Jones, L., Langbein, J., Lin, J., Roeloffs, E., Simpson, R., Stark, K., Stein, R., and J. Tinsley, Preliminary report on the 22 December 2003 M6.5 San Simeon, California, earthquake, Seismol. Res. Lett., 75, 155-172, 2004.

Ichinose, G., J. Anderson, K. Smith, and Y. Zeng, Source parameters of eastern California and western Nevada earthquakes from regional moment tensor solutions, Bull. Seis. Soc. Am., 93, 61-84, 2003.

Ritsema, J., and T. Lay, Long-period regional waveform moment tensor inversion for earthquakes in the western United States, J. Geophys. Res., 100, 9853-9864, 1995.

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.

Thio, H. K., and H. Kanamori, Moment-tensor inversions for local earthquakes using surface waves recorded at TERRAscope, Bull. Seis. Soc. Am., 85, 1021-1038, 1995.

Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
Questions or comments? Send e-mail: www@seismo.berkeley.edu
© 2004, The Regents of the University of California