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. Starting in the mid 1990s, the BSL invested in the development of the hardware and software necessary for an automated earthquake notification system (Gee et al., 1996; 2003a) called the Rapid Earthquake Data Integration (REDI) project. This system provides rapid determination of earthquake parameters: near real-time locations and magnitudes of Northern and Central California earthquakes, estimates of the rupture characteristics and the distribution of ground shaking following significant earthquakes, and tools for the rapid assessment of damage and estimation of loss. In 1996, the BSL and the USGS began collaborating on a joint notification system for Northern and Central California earthquakes. This 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 joint BSL and USGS system forms the Northern California Earthquake Management Center (NCEMC) of the California Integrated Seismic Network (Section 2), and development is proceeding on the next generation of earthquake reporting software based on Southern California's Trinet system. With partial support from the USGS, the BSL has also embarked on the development and assessment of a system to warn of imminent ground shaking in the seconds after an earthquake has initiated but before strong motion begins at sites that may be damaged (Research Study 26.).

Northern California Earthquake Management Center

Details of the Northern California processing system and the REDI project have been described in previous annual reports. In this section, we describe how the Northern California Earthquake Management Center fits within the CISN system. Figure 3.8 in Section 2 illustrates the NCEMC as part of the the CISN communications ring. The NCEMC is a distributed center, with elements in Berkeley and in Menlo Park. The 35 mile separation between these two centers is in sharp contrast to the Southern California Earthquake Management Center, where the USGS Pasadena is located across the street from the Caltech Seismological Laboratory. As described in Section 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 communications link to provide bandwidth for shipping waveform data and other information between their processing systems. Figure 3.26 provides more detail on the current system at the NCEMC. 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 standard detection and location and estimate $M_{d}$, $M_{L}$, and $M_{w}$ as well as processing ground motion data. The computation of ShakeMaps is also performed on two systems, one in Menlo Park and one in Berkeley. An additional system at the BSL performs finite-fault processing and computes higher level ShakeMaps.

Figure 3.26: 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.

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 small quakes. The high dynamic range data loggers, digital telemetry, and broadband and strong-motion sensors of the BDSN along with the 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 3.27). 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 3.27: Illustration of the earthquake products timeline for the $M_{w}$ 5.4 Alum Rock earthquake of October 30, 2007. Note that all processing was complete within 10 minutes of the origin time.

2007-2008 Activities

System Development

As part of ongoing efforts to improve the monitoring systems in Northern California and to unify the processing systems within the CISN, the BSL and the USGS Menlo Park made progress in the development of the next generation of the Northern California joint notification system for the Northern California Seismic System (NCSS). Figure 3.26 illustrates the current organization of the system. Although this approach functions reasonably well, there are potential problems associated with the separation of critical system elements by $\sim$35 miles of San Francisco Bay. Since FY01-02, we have been working to design and implement software for Northern California operations so that identical, complete systems operate independently at the USGS and UC Berkeley. When CISN started, independently developed systems for monitoring earthquakes operated in Southern and Northern California, Trinet and Earthworm/REDI, respectively. Each of these systems has its strengths and weaknesses, and choices had to be made. The current design for the new Northern California system draws strongly on the development of TriNet in Southern California (Figure 3.28), with 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. One exception is the database program. 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.

Figure 3.28: Schematic diagram of the planned NCSS system. The design combines elements of the Earthworm, TriNet, and REDI systems

During the last few years, BSL staff members, particularly Pete Lombard, have become extremely familiar with portions of the TriNet software. We have continued 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 now 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 biggest programming efforts in the past year has been to make the package leap second compliant. The BSL and the USGS Menlo Park have implemented a system to exchange ``reduced amplitude timeseries." One of the important innovations of the TriNet software development is the concept of continuous processing (Kanamori et al., 1999). Waveform data are constantly processed to produce Wood Anderson synthetic amplitudes and peak ground motions. 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. The BSL and the USGS Menlo Park have collaborated to establish the tools for ADA-based exchange. As part of the software development in Northern California, a number of modules have been developed.

Event Review with Jiggle

CUSP was finally retired as the event review system in the NCEMC in late November, 2006. This program was initially developed in Southern California during the late 1970s - early 1980s and has been used to time earthquakes for a number of years in Northern California. However, the CUSP system became increasingly outdated, as it relied on obsolete hardware. The primary responsibility for the programming and development necessary to make the transition has rested on BSL staff. They implemented the RequestCardGenerator (a module that decides which channels to archive, given a particular earthquake), a waveform archiving module, and Jiggle (the earthquake timing interface) within the Northern California system. The entry of all parameteric earthquake data from realtime processing into the Oracle database and the preparation of station and instrument metadata for insertion into the database were important prerequisites for the transition. The NCEMC and SCEMC collaborated on modifications to Jiggle for use in Northern California, such as the computation of $M_{d}$.

$M_{L}$ and $M_{w}$

The REDI system has routinely produced automatic estimates of moment magnitude ($M_{w}$) for many years. However, wary of complications caused by the publication of multiple magnitudes, 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). Since then, 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; 2004) and the comparison between $M_{w}$ and $M_{L}$. Two methods of determining regional moment tensor (RMT) solutions were originally 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). In FY05-06, processing for the SW algorithm was discontinued; however, CW moment tensors continue to be calculated, reviewed, and reported. Comparison between the results of 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 $M_{w}$. As we transition toward statewide reporting of earthquake information, a comparison of magnitudes calculated for Southern and Northern California becomes important. We have collected a set of events recorded well by digital broadband and and strong motion stations of the Northern California (NC), Berkeley, (BK) and Southern California (CI) networks. Research Study 34. reports on these activities. A new log$A_o$ function has been developed that is valid throughout the state, and a corresponding set of corrections calculated for the collocated broadband and strong motion stations. Research Study 34. reports on the validation of these parameters for Northern and Southern California.

Table 3.14: Moment tensor solutions for significant events from July 1, 2007 through June 30, 2008 using a complete waveform fitting inversion. Epicentral information is from the UC Berkeley/USGS Northern California Earthquake Management 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
Aromas	7/2/2007	19:58:53	36.882	-121.622	8	4.5	4.3	3.06E+22	143	88	-174
Oakland	7/20/2007	11:42:22	37.8	-122.18	5	4.1	4.2	2.52E+22	321	89	168
Geysers	7/20/2007	17:50:20	38.81	-122.8	11	3.7	3.9	9.09E+21	61	75	-41
Chatsworth	8/9/2007	07:58:48	34.258	-118.635	8	4.4	4.4	5.86E+22	298	60	111
Oakland	8/15/2007	07:13:10	37.8052	-122.189	5	3.2	3.2	8.54E+20	140	88	-178
Petrolia	9/8/2007	08:16:28	40.3193	-124.654	14	4.3	4.3	3.30E+22	105	88	177
Clear Lake	9/17/2007	14:43:21	38.86	-122.388	8	3.6	3.6	2.73E+21	91	77	-23
Cuca	10/16/2007	08:53:44	34.356	-117.629	11	4	4	1.31E+22	295	58	114
Alum Rock	10/31/2007	03:04:54	37.4323	-121.776	11	5.6	5.4	1.85E+24	323	87	180
Ferndale	11/9/2007	03:37:11	40.5398	-124.404	21	4.1	4.1	1.80E+22	149	77	157
Pinnacles	11/28/2007	02:30:12	36.5998	-121.208	8	5.1	3.4	1.40E+21	49	78	-15
Geysers	12/1/2007	20:50:12	38.7333	-122.933	4	3.9	3.9	1.97E+22	149	77	152
Pinnacles	12/11/2007	08:04:45	36.554	-121.122	5	3.4	3.5	2.44E+21	287	80	23
Offshore of Mendocino	12/11/2007	19:17:20	40.415	-126.384	8	4.8	4.8	1.98E+23	271	88	-134
Redbluff	1/19/2008	17:18:46	40.175	-122.755	11	4.4	4.5	6.49E+22	352	68	-112
Redbluff	1/19/2008	23:13:05	40.125	-122.759	11	4.9	4.7	1.26E+23	14	90	-10
Nevada	2/7/2008	12:53:09	38.236	-117.855	5	3.28	3.6	2.79E+21	190	83	-148
Geysers	2/24/2008	05:32:10	38.817	-122.809	5	4.01	4	1.30E+22	50	63	-44
Willits	3/10/2008	05:14:27	39.355	-123.277	8	3.44	3.7	4.35E+21	140	86	151
Offshore of Oregon	3/15/2008	14:44:38	42.487	-126.663	18	5.6	5.6	3.48E+24	25	73	-91
Geysers	3/27/2008	21:04:36	38.817	-122.786	8	3.6	3.6	3.04E+21	244	84	23
Offshore of Mendocino	4/10/2008	14:17:35	40.404	-125.639	30	3.89	4.2	2.46E+22	175	89	-174
Eureka	4/21/2008	22:00:54	40.78	-124.208	21	4.2	4.2	2.38E+22	301	61	-79
Reno, NV	4/24/2008	22:47:04	39.525	-119.223	5	3.8	3.7	5.10E+21	236	81	-11
Reno, NV	4/24/2008	22:55:49	39.527	-119.929	5	4.2	4.3	3.81E+22	153	74	-166
Reno, NV	4/26/2008	06:40:11	39.524	-119.932	5	5.13	4.9	3.35E+23	60	85	25
Reno, NV	4/28/2008	11:33:18	39.533	-119.931	5	4.2	4.1	1.77E+22	315	87	-170
Templeton	4/29/2008	17:45:17	35.499	-120.783	8	3.6	3.6	2.95E+21	119	55	93
Willow Creek	4/30/2008	03:03:07	40.836	-123.497	30	5.4	5.4	1.46E+24	359	60	-88
Lake Isabella	5/1/2008	08:11:43	35.471	-118.42	8	4.1	4.1	1.96E+22	291	89	-172
Offshore of Mendocino	5/6/2008	17:18:32	43.171	-126.447	14	4.5	4.5	6.70E+22	123	83	165
Reno, NV	5/8/2008	05:55:01	39.542	-119.92	5	3.5	3.6	3.40E+21	118	82	-167
Ferndale	5/13/2008	04:07:39	40.801	-125.374	11	4.2	4.3	3,29e22	314	86	167
Geysers	5/30/2008	04:48:36	38.776	-122.764	5	3.9	4.1	2.01E+22	250	82	-14
Green Valley	6/4/2008	02:29:04	38.242	-122.184	8	3.9	3.9	8.27E+21	63	87	-12
Oakland	6/6/2008	09:02:54	37.816	-122.075	8	3.5	3.5	2.00E+21	339	84	-162
Reno, NV	6/8/2008	17:53:41	39.546	-119.918	5	3.6	3.7	3.66E+21	195	-54	56
Tres Pinos	6/19/2008	23:57:51	36.681	-121.308	8	3.76	3.8	5.45E+21	40	88	-24
Tom's Place	6/28/2008	14:44:10	37.586	-118.819	11	4.3	3.9	9.89E+21	351	83	-32

Routine Earthquake Analysis

In fiscal year 2007-2008, more than 26,000 earthquakes were detected and located by the automatic systems in Northern California. This compares with over 23,000 in 2006-2007, 30,000 in 2005-2006 and 38,800 in 2004-2005. Many of the large number of events in 2004-2005 are aftershocks of the 2004 Parkfield earthquake. The number of events continues to remain high, because we now receive and process data from a network of seismometers in the Geysers, a region with a high level of small magnitude seismicity. Of the more than 26,000 events, over 200 had preliminary magnitudes of three or greater. Thirty-two events had $M_{L}$ greater than 4. The largest event recorded by the system was the Alum Rock earthquake which occurred on 31 October 2007 with $M_{w}$ 5.4. Other earthquakes with magnitudes greater than 5 occurred off the coast of northernmost California. As described in the 2003-2004 Annual Report, the BSL staff are no longer reading BDSN records for local and regional earthquakes (as of March 2004). This decision was in part intended to reduce duplication of effort between Berkeley and Menlo Park.

Moment Tensor and Finite Fault Analysis

The BSL continues to focus on the unique contributions that can be made from the broadband network. From July 2007 through June 2008, BSL analysts reviewed many earthquakes in Northern California and adjoining areas of magnitude 3.2 and higher. Reviewed moment tensor solutions were obtained for 39 of these events (through 6/30/2008). Figure 3.29 and Table 3.14 display the locations of earthquakes in the BSL moment tensor catalog and their mechanisms.

Figure 3.29: Map comparing reviewed moment tensor solutions determined by the BSL from past years (black) with those from the fiscal year 2007-2008 (grey).

In the past, moment tensor information has been stored in a flat file, with only the fault planes and the moment recorded. The database associated with the CISN software system allows all the information to be stored that is necessary to recalculate the moment tensor. It includes the moment tensor components as well. During this year, we made an effort to enter information for past Northern California events into the database (see Research Study 36.). This project is nearly completed, thanks to the hard work of Angie Chung, Rick McKenzie, and Jennifer Taggart. During this year, two earthquakes were large enough to allow finite fault inversions to be performed: the Alum Rock earthquake (October 31, 2007) (see Research Study 22.) and the Wells, NV earthquake of February 7, 2008 (see Research Study 24.).

Acknowledgements

Peggy Hellweg oversees the REDI system and directs the routine analysis. Peter Lombard and Doug Neuhauser contribute to the development of software. Rick McKenzie, Doug Dreger, Aimin Cao, Sean Ford, Aurelie Guilhem, Ayhi Kim, Ved Lekic, Angie Chung, and Jennifer Taggart contribute to the routine analysis of moment tensors. Peggy Hellweg, Doug Neuhauser, and Bob Uhrhammer contributed to the writing of this section. Partial support for the development and maintenance of the REDI system is provided by the USGS.

References

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

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.

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

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.