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Figure 5.1 shows the location of the network, its relationship to the San Andreas fault, sites of significance from previous and ongoing research using the HRSN, relocated earthquake locations from 1987-1998.5, routine locations of seismicity since August 2002, and the epicenter of the 1966 M6 earthquake that motivated the PPE. The HRSN records exceptionally high-quality data, owing to its 13 closely spaced three-component borehole sensors (generally emplaced in the extremely low attenuation and background noise environment at 200 to 300 m depth (5.1)), its high-frequency wide bandwidth recordings (0-125 Hz), and its low magnitude detection threshold (recording events below magnitude -1.0).
Several aspects of the Parkfield
region make it ideal for the study of small earthquakes and their
relationship to tectonic processes. These include the fact that the
network spans the expected nucleation region of a repeating
magnitude 6 event and a significant portion of the transition from locked to creeping
behavior on the San Andreas fault, the availability of
three-dimensional P and S velocity models (Michelini and McEvilly, 1991), a
seismicity catalogue that is complete to very low magnitudes and
that includes at least half of the M6 seismic cycle, a well-defined and
simple fault segment,
a homogeneous mode of seismic energy release as indicated by the
earthquake source mechanisms (over 90
right-lateral strike-slip),
and the planned drilling zone and penetration and instrumentation site
of the San Andreas Fault deep observatory at depth experiment
(SAFOD) (see: http://www.earthscope.org/safod/index.html or
http://www.iris.iris.edu/HQ/EarthScope/EarthScope.saf.html).
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In a series of journal articles and Ph.D. theses, we have presented the cumulative, often unexpected, results of U.C. Berkeley's HRSN research efforts (see: www.seismo.berkeley.edu/seismo/faq/parkfield_bib.html). They trace the evolution of a new and exciting picture of the San Andreas fault zone responding to its plate-boundary loading, and they are forcing new thinking on the dynamic processes and conditions within the fault zone at the sites of recurring small earthquakes.
about 4.5. The
data acquisition system operated quite reliably until late 1996,
when periods of unacceptably high down time developed. During this period
as many as
7 of the remote, solar-powered telemetered stations were occasionally
down simultaneously
due to
marginal solar generation capacity, old batteries, and
recording system outages of a week or more were not uncommon. In July of 1998
the original data acquisition system
failed permanently. This system
was a modified VSP recorder acquired from LBNL, based on a 1980-
vintage LSI-11 cpu and a 5 MByte removable Bernoulli system disk
with a 9-track tape drive, configured to record both triggered
microearthquake and Vibroseis data (discontinued in 1994, Karageorgi et
al., 1997). The system was remote and completely autonomous, and
data
tapes were mailed about once a month to Berkeley for processing and analysis. The old
system also had a one-sample timing
uncertainty and a record length limitation because the tape write system
recovery after event detection was
longer than the length of the record, leaving the system off-line after record
termination and until write recovery was completed.
In July, 1999 we had to reduce the network to four RefTeks at critical sites that would ensure continuity in monitoring at low magnitudes and the archive of characteristic events for studying the evolution of their recurrence intervals. Properties of the 10 original sites are summarized in Table 5.2.
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We have also added three new borehole stations at the NW end of the network as part of the SAFOD project, with NSF support, to improve resolution at the planned drilling target on the fault. Figure 5.1 illustrates the location of the proposed drill site (star), the new borehole sites, and locations of earthquakes recorded by the initial and the upgraded/expanded HRSN.
These three new stations use similar hardware to the main network, with the addition of an extra channel for electrical signals. Station descriptions and instrument properties are summarized in Tables 5.1 and 5.2. All HRSN Q730 data loggers employ FIR filters to extract data at 250 and 20 Hz (Table 5.3).
The remoteness of the drill site and new stations require an intermediate data collection point at Gastro Peak, with a microwave link to the CDF facility. The HRSN stations use SLIP to transmit TCP and UDP data packets over bidirectional spread-spectrum radio links between the on-site data acquisition systems and the central recording system at the CDF. Six of the sites transmit directly to a router at the central recording site. The other seven sites transmit to a router at Gastro Peak, where the data are aggregated and transmitted to the central site over a 4 MBit/second digital 5.4 GHz microwave link. All HRSN data are recorded to disk at the CDF site. A modified version of the REDI real-time system detects events from the HRSN data, creates event files with waveforms from the HRSN and sends the event data in near real-time to UC Berkeley. Currently the continuous data is being migrated to DLT tape when local disk space fills up, and the tapes are mailed to the BSL for long-term storage. Efforts are being made to acquire funding to make this data Internet accessible to the research community through the NCEDC.
The upgraded system is compatible with the data flow and archiving common to all the elements of the BDSN/NHFN and the NCEDC, and is providing remote access and control of the system. It is also providing data with better timing accuracy and longer records, which are to eventually flow seamlessly into NCEDC. The new system also solves the problems of timing resolution, dynamic range, and missed detections, in addition to providing the added advantage of conventional data flow (the old system recorded SEGY format).
A feature of the new system that has been particularly useful both for routine maintenance and for pathology identification has been the Internet connectivity of the central site processing computer and the station data loggers with the computer network at BSL. Through this connection, select data channels and on-site warning messages from the central site processor are sent directly to BSL for evaluation by project personnel. If, upon these evaluations, more detailed information on the HRSN's performance is required, it can also be directly accessed. Analysis of this remotely acquired information has been extremely useful for trouble shooting by allowing field personnel to schedule and plan the details of maintenance visits to Parkfield. The connectivity also allows certain data acquisition parameters to be modified remotely when needed, and commands can be sent to the central site computer and data loggers to modify or restart processes when necessary.
The network connectivity allows analysts at the BSL to routinely perform checks on the system health of the HRSN and its data quality. One example of a technique used by BSL analysts involves the use of teleseismic arrivals from deep focus earthquakes. Since seismic waves from such events impose a near simultaneous and vertically incident plane wave of relatively uniform amplitude on all HRSN stations, seismograms from these events can be used to assess relative station responses across the network and help identify pathologies in station polarities, individual component failures and other response characteristics.
Figure 5.2 shows an example of a recent teleseism recorded on the DP1 (vertical) channel across the network. Not shown are recordings from stations MMNB and VARB. The initial display of seismograms from this teleseism showed these station components to be responding abnormally at the time of the earthquake. Based on this teleseismic result other remotely acquired information was uploaded from the HRSN and it was determined that these components were indeed malfunctioning. Subsequent field visits were then scheduled and the necessary repairs made.
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The network connectivity also allows remote monitoring of the background noise levels being recorded by the HRSN stations. For example shown in Figure 5.3 are power spectral density plots of background noise for vertical components of the 7 HRSN stations that are most critical for monitoring seismicity in the region containing SAFOD. The PSD analysis gives a rapid assessment of the HRSN seismometer responses across their wide band-width. By routinely generating these plots with data telemetered from Parkfield, changes in the seismometer responses, often indicating problems with the acquisition system, can be easily identified, and corrective measures can then be planned and executed on a relatively short time-frame.
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Triggered event data for the HRSN is also telemetered in near real time to the BSL, and this allows for rapid evaluation of the triggered data. This year we have implemented a semi-automated waveform and trigger review procedure using a graphical user interface (GUI). This procedure is now being used to review the triggered waveform data daily to discriminate between earthquake and non-earthquake events and to pick P and S phases of the local events. In the process, our analyst/field technician also makes note of obvious problems with station/component specific earthquake recording, and this malfunction information is used to identify maintenance needs for the HRSN.
The sophisticated HRSN data acquisition involves integration of a number of distinct components at each station (i.e., sensor, preamp, solar panels, solar regulator, batteries, Freewave radio, antenna, lightening arresters, and associated cabling, connectors and grounds) and radio telemetry apparatus between the seismic stations, telemetry relay stations, and the central processing site on the CDF site in Parkfield.
This complex integration of station and communication components combined with a variety of associated concerns (e.g., ground loops, cable resistances, radio feedback into recording equipment at stations, radio interference between stations, marginal line of site paths, cloud cover and solar power, the integration of older (pre-upgrade) hardware components with new components, old component deterioration and failures, and malfunctioning and unexpected performance characteristics of newer components) all make identification of specific causes of network generated (i.e. artificial) noise difficult to identify.
Exhaustive and iterative testing of HRSN performance has identified two primary causes for observed artificial noise remaining in the system (i.e. solar regulator spiking and preamp self-noise generation). We have designed and have implemented or are in the process of implementing fixes for these problems. We are also continuing to improve the HRSN event detection sensitivity by refining the HRSN triggering scheme.
Figure 5.4 shows the preamp noise reduction effect observed on background noise signals at three vertical components of the HRSN when gains are raised from x80 to x1,000. Considerable signal hash is seen at gain levels of x80 (top waveform in each station pair), and significantly reduced when gains are increased to x1,000 (lower waveforms). Since we are also interested in recording large earthquakes on-scale, simply increasing gain levels on all stations is not the preferred solution, since doing so causes the recording system to saturate at much lower magnitudes. Instead we are attempting to redesign the preamps using modern components to reduce the noise levels at the lower gain levels. However our attempts at redesign have not yet yielded satisfactory results.
Since a primary objective of the HRSN is to monitor the evolving patterns of the numerous small earthquakes that occur at very low magnitudes, and since this objective also complements the scientific objectives of the recently funded SAFOD experiment, it is important to address the preamp noise problem in a timely manner. We have opted, therefore, to raise the gain levels for the near-term on all the station preamps from x80 to x1,000. These gain changes are currently (late August, 2003) being implemented, and we estimate that the number of small earthquakes we will detect will increase by a factor of 2 to 3. We will continue investigating preamp redesigns until a suitable alternative is found at which time we will install the new preamps and lower the preamp gain back to x80-allowing both the increased detection of small events and the on-scale recording of events up to about magnitude 4 to 4.5.
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Shortly after being recorded to disk, event triggers for the individual station data are determined and a multi-station trigger association routine then processes the station triggers and identifies potential earthquakes. For each potential earthquake trigger, 30 second waveform segments are then collected for all stations and components, assigned a unique event identifier (compatible with the NCEDC classification scheme) and saved as an event gather. Event gathers are then periodically telemetered to BSL and included directly into the NCEDC earthquake database (dbms) for analysis and processing.
An ongoing effort has been the development of a new earthquake triggering scheme, with the goal of routinely detecting SAFOD area events to magnitudes below -1.0. A first cut version of the new scheme has been implemented and is already detecting earthquakes at an increased rate-nearly 3 times the number of earthquakes detected before the upgrade.
In order to facilitate the processing and archiving of this large number of events (approx. 150 per month), BSL personnel have recently developed a Graphical User Interface (GUI). The GUI is integrated with the NCEDC dbms and allows review of the waveforms from every triggered event. Initial analysis of the data using the GUI involves review of the waveforms and classification of the event as an earthquake or non-earthquake event. The GUI also allows the analyst to log potential network problems that become apparent from the seismograms. The HRSN analyst then classifies the event as a local, distant-local, regional, or teleseismic event and then systematically hand picks the P- and S-phases for the local and distant local events.
Picking of the numerous microearthquake events is no mean task. On average about 7 P-phases and 4 S-phases are picked for each event, putting the total number of annual phase picks for the HRSN data on the order of 19,000 to 20,000. We have experimented with algorithms that make initial auto-picks of the phase arrivals, but have so far found picking by hand to be an advantage since it forces the analyst to review each pick carefully while at the same time allowing him to assess the state of health of recording on each station-component in detail. In all our tests, repicked autopicks have also invariably resulted in catalog locations that are significantly more scattered and that have higher residuals than locations done with purely hand-picked data.
A peculiarity of processing very small earthquake data, is that multiple events
commonly occur within a few seconds of one another (Figure 5.5).
The close timing of these events does not allow the local triggering algorithm
to recover from one event before another occurs. As a result, the central site
processor often does not trigger uniquely for each event. In such cases only one,
30 sec waveform gather and one earthquake identifier will be created for all the
events. These multiple earthquake records (MER) account for only 3 to 5 of the total
seismicity recorded by the HRSN. However, there are times when this rate rises
to over 10. In order to assign each event in an MER a unique event identifier
for the NCEDC dbms and to make picking and automated processing of these events more manageable
an additional feature of the GUI was developed that allows the analyst to "clone"
MER into separate gathers for each event.
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Initial locations are then performed and phase residuals analyzed in order to determine whether severe pick outliers must be removed or adjusted. Unstable location solutions based on events with few picks are also assessed to see if the addition of marginal phases will improve the stability of the location determination.
After any required pick adjustments have been made, the events are then relocated, and combined with error information to allow ranking of the confidence of location quality.
These procedures have all been put in place and tested over the past year for the new HRSN configuration. Currently we have located 9 months of data recorded by the new HRSN (over 1300 events) and are staying current with ongoing seismicity and also moving backwards in time to pick and locate the earlier data collected since early 2001.
We now have enough data and are confident enough with the procedures to begin organizing the locations for formal inclusion into the NCEDC dbms and dissemination to the community. These efforts are now underway. We are also in the early stages of establishing a scalar seismic moment catalog for the new HRSN events that is also to be included in the NCEDC dbms.
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Under Bob Nadeau's and Doug Dreger's general supervision, Rich Clymer, Wade Johnson, Bob Uhrhammer, Doug Neuhauser, Don Lippert, Bill Karavas, John Friday, Pete Lombard, and Lane Johnson all contribute to the operation of the HRSN. Bob Nadeau prepared this chapter with the assistance of Bob Uhrhammer and Wade Johnson.
During the period of this report, the operation and maintenance of the HRSN and the processing and archiving of its data was supported in large part by the USGS, through the NEHRP External Grants Program (grants: 02HQGR0067 and 03HQGR0065). NSF also provided support for the expansion of the HRSN near the SAFOD drill site through grant EAR-9814605.
Daley, T.M. and T.V. McEvilly, Shear wave anisotropy in the Parkfield Varian Well VSP, Bull. Seism. Soc. Am., 80, 857-869, 1990.
Karageorgi, E., R. Clymer and T.V. McEvilly, Seismological studies at Parkfield. IV: Variations in controlled-source waveform parameters and their correlation with seismic activity, 1987-1994, Bull. Seismol. Soc. Am., 87, 39-49, 1997.
Michelini, A. and T.V. McEvilly, Seismological studies at Parkfield: I. Simultaneous inversion for velocity structure and hypocenters using B-splines parameterization, Bull. Seismol. Soc. Am., 81, 524-552, 1991.
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