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, double-difference 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 (Table 5.1)), its high-frequency wide bandwidth recordings (0-100 Hz), and its low magnitude detection threshold (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), the existing long-term HRSN 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, and a homogeneous mode of seismic energy release as indicated by the earthquake source mechanisms (over 90 right-lateral strike-slip).
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: http://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.
More recently, the Parkfield area has become an area of focus of the Earthscope Project (http://www.earthscope.org) through the San Andreas Fault Observatory at Depth (SAFOD) experiment (http://www.icdp-online.de/sites/sanandreas/news/news1.html). SAFOD is a comprehensive project to drill into the hypocentral zone of repeating M 2 earthquakes on the San Andreas Fault at a depth of about 3 km. The goals of SAFOD are to establish a multi-stage geophysical observatory in close proximity to these repeating earthquakes, to carry out a comprehensive suite of downhole measurements in order to study the physical and chemical conditions under which earthquakes occur and to exhume rock and fluid samples for extensive laboratory studies (Hickman et al., 2004).
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 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 (Vibroseis 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, the network was reduced 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.
We have also added three new borehole stations, with NSF support, at the NW end of the network as part of the SAFOD project to improve resolution of the structure, kinematics and monitoring capabilities in the SAFOD drill-path and target zones. Figure 5.1 illustrates the location of the drill site, the new borehole sites, and locations of earthquakes recorded by the initial and upgraded/expanded HRSN.
The three new SAFOD stations have a similar configuration as the original upgraded 10 station network and include an additional 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 required an installation of 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.
The upgraded and expanded 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).
Because of limitations in bandwidth, a modified version of the REDI system (Chapter 9) is used to detect events in the HRSN data, extract waveform triggers, and transmit the waveform segments to the BSL. However, the December 22, 2003 San Simeon earthquake and its aftershocks sent the HRSN into nearly continuous triggering. As a result, BSL staff disabled the transmission of triggered data.
At present, all continuous 20 sps data streams and 7 vertical component channels at 250 sps are telemetered to the BSL and archived on the NCEDC in near-real-time. All continuous 250 sps data are migrated periodically from HRSN computer in Parkfield to DLT tape. These tapes are then mailed periodically to the BSL and then are processed for archiving at the NCEDC.
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, additional information can also be remotely accessed from the central site processing computer at Parkfield. 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 also allows remote monitoring of the background noise levels being recorded by the HRSN stations. For example shown in Figure 5.2 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.
Smaller scale maintenance issues addressed this year include cleaning and replacement of corroded electrical connections, grounding adjustments, cleaning of solar panels, re-seating, resodering and replacement of faulty pre-amp circuit cards, and the testing and replacement of failing batteries. Larger efforts included the implementation of periodic emergency generator tests, replacement of the central site air conditioning unit, a major insulation, painting and power enhancement effort at our Gastro Peak repeater site to address problems with outages and low power during cold weather, and a switch to an alternative sensor on the VARB station deep string due to the failure of one of the 1877' deep sensor components.
The HRSN data acquisition involves integration of a number of distinct components at each station (i.e., sensor, pre-amp, 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 pre-amp self-noise generation). Over the past year we have designed and have implemented fixes for these problems.
Figure 5.3 shows the pre-amp 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 pre amps 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, the pre-amp noise problem was a priority maintenance item. We have opted, therefore, to raise the gain levels for the near-term on all the station pre-amps from x80 to x1,000. By early October of 2003, these gain changes were implemented at all 13 HRSN stations. Plans are to continue investigating pre-amp redesigns until a suitable alternative is found at which time we will install the new pre-amps and lower the pre-amp 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.
During joint planning of the cleanup effort, we made it clear that it was important to retain connectivity to the seismic sensors at VARB, and the importance of this fact was acknowledged by the USGS personnel managing the effort. Subsequent plans for the cleanup specified coordination of the effort with field personnel from UC Berkeley. Unfortunately, BSL staff were not notified of the cleanup date, nor the specifics of the cleanup plan (which apparently included severing the deep strings cable at the well head and of course we would have loudly objected to).
Because VARB has the deepest HRSN borehole sensor and is centrally located within the network, it is a critical site for the HRSN. In addition at that time, we were in the process of preparing VARB and all the HRSN stations to make high-gain recordings of the controlled source shots from the SAFOD related 50 km line experiment to aid in characterizing the velocity and Fault Zone Guided Wave propagation structure in the region around SAFOD. Needless to say, the loss of VARB severely hampered these efforts. The situation was further complicated by the details of how the Varian well cable disconnect was made.
It was important for a number of scientific reasons to reconnect our VARB acquisition system to a sensor of known depth. However, there are 48 pairs of wires in the severed seismic string cable, and unfortunately, the mapping of these wires to their corresponding sensors was not documented when the cable was severed. In addition because the cable was severed at the well head, some 100' of trenching and cable was needed to reconnect the sensor and recording installation.
The estimated cost for the additional man-hours, parts, travel and lodging needed to do the necessary repairs at VARB was several thousand dollars. With emergency funding from NEHRP and assitance from the USGS field technician at Parkfield, we put VARB back on-line and made the high-gain HRSN adjustments in time for the 50 km line experiment shots.
In a special section of Geophysical Research Letters from May of 2004, several papers make significant use of the HRSN data for characterizing the SAFOD area and illustrate the role that the HRSN data have played in the SAFOD effort over the past year (e.g., Oye et al., 2004; Roecker et al., 2004; Thurber et al., 2004; Nadeau et al., 2004).
Most of our efforts during the 2000-2002 were spent on implementing the emergency upgrade and SAFOD expansion of the HRSN, and routine processing of the data collected during that period was deferred until after upgrade and installation efforts were completed. In 2003 we began in earnest the task of routine processing of the ongoing data that was being collected. Our initial focus was on refining and developing our processing procedures to make the task more efficient and to ensure quality control of the processed catalogs. We also began working back in time to fill in the gap that developed during the deferment period. Because routine processing of the post-San Simeon data is effectively impossible at this time, because of the overwhelming number of aftershocks, we have suspended our efforts at processing the ongoing data and focused our efforts at filling in the complete gap of unprocessed data (i.e., back to March of 2001). Outlined below in the "Pre-San Simeon Processing" subsection are the procedures and issues related to this effort. In the subsequent subsection we illustrate and discuss briefly the issues that need to be addressed in order to process the post-San Simeon event data.
Shortly after being recorded to disk on the central site HRSN computer, 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 that is detected, a unique event identification number (compatible with the NCEDC classification scheme) is assigned. Prior to San Simeon earthquake of December 22, 2003, 30 second waveform segments were then collected for all stations and components and saved to local disk as an event gather, and event gathers were 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 detection 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 currently detecting real 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 the increased number of potential earthquakes ( 350 per month), the BSL has recently developed a Graphical User Interface (GUI). The GUI is integrated with the NCEDC dbms and allows review of the waveforms from every potential 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. The GUI also allows the analyst to log potential network problems that become apparent from the seismograms. The HRSN analyst then classifies the earthquakes as either 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 (for the period Sept. 2002 - Aug. 2003 the number of picked events was 2000).
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 to 22,000. We have experimented with algorithms that make initial auto-picks of the phase arrivals, but have so far found picking by hand to be significantly more accurate and has the added advantage of allowing the analyst to assess the state of health of each station-component. In all our tests, autopicks have also invariably resulted in some missed events and catalog locations that are significantly more scattered and with 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. 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.
Quality Control. Once false triggers have been removed and picks for the local and distant local events have been completed, quality control on the picks is made to ensure that all picks have phase and weights assigned, that extraneous characters have been removed from the pick files, that double station-phase picks have not inadvertently been made, and that no repicks of the same event had been accidentally made during any cloning that was performed. 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 for the new HRSN configuration. Currently we have located 13 months of local earthquakes recorded by the new HRSN (over 2200 events) and are moving backwards in time to pick and locate the earlier data collected since March of 2001. We currently 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.
Catalog Assessment. We continue to examine the earthquake data in search of possible earthquake precursors. This includes quality control and evaluation of the routine earthquake catalog locations and analyses of the spatial and temporal distribution of the microseismicity in relation to the occurrence of larger earthquakes in the area and heightened alert levels declared as part of the Parkfield Prediction Experiment. The new data and event detection scheme allows complete event detection down to magnitude 0.0. As a result, the rate of earthquake detection by the HRSN exceeds that of the NCSN by about a factor of 5 in the 30 km stretch of the SAF centered at the location of the 1966 M6 Parkfield event (Figure 5.1). The additional rate of HRSN event detection significantly increases both the spatial and temporal resolution of the changing seismicity patterns and provide unique additional information on the earthquake pathology at very low magnitudes.
Following the occurrence of the M6.5 San Simeon earthquake on December 22, of 2003, the long-standing data handling procedure outlined in the previous section was no longer viable due to the enormous rate of San Simeon aftershock detections (Figures 5.5 and 5.6). In the first 5 months following the mainshock, over 70,000 event detections were made by the HRSN system (compared to a yearly average detection rate of 6000 prior to San Simeon), and spot checks of the continuous 20 sps data revealed that the overwhelming majority of these detections resulted from seismic signals generated by San Simeon's aftershocks.
Data from the California Integrated Seismic Network (CISN) show that there were 1,150 San Simeon aftershocks with magnitudes 1.8 occurring in the week following the mainshock. During this same period, the number of event detections from the HRSN was 10,500 (compared to an average weekly for the year prior to San Simeon of 115 detections/per week). This suggests that the HRSN is detecting San Simeon aftershocks well below magnitude 1, despite the network's 50 km distance from the mainshock (Figures 5.5 and 5.6).
The dramatic increase in event detections vastly exceeded the HRSN's capacity to process both the continuous and triggered event waveform data. To prevent the loss of seismic waveform coverage, processing of the triggered waveform data has been suspended to allow archiving of the 250 sps continuous data to tape to continue uninterrupted. Cataloging of the event detection times from the modified REDI real-time system algorithm is also continuing, and the 250 sps waveform data is currently being periodically uploaded from the DLT tapes onto the NCEDC for access to the research research community. Research funding has also been requested from NSF-EarthScope to develop and apply new techniques to process these continuous data with the aim of identifying the Parkfield local events from among the San Simeon aftershocks and of compiling waveform and location catalogs for the local earthquakes.
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