Figure 3.16 shows the location of the network, its relationship to the San Andreas fault, sites of significance from previous and ongoing experiments using the HRSN, double-difference relocated earthquake locations from 1987-1998, routine locations of seismicity from August 2002 to July 2003, nonvolcanic tremor locations from 27 July 2001 through 21 February 2009, and the epicenters of the 1966 and 2004 M6 earthquakes that motivated much of the research. 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 3.8], its high-frequency, wide bandwidth recordings (0-100 Hz; 250 sps), and its sensitivity to very low amplitude seismic signals (e.g., recording signals from micro-earthquakes with magnitudes below ).
Several aspects of the Parkfield region make it ideal for the study of small earthquakes and nonvolcanic tremors and their relationship to tectonic processes and large earthquakes. These include the fact that the network spans the SAFOD (San Andreas Fault Observatory at Depth) experimental zone, the nucleation region of earlier repeating magnitude 6 events 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 long-term HRSN seismicity catalog (complete to very low magnitudes and that includes at least half of the M6 seismic cycle); a well-defined and simple fault segment; the existence of deep nonvolcanic tremor (NVT) activity; and a relatively 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, the cumulative, often unexpected, results of UC Berkeley's HRSN research efforts (see: http://seismo.berkeley.edu/seismo/faq/parkfield_bib.html) 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 and deep nonvolcanic tremors.
The Parkfield area has also become an area of focus of the EarthScope Project (http://www.earthscope.org) through the deep borehole into the San Andreas Fault, the SAFOD experiment (http://www.earthscope.org/observatories/safod), and the HRSN is playing a vital role in this endeavor. 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 down-hole measurements in order to study the physical and chemical conditions under which earthquakes occur, and to monitor and exhume rock, fluid, and gas samples for extensive laboratory studies (Hickman et al., 2004).
The 3 newest borehole stations (CCRB, LCCB, and SCYB) were added, with NSF support, at the northwest 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 3.16 illustrates the location of the drill site and the new borehole sites, as well as locations of earthquakes recorded by the initial and upgraded/expanded HRSN.
These 3 new stations have a similar configuration to the original upgraded 10 station network and include an additional channel for electrical signals. Station descriptions and instrument properties are summarized in Tables 3.8 and 3.9. All the HRSN data loggers employ FIR filters to extract data at 250 and 20 Hz (Table 3.10).
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 our facility on the California Department of Forestry's (CDF) property in Parkfield. 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. Prior to June, 2008, six of the sites transmitted directly to a router at the central recording site. The other seven sites transmitted 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. Due to disproportionately increasing landowner fees for access to the Gastro Peak site, we reduced our dependence on that site in the summer and fall of 2008 in cooperation with the USGS, and data from five of the stations previously telemetering through Gastro Peak have now been re-routed through an alternative site at Hogs Canyon (HOGS).
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 (Northern California Earthquake Data Center), and is providing remote access and control of the system. It has also provided event triggers with better timing accuracy and is in addition now recording continuous 20 and 250 sps data for all channels of the HRSN, which flow seamlessly into both the USGS automated earthquake detection system and into Berkeley's NCEDC for archiving and online access by the community. The new system also helps minimize the problems of timing resolution, dynamic range, and missed detections, in addition to providing the added advantage of conventional data flow (the old system (1987-2001) recorded SEGY format).
Another 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 and seamless data flow to the NCEDC also provide near-real-time monitoring capabilities that are useful for rapid evaluation of significant events as well as the network's overall performance level. For example, shown in Figure 3.17 are P-wave seismograms of the teleseismic 8.8 earthquake offshore of Maule, Chile (Lat.: 35.909S; Lon.: 72.733W; Depth: 35 km) occurring on February 27, 2010 03:34:14 (UTC) recorded on the DP1 (vertical) channels of the 9 HRSN borehole stations in operation at the time. The seismic data from the quake was telemetered to Berkeley and available for analysis by the Northern California Seismic System (NCSS) real-time/automated processing stream within a few seconds of being recorded by the HRSN.
This is a good signal source for examining the relative responses of the
BP borehole network station/components to
seismic ground motion, and
these and corresponding waveform plots for the horizontal (DP2 and DP3
channels) indicate that the following stations were not
responding normally to seismic ground motions at the time of this event:
FROB.BP.DP1 - anomalous, weak signal
SMNB.BP.DP1 - no seismic response, telemetry outage
SMNB.BP.DP2 - no seismic response, telemetry outage
SMNB.BP.DP3 - no seismic response, telemetry outage
MMNB.BP.DP1 - no seismic response, telemetry outage
MMNB.BP.DP2 - no seismic response, telemetry outage
MMNB.BP.DP3 - no seismic response, telemetry outage
CCRB.BP.DP1 - no seismic response, telemetry outage
CCRB.BP.DP2 - no seismic response, telemetry outage
CCRB.BP.DP3 - no seismic response, telemetry outage
JCNB.BP.DP1 - no seismic response, signal cable cut
JCNB.BP.DP2 - no seismic response, signal cable cut
JCNB.BP.DP3 - no seismic response, signal cable cut
By rapidly generating such plots following large teleseismic events, quick assessment of the HRSN seismometer responses to real events is easily done and corrective measures implemented with relatively little delay.
Shortly after being recorded to disk on the central site HRSN computer, event triggers for the individual station data are also determined, and a multi-station trigger association routine then processes the station triggers and generates a list of potential earthquakes. For each potential earthquake that is detected, a unique event identification number (compatible with the NCEDC classification scheme) is also assigned. Prior to the 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.
Because of its mandate to detect and record very low magnitude events in the Parkfield area, the HRSN is extremely sensitive to changes in very low amplitude seismic signals. As a consequence, in addition to detecting very small local earthquakes at Parkfield, the HRSN also detects numerous regional events and relatively distant and small amplitude nonvolcanic tremor events. For example, spot checks of aftershocks following the M6.5 San Simeon earthquake of December 22, 2003 using continuous data and HRSN event detection listings have revealed that the overwhelming majority of HRSN detections following San Simeon resulted from seismic signals generated by San Simeon's aftershocks, despite the HRSN's 50 km distance from the events. Data from the California Integrated Seismic Network (CISN) show that there were 1,150 San Simeon aftershocks with magnitudes 1.8 in the week following San Simeon, and during this same period, the number of HRSN event detections was 10,500 (compared to an average weekly rate before San Simeon of 115 detections). This suggests that, despite the 50 km distance, the HRSN is detecting San Simeon aftershocks well below magnitude 1.
Current Processing. Since the beginning of the network's data collection in 1987, and up until 2002, the local and regional events were discriminated based on analyst assessment of S-P times, and only local events with S-P times less than 2.5 s at the first arriving station were picked and located as part of the HRSN routine catalog. However, because of the network's extreme sensitivity to the large swarm of aftershocks from the San Simeon and M6 Parkfield earthquakes of September 2004 (e.g., in the first 5 months following the San Simeon mainshock, over 70,000 event detections were made by the HRSN system, compared to an average 5 month detection rate of 2500 prior to San Simeon) and because of ever declining funding levels, this approach has had to be abandoned.
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 the telemetry and archiving of the 20 and 250 sps continuous data to continue uninterrupted. Subsequent funding limitations have precluded reactivation of the processing of triggered waveform data. Cataloging of the event detection times from the modified REDI real-time system algorithm is continuing, however, and the continuous waveform data is currently being telemetered directly to the BSL and USGS over the T1 link for near-real-time processing and archiving at the NCEDC, for access by the research community.
Funding to generate catalogs of local events from the tens of thousands of aftershock detections has not been forthcoming, and, as a consequence, major changes in our approach to cataloging events have been implemented. The HRSN data is now integrated into NCSN automated event detection, picking, and catalog processing (with no analyst review). In addition, a high resolution procedure is now being developed to automatically detect, pick, locate, double-difference relocate, and determine magnitudes for similar and repeating events down to very low magnitudes (i.e., below magnitude -1.0). These new schemes are discussed in more detail in the activities section below.
Station MMNB Failure.
Station MMNB is situated directly in the fault zone over the epicenter of the Parkfield 1996 mainshock, 5 km southeast of SAFOD, and plays a key role in a variety of scientific investigations, including studies of fault zone guided waves (FZGWs), monitoring of seismicity and non-volcanic tremor, seismic source and scaling studies and SAFOD related research. The station also contributes real-time data to the Northern California Seismic System (NCSS) real-time/automated processing stream for earthquake detection and location.
On August 13, 2009 (DOY 225, UTC) the flow of seismic signals from borehole sensors of the station ceased at between 16:13 and 16:14 (UTC) (local 11:13 and 11:14 AM) (Figure 3.18). It was discovered, only after inquiring about the site after observing the failure remotely, that an effort to clean-up the abandoned USGS water well gear at middle mountain took place on the same day. After the sensor failure, only instrument/pre-amp noise was recorded at the station up until Sept. 16 at 17:37 (UTC), at which time our field engineer removed the pre-amp electronics after confirming no response from the downhole sensors. Our engineer also found that what we believed to be the well head of the MMNB borehole had been demolished. This was apparently done on Sept. 15th as part of the subsequent filling-in of the USGS water well vault pit by the landowner due to safety concerns.
It is now clear that the MMNB borehole well-head was mistakenly assumed to be part of the USGS water well installation during the clean-up effort and that during this effort the scientifically important HRSN station was inadvertently disabled. Significant and scarce resources were expended to track down the cause of the MMNB failure, to assess the degree of damage, and to devise a plan for possible recovery of the station's operation. Fortunately, the recovery efforts have proven successful and data is once again being collected from this vital installation.
Nonetheless, we were disappointed at not having been notified of the clean-up plans at the Middle Mountain site, and we have asked for and received assurances from the USGS that closer coordination between USGS and Berkeley during activities of this kind will be implemented to avoid similar catastrophes in the future.
Station JCNB Status.
In the spring of 2008, signals from HRSN station JCNB began showing signs of deterioration. Shortly thereafter, data flow from this station stopped completely. Field investigation showed that the borehole sensor and cable had been grouted to within 34 feet of the surface and that a rodent had found itself trapped in the upper 100 foot void space and chewed through the cable, thus severing the connection to the deep borehole package. At this time, costs for reestablishing connection to the cable at depth are prohibitive, and it is also likely that the grouted-in sensor has been compromised by fluids running down the exposed cable. Hence, plans are being made to substitute either a surface seismometer or a borehole sensor package within the open 34 foot section of the borehole to provide continued seismic coverage at the JCNB site. An surplus mPBO sensor package in storage at the BSL has been identified as a possible replacement in the remaining void space of the JCNB hole and the sensor and feasibility of installation are now being assessed by BSL's engineering group.
Remote SOH Monitoring.
The network connectivity over the T1 circuit also allows remote monitoring of various measures of the state of health (SOH) of the network in near-real-time, such as background noise levels. Shown in Figure 3.19 are power spectral density (PSD) plots of background noise for the 12 operational vertical components of the HRSN for a 1000 second period beginning at 00:00 AM local time on 6/7/2010 (a Monday morning). By periodically generating such plots, we can rapidly evaluate, through comparison with previously generated plots, changes in the network's station response to seismic signals across the wide band high-frequency spectrum of the borehole HRSN sensors. Changes in the responses often indicate problems with the power, telemetry, or acquisition systems, or with changing conditions in the vicinity of station installations that are adversely affecting the quality of the recorded seismograms.
Once state of health issues are identified with the PSD analyses, further remote tests can be made to more specifically determine possible causes for the problem, and corrective measures can then be planned in advance of field deployment within a relatively short period of time.
One action taken to help address this problem has been to integrate HRSN data streams into the NCSN event detection and automated cataloging process. This approach has been successful at detecting and locating a significantly greater number of micro-earthquakes over the previous NCSN detection and location rate (essentially doubling the number of events processed by the NCSN). However, the HRSN sensitized NCSN catalog is still only catching about 1/2 the number of local events previously cataloged by the HRSN using the old HRSN-centric processing approach. Furthermore, triggered waveforms for the additional small NCSN processed events are not typically made available; they are not reviewed by an analyst, nor do they generally have NCSN magnitude determinations associated with them.
These limitations severely hamper research efforts relying on similar and characteristically repeating micro-seismicity such as earthquake scaling studies, SAFOD-related research, deep fault slip rate estimation, and the compilation of recurrence interval statistics for time-dependent earthquake forecast models. They also reduce, to some degree, the use of recurring micro-seismicity as a tool for monitoring the state-of-health (SOH) of either the HRSN or NCSN.
To help overcome these limitations, this year, we have begun implementing an automated similar event cataloging scheme based on pattern matching (match filter) scans using cross-correlation of the continuous HRSN data. The method uses a set of reference events whose waveforms, picks, locations, and magnitudes have been accurately determined, and it automatically detects, picks, locates, and determines magnitudes for events similar to the reference event to the level of accuracy and precision that only relative event analysis can bring.
The similar event detection is also remarkably insensitive to the magnitude of the reference event used, allowing similar micro-events ranging over about 3 magnitude units to be fully cataloged using a single reference event, and it does a remarkably good job at discriminating and fully processing multiple superposed events.
Once a cluster of similar events has been processed, an additional level of resolution can then be achieved through the identification and classification of characteristically repeating microearthquakes (i.e., near identical earthquakes) occurring within the similar event family (Figure 3.20). The pattern scanning approach also ensures optimal completeness of repeating sequences owing to scans of the matching pattern through all available continuous data (critical for applications relying on recurrence interval information). For example, while only about half of the events shown in Figure 3.20 were picked up by the NCSN-HRSN integrated network, the pattern scanning approach we employ picked up all of the near identical events.
Figure 3.20 shows how stable the performance of the VCAB.BP.DP1 channel has remained over the 4 year period analyzed. This is not necessarily the case for all the other HRSN channels being recorded. These repeating events can generally be identified using as few as 4 of the 38 HRSN channels, so, once they are identified, assessment of the channel responses for all the remaining HRSN channels can be carried out repeatedly through time and with time resolutions dependent on the number of repeating sequences used and the frequency of their repeats. Armed with this type of information, field engineers can quickly identify and address major problems. In addition to a visual assessment, the extreme similarity of the events lends itself to the application of differencing techniques in the time and frequency domains to automatically identify even subtle SOH issues.
Repeating sequences of this magnitude typically repeat every 1 to 2 years, and we are currently monitoring 34 of these sequences. Hence, on average, evaluations of this type can be made approximately every few weeks on an automated basis. However, there are on the order of 200 such sequences known in the Parkfield area, leaving open the possibility that automated SOH analyses could take place every 2 to 3 days.
For other networks recording continuously in the Parkfield area (e.g., NCSN, BDSN) it is also a relatively simple process to extend the SOH analysis using characteristic repeating events to their stations. Furthermore, numerous repeating and similar event sequences are also known to exist in the San Francisco Bay and San Juan Bautista areas, where continuous recording takes place. Hence, application of the repeating event SOH technique to these zones should also be feasible.
We are continuing to expand the number of pattern events and resulting multi-year scans in the Parkfield area to increase the frequency of sampling of similar and repeating event sequences for SOH purposes and for expanding the catalog of very small similar and repeating microearthquakes (down to of -0.5). We are also adapting the codes to take advantage of faster computing now available.
Further development of the similar event processing approach also holds promise in other applications where automated and precise monitoring of bursts of seismic activity to very low magnitudes is desirable (e.g. in aftershock zones or in volcanic regions) or where automated updates of preexisting repeating sequences and their associated deep slip estimates are desired.
To help alleviate the problem, we have completed implementation (through cooperation with the USGS) of plans to minimize our dependence on access to private lands. This primarily involved establishing alternative telemetry paths for roughly half of the HRSN sites through Gastro Peak.
To date, telemetry paths for five HRSN sites (SMNB, CCRB, MMNB, VARB, and SCYB) have been redirected from the Gastro Peak relay site to an alternative relay site at Hogs Canyon (HOGS) through an agreement with the USGS. Telemetry of GHIB data has also been redirected from Gastro Peak through an alternative path. Plans to redirect telemetry of an additional site from Gastro Peak (LCCB) are being examined and field tested for viability. Last year, the landowner also chose not to renew our access agreement for Gastro Peak, saving us approx. in annual fees. However, the owner did allow us to temporarily continue operating one station (RMNB) located at the Gastro Peak site free of charge for an unspecified period of time. This past summer the landowner had suggested that he was going to remove the RMNB site. We immediately began re-negotiations, and the site is still operating, however resolution of the issue has still not been forthcoming. Adding to the seriousness of the situation, until alternative telemetry to low lying station LCCB can be worked out (a difficult task given the limited telemetry options available), the RMNB station is also serving as a repeater for LCCB.
As of early September 2007, SAFOD drilling had penetrated the fault near the HI repeating target sequence and collected core samples in the fault region that presumably creeps and surrounds the repeatedly rupturing HI patch. Unfortunately, due to complications during drilling, penetration and sampling of the fault patch involved in repeating rupture was not possible, though core samples and installation of seismic instrumentation in the region adjacent to the repeating patch was achieved. Current efforts are focused on long-term monitoring of the ongoing chemical, physical, seismological, and deformational properties in the zone (particularly any signals that might be associated with the next repeat of the SAFOD repeating sequences).
HRSN activities this year have contributed in three principal ways to
these and longer-term SAFOD monitoring efforts:
1) Integration and processing of the HRSN data streams with those from the NCSN in the Parkfield area continues, effectively doubling the number of small events available for monitoring seismicity in the target zone and for constraining relative locations of the ongoing seismic activity.
2) Telemetry of all HRSN channels (both 20 and 250 sps data streams) continues to flow directly from Parkfield, through the USGS Parkfield T1 and the NCEMC T1, to the USGS and the BSL for near-real-time processing, catalog processing, and data archiving on the Web-based NCEDC. This also provides near immediate access of the HRSN data to the SAFOD community without the week- or month-long delay associated with the previous procedure of having to transport DLT tapes to Berkeley to upload and quality check the data.
3) We have also continued to apply our prototype similar event automated catalog approach to the primary (HI), secondary (SF), and tertiary (LA) SAFOD target zones as a continued effort to monitor the SAFOD target zone activity at very high relative location precision, and to notify the SAFOD community of repeats of M2 target events. The most recent repeats of the SAFOD HI, SF, and LA sequences occurred on (UTC): August 29, 2008; December 20, 2008; and December 19, 2008 (respectively). Of particular interest were the SF and LA repeats, which were recorded on the SAFOD main hole seismometer that had been installed in October.
Hickman, S., M.D. Zoback and W. Ellsworth, Introduction to special section: Preparing for the San Andreas Fault Observatory at Depth, Geophys. Res. Lett., 31, L12S01, doi:10.1029/2004GL020688, 2004.
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.
Nadeau, R.M. and D. Dolenc, Nonvolcanic Tremors Deep Beneath the San Andreas Fault, SCIENCE, 307, 389, 2005.
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