Preliminary Report on Aftershock Sequence
edited by
sponsored by
Open-File Report 86-181
for Earthquake of January 31, 1986
near Painesville, Ohio
(Time Period: 2/1/86 - 2/10/86)
R. D. Borcherdt
U. S. Geological Survey
Electric Power Research Institute
U. S. Geological Survey
This report is preliminary and has not been reviewed for conformity with U. S. Geological Survey editorial standards and stratigraphic nomenclature.
R. D. Borcherdt
Recording System
Data Playback System
G. Maxwell, J. Sena, J. VanSchaack, R. Warrick, R. Borcherdt, C. Dietel, J. Fletcher, J. Gibbs, and G. Jensen
R. D. Borcherdt, C. Dietel, G. Sembera, J. Gibbs, and C. Nicholson
G. Glassmoyer, E. Roeloffs, C. Valdes, R. Borcherdt, and C. Mueller
C. Valdes, E. Roeloffs, G. Glassmoyer, C. Langer, R. Borcherdt, and C. Nicholson
A ten-station array of broad-band digital instrumentation (GEOS) was deployed by the U. S. Geological Survey with partial support provided by Electric Power Research Institute to record the aftershock sequence of the moderate (mb ~ 4.9) earthquake that occurred on January 31, 1986 (16:46:43 UTC) near Painesville, Ohio. The occurrence of the event has raised questions concerning possible contributory factors to the occurrence of the event and questions concerning the character of earthquake-induced high-frequency ground motions in the area. To aid in the timely resolution of the implications of some of these questions, this preliminary report provides copies of the ground motion time-histories and corresponding spectra for the six identified aftershocks and two events, thought to be quarry blasts, recorded as of February 10, 1986. Recording station locations and epicenter locations based on two preliminary estimates of local seismic velocity structure are provided.
The moderate earthquake (mb ~ 4.9) that occurred near Painesville, Ohio (41.650°N, 81.162°W; J. Dewey, pers. comm., 1986) on January 31, 1986 (16:46:43 GMT) was felt over a broad region including eleven states, the District of Columbia, and parts of Ontario, Canada. Isoseismals for the event as inferred by field investigation (M. Hooper, pers. comm., 1986) were centered in the general area of the small communities of Chardon, Mentor, and Hambden, Ohio. The largest observed intensities were in the VI-VII range with only minor building damage (M. Hooper, pers. comm., 1986). Fifteen people were reported to have experienced minor injuries.
Considerable scientific and engineering interest in the event resulted in a team of five seismologists being dispatched from Menlo Park, California on the evening of January 31 to install ten digital event recorders (GEOS) in the epicentral area (USGS: R. D. Borcherdt, C. Dietel, J. Gibbs, G. Sembera; EPRI: J. King). Seven of these stations were installed subsequent to arrival on February 1 (6:00 a.m.; -8°C) and the remaining three stations were installed on February 2, 1986. This preliminary report provides the data recorded as of February 10, 1986 for six aftershocks and two events thought to be quarry blasts. Station locations, time histories, Fourier amplitude spectra, and epicentral locations of the aftershocks based on two different preliminary velocity models are provided.
On February 10, 1986 the number of stations deployed in the area was reduced to five. As of this writing, these stations are planned for removal during the week of March 23, 1986. Any data recorded at these stations subsequent to February 10, 1986 will be reported in subsequent, more thorough reports.
Data presented in this report were recorded, using General Earthquake Observation Systems (GEOS). A detailed description of the systems is provided by Borcherdt et al., 1985. A brief summary is provided of general recording and playback system characteristics of interest for this report. Recording system parameters chosen for this experiment are provided.
The signal conditioning module for the GEOS is configured with six input channels, selectable under software control, to permit acquisition of seismic signals ranging in amplitude from a few nanometers of seismic background noise to 2 g in acceleration for ground motions near large events. The analog-to-digital conversion module is equipped with a 16-bit CMOS analog-to-digital converter which affords 96 dB of linear dynamic range or signal resolution; this, together with two sets of sensors, implies an effective system dynamic range of about 180 dB. A data buffer with direct memory access capabilities allows for maximum throughput rates of 1200 sps. With sampling rates selectable under software control as any integral quotient of 1200, broad and variable system bandwidth ranging from 10-5 to 6 × 102 Hz is achieved for use of recorders with a wide variety of sensor types.
Modern high-density (1600 - 6400 bpi) compact tape cartridges offer large data storage capacities (1.25 - 33 Mbyte) in ANSI standard format to facilitate data accessibility via minicomputer systems. Read capabilities of cartridge tape recorders is utilized to allow recording parameters and system operational software to be changed automatically. Read capablity also allows systems equipped with modems to transmit data via telecommunications to a central data processing laboratory.
Microcomputer control of time-standard provides capability to synchronize internal clock via internal receivers (such as WWVB and satellite), external master clock, or conventional digital clocks. Microcomputer control of internal receivers permits systems on command to determine time corrections with respect to external standard. This capability permits especially accurate correction for conventional drift of internal clocks.
Convenient system set-up and flexibility to modify the system in the field for a wide variety of applications is achieved using a 32-character alphanumeric display under control of the microcomputer. English-language messages to the operator executed in an interactive mode, reduce operator field set-up errors. A complete record of recording system parameters is recorded on each tape together with calibration signals for both the sensor and the recorders. These records assure rapid and accurate interpretation via computer of signals, both in the field and in the laboratory.
Flexibility to modify the system to incorporate future improvements in technology is achieved using a ringed software architecture and modular hardware components. Incorporation of new hardware modules is accomplished in a straightforward manner by replacing appropriate module and corresponding segments of controlling software.
The system response designed for strong-motion and aftershock applications of the GEOS was intended to allow large-amplitude near-source signals of 1 - 10 Hz as detected by a force-balance accelerometer (FBA) to be recorded on scale, while at the same time permitting much smaller-amplitude high-frequency signals (50 - 100 Hz) as might be detected on FBAs or velocity transducers to be recorded with high signal resolution. The design system response, together with that for two types of sensors frequently used for aftershock studies in the near-source region of large earthquakes, is shown in Figure 2.
For these experiments only analog playback capability was utilized in the field. Analog playbacks on light-sensitive paper were utilized in the field to identify seismic events, trigger parameters, and evaluate instrument performance. Digital playback of the data was conducted with a Tandberg serpentine tape drive attached to the PDP 11/70 at the National Strong Motion Data Center in Menlo Park, using a variety of software packages, with principal components developed in large part by G. Maxwell, E. Cranswick, and C. Mueller.
Locations for the stations deployed in the 10-station array are shown in Figure 3 together with the location of the main shock. Objectives in choice of the locations included event location, source parameter determination, attenuation of high-frequency ground motion along a linear north-south array, and effects of local site conditions at stations 001 and 002.
Due to the suspected low seismicity and expected small magnitudes for aftershocks, attempts were made to locate the stations at sites with anticipated low seismic background noise levels in areas (with the exception of station 001) where the effects of local soil conditions were expected to be minimized. To reduce the effect of the adverse environmental conditions (-15°C; snow and ice) on the recording equipment, each unit was located in an unheated shelter (small tool sheds or abandoned animal shelters some distance from local sources of cultural noise were chosen).
Stations 001, 002, 003, 004, 005, 006, and 007 were installed in the time period 1400 - 1800 hours EST on February 1, 1986. Stations 008, 009, and 011 were installed on February 2, 1986. Station 005 was moved to location 055 on February 4, 1986 at 1800 hours EST.
In anticipation of recording only small events, some of which had signal levels just above seismic background noise levels, the GEOS recorders with the exception of station 005 and 055 were operated as high-gain, three-channel recorders, using only L-22 (2 Hz) three-component velocity transducers. Station 005 (055) in the expected epicentral region was operated as a six-channel recorder with both three-component force-balance accelerometers (FBA-13; USGS case) and similar three-component velocity transducers to permit on-scale recordings of any larger events. The channels used to record the output of the L-22 seismometers were operated at relatively high gain of 48 and 54 dB initially and later reduced to 42 and 48 dB gain. The output of the FBA channels were recorded at 12 dB gain. Sampling rates for the three-component stations with velocity transducers were chosen at 400 sps/channel with a resultant Nyquist frequency of 200 Hz. No anti-aliasing filters were utilized for these stations because of the anticipated low background noise level near the Nyquist. Sampling rates of 200 sps/channel and anti-aliasing filters of 50 and 100 Hz were utilized for the stations recording six-component data.
Timing at each of the stations was obtained using the automatic capability of the recorders to synchronize to WWVB. Timing corrections at 12-hour intervals with an accuracy of generally less than 5 ms are automatically recorded and used to correct the internal clock for each recorder, which has a temperature stability specification of ± 1 × 10-6 -20°C to +70°C.
The recorders were operated in trigger mode with activation to record sensor outputs on digital cartridge tape for the various seismic events being determined from a variety of various trigger parameters programmed into the systems, depending on characteristics of local background noise at the sites. In general, the trigger parameters were set to ensure that essentially all seismic events with peak amplitudes more than a factor of five above background noise would be recorded. Subsequent comparison of recorded events with those apparent on visible recorders (C. Langer, pers. comm., 1986, and H. Seeber, pers. comm., 1986) confirms that all events identified on visible recorders were detected and recorded on at least the three stations closest to the epicenters.
Surprisingly few instrument malfunctions, considering the cold environmental conditions, have been encountered to date. Station 004 experienced a varmit with active molars resulting in a semi-severed seismometer cable on February 3, 1986. Intermittent behavior of station 005 from February 1 through February 4 was attributed to a poor shelter with a leaky roof and no endwall. System performance stabilized upon moving this system to location 055 within a new barn (unheated). Two short interruptions in station operation at stations 006 and 007 were corrected on February 3 and 4 with replacement of defective automobile batteries. Some difficulties in reading two of the tapes from station 004 have been encountered in Menlo Park, even though no difficulties were encountered in reading the same tapes in the field. At this point, it appears the difficulty is attributable to deterioration of magnetic flux on the affected tapes due either to shipping environment or playback procedures.
Time histories recorded at each of the stations for the six aftershocks and two suspected quarry blasts detected in the time period February 1 through February 10, 1986 are presented as Figures A-1 to A-37 (Appendix A). The three-component time histories are identified by universal time and station number. Six seconds of time history in units of ground velocity (cm/s) are plotted for each station with additional six-second intervals plotted for the more distant stations. The time scale for each event expressed in universal time permits accurate absolute timing of various phases from the figures presented. The time scale is the same for each figure, permitting straightforward comparisons of frequency content. (The time histories are plotted using a basic software package developed by C. Mueller, with modifications for format implemented by G. Glassmoyer.)
The amplitude scale presented for each time history was determined using the gain factors and sensor coil constants recorded automatically by GEOS near the time of event detection. With the instrument response taken into account the time histories as presented can be interpreted in terms of ground velocity. Exceptions are the peak amplitudes on trace H=0 (Figure A-8), three traces (Figure A-18), and traces H=0, H=90 (Figure A-19), for which the peak signal amplitudes for the S waves and one P wave exceeded full-scale resolution level at the gain level of 54 dB used to record these traces. It should be noted that the high sampling rates of 400 sps used to record the velocity time histories (200 sps for stations 005 and 055), resulted in accurate resolution of the peak amplitudes, many of which occur at relatively high frequencies. Lower sampling rates (e.g., 100 sps) with anti-aliasing filters set near 30 Hz would be expected to yield time histories for the nearest stations having smaller peak amplitudes.
To summarize the relative frequency content of the recorded signals, Fourier amplitude spectra were computed for 10.24 seconds of each of the time histories. The resulting amplitude spectra, shown in Figures B-1 to B-37 (Appendix B) were computed using a software package developed by C. Mueller for a time interval, commencing approximately 2 seconds prior to onset of the P wave, with a 10 percent cosine taper applied to correct for leakage. The amplitude spectra were smoothed with a 0.25 Hz Hanning window and plotted for the interval 0.5 Hz to 200 Hz (Nyquist). The spectra for several of the stations closer to the epicenter (e.g., 003, 055, 006, and 002) show that the recorded time histories are relatively rich in frequency content in the high-frequency interval (30 - 70 Hz). Interpretation of the spectra in terms of source parameters, attenuation, and local site conditions is planned for subsequent reports.
This summary covers aftershocks recorded between 1 February 19:45 GMT (1 February 14:45 EST) and 10 February 20:07 GMT (12 February 15:07 EST). During this period, six aftershocks occurred that were detected by three or more GEOS stations. Visual examination of time histories for all triggers that occurred within 20 seconds of each other at two stations only did not reveal any seismic events other than those that were recorded at three or more stations. The occurrence times of the six aftershocks and the number of stations detecting each one are listed in Table 1a. Table 1a also lists coda duration magnitudes determined from smoked-paper records obtained by Lamont-Doherty Geological Observatory (J. Armbruster and L. Seeber, pers. comm., 1986).
In addition to the aftershocks, two events believed to be quarry blasts were recorded at three stations (Table 1b). These events occurred on a weekday during working hours. Preliminary locations for the events near a sand and gravel pit about 5.5 km north-northeast of station 006, and the substantially different nature of the time histories (see Figures A-32 to A-37) suggest that the events are quarry blasts. Additional confirmation with quarry owners is being pursued.
Station locations, determined from 7.5-minute series topographic maps, were independently checked by a second interpreter and are believed to be accurate to within 60 meters. Station locations are listed in Table 2 and plotted together with the location of the main shock in Figure 3. Expansion of the digital traces on a graphics terminal permitted picking of P and S arrival times to within 0.02 seconds by two independent observers. The automatic clock corrections provided every 12 hours and recorded on the GEOS tapes indicate the clock errors for the GEOS recordings are within ±5 ms.
Two preliminary velocity models were used to locate the events using the computer program HYPOINVERSE (Table 3). The first model is essentially a half-space with a P wave velocity of 5.5 km/s and an S wave velocity of 3.27 km/s. The second model contains five sedimentary layers overlying Precambrian basement at a depth of 2.1 km. The interfaces included in the flat layered model are based on a compilation of information from wells drilled as far as the top of the Precambrian basement (Cleveland Electric Illuminating Company, 1982). An average of down-hole and cross-hole velocity logs was used to determine the P velocity in the upper 0.5 km. The P velocity in the Precambrian basement is based on regional earthquake travel-time studies (Nuttli et al., 1969). Average P and S velocities for the other sedimentary layers were estimated from a geologic cross-section using a handbook of physical constants. The boundary between the Precambrian basement and the lower crust was chosen arbitrarily at 20 km depth and has no influence on the derived locations. The two velocity models should be considered as preliminary. With the exception of the near-surface P velocities, they are not based on in-situ measurements in the area, and they do not take into account dip of the Precambrian interface from about 6000 feet at the shore of Lake Erie to about 7000 feet in the epicentral region.
In addition to the GEOS recording locations shown in Figure 3, C. Langer (pers. comm., 1986) provided preliminary recording locations for 10 smoked-paper recorders deployed in the area on February 2, 1986. He also provided preliminary P and S times for the five aftershocks recorded at these stations as of February 10, 1986. The station locations provided by C. Langer are referred to subsequently using a three-letter code to distinguish them from the GEOS locations identified by a three-digit code.
Locations for the six aftershocks and two quarry blasts are summarized on Figure 4. The aftershock locations also are shown at an expanded scale in Figure 5. The event locations shown in Figures 4 and 5 have been derived using the layered velocity model summarized in Table 3b. Within the accuracy of the location for the main shock (±4 km; 90% confidence interval, J. Dewey, pers. comm., 1986) the aftershock locations are in the epicentral region of the main shock. Procedures utilized to locate the events based on the preliminary velocity models are discussed subsequently.
Four of the aftershocks (2 February 03:22, 3 February 19:47, 5 February 06:34, and 6 February 18:36) triggered four or more GEOS recorders with an appropriate station distribution to permit location of the events based only on P arrival times. For each of these events, the two velocity models yield epicenter locations that differ by at most 0.2 km. Compared to the uniform crust, the layered crust gives depths as much as 0.85 km shallower and with somewhat smaller estimated errors. The maximum value of ERZ for the layered model was 2.65 km. The epicenters for these four events, located with the layered crustal model (Table 3b) and P arrival times only, are within 0.44 km of 41°38.85'N and 81°9.51'W, and at depths between 4.0 km to 6.5 km.
The preferred locations (Figure 5) for these four events were derived using the layered model (Table 3b) and both P and S arrival times as determined from the GEOS recordings of the events. Output of HYPOINVERSE for this set of locations is presented in Appendix C. The epicenters based on both the P and S arrival times are displaced at most 80 meters from those determined on the basis of P information alone. Incorporation of S arrivals changed the depths by a maximum of 80 meters, with two depths becoming shallower and two becoming deeper.
For the last two events (7 February 15:20 and 10 February 20:06), the distribution of the digital stations for which digital tapes were available as of this writing was not considered adequate for determining locations, as illustrated by large uncertainties in preliminary locations based only on these data. The uncertainties were reduced significantly (as measured by ERZ, GAP) upon incorporation of the P arrival data provided by C. Langer. Although the uncertainties in the epicentral locations (ERH) are comparable to those for the other four events, the uncertainty in depth (ERZ) is large for the two events (5.21 and 6.33 km, respectively), suggesting that these events are not as well located as the four other events (see Appendix C). The epicenters for these two events are within 0.42 km of the average location for the epicenters of the four better-located events. Locations for these events are also summarized in Table 4.
Approximate locations for the two events throught to be quarry blasts (Table 1b) as determined from two sets of P and S times and one S time are shown on Figure 4 and tabulated in Appendix C.
In summary, the epicenters of the six aftershocks are reasonably tightly constrained, with the depths for the four better-located events ranging between 4.0 and 6.2 km. The locations of the epicenters are not particularly sensitive to the velocity model, but further work to improve the velocity model would better constrain inferred depths.
The data set reported in this investigation represents the combined efforts of several individuals, as indicated in part by the distributed authorship of the various sections. The rapid release of this data set was facilitated by efficient processing and analysis software, major portions of which were developed by G. Maxwell, E. Cranswick, and C. Mueller. The determination of preliminary event locations was facilitated with software and hardware implemented with contributions from J. Fletcher, H. Bundock, L. Baker, F. Klein, and L. Haar. C. Stepp (Electric Power Research Institute) and J. Filson (U. S. Geological Survey) applied professional management expertise to initiate and coordinate the investigation in its early stages. C. Langer (USGS, Golden, CO) provided preliminary event locations in the field and made his initial travel time and event locations available for location of some of the smaller events. J. Armbruster and L. Seeber (Columbia Univ., NY) kindly provided preliminary estimates of coda magnitude.
Borcherdt, R. D., J. P. Fletcher, E. G. Jensen, G. L. Maxwell, J. R. VanSchaak, R. E. Warrick, E. Cranswick, M. J. S. Johnston, and R. McClearn (1985). A General Earthquake Observation System (GEOS), Bull. Seism. Soc. Am. 75, 1783-1825.
Cleveland Electric Illuminating Company (1982).
The Perry Nuclear Power Plant Units I and II,
Final Safety Analysis Report,
Cleveland, Ohio.
Nuttli, 0. W., W. Stauter, and C. Kisslinger (1969).
Travel time tables for earthquakes in the central United States,
Earthquake Notes, 40, 19-28.
Figure 1: General Earthquake Observation System (GEOS)
Figure 2: Unit-impulse response designed for the GEOS recorder
Figure 3: Map showing all GEOS station locations
Figure 4: Map of epicentral GEOS station locations and aftershock epicenters
Figure 5: Detail map of aftershock epicenters
Table 1a: Occurence time and magnitude for aftershocks
recorded 2/2 - 2/10, 1986
Table 1b: Occurence times for apparent quarry blasts
Table 2: Recording station locations
Table 3: Preliminary models used to locate events listed in
Tables 1a and 1b
Table 4: Origin time and locations for seismic events
Appendix A: Time histories for seismic events
Appendix B: Fourier Amplitude spectra
Appendix C: Location parameters (HYPOINVERSE) for seismic events
The GEOS Home Page is at /GEOS/geos.html
Figures
Tables
Appendix
The USGS Home Page is at http://www.usgs.gov
The USGS Geologic Division Home Page is at http://geology.usgs.gov
The USGS National Strong Motion Program Home Page is at http://earthquake.usgs.gov/monitoring/nsmp
The URL of this page is /GEOS/CLV/OFR_86-181/Cleveland.html
e-mail: glassmoyer@usgs.gov
or address:
Gary Glassmoyer
345 Middlefield Road M/S 977
Menlo Park, California 94025