Auxiliary material for paper 2010GL044660.

High-frequency identification of non-volcanic tremor triggered by regional earthquakes
Aurélie Guilhem1, Zhigang Peng2, Robert M. Nadeau1
1 Berkeley Seismological Laboratory, University of California, Berkeley, 225 McCone Hall, Berkeley, CA 94720-4670
2 School of Earth and Atmospheric Sciences, The Georgia Institute of Technology, ES&T Building, Rm. 2256, 311 Ferst Drive, Atlanta, GA, 30332-0340


Introduction:
This auxiliary material contains a section called Methods, one table, ten figures, and their figure captions.

Methods:

Filtering
Filtering is an important step to identify non-volcanic tremors. In this study we first deconvolved the borehole HRSN seismograms as well as the surface PKD and PST stations shown in Figures 2, S1-S3 and S6-S8. We applied filtering routines in SAC, Seistool and Matlab and found that the obtained band-pass-filtered seismograms are similar to each other (Figure S11), suggesting that the observed triggered tremor signals are not generated by the analysis procedure. The band-pass-filtered seismograms showed in this study are generated by the Butterworth filter (command bp) with two-pass and 4th pole in the Seismic Analysis Code (SAC) [Goldstein et al., 2003]. The HRSN instruments record signals with a sampling rate of 250 samples per sec and have instrument corner frequencies of 2 or 4.5 Hz. Filtering their non-deconvolved records above 15 Hz gives also the same signal with a difference in the gain.
The same filtering procedures were applied to the surface stations. However, because their sampling rates are lower than those of the HRSN stations and because they are installed at shallower depths where attenuation can be a significant issue, they show a lower signal to noise ratio (SNR) in the higher recorded frequency range. In addition, the majority of the surface stations in the area use analog telemetry in which a discriminator is incorporated to prevent aliasing during transmission. This effectively reduces the usable frequency range of the data to less than 25 Hz. However triggered tremors are still observed at some of the surface stations such as PKD and PST that are not subject to the telemetry discriminator (Figure 2, S1-S3)
 
Location
We used an adapted envelope based location algorithm previously used for locating ambient tremors in the same area [Nadeau and Guilhem, 2009] to locate the four regionally triggered tremors. Because the SNRs are low above 20 Hz for most surface stations, the obtained tremor locations are primarily based on the borehole HRSN data but additional data from selected NCSN, SCSN, and BDSN surface stations are also used. Hence, the station coverage is limited.
Locations were obtained after filtering the data between 10 and 30 Hz for the 2006 and 2010 Baja California earthquakes and between 20 and 40 Hz for the 2005 Mendocino and 2009 Baja California. Eight HRSN stations and the NCSN surface station PST were used for locating the 2005 Mendocino tremor. In comparison 16 stations including 6 surface stations located the 2006 Baja California tremor and 9 HRSN stations with 13 surface stations were used for the 2009 Baja California tremor. Finally 11 stations including 2 surface stations permitted the location of the 2010 Baja California tremor.

Peak ground velocities (PGVs)
PGVs (Table S1) were measured on the transverse and vertical velocity seismograms of the surface PKD station (BH channels) between the arrival time of the earthquake waves traveling at a velocity of 5.5 km/s and 1.5 km/s. Measurements were made on unfiltered and filtered (30-200 sec periods) PKD records. The PGV threshold and dynamic stress threshold were defined after comparing the overall PGVs of the regional and teleseismic earthquakes [Peng et al., 2009]. All the regional earthquakes that triggered tremors in this study show a PGV above the threshold placed at 10-2 cm/s in the 30-200 s periods on the transverse component (Figure 3).  The only events that did not satisfy the threshold criteria are teleseismic events. Regarding the vertical component (Figure S5), one regional earthquake that triggered tremors (i.e. the 2009 M6.9 Baja California) appears below the threshold while one that did not trigger (i.e. the January 2010 M6.5 Mendocino) is above the threshold. 

References (those not included in the main text):
Peng, Z., L. T. Long, and P. Zhao (2010), High-frequency artifact caused by the analysis procedure during large-amplitude surface waves, Seismol. Res. Lett., submitted.
Goldstein, P., D. Dodge, M. Firpo, and L. Minner (2003), SAC2000: Signal processing and analysis tools for seismologists and engineers, In The IASPEI International Handbook of Earthquake and Engineering Seismology, Part B (ed. Lee, W.H.K., Kanamori, H., Jennings, P.C., and Kisslinger C.) (Academic Press, London 2003) Chapter 85.5

Table:
2010gl044660-ts01_orig.txt (Table S1). Table S1. List of the regional and teleseismic earthquakes considered in this study and their peak ground velocities (PGV). The earthquake catalog information (origin time, location, and magnitude) is obtained from the Advanced National Seismic System (ANSS) catalog. The epicentral distance (in km) is computed relative to the broadband station PKD (stlo: -120.5416, stla: 35.9452). The peak ground vertical (PGV_BHZ) and transverse (PGV_BHT) velocity are measured from the broadband recording by station PKD, and are in the unit of cm/s. The BHT30_200 and BHZ30_200 are computed after filtering the records between 30 and 200 sec period. The quality (Q) 1 marks events that trigger tremor around Parkfield. 0 marks those that do not. The parameters of the teleseismic events are from Peng et al., JGR 2009.
 
Figure Captions:
 
2010gl044660-fs01_orig.eps (Figure S1). a) Velocity borehole seismic records (relative scaling) of the 2006 Baja California triggered tremor filtered at several frequency bands (3-15, 15-30 and 25-40 Hz from top to bottom respectively) and surface PKD station filtered between 3-15 Hz and 15-30 Hz. The HRSN traces are ordered from northwest to southeast along the strike of the SAF. The triggered tremor is best observed between 350 and 550 sec after 08:32:00 UTC. 
b) Unfiltered transverse (BHT) and radial (BHR) components of the broadband surface PKD station showing the surface wave train between 350 and 550 sec after 08:32:00 UTC. 

2010gl044660-fs02_orig.eps (Figure S2). a) Velocity borehole seismic records (relative scaling) of the 2009 Baja California triggered tremor filtered at several frequency bands (3-15, 15-30 and 25-40 Hz from top to bottom respectively) and surface PKD station filtered between 3-15 Hz and 15-30 Hz. The HRSN traces are ordered from northwest to southeast along the strike of the SAF. The triggered tremor is best observed between 350 and 500 sec after 17:59:00 UTC. 
b) Unfiltered transverse (BHT) and radial (BHR) components of the surface broadband PKD station showing the surface wave train between 350 and 500 sec after 17:59:00 UTC.

2010gl044660-fs03_orig.eps (Figure S3). a) Velocity borehole seismic records (relative scaling) of the 2010 Baja California triggered tremor filtered at several frequency bands (3-15, 15-30 and 25-40 Hz from top to bottom respectively) and surface PKD station filtered between 3-15 Hz and 15-30 Hz.  The HRSN traces are ordered from northwest to southeast along the strike of the SAF. The triggered tremor is best observed between 200 and 350 sec after 22:40:00 UTC. 
b) Unfiltered transverse (BHT) and radial (BHR) components of the surface broadband PKD station showing the surface wave train between 200 and 350 sec after 22:40:00 UTC.

2010gl044660-ps04_orig.eps (Figure S4). a) Unfiltered (top) and 25-40 Hz band-pass-filtered seismograms (middle) showing the broadband signals of the Mw7.2 Mendocino earthquake and triggered tremor observed at the borehole HRSN station FROB after 2006/06/15 02:50:00 UTC. The bottom panel shows the corresponding spectrogram in the frequency range of 0 and 60 Hz. The spectrogram is computed from a 0.5 Hz high-pass-filtered seismogram in order to suppress potential artifact from the short time windows in computing the spectra for long-period signals [Peng et al., 2010]. 
b) Unfiltered (top) and 15-30 Hz band-pass-filtered seismograms (middle) of the Mendocino earthquake and triggered tremor observed at the surface broadband station PKD after 02:50:00 UTC. The bottom panel shows the corresponding spectrogram in the frequency range of 0 and 40 Hz.

2010gl044660-ps05_orig.eps (Figure S5). a) Peak ground velocities (PGV) recorded at the surface Berkeley broadband seismic station PKD on the unfiltered vertical seismograms for the regional (circle symbols) and teleseismic (triangle symbols) earthquakes. Tremors triggered by regional and teleseismic events are shown by the red circles and the blue triangles, respectively. 
b) Same as a) but with the transverse components filtered between 30 and 200 sec period. The dashed line at 0.01 cm/sec marks the PGV threshold for tremor triggering in this pass-band. The earthquake with a PGV above that dashed line that did not trigger tremor is the January 2010 M6.5 Mendocino earthquake.

2010gl044660-fs06_orig.eps (Figure S6). a) Velocity borehole seismograms (relative scaling) showing non-volcanic tremors triggered by the Mw7.9 2002 Denali Fault earthquake observed at two frequency bands (3-15 and 25-40 Hz). The HRSN traces are ordered from northwest to southeast along the strike of the SAF. The triggered tremor is well observed between 900 and 1400 sec after 22:12:00 UTC at the two frequency ranges. 
b) Unfiltered transverse and radial components of the surface broadband PKD station showing the surface wave train between 900 and 1400 sec after 22:12:00 UTC.

2010gl044660-fs07_orig.eps (Figure S7). a) Velocity borehole seismograms (relative scaling) showing non-volcanic tremors triggered by the Mw7.9 2008 Wenchuan earthquake observed at two frequency bands (3-15 and 25-40 Hz). The HRSN traces are ordered from northwest to southeast along the strike of the SAF. The triggered tremor is well observed between 2500 and 3300 sec after 06:28:00 UTC at the two frequency ranges. 
b) Unfiltered transverse and radial components of the surface broadband PKD station showing the surface wave train between 2500 and 3500 sec after 06: 28:00 UTC.

2010gl044660-fs08_orig.eps (Figure S8). Velocity borehole seismic records (relative scaling) of the 12 April 2005 20-minutes-long ambient tremor filtered between 3-15 and 25-40 Hz. The HRSN traces are ordered from northwest to southeast along the strike of the SAF. The triggered tremor is best observed between 550 and 1700 sec from 03:00:00 UTC. 

2010gl044660-ps09_orig.eps (Figure S9). Comparison of the velocity spectra of the vertical components at the HRSN station FROB for the 99 regional earthquakes of the study (gray: earthquakes that did not trigger tremors, green: 2005 Mendocino, red: 2006 BC, blue: 2009 BC, and orange: BC 2010), and the 2002 Denali Fault earthquake that triggered tremors (light blue), and of a 20-min-long ambient tremor (pink). Spectra are computed from the instrumented corrected borehole velocity seismograms with apparent velocities between 5 km/s and 1.5 km/s. The earthquakes that triggered tremors have peak energies observed around 20 s (or 0.5 Hz), which correspond to the surface waves. One earthquake that did not trigger tremor has a similar energy at 20 s as compared with other triggering events. It corresponds to a foreshock occurred about 4 minutes before the 2010 BC earthquake. The spectra of the foreshock overlaps with the upcoming surface waves of the mainshock, which triggered tremors.

2010gl044660-ps10_orig.eps (Figure S10). a) Comparison of the velocity spectra between 1 and 100 Hz of the vertical components at the HRSN station FROB for the 99 regional earthquakes of the study (see Figure S9).
b) Velocity spectra for the 2006 Baja California (red) and its pre-event noise (black).
c) Velocity spectra for the 2002 Denali (blue) and its pre-event noise (black).
d) Velocity spectra for a 2005 ambient tremor (pink) and its pre-event noise (black).

2010gl044660-fs11_orig.eps (Figure S11). a) Comparison of three filtering routines in SAC, Seistool and Matlab for the 2005 Mendocino earthquake and tremor at the borehole SMNB station. For each frequency bands the amplitudes (in m/s) of the three traces are the same. Time scale is given in seconds after 02:50:00 UTC, and the triggered tremor is clearest between 250 and 350 s.
b) Unfiltered, instrument-correlated transverse (BHT) and radial (BHR) components of the broadband surface station PKD showing the surface wave train (time scale same as in a).