Seismic signals from tsunamis in the Pacific Ocean

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L03305, doi: /2007gl032601, 2008 Seismic signals from tsunamis in the Pacific Ocean Gordon Shields 1 and J. Roger Bowman 1 Received 8 November 2007; revised 19 December 2007; accepted 31 December 2007; published 6 February [1] We examine seismic signals from tsunamis for 14 earthquakes of Mw 7.1 to 8.4 in the circum-pacific region that generated tsunamis measured previously from tide gauges or coastal runup. Low-frequency signals of 0.5 mhz up to 2 8 mhz are observed at the expected tsunami arrival time on the horizontal components of low elevation seismic stations near coastlines for eight of the 14 earthquakes. Lower amplitude signals are observed for the other six. For two earthquakes (Mw ) we observe a dispersed tsunami signal between 1 and 8 mhz, with lower frequencies arriving earlier. This dispersion is consistent with predictions from the standard model for gravity waves for the bathymetry along these source-station paths. These observations, along with earlier observations from the Indian Ocean, suggest that seismic data could be used to complement tide gauges and ocean bottom pressure recorders to indicate the arrival of a tsunami. Citation: Shields, G., and J. R. Bowman (2008), Seismic signals from tsunamis in the Pacific Ocean, Geophys. Res. Lett., 35, L03305, doi: /2007gl Introduction [2] The great Sumatra-Andaman earthquake (Mw ) on 26 December 2004 [Lay et al., 2005] was the largest in 40 years and since the advent of modern digital geophysical sensor networks. As a consequence a variety of unprecedented signals were observed. In particular, clear dispersive tsunami signals were seen at both hydrophones and coastal seismic stations. Graeber et al. [2005] observed dispersed signals in hydrophone data. Hanson and Bowman [2005] found hydrophone and seismic signals dispersed between 1 and 25 mhz, consistent with predictions from the model of gravity waves. Yuan et al. [2005] observed low-frequency signals below 1 mhz at seven coastal seismic stations corresponding to the expected tsunami arrival time. [3] The distinctive Sumatra tsunami signals motivated investigations of tsunami signals from other events. Hanson et al. [2007] extended their observations to the Nias earthquake of 28 March 2005, for which dispersive tsunami signals were also observed. In addition to extending the number of seismic stations that recorded the Sumatra tsunami to 16, Okal [2007] also found such signals for five other events of magnitude 8.0 or larger, including the Nias earthquake, and three earthquakes in the Pacific Ocean basin. [4] These studies raise the questions of how common these tsunami signals are, and whether they can be seen for 1 Monitoring Systems Division, Science Applications International Corporation, San Diego, California, USA. Copyright 2008 by the American Geophysical Union /08/2007GL events smaller than magnitude 8. The objective of this paper, therefore, is to determine whether seismic signals are recorded for large (Mw ) earthquakes in the Pacific Ocean. If seismic signals from tsunamis are common, then they could be used to confirm the arrival of a tsunami as part of a basin-wide tsunami warning system. 2. Data [5] We selected earthquakes from the Tsunami Event Database of the National Geophysical Data Center through We first looked for events with surface-wave magnitude 7.0 or larger in various Pacific Ocean seismic regions having at least one documented tsunami run-up measurement. We then selected 14 of the 40 events between 1990 and 2004 for analysis (Figure 1 and Table 1) based on magnitude, run-up, and date (recent events have more seismic data available), while also sampling different source regions. The selected events range between Mw 7.1 and 8.4. [6] We consider data from all 18 broadband seismic stations (Figure 1) of the Global Seismic Network in the Pacific Ocean basin that have longitudes between 120 E and 290 E, are less than 10 km from the coast, and have elevations less than 200 m. For each selected event, we request from the IRIS Data Management Center 48 hours of very long period (VLP) data (0.1 Hz sample rate) starting at the event origin time for the nearest eight stations from the list of 18 (excluding stations in the immediate source region). For most events data are only available for between two and five stations, because some stations were not yet installed or were not operational at the time of the events. In some cases quality problems render the returned waveforms unusable. [7] For three of the 14 earthquakes, we also acquire sealevel measurements from tide gauges (Figure 1) near seismic stations from the National Ocean Service (NOS) of the National Oceanic and Atmospheric Administration. These data are used to corroborate the observed seismic tsunami signals. 3. Observations [8] To identify tsunami signals, we filter the horizontalcomponent seismic waveforms in five different frequency bands and compare the signal with expected body-wave (8 km/s) and tsunami (200 m/s) arrival times. Figure 2 shows two examples of seismic tsunami signals. Figure 2 (left) shows an unmistakable signal from a magnitude 8.4 earthquake off the coast of Peru (event 9), which is one of the clearest signals observed. The tsunami from this earthquake caused local runups up to 8 m [Okal et al., 2002], and waves up to 30 cm were observed on tide gauges throughout the Pacific Ocean. Figure 2 (right) shows a less clear signal from a magnitude 7.9 earthquake in the Andreanof Islands L of6

2 Figure 1. Map of the Pacific Ocean showing the 14 analyzed events (red circles) and the pool of candidate seismic stations (triangles). Green triangles indicate sites from which tide gauge data were also retrieved. Event numbers correspond to entries in Table 1. of Alaska (event 4). Local tide gauges recorded up to 1 m of runup from the tsunami (at Adak, Alaska), while tide gauges on the west coast of the United States and various Pacific islands observed runups of a few centimeters. For both events, the tsunami signal only appears on the horizontal components. [9] Figure 3 shows the best single seismic signal from each of the 14 events, aligned on the expected tsunami Table 1. Summary of Tsunamigenic Earthquakes Analyzed and Tsunami Signals Observed a Max Seismometer Tide Gauge # Date Location Mw runup, m # Sta Obs No Obs # Sta Obs No Obs 1 4 Oct 1994 Kuril Islands GUMO RAR Oct 1995 Mexico KIP RAR Dec 1995 Kuril Islands GUMO RAR Jun 1996 Andreanof Islands GUMO ERM Nov 1996 Peru RPN KIP RAR Apr 1997 Solomon Islands GUMO RAR Dec 1997 Kamchatka GUMO KIP Nov 2000 Papua New Guinea KWAJ ERM GUMO JOHN KIP 3 KWAJ GUMO 9 23 Jun 2001 Peru JOHN PAYG H2O 1 JOHN PTCN RAR 10 5 Mar 2002 Philippines GUMO KWAJ PTCN Mar 2002 Taiwan GUMO HNR JOHN KIP Jan 2003 Solomon Islands GUMO KWAJ MIDW RAR Sep 2003 Japan GUMO MIDW KWAJ RAR 4 GUMO MIDW KWAJ Nov 2003 Aleutian Islands KIP KWAJ ERM, Totals a Observations are divided into seismometer and tide gauge. Mw and maximum runup are from the Tsunami Event Database. The # Sta column specifies the number of stations for which usable data were retrieved. The Obs and No Obs columns specify which stations did and did not observe the tsunami signal. The three underlined stations for events 8 and 9 also exhibit dispersion in the spectrograms. We searched for tide gauge data for only four events. 2of6

3 Figure 2. Tsunami signals in filtered three-component seismic waveforms for (left) station JOHN for the Mw 8.4 earthquake off the coast of Peru on 23 June 2001 (event 9), and for (right) station GUMO for the Mw 7.9 earthquake in the Andreanof Islands on 10 June 1996 (event 4). The filter bands used are shown at the top of each panel. Also shown are the expected P-wave (dashed lines) and tsunami (dotted lines) arrival times, based on 8 km/s and 200 m/s, respectively. arrival time. Each waveform is a horizontal component rotated to the direction that maximizes the amplitude of the tsunami signal and is filtered in the band shown along the right. The events are ordered by clarity of the tsunami signal, and do not use the same amplitude scale. Clear tsunami signals are evident in the filtered seismic waveforms from the top eight of the 14 events. For the other six events, less clear signals are seen that are possibly from the tsunami. For most of the 14 events the tsunami signal is most clear in the 2000 to 1000 second period band, which contains the spectral peak of the signals. Some of the signals have energy that extends to higher frequencies, as in Figure 2 (left). [10] For the Mw 9.1 to 9.3 December 2004 Sumatra earthquake, spectrograms showing the dispersed tsunami arrivals provide compelling evidence of the seismic tsunami signals [Hanson et al., 2007]. Traces of high energy observed in the spectrograms up to 25 mhz (40 s) and more than 24 hours after signal onset closely match the dispersion predicted for ocean gravity waves [Hanson et al., 2007]. [11] Spectrograms of the horizontal components stacked together, with the predicted dispersed arrival curves overlain [e.g., Lamb, 1932], are used to look for evidence of dispersion in the tsunami signals. The spectrogram for station JOHN for the 2001 Peru earthquake in Figure 4 exhibits clear dispersion between 1 and 5 mhz and a less clear signal up to 8 mhz. The 11,000 km distance between the tsunami source and JOHN results in significant dispersion even below 5 mhz, which matches the prediction well. Dispersion is also observed at station PTCN for the Peru earthquake and at KWAJ for the 2000 Papua New Guinea earthquake (#8). However, the other events examined do not appear to have generated enough tsunami energy above 2 3 mhz to produce discernible dispersion in the spectrograms. [12] Sea level data from tide gauges within 30 km of the seismic stations were retrieved for three events. Figure 5 compares the seismic and tide gauge data for the 2001 Peru event (#9) at stations separated by 0.6 km on Johnson Atoll. The sea level data corroborate the presence of a tsunami. The presence and absence of the tsunami signal is consistent between the two data types, except for event 13 at station. This observation lends confidence to the seismic observations of the tsunami signal. [13] Table 1 summarizes the results for all 14 analyzed events. The event numbers in the first column correspond to the numbers on the map in Figure 1 and Figure 3. The first four columns describe the event, including the date, location, moment magnitude, and maximum tsunami run up. The rest of the columns specify the total number of stations for which data were retrieved and list the stations at which the tsunami signal is and is not observed. At least one station for each event contains evidence of the tsunami signal in the filtered seismic waveforms. However, as can be seen in Figure 3, the presence of tsunami signals for the bottom six events is not as clear as for the others. There are three instances in which the signal extends over a wide enough frequency range to exhibit dispersion in the spectrograms (underlined stations in Table 1). 4. Discussion and Conclusion [14] This paper extends recent work that found tsunami signals in seismic data from the great Indonesian earthquakes of 26 December 2004 and 28 March 2005 [Yuan et al., 2005; Hanson and Bowman, 2005; Hanson et al., 2007; Okal, 2007], as well as four other events larger than magnitude 8.0 [Okal, 2007]. We analyze seismic data from 14 earthquakes (Mw ) in the Pacific Ocean region that generated tsunamis measured by tide gauges or coastal runup. We find clear seismic signals at the expected time of arrival for the tsunami for eight events, and less clear signals for six others. The spectrogram for one Mw 8.4 earthquake shows one tsunami signal that disperses over a range of 1 to 3of6

4 Figure 3. Examples of seismic tsunami signals from each of the 14 analyzed events. All waveforms are aligned on the expected tsunami arrival time based on 200 m/s, indicated by the edge of the shaded region. Events are ordered by clarity of the tsumami signal, and event numbers on the left axis correspond to numbers on the map in Figure 1 and Table 1. The station names for each waveform are shown along the left axis, and the filter bands used are shown on the right. 8 mhz and over 18 hours, consistent with predictions from the standard model for gravity waves in deep water. Two other signals exhibit discernable dispersion, but over more limited time and frequency ranges. Earthquakes smaller than Mw 8 may not generate sufficient tsunami energy above 3 5 mhz to be seen above the ambient seismic noise. [15] There are several possible mechanisms by which tsunamis could generate the observed seismic signals. Yuan Figure 4. Spectrogram computed from the stacked horizontal waveform components for the data in Figure 2 (left). The spectrogram is shown twice, with and without the predicted dispersion curve overlain. The color scale indicates SNR in db, according to the color bar on the right. The red regions that span the frequency range correspond to direct seismic arrivals from the main event or aftershocks. 4of6

5 Figure 5. Comparison of seismic and tide gauge data for the 2001 Peru event (#9) at stations separated by 0.6 km on Johnston Atoll. The seismic data in the upper trace are bandpass filtered between 0.5 and 1.4 mhz. The tide gauge data in the lower trace, with 1.4 mhz Nyquist frequency, are highpass filtered at 0.14 mhz (2 hr) to remove the tide signal. The dotted line indicates the expected tsunami arrival time (200 m/s). Separate amplitude scales for each signal are on the right axis. et al. [2005] note that the absence of tsunami energy on the vertical component and the rapid decay from the coast implies that the tsunami induced seismic signal in the Indian Ocean may not be related to a propagating seismic wave. They suggest that tilt of island and coastal areas due to sea level changes from the tsunami is the dominant mechanism. A contributing factor to this mechanism could be harbor oscillations, though dispersion consistent with deep-water propagation would not be expected in this case. On the other hand, analysis of tide gauge data led Van Dorn [1984] to suggest that tide station response is caused primarily by linear normal-mode forcing of the continental shelf by the isotropic tsunami spectrum in deep water. This forcing might couple tsunami energy into the solid earth and produce propagating seismic waves. [16] Okal [2007] supports the idea that the propagating tsunami wave directly causes the observed signals by demonstrating that the seismic moment can be recovered quite accurately from the tsunami signals. To accomplish this, stations that record the tsunami are approximated as ocean-bottom seismometers responding to the progressive tsunami over the ocean basin. In this model, seismic signals represent the response to a deformation of the ocean floor from lateral displacement, tilt, and gravitational potential, and island or coastal structure is ignored. The presence of dispersion in three of our observed signals supports this model, but since most of our signals do not exhibit dispersion, other mechanisms such as described by Yuan et al. [2005] may contribute. [17] Tsunami warning systems typically consist of a rapid warning component that estimates seismic source size using body waves (e.g., M wp ) or surface waves (e.g., M m ) and a second, less rapid component that confirms or refutes the presence of a tsunami through direct observation of sea level changes, as measured by tide gauges or bottom pressure recorders. The second component is important for reinforcing or canceling warnings made on the basis of seismic source size alone. Seismic observations of the tsunami itself, as described here, could provide a complementary method for confirming the presence of a tsunami. Changes in sea level or runup heights might be estimated from the seismic data with careful calibration. [18] Application to tsunami warning would require a very low false-alarm rate. We assess the uniqueness of the tsunami signal by running a detector based on the ratio of the short-term-average amplitude to the long-term-average amplitude. Using a short-term window of 5 hours, a longterm window of 50 hours, and a threshold ratio of 10, the algorithm detects only the tsunami signal for event 9 in the month of that event, and two signals, including the tsunamis, in each of the months containing events 3 and 13. This suggests that the false alarm rate is sufficiently low for these seismic signals to be used to confirm tsunamis if observations from two or more stations are combined. [19] Several factors contribute to the absence of reported observations of seismic signals from tsunamis prior to those from the 2004 Sumatra-Andaman earthquake. First, the signals are in a neglected part of the spectrum with frequencies lower than those commonly analyzed for surface waves (>10 mhz), but higher than those analyzed for normal modes (<0.5 mhz). Second, significant tsunami dispersion is seen in our event set only for earthquakes of Mw greater than 8 and is most clear for great earthquakes that generate tsunamis with energy exceeding background levels at frequencies above 5 10 mhz. Finally, the growth of the Global Seismograph Network in the last 10 to 20 years affords more opportunity to observe these signals throughout the ocean basins than existed previously. [20] Acknowledgments. Jeff Hanson and two anonymous reviewers provided constructive reviews. Seismic data were provided by the IRIS Data Management Center, and sea-level data by the National Ocean Service. References Graeber, F. M., P. Grenard, and K. Koch (2005), Observations at IMS hydrophone stations from the December 2004 Indian Ocean tsunami event, Geophys. Res. Abstr., 7, Hanson, J. A., and J. R. Bowman (2005), Dispersive and reflected tsunami signals from the 2004 Indian Ocean tsunami observed on hydrophones 5of6

6 and seismic stations, Geophys. Res. Lett., 32, L17606, doi: / 2005GL Hanson, J. A., C. Reasoner, and J. R. Bowman (2007), High frequency tsunami signals of the great Indonesian earthquakes of 26 December 2004 and 28 March 2005, Bull. Seismol. Soc. Am., 97, S232 S248, doi: / Lamb, H. (1932), Hydrodynamics, 6th ed., 738 pp., Cambridge Univ. Press, New York. Lay, T., et al. (2005), The great Sumatra-Andaman earthquake of 26 December 2004, Science, 308, , doi: /science Okal, E. (2007), Seismic records of the 2004 Sumatra and other tsunamis: A quantitative study, Pure Appl. Geophys., 164, Okal, A., et al. (2002), Field survey of the Camana, Peru tsunami of June 23, 2001, Seismol. Res. Lett., 73, Van Dorn, W. G. (1984), Some tsunami characteristics deducible from tide records, J. Phys. Oceanogr., 14, Yuan, X., R. Kind, and H. A. Pedersen (2005), Seismic monitoring of the Indian Ocean tsunami, Geophys. Res. Lett., 32, L15308, doi: / 2005GL J. R. Bowman and G. Shields, Monitoring Systems Division, Science Applications International Corporation, Campus Point Drive A-3, San Diego, CA 92121, USA. (shieldsg@saic.com) 6of6