Tsunami Early Warning Within Five Minutes
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- Lomax, A. & Michelini, A. Pure Appl. Geophys. (2013) 170: 1385. doi:10.1007/s00024-012-0512-6
Tsunamis are most destructive at near to regional distances, arriving within 20–30 min after a causative earthquake; effective early warning at these distances requires notification within 15 min or less. The size and impact of a tsunami also depend on sea floor displacement, which is related to the length, L, width, W, mean slip, D, and depth, z, of the earthquake rupture. Currently, the primary seismic discriminant for tsunami potential is the centroid-moment tensor magnitude, MwCMT, representing the product LWD and estimated via an indirect inversion procedure. However, the obtained MwCMT and the implied LWD value vary with rupture depth, earth model, and other factors, and are only available 20–30 min or more after an earthquake. The use of more direct discriminants for tsunami potential could avoid these problems and aid in effective early warning, especially for near to regional distances. Previously, we presented a direct procedure for rapid assessment of earthquake tsunami potential using two, simple measurements on P-wave seismograms—the predominant period on velocity records, Td, and the likelihood, T50Ex, that the high-frequency, apparent rupture-duration, T0, exceeds 50–55 s. We have shown that Td and T0 are related to the critical rupture parameters L, W, D, and z, and that either of the period–duration products TdT0 or TdT50Ex gives more information on tsunami impact and size than MwCMT, Mwp, and other currently used discriminants. These results imply that tsunami potential is not directly related to the product LWD from the “seismic” faulting model, as is assumed with the use of the MwCMT discriminant. Instead, information on rupture length, L, and depth, z, as provided by TdT0 or TdT50Ex, can constrain well the tsunami potential of an earthquake. We introduce here special treatment of the signal around the S arrival at close stations, a modified, real-time, Mwpd(RT) magnitude, and other procedures to enable early estimation of event parameters and tsunami discriminants. We show that with real-time data currently available in most regions of tsunami hazard, event locations, mb and Mwp magnitudes, and the direct, period–duration discriminant, TdT50Ex can be determined within 5 min after an earthquake occurs, and T0, TdT0, and Mwpd(RT) within approximately 10 min. This processing is implemented and running continuously in real-time within the Early-est earthquake monitor at INGV-Rome (http://early-est.rm.ingv.it). We also show that the difference mb − log10(TdT0) forms a rapid discriminant for slow, tsunami earthquakes. The rapid availability of these measurements can aid in faster and more reliable tsunami early warning for near to regional distances.
KeywordsEarthquake tsunami early warning earthquake location real-time seismology body waves
Tsunamis are most destructive at near to regional distances (i.e. <1,000 km) from the causative earthquake, arriving within 20–30 min after the earthquake origin time (OT); effective early warning at these distances requires notification within 15 min or less after OT (Tsushimaet al. 2011; Newmanet al. 2011; Madlazim2011). Currently, rapid assessment of the tsunami potential of an earthquake by organizations such as the Japan Meteorological Agency (JMA), the German–Indonesian tsunami early warning system (GITEWS), or the West Coast and Alaska (WCATWC) and Pacific (PTWC) Tsunami Warning Centers relies mainly on initial estimates of earthquake location, depth, and moment, M0, or the corresponding moment magnitude, Mw. For the regional scale, WCATWC and PTWC issue warning messages within approximately 5–10 min after OT for shallow, underwater events using the P-wave moment-magnitude discriminant, Mwp (Tsuboiet al. 1995, 1999) if Mwp ≥ 7.5 (e.g., Hirshorn and Weinstein2009).
M0 is of interest for tsunami warning because the efficiency of tsunami generation by a shallow earthquake depends on the amount of sea floor displaced, which can be related to a finite-faulting model expressed by the seismic potency, LWD, where L is the length, W the width, and D the mean slip of the earthquake rupture (Kanamori1972; Abe1973; Kajiura1981; Lay and Bilek2007; Polet and Kanamori2009). Then, because M0 = μLWD, where μ is the shear modulus at the source, the sea-floor displacement and thus tsunami potential should scale with LWD = M0/μ.
Mw is found to be a good discriminant for many past tsunamigenic earthquakes, but not all, especially not for slow “tsunami earthquakes” (TsE), which, by definition, cause larger tsunami waves than would be expected from their Mw (Kanamori1972; Satake2002; Polet and Kanamori2009; Newmanet al. 2011). The discrepancy for these earthquakes can be related to rupture at shallow depth where μ can be very low, an-elastic deformation, for example ploughing and uplift of sediments may occur, and the fault surface may be non-planar with splay faulting into the accretionary wedge (Fukao1979; Mooreet al. 2007; Lay and Bilek2007). Additionally, a reliable estimate of Mw for large earthquakes is usually provided by a centroid-moment tensor magnitude, MwCMT (Dziewonskiet al.1981; Ekströmet al.2005), which requires waveform inversion, varies with rupture depth, earth model, and other factors, and is only available 20–30 min or more after an earthquake (Hayeset al. 2011; Duputelet al. 2011). Thus rapid magnitude estimators such as Mwp are used for tsunami warning, but Mwp performs poorly compared with MwCMT and other discriminants for tsunami potential (Lomax and Michelini2011a, LM2011 hereinafter).
To avoid these problems and aid in effective early warning, especially for near to regional distances, we suggest the use of more direct and rapid discriminants for tsunami potential and for identification of TsEs. We have presented (Lomax and Michelini2009b; LM2011; Lomax and Michelini2011b) a direct procedure for rapid assessment of earthquake tsunami potential using two, simple measurements on P-wave seismograms—the predominant period on velocity records, Td, and the likelihood, T50Ex, that the high-frequency, apparent rupture-duration, T0, exceeds 50–55 s. T0 for large earthquakes is related primarily to rupture length, L, and both Td and T0 will increase as rupture depth, z, decreases, because of the effects of a reduced shear modulus and rupture velocity, vr. We show in LM2011 that either of the period–duration products TdT0 or TdT50Ex gives more information on tsunami impact and size than MwCMT, Mwp, Mwpd (Lomax and Michelini2009a, LM2009A hereinafter), and other currently used discriminants. These results imply that tsunami potential is not directly related to the product LWD from the “seismic” faulting model, as is assumed with the use of Mw discriminants, and suggest instead that information on rupture length and depth can constrain well the tsunami potential of an earthquake. This information on rupture length and depth is provided by TdT0 and TdT50Ex, in which case explicit estimates of rupture length and depth, which are difficult or impossible to determine rapidly, are not required.
We introduce here special treatment of the signal around the S arrival at close stations, a modified, real-time, Mwpd magnitude (Mwpd(RT)), and other procedures to enable early estimation of multiple event parameters. We show that, with current, real-time seismogram data, rapid calculation of the direct, period–duration discriminant, TdT50Ex, can be completed within 5 min after OT, and calculation of T0, TdT0, and Mwpd(RT) magnitudes within approximately 10 min. We also show that the difference mb − log10(TdT0) is a useful, rapid discriminant for TsEs. We illustrate that the rapid availability of these measurements can aid in faster and more reliable tsunami early warning for near to regional distances.
2 Rapid, Direct Assessment of Tsunami Potential
We define the dominant period, Td, for an event as the median of the dominant period values for each station given by the peak of the τc algorithm (Nakamura1988; Wu and Kanamori2005) applied with a 5 s sliding time-window from 0 to 55 s after the P arrival on velocity seismograms (Fig. 2; LM2011). Then, taking T50Ex as a substitute for T0, the TdT0 discriminant for tsunami potential becomes TdT50Ex (i.e., TdT50Ex ≥ 8.0 s).
For distances less than 30° significant S signal may remain on the HF seismograms (LM2009A). Consequently, if the raw, HF duration (T0raw) determined following LM2011 ends later than (TS − TP)/4 after TS, we assume that the HF signal after TS is primarily S wave energy, and that a best estimate of T0 is measured starting at TS. That is, if T0raw > (TS − TP) + (TS − TP)/4 then T0 = T0raw − (TS − TP), (Fig. 2).
To prevent including S energy in the magnitude estimates, the Mwp and Mwpd analysis windows are truncated at TS.
applying the depth correction but no other geometrical or attenuation corrections from LM2009A; and
applying the moment correction of LM2009A to all event types if T0 > 90 s; to prevent discontinuous jumps in magnitude, this correction is applied using a linear ramp, weight factor from 0 at 90 s to 1.0 at and above 110 s.
We identify this magnitude as Mwpd(RT).
We also calculate mb on simulated WWSSN-SP velocity traces, following the Vmax procedure of Bormann and Saul (2008), we identify this magnitude as mb(Vmax).
The processing procedures discussed here are implemented and running continuously in real-time within the Early-est software and earthquake monitor at INGV-Rome (http://early-est.rm.ingv.it), including rapid calculation of event locations, TdT50Ex and mb(Vmax) and Mwp magnitudes within approximately 5 min, and T0, TdT0, Mwpd(RT) magnitudes and P first-motion, fault-plane solutions within approximately 10 min.
3 Application to Recent Large Earthquakes
We consider a reference set of 120 large earthquakes (6.4 ≥ Mw ≥ 9.1; 103 shallow, under water) since 1992, when high-quality, broadband seismograms became widely available, along with measurements of the impact and size of any generated tsunamis (Table S1). This reference set includes most tsunamigenic earthquakes listed in the NOAA/WDC Historical Tsunami Database (http://www.ngdc.noaa.gov/hazard/tsu_db.shtml), most events of Mw ≥ 7 in the past few years, and several events of regional importance.
As a measure of tsunami impact we define an approximate measure of tsunami importance, It, for the reference earthquakes based on 0–4 descriptive indices of tsunami effects and maximum water height h in metres from the NOAA/WDC database (see LM2011 for details). It is approximate because it is highly dependent on the instrumentation available, coastal bathymetry, and population density in the event region. It ≥ 2 corresponds approximately to the JMA threshold for issuing a “Tsunami Warning”; the largest or most devastating tsunamis typically have It ≥ 10.
Tsunami discrimination results
Available (min after OT)
It ≥ 2
It < 2
It ≥ 2
It < 2
Results at OT + 15 minb
Results at OT + 5 minc
For the reference events, the period–duration discriminant TdT0 shows a better correspondence with It than either MwCMT or Mwp (Fig. 3). At OT + 15 min both TdT0 and TdT50Ex correctly identify 72–74 % of tsunamigenic events with It ≥ 2 (Table 1), more than the Mwp(44 %), MwCMT (68 %), and T0 (68 %) discriminants, with fewer false positive identifications of events with It < 2. All discriminants are poor at identifying the tsunami potential of oceanic, strike-slip events, and give mixed results for back-arc intraplate earthquakes (see LM2011 for details). At OT + 5 min, with fewer available events (because of lack of data, mainly for earlier events), correct classification of events by the TdT50Ex discriminant is approximately the same as at OT + 15 min. These results show that use of the TdT50Ex discriminant for tsunami potential at OT + 5 min and perhaps earlier should be reliable and useful, especially as more real-time stations become available in regions of high tsunami hazard.
In LM2011 the tsunami discriminants are also compared using a physical measure of tsunami size—a tsunami wave amplitude at 100 km distance from the source, At, estimated from water height data in the NOAA/WDC database. As above for tsunami impact, It, comparisons in LM2011 show that TdT0 or TdT50Ex give more information on tsunami size, At, than MwCMT, Mwp and other currently used discriminants (Table S1). Moreover, there is indication of a linear, and thus possibly a physical, relationship between log(TdT0) and At.
4 Example Timelines for Assessment of Tsunami Potential
For the 2009 Tonga event the PTWC (2009) issued a tsunami warning at OT + 12 min based on a magnitude measure of M 7.7; the tsunami warning was cancelled 1.5 h later. This earthquake generated a mild, non-destructive tsunami with observed water heights mostly less than 0.1 m (NOAA/WDC database). For this event (Fig. 5), Early-est determines at OT + 4 → 5 min the epicentre with an error of approximately 35 km, along with Mwp 7.6 and at OT + 7 min Mwpd(RT) 7.7, both in good agreement with the final MwCMT. At OT + 5 → 7 min, all three discriminants for tsunami potential, TdT50Ex, T0, and TdT0 are available and indicate a low tsunami potential for this event. All Early-est measurements have stabilized to near-final values within OT + 8 → 10 min.
For the devastating 2011 Tohuku earthquake and tsunami JMA first issued a tsunami warning at approximately OT + 3 min, but a near-final Mw and the true size of this event were not determined until OT + 20 → 30 min (Ozaki2011; Hayeset al. 2011). For this earthquake (Fig. 6), Early-est determines the epicentre at OT + 2 → 3 min with an error of approximately 60 km, Mwp 8.0–8.3 at OT + 4 → 6 min, and Mwpd(RT) 9.0–9.2 at OT + 6 → 8 min. These epicentre and Mwp results compare favourably with the JMA and NEIC response timelines (Ozaki2011; Hayeset al. 2011), and the near-final Mwpd(RT) determination precedes the earliest MwCMT estimation (NEIC Mww) by approximately 12 min. The TdT50Ex discriminant for tsunami potential is available at OT + 4 → 5 min and the T0 and TdT0 measurements are available at OT + 6 → 7 min; all three of these discriminants indicate a very high likelihood that a tsunami was generated. Almost all Early-est measurements have stabilized to near-final values within OT + 7 → 8 min, one exception being the epicentre, which only stabilizes at OT + 9 → 10 min because of the lack of station coverage off-shore to the east of the epicentre. For this event it is notable that the Mwpd(RT) 9.2 magnitude and T0 = 160 s duration estimates provide early information about the true size and extent of the earthquake rupture and tsunami source; these rapid measurements should be useful during future large earthquakes not only for tsunami warning, but also for early shake-map, finite-fault, and tsunami forecast modelling.
The 2010 Mentawai earthquake generated a large and destructive, local tsunami and has been identified as a TsE (Newmanet al. 2011; Madlazim2011). The timeline for Early-est characterization of this event (Fig. 7) is similar to that for 2011 Tohoku, except that the Mwp, Mwpd(RT), and TdT50Ex values are available approximately 1 min earlier, at OT + 3 → 4 min, and the epicentre is better constrained in the first minutes. These differences are mainly because of denser station coverage near this event, though there is still a lack of stations off-shore to the southwest of the epicentre. All three discriminant for tsunami potential, TdT50Ex at OT + 3 → 4 min, and T0 and TdT0 at OT + 5 → 6 min, indicate a high likelihood that a tsunami was generated. All measurements have stabilized to near their final values within OT + 7 → 8 min, although the large, early values for TdT50Ex indicate that the determination of this measure for near stations might be improved. For this event, in addition to early indication of high tsunami potential at OT + 3 → 6 min, Early-est gives at OT + 6 → 9 min a stable Mwpd(RT) 7.8–7.9 that matches final MwCMT, and mb − log10(TdT0) ≈ 3.0, suggesting this event is a TsE.
We have presented rapid determination of tsunami potential by use of two direct and simple measurements on P-wave seismograms, the predominant period, Td, and the likelihood, T50Ex, that the T0, exceeds 50–55 s (LM2011). We have also introduced a modified, real-time, Mwpd(RT) magnitude, and special treatment of the signal around the S arrival at close stations to enable early estimation of all event parameters. We find that either of the period–duration products TdT0 or TdT50Ex give more information on tsunami impact and size than MwCMT, Mwp, and other currently used discriminants. This result follows from T0 and Td being most sensitive to rupture length, L, and depth, z, which determine total seafloor uplift and tsunami potential.
We show that the TdT50Ex discriminant can be obtained within 5 min after an earthquake occurs with real-time data currently available in most regions of tsunami hazard. We also show that other critical event parameters can be obtained within 5–10 min, including: the T0 and TdT0 tsunami potential discriminants, an Mwpd(RT) that matches closely final MwCMT, and the difference mb − log10(TdT0) which is a rapid discriminant for slow, tsunami earthquakes. The rapid availability of these direct and simple measurements can aid faster and more reliable tsunami early warning for near to regional distances.
We thank two reviewers for critical comments that greatly improved the clarity of the manuscript. This work is supported by INGV—Centro Nazionale Terremoti institutional funds and by the EC n.262330 NERA 2010-2014 project. The IRIS DMC (http://www.iris.edu) and GFZ Data Archive (http://geofon.gfz-potsdam.de) provided access to waveforms used in this study.