Earthquake Detec-on and Loca-on using Cascadia Ini-a-ve data. Susan L. Bilek and Emily Morton Earth and Environmental Science Dept.

Transcription

1 Earthquake Detec-on and Loca-on using Cascadia Ini-a-ve data Susan L. Bilek and Emily Morton Earth and Environmental Science Dept. New Mexico Tech

2 Mo-va-on Use unique Cascadia Ini-a-ve dataset to explore subduc-on zone and broad spectrum of fault slip Cascadia subduc-on zone: host of past great earthquakes, small earthquakes, LFEs, tremor, slow slip Ques-ons about processes and condi-ons needed for the range of slip behaviors For answers, need understanding of spa-al palerns of diverse slip behaviors On- shore/off- shore seismic network cri-cal to recording the smaller events, used for loca-on and source parameter determina-on

3 50 45 OBS Stations: Year 1 (7/25/11-8/7/12) Year 2 (7/14/12-8/10/13) Year 3 (8/2/13-7/3/14) Year 4 (7/11/14-9/1/15) Years 1 & 2 Years 1 & 3 Years 2 & 4 Years 1-3 Years 1, 2, & 4 All 4 Years Objec-ves Iden-fy earthquakes to contribute to catalog of events Locate events, compare with regional geology/tectonics/ tremor/slow slip CI Affiliated Land Stations Determine source parameters for repea-ng earthquakes

4 Modified from Harris and Paik [2006] Event Detec-on Methodology Scanning Waveform Waveform 1 Waveform 2 M08A Correlation Coefficient = Correlation Coefficient = Waveform cross- correla,on Use template waveform to cross- correlate with con-nuous data at given sta-on Similar -me windows highlighted by large correla-on coefficient Subspace Detec,on Assume signal (s) is described by linear combina-on of basis vectors that span subspace (U) with weights (a) Use level of match between windowed data and best approxima-on of windowed data by subspace to make detec-on Sta-on: M08A Used to detect LFEs in Japan [Maceira et al., 2010]

5 Analysis Strategy Iden-fy possible subduc-on zone events in all na-onal/regional catalogs recorded with land- based sta-ons ANF/UW/NEIC/CNDC catalogs Define as template events Find corresponding event in OBS record if possible Waveform used as template for specific OBS Mul-ple templates used in subspace detector

6 50 Year 1 Earthquake Templates 95 events chosen as templates for year 1 scanning 45 Results shown here focus on 4 template events located near OBS sta-ons J25A and M08A offshore central Oregon J25A M08A Template events in subspace located near area of previously iden-fied megathrust/seamount earthquakes [Tréhu et al., 2008, 2012; 2015; Williams et al., 2011], in some cases templates are previously iden-fied events from Tréhu et al [2015] Stations (Year 1) Earthquakes During Year 1, M Earthquakes Chosen as Templates Plate Boundaries

7 Central Oregon Templates M08A x10 4 Digital Counts x10 3 x10 4 x10 5 P S 2 M08A BHZ Mar 12, :37:22 26 km M 3.1 Mar 20, :43:47 30 km M 3.2 Mar 20, :28:37 33 km M 3.7 Mar 26, :15:53 5 km M 1.9 Template waveforms cut to 15 s windows following P wave, high- pass filtered with corner at 5 Hz All 4 templates used in subspace; subspace was not truncated Time (s)

8 October M08A Z- sta-s-c used to determine threshold for posi-ve detec-on. Large peak at low values = background noise. Fit normal PDF to z- sta-s-cs, choose detec-on threshold where PDF nearly 0 (example, October 2012, threshold = 0.05, PDF = 4x10-60 )

9 Example - M08A Z-Statistic One week (October 1-7, 2012) results from subspace detector Event detec-ons at spikes Threshold of z- sta-s-c = 0.05 applied x10 7 Sample Position During this week, 2474 detec-ons, 498 aler elimina-ng mul-ple detec-ons within sliding window. Aler review, 5 were real events

10 Time (s) Example Detec-ons: Central Oregon Cluster Subset (31 of 175) of detected events on M08A during year 1, aligned on P wave

11 Year 1 Detec-ons/Associa-ons Central Oregon Cluster OBS sta-ons: M08A: 4 templates, 175 detec-ons (Oct 21, July 17, 2012) J25A: 4 templates, 80 detec-ons (Oct 21, July 18, 2012) Land sta-on: I02D: 5 templates, 48 detec-ons (July 26, July 21, 2012) 4 of the 5 templates are in Tréhu et al. [2015] repea-ng earthquake sequence Total detected & associated unique events: 263

12 Earthquake Loca-ons Loca-on Algorithm: Hypoinverse2000 (USGS- Klein, 2013 version) Vp Velocity model: ORWELL1 (used in Tréhu et al., 2015, Williams et al., 2011) Based on seismic lines Depth (km) ORWELL in Gerdom et al. [2000] Velocity (km/s)

13 Earthquake Loca-ons Picks: used nearest 10 sta-ons, picked P and S arrivals where possible, phase weigh-ng for quality Of the 263 detected/associated events 154 had picks only on 1-2 sta-ons, 6 only 3 picks 103 located

14 Example Event: May 17, 2012 Set of 10 sta-ons used for events in this cluster. P and S arrivals picked where possible and weighted. Some of OBS have higher data quality than coastal land sta-ons

15 50 Earthquake Loca-ons: Central Oregon Cluster 106 events located, majority between trench and coastline Epicentral errors: median 6.3 km Depths: s-ll being refined Number of Events Horizontal Error (km)

16 Earthquake Loca-ons: Central Oregon Cluster Lowest epicentral errors in cluster within array 10 sta-ons used for ini-al loca-ons well distributed around templates and newly located events

17 Majority of new events found occur in area of previously iden-fied thrust events [e.g, Williams et al.,2011; Tréhu et al., 2012; 2015] that may be related to seamount interac-on with overriding plate Tréhu et al., 2012

18 Ini-al Results Summary Over 260 unique events detected in central OR cluster during year 1 based on subspace detec-on using 4 template events 103 events located, majority near previously iden-fied cluster possibly related to seamount interac-on Addi-onal OBS sta-ons in this area would have been beneficial, as we lost over 50% of unique detec-ons because of too few phase picks available for loca-on

19 50 Ongoing Work Currently refining loca-ons, including depth Use subspace detector on the remaining template events for Year 1, verifying detec-ons, and loca-ng events 45 Con-nue with Years YR1 Templates 125